MS Nahema Ledard

BS Emilie Artru

Patricia Colmenarez Sayago

MD, PhD Stefania Redolfi

MD, PhD Jean-Louis Golmard

MD Marisol Carrillo-Solano

MD, PhD Isabelle Arnulf
email: isabelle.arnulf@aphp.fr

; Patricia

MD Colmenarez Sayago

Adrenergic reactions during N3 sleep arousals in sleepwalking and sleep terrors: The chicken or the egg?

Keywords: Sleepwalking, night terrors, parasomnia, autonomic system, tachycardia, nightmare

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Patients with sleepwalking episodes or sleep terrors (SW/ST) exhibit sudden abnormal behaviours arising from sleep, mostly from N3 sleep (American Academy of Sleep Medicine, 2014). Patients usually open their eyes, look around with a confused gaze, sit, stand, walk, talk, scream or flee their bed [START_REF] Derry | NREM arousal parasomnias and their distinction from nocturnal frontal lobe epilepsy: A video EEG analysis[END_REF].

There is a continuum between the different behavioural patterns of arousal parasomnias. Specifically, the behavioural patterns of NREM parasomnias appear with a hierarchical order, with arousal behaviours being the fundamental component, possibly followed by abnormal agitated conducts which can be further accompanied by distressed emotional manifestations [START_REF] Derry | NREM arousal parasomnias and their distinction from nocturnal frontal lobe epilepsy: A video EEG analysis[END_REF]. Several clinical, polygraphical and brain functional imaging studies support the concept that these behaviours occur during local (primarily thalamo-amygdalo-cingulo-cortical) arousals from N3 sleep, disengaged from the control of the prefrontal and frontal associative cortex [START_REF] Bassetti | SPECT during sleepwalking[END_REF][START_REF] Terzaghi | Evidence of dissociated arousal states during NREM parasomnia from an intracerebral neurophysiological study[END_REF][START_REF] Terzaghi | Dissociated local arousal states underlying essential clinical features of non-rapid eye movement arousal parasomnia: an intracerebral stereo-electroencephalographic study[END_REF][START_REF] Flamand | Confusional arousals during non-rapid eye movement sleep: evidence from intracerebral recordings[END_REF]. The mental content is poor, often amnestic, but adult patients occasionally report a brief visual scene, containing imminent danger or misfortune [START_REF] Schenck | A polysomnographic and clinical report on sleep-related injury in 100 adult patients[END_REF][START_REF] Oudiette | Dreamlike mentations during sleepwalking and sleep terrors in adults[END_REF][START_REF] Zadra | Somnambulism: clinical aspects and pathophysiological hypotheses[END_REF] and sometimes longer scenarios [START_REF] Uguccioni | Fight or flight? Dream content during sleepwalking/sleep terrors vs rapid eye movement sleep behavior disorder[END_REF]. Regular episodes of SW/ST persists in 2-4% of adults [START_REF] Ohayon | Night terrors, sleepwalking, and confusional arousals in the general population: their frequency and relationship to other sleep and mental disorders[END_REF][START_REF] Ohayon | Prevalence and comorbidity of nocturnal wandering in the U.S. adult general population[END_REF] and leads to a high risk of self-induced injury or violence to other persons, disturbed sleep, fatigue, excessive daytime sleepiness, shame, anxiety and altered quality of life [START_REF] Lopez | Functional impairment in adult sleepwalkers: a case-control study[END_REF][START_REF] Zadra | Somnambulism: clinical aspects and pathophysiological hypotheses[END_REF][START_REF] Arnulf | A scale for assessing the severity of arousal disorders[END_REF][START_REF] Carrillo-Solano | Sleepiness in sleepwalking and sleep terrors: a higher sleep pressure?[END_REF].

The cause of SW/ST remains unknown, although familial vulnerability and factors that increase sleep depth (e.g., sleep debt or fever) and that cause arousals during N3 sleep (e.g., noise or contact) promote abnormal behaviours [START_REF] Pressman | Factors that predispose, prime and precipitate NREM parasomnias in adults: Clinical and forensic implications[END_REF]. However, most N3 arousals appear to be spontaneous in SW/ST, including no external triggers, and no change preceding the arousal whether in the surface EEG [START_REF] Schenck | Analysis of polysomnographic events surrounding 252 slow-wave sleep arousals in thirty-eight adults with injurious sleepwalking and sleep terrors[END_REF][START_REF] Gaudreau | Dynamics of slow-wave activity during the NREM sleep of sleepwalkers and control subjects[END_REF][START_REF] Pilon | Hypersynchronous delta waves and somnambulism: brain topography and effect of sleep deprivation[END_REF][START_REF] Jaar | Analysis of slow-wave activity and slowwave oscillations prior to somnambulism[END_REF] or stereo EEG [START_REF] Terzaghi | Dissociated local arousal states underlying essential clinical features of non-rapid eye movement arousal parasomnia: an intracerebral stereo-electroencephalographic study[END_REF]. Nevertheless, rare hypersynchronous delta waves [START_REF] Guilleminault | Sleep and wakefulness in somnambulism: a spectral analysis study[END_REF] and relatively short-duration increases in slow wave oscillations in the two minutes preceding behavioural (but not quiet) N3 interruptions have been found in patients with arousal disorders [START_REF] Espa | Sleep architecture, slow wave activity, and sleep spindles in adult patients with sleepwalking and sleep terrors[END_REF].

In addition, recent EEG tools allowed to observe more gradual and local changes in the brain activity, within up to 20 sec before a confusional arousal. The activity of the primary motor cortex and cingulate cortex increased 5 sec before the onset of confusional arousal in stereo EEG in an adult patient [START_REF] Terzaghi | Evidence of dissociated arousal states during NREM parasomnia from an intracerebral neurophysiological study[END_REF]. There was an "arousing" (increased beta band) brain activation in the cingulate motor area during the 4 sec preceding the N3 interruption, using current density EEG in 20 adults with arousal disorders [START_REF] Januszko | Sleepwalking episodes are preceded by arousal-related activation in the cingulate motor area: EEG current density imaging[END_REF]. Changes in EEG functional connectivity were also observed in the 20 sec preceding a parasomniac event (compared to 2 min before the event), suggesting that both arousing (increased functional connectivity in the alpha and beta bands over a wide anteroposterior network) and sleep (increased delta and theta power) processes coexisted before the parasomniac behaviour [START_REF] Desjardins | EEG Functional Connectivity Prior to Sleepwalking: Evidence of Interplay Between Sleep and Wakefulness[END_REF]. In 19 confusional arousals monitored with stereo EEG, [START_REF] Flamand | Confusional arousals during non-rapid eye movement sleep: evidence from intracerebral recordings[END_REF] identified an increase in delta activity, predominantly in the frontal regions, in the last few seconds before behaviour onset.

When looking at cardiac and respiratory activity surrounding ST episodes in adults, Fisher et al. observed that the episodes of tachycardia typically arose from "physiological quiescence, with no gradual build-up" [START_REF] Fisher | A psychophysiological study of nightmares and night terrors: physiological aspects of the stage 4 night terror[END_REF]. ST episodes but also SW episodes and confusional arousals are commonly associated with increased sympathetic arousal [START_REF] Schenck | Analysis of polysomnographic events surrounding 252 slow-wave sleep arousals in thirty-eight adults with injurious sleepwalking and sleep terrors[END_REF]. However, it was unclear whether autonomic arousal preceded or accompanied the behavioural episode. The heart rate increased one heart beat interval before the phases of transitory activation in N3 (phases which included EEG changes, eye movements, increased respiration or EMG activation) in 10 sleepwalkers [START_REF] Busek | Spectral analysis of the variations in heart rate and cardiac activation on waking up in sleepwalking[END_REF]. Therefore, we aimed at determining more precisely whether the autonomic arousal preceded (the "egg") or accompanied (the "chicken") the behavioural episode. For this purpose, we measured the profile of heart rate changes, prior to and during N3 arousals in patients with SW/ST and in controls. In addition, as the respiratory reaction and vascular tone changes had not been quantified in these three previous studies, we also measured the profile of respiratory rate and amplitude changes, as well as vascular tone, prior and during the pathological and non-pathological arousals in N3 sleep.

METHODS

Participants

We recruited 20 consecutive patients referred to the Sleep Disorder Unit for SW, ST, or both. They met the general diagnostic criteria for disorders of arousal (American Academy of Sleep Medicine, 2014), which include (i) recurrent episodes of incomplete awakening from sleep, (ii) inappropriate or absent responsiveness to efforts of others to intervene or redirect the person during the episode, (iii) limited or no associated cognition or dream imagery, (iv) partial or complete amnesia for the episode and (iv) the disturbance was not better explained by another sleep disorder, mental disorder, medical condition, medication, or substance use. In addition, SW was defined as (i) a history of ambulation during sleep, and (ii) the persistence of sleep or impaired judgment during ambulation. Sleep terror was defined as (i) a history of sudden episodes of terror occurring during sleep, usually initiated by a cry or loud scream, with sympathetic and behavioural manifestations of intense fear; and

(ii) difficulty in arousing the person, mental confusion when awakened from an episode, complete or partial amnesia of the episode, or dangerous behaviours. Additional inclusion criteria were (i) at least one motor episode occurring during N3 sleep, and (ii) no treatment for more than one month. The control group comprised 20 healthy subjects who had visited the sleep disorder unit to take part in various trials. Only healthy subjects with at least one awakening episode during N3 sleep without any treatment for more than 1 month and without any sleep disorders were included. These subjects were matched for age and sex with the SW/ST patients.

The patients gave their written consent to take part in the study, which was approved by the ethics committee Ile de France-06 as part of a larger study on hypnosis in sleepwalking (NCT02648568 on clinicaltrial.gov). The healthy subjects also signed a consent (depending on the study they were control for, studies which had been approved by the ethics committee Ile de France-06), which allowed the investigators to reuse their data in other researches.

Evaluations

Participants had a medical interview with a sleep specialist and completed the Epworth sleepiness scale [START_REF] Johns | A new method for measuring daytime sleepiness: the Epworth Sleepiness Scale[END_REF], and the Paris Arousal Disorder Severity Score [START_REF] Arnulf | A scale for assessing the severity of arousal disorders[END_REF]. Patients underwent a videopolysomnography during two consecutive nights, whereas controls underwent a single night. Only the first night was used for this study. The videopolysomnography included recordings from 9 EEG leads (Fp1, C3, T3, O1, Fp2, C4, T4, O2, and A2), two bilateral electro-oculograms, surface EMG electrodes of the mentalis and right and left anterior tibialis muscles, nasal pressure, tracheal sounds, chest and abdomen movements measured via plethysmography, electrocardiogram (EKG), pulse oximeter and infrared video and audio (Medatec Ltd, France). The sleep stages and the motor and respiratory events were scored by experienced neurologists following standard methods [START_REF] Iber | The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specifications[END_REF]. N3 arousals were defined as "any increase in EMG on the chin channel, which is accompanied by a change in pattern on any additional channel" [START_REF] Rechstchaffen | A manual of standardized terminology, techniques and scoring system for sleep stages of human subjects[END_REF]) lasting for more than 3 s. The classical definition of arousal (any increase in EEG rhythm lasting more than 3 s) could not be used because it did not fit with the N3 arousals, which may combine an apparently awake behaviour and a hypersynchronous, desynchronized or mixed (on the same or on different channels) EEG rhythm. For the same reasons, the exact time of the N3 arousal was set at the precise moment of sudden chin muscle tone enhancement (30% greater than stable, previous muscle tone). The associated behaviour was analyzed on the video/audio clip. If there were no visible movements or quiet, routine movements (scratching the nose, touching the nasal sensor, repositioning oneself, all behaviours performed with closed eyes), the episode was qualified as a "quiet arousal". If the motor behaviour was sudden, including an immediate opening of the eyes, staring or looking around, raising head or torso, any facial or vocalizations expression of fear or surprise, sitting in bed, standing, or screaming, the episode was qualified as "parasomniac arousal".

Although not formally measured, the duration of quiet arousals was similar in patients and in healthy controls and around 15 s, with increased chin muscle activity at the beginning of the arousal in both cases.

The autonomic (heart rate, respiration and vessel tone) changes concomitant to each N3 arousal were measured as indicated in Figure 1. The instantaneous heart rate was automatically calculated by the Compumedics Profusion Software, calculation which is based on the pulse wave signal from the oximetry and not on the EKG. The absence of artefactual measures was checked via manual measure of the EKG signal. Pulse oximetry wave amplitude was manually measured, wave by wave, as the minimum to maximum of each wave of the percutaneous pulse signal. This signal is automatically calibrated by the computer and is used here as a relative measure (prior to and during the arousal), proper to each subject and each episode of arousal. This is a finger plethysmography signal, which can be considered as a surrogate measure of blood pressure. A decrease in pulse wave amplitude indicates a vasoconstriction. Only arousals with no concomitant movement of the finger (as check on the concomitant video) were kept in the analysis, to ensure that the signal changes were not artefacts of movement. Heart rate and pulse wave amplitude were measured beat to beat on a segment comprising 10 beats before the episode and 15 beats after the onset of the episode. Typically, an increase in heart rate (tachycardia) and a decrease in pulse wave amplitude (vasoconstriction) indicate a sympathetic reaction [START_REF] Shelley | Arterial-pulse oximetry loops: a new method of monitoring vascular tone[END_REF]. The respiratory rate (peak to peak measure of the signal on the thoracic belt) and amplitude (in mL) were measured based on the thoracic plethysmography signal during the 3 respiratory cycles preceding the episode, during the cycle when the episode started (the episode could start during inspiration, expiration or post-expiration pause), and during 5 cycles after the initiation of the episode. This duration was chosen to evaluate the stability of the autonomic system prior to awakening and its changes during the early onset of awakening. We did not extend the analysis after 15 s post-arousal because the behaviour evolution was variable, with some subjects resuming sleep while others evolving toward complete awakening. Because the measures were manually performed, artifacts (e.g., contamination of the EKG signal by myographic signals, contamination of all signals by movements, etc.) were carefully excluded. In addition, any arousal secondary to a noise in the room (based on a careful ambient audio listening), a leg movement or a respiratory event (flow limitation, hypopnea, apnea) was discarded from the analysis.

Statistical analysis

The raw measures were transformed as relative changes (in %) from the individual baseline measurement performed 10 s before awakening, in order to (i) allow for comparisons between subjects who had different heart and respiratory measures in N3 stage, as well as within subjects when measures were taken at different N3 arousals, and (ii) to get rid of calibration problems. The clinical measures were compared between groups using chi-square tests for qualitative measures and Student's t tests for quantitative measures using SAS version 9 (SAS Institute, Cary, NC). Because the number of N3 arousals was different among participants, the measurements were adjusted for each subject and analyzed (patients vs. controls, pre-vs. post-arousal periods [within subjects], parasomniac vs. quiet arousals [within-subjects]) using a mixed linear model on SAS V9, including interactions.

When this first analysis yielded significant differences, adjusted Tukey-Kramer tests were performed for between-groups analysis. In order to estimate the pre-arousal, post-arousal, and the time of onset of the increase of heart rate and pulse wave amplitude curves after the stability period, we fitted the following non-linear model: y(t)=a0+amplitude*(1-E -lamba (t-t0) ) if t>t0; y(t) = a0 if t<t0; where y(t) is the value of the heart rate or the pulse wave amplitude, a0 is the mean pre-arousal value, amplitude is the difference between the mean post-and pre-arousal values, lambda is a parameter reflecting the speed of the change, and t0 is the time of onset of the increase.

RESULTS

Sample and sleep characteristics

The 20 patients with disorders of arousal suffered from isolated SW (n = 8, 40% of the sample), isolated ST (n = 4, 20%) or a combination of SW and ST (n = 8, 40%).

Seizure-like activity was not detected during polysomnographic studies, and none of the patients had any personal or family history of seizure. The mean age, sex, body mass index and score of the Epworth sleepiness scale were not different between SW/ST and control groups (Table 1). Patients with SW/ST slept longer, had longer wakefulness after sleep onset, and more arousals from N3 despite lower sleep fragmentation (reduced number of arousals plus awakenings per hour of sleep), whereas other sleep measures were not different from those in controls (Table 1).

Autonomic changes surrounding N3 arousals in patients

There were 1 to 18 arousals from N3 sleep per patient (50 quiet and 55 parasomniac arousals) and 1 to 4 arousals from N3 sleep per control (38 quiet and no parasomniac arousals). The raw heart rate profiles across time (Figure 2) were best estimated using an S curve with a low and stable rate from the -10 th to the -6 th heartbeats and a gradual increase from the -4 th heartbeat before the onset of N3 arousal. The estimates of the curve were a0 = 62.1 ± 0.9 beats per min, time of onset = 10.8 ± 0.2 s, amplitude = 23.7 ± 1.4 beats per min, and lambda = 6.8 ± 1.9 s, which indicated that the inflexion of the curve occurred 4 seconds before the arousal. Heart rates were higher (+33% on average) after arousal than before arousals (Table 2). A mirroring pattern was observed for the pulse wave amplitude (Figure 2), starting with a stable amplitude, a high amplitude during the stable N3 stage, followed by a gradual decrease in amplitude from the -4 th pulse wave before the arousal to the +5 th pulse wave, and finally a relatively stable, low amplitude from the 5 th to the 15 th pulse waves. The amplitude was greater (+57%) before arousals than after arousals onset, indicating vasoconstriction after arousals. The respiratory rate (+24%) and amplitude (+100%) were greater after arousals than before arousals (Table 2, Figure 3).

Comparison with controls

The raw heart rate curve across time (Figure 2) was best estimated in controls using an S curve with the following estimates: a0 = 65.4 ± 1.6 (higher than in the patient group, p = 0.02), time of onset = 10.3 ± 0.5 seconds (not different from patient group), amplitude = 21.4 ± 2.6 beats per min (not different from patient group), and lambda = 6.4 ± 4.6 (not different from patient group). In controls, the heart rate and respiratory amplitude were greater after arousals than before arousals, the pulse wave amplitude was lower after arousals than before arousals, and the respiratory rate remained unchanged before and after arousals (Table 2). Patients had more marked changes than controls before vs. after N3 arousals for all measures (heart rate, pulse wave amplitude, respiratory rate and amplitude), as they had lower values before arousals (heart rate, respiratory rate) or higher values after arousals (heart rate, respiratory amplitude). The pulse wave amplitude was lower before arousals than after arousals.

Parasomniac vs. quiet arousals in sleepwalkers

The heart rate exhibited a greater increase from baseline to parasomniac arousals than from baseline to quiet arousals, because it was lower before parasomniac than before quiet arousals (Table 2, Supplemental Figure A). Conversely, the pulse wave amplitude decreased more after parasomniac arousals than after quiet arousals, because it was higher before (and was not different after arousals) parasomniac arousals than before quiet arousals. The respiratory rate increased after arousals in both groups, with a higher respiratory rate after parasomniac arousals than after quiet arousals (Supplemental Figure B). In contrast, the respiratory amplitude was not changed by the nature of the arousals.

Quiet arousals in sleepwalkers and in healthy controls

The autonomic profile associated with the 50 quiet arousals in sleepwalkers was compared to the profile in 38 quiet arousals in healthy controls. The heart rate was similar before and after the arousals in both groups, with a non-different increase after the arousal (Table 2 andSupplemental Figure C). In contrast, the pulse wave amplitude (which was similar before the quiet arousals in the sleepwalking and in control groups) was lower after quiet arousals in sleepwalkers, hence the postarousal drop was greater in the sleepwalking group. The respiratory rate (but not the respiratory amplitude) had a greater increase after the quiet arousals in sleepwalkers than in controls (Supplemental Figure D).

DISCUSSION

Main findings

In patients with SW/ST, the N3 arousals were associated with a 33% increase in heart rate, a 57% decrease in pulse wave amplitude (indicating a major vasoconstriction), a 24% increase in respiratory rate and a doubling of respiratory amplitude. Notably, tachycardia and vasoconstriction started 4 seconds before motor arousals. The same profile (tachycardia and vasoconstriction gradually increasing from the 4 s preceding arousal, post-arousal increase of respiratory amplitude, but no polypnea) was also observed, to a lesser degree, during the less frequent N3 arousals in control subjects. Parasomniac arousals were associated with greater tachycardia, vasoconstriction and polypnea than quiet arousals, with the same prearousal gradual increases in heart rate and vasoconstriction.

Autonomic reactions are associated with various types of arousals

Cortical arousals caused by acoustic stimuli [START_REF] Catcheside | Noninvasive cardiovascular markers of acoustically induced arousal from non-rapid-eye-movement sleep[END_REF], periodic leg movements [START_REF] Pennestri | Blood pressure changes associated with periodic leg movements during sleep in healthy subjects[END_REF], or apnea/hypopnea [START_REF] Pitson | Autonomic markers of arousal during sleep in patients undergoing investigation for obstructive sleep apnoea, their relationship to EEG arousals, respiratory events and subjective sleepiness[END_REF] are all associated with enhanced sympathetic drive (increased heart rate and blood pressure as well as decreased pulse transit time). The highest response to noises has been observed in N3 arousals, as compared to waking [START_REF] Griefahn | Autonomic arousals related to traffic noise during sleep[END_REF].

Increases in cardiovascular markers are also observed during spontaneous arousals in young subjects [START_REF] Bonnet | The scoring of arousal in sleep: reliability, validity, and alternatives[END_REF], as found here in young subjects, with and without parasomnia. Arousals caused by noise, flow limitations, limb movements or nurse interventions were carefully discarded from the analysis, such that the remaining N3 arousals were apparently spontaneous.

Autonomic reactions during parasomniac arousals

Enhanced sympathetic drive is a normal reaction to any N3 arousal [START_REF] Janackova | Neurobiology of sleep fragmentation: cortical and autonomic markers of sleep disorders[END_REF], but here this reaction was more increased in patients with SW/ST than in healthy controls, and was further increased when the arousal was associated with abnormal behaviours, facial expressions of surprise or fear, or simply sudden eye opening. Fisher et al observed that the heart rate increased after, but not before, the arousals onset [START_REF] Fisher | A psychophysiological study of nightmares and night terrors: physiological aspects of the stage 4 night terror[END_REF], but the measures were averaged over 2.5 min before the N3 arousal, without paying attention towards the few seconds preceding the N3 arousal. Signs of intense autonomic discharge, such as mydriasis, tachycardia, tachypnea, flushing of the skin, and sweating, are part of the sleep terror phenotype and appear to be congruent with the apparent fear of the subject (American Academy of Sleep Medicine, 2014). Two groups found that heart rate increased after N3 arousals in patients with sleepwalking [START_REF] Schenck | Analysis of polysomnographic events surrounding 252 slow-wave sleep arousals in thirty-eight adults with injurious sleepwalking and sleep terrors[END_REF][START_REF] Busek | Spectral analysis of the variations in heart rate and cardiac activation on waking up in sleepwalking[END_REF] and in controls [START_REF] Schenck | Analysis of polysomnographic events surrounding 252 slow-wave sleep arousals in thirty-eight adults with injurious sleepwalking and sleep terrors[END_REF]. Our results confirm these previous observations and extend the findings to vasoconstriction as well as respiratory rate and amplitude (not measured before) and to quiet N3 arousals in patients with ST/SW (which contained a more marked vasoconstriction and polypnea than the quiet arousals in controls), suggesting that the sympathetic discharge associated with N3 arousals is higher in patients than in controls. It is here not possible to determine whether this increased cardiorespiratory activity is the consequence of an increased sympathetic activity or of a decreased parasympathetic activity, although the former seems more plausible than the latter.

Because patients have more frequent N3 arousals than controls, they have more frequent sympathetic activation. Such activation could have long-term deleterious consequences on the cardiovascular system. In this direction, Busek et al found an increased sympathetic response to standing up in sleepwalkers compared to controls, although the heart rate variability did not differ between groups across sleep and quiet waking [START_REF] Busek | Spectral analysis of the variations in heart rate and cardiac activation on waking up in sleepwalking[END_REF].

Autonomic arousals preceding motor arousals

Here, the sympathetic discharge started approximately 4 seconds before the motor arousal. When studying N3 arousals, Schenck et al. noted that the RR intervals on EKG were unchanged during N3 sleep, and then progressively decreased from the -3 rd heartbeat prior to N3 arousal to the +2 nd heartbeat after arousal [START_REF] Schenck | Analysis of polysomnographic events surrounding 252 slow-wave sleep arousals in thirty-eight adults with injurious sleepwalking and sleep terrors[END_REF]. Busek et al. found an increased heart rate on average one second prior to arousals in sleepwalkers, possibly because they chose as onsets other markers of activation (increased respiration, EEG changes, eye movements) than just an EMG increase [START_REF] Busek | Spectral analysis of the variations in heart rate and cardiac activation on waking up in sleepwalking[END_REF].

What could evoke these autonomic arousals?

This autonomic arousal preceding spontaneous motor and cortical N3 arousal may reflect an internal stimulus (an alarming event) provoking the arousal. The nature of the alarming event is unknown, but one may imagine that it could be some troubling sleep mentation. Indeed, when not amnestic, most patients with SW/ST report a brief nightmarish (baby in danger, ceiling collapse, being buried alive, not finding the exit, intruders, tsunami, poisoned air) or concerning (unfinished homework, clothes to be washed) scene congruent with the parasomnia episode [START_REF] Fisher | A psychophysiological study of nightmares and night terrors. 3. Mental content and recall of stage 4 night terrors[END_REF][START_REF] Schenck | A polysomnographic and clinical report on sleep-related injury in 100 adult patients[END_REF][START_REF] Oudiette | Dreamlike mentations during sleepwalking and sleep terrors in adults[END_REF][START_REF] Mwenge | Sleepwalking: long-term home video monitoring[END_REF][START_REF] Arnulf | A scale for assessing the severity of arousal disorders[END_REF]. These threatening dreams in SW/ST contrast with the low frequency (34%) of emotions found in dream reports collected after provoked arousals from N3 in normal subjects [START_REF] Cavallero | Slow wave sleep dreaming[END_REF]. One may wonder whether the dream scenario that occurs during N3 sleep (and that is poorly recalled upon awakening) does not gradually develop to the point of being so emotionally charged that it evokes an arousal.

Conversely, one may also imagine the reverse scenario: a spontaneous activation of the autonomic nervous system occurring in N3 could then trigger the "troubling dream mentation", as some physical sensation can be incorporated into the ongoing dream scenario [START_REF] Dement | The relation of eye movements, body motility, and external stimuli to dream content[END_REF]. In this direction, the SW/ST patients report frightening/threatening dream fragments (i.e., short time course) and not welldeveloped dream plots (as more commonly found in REM sleep). If true, patients with SW/ST should benefit from drugs that reducing sympathetic activity during sleep (e.g., alpha-blockers like prazosin), which are commonly used to alleviate the nightmares associated with post-traumatic stress disorder. A similar alert would be at play in patients with SW/ST and in healthy subjects (as both present sympathetic activation prior to N3 arousal), but the inability to fully, consciously awake patients with SW/ST would prevent them from realizing that they just had a bad dream, and hence their inappropriate behaviour. Alternatively, this early autonomic arousal could be an external marker of the local, subcortical arousal preceding by 3-5 s the surface EEG arousal [START_REF] Terzaghi | Evidence of dissociated arousal states during NREM parasomnia from an intracerebral neurophysiological study[END_REF][START_REF] Januszko | Sleepwalking episodes are preceded by arousal-related activation in the cingulate motor area: EEG current density imaging[END_REF][START_REF] Desjardins | EEG Functional Connectivity Prior to Sleepwalking: Evidence of Interplay Between Sleep and Wakefulness[END_REF]. Both hypotheses are not mutually exclusive. It would be interesting to correlate the autonomic analysis with all these recent connectivity and stereo EEG analysis measures in confusional and non confusional arousals.

Parasomniac events triggered by external stimuli

It would be interesting to elucidate whether the onset of tachycardia and vasoconstriction shifts toward motor arousal onset (rather than prior to it) when N3 arousals are evoked by noise in patients with SW/ST and in controls. Indeed, sudden noise (or contact) may also trigger parasomniac episodes in predisposed subjects [START_REF] Pilon | Precipitating factors of somnambulism: Impact of sleep deprivation and forced arousals[END_REF]. Two types of abnormal N3 arousals can be considered: those precipitated by nightmarish or alarming sleep mentation (internal, mental trigger, with autonomic reaction preceding the motor arousal), and those precipitated by ambient noise, apnea, movement, contact, pain or esophageal reflux (external triggers, with autonomic reaction associated with the trigger). Both would result in inappropriate behavioural responses in patients and in appropriate behavioural responses (quiet arousal, repositioning, resuming sleep) in controls.

This study has several limitations, including the small number of patients and controls that were studied. This size limitation is counterbalanced by the manual measurements of more than 10,000 autonomic events and the careful exclusion of any artifact caused by movement, or arousal caused by noise, nurse intervention or flow limitation. The measure of vasoconstriction via the pulse oximetry was here a surrogate, as the gold standard continuous measure of blood pressure is a finger plethysmography (e.g., Portapres®), a non-routine, expensive device, which may alter sleep continuity. Similarly, the measure of respiratory reaction was based on plethysmography band, which is not a calibrated measure of ventilation as obtained using a pneumotachograph. However, it would be difficult to obtain normal sleep in young SW/ST patients unused to wear a facial mask.

In conclusion, a sympathetic arousal comes 4 seconds before any motor or cortical The measures are the least squares means estimate ± standard error; a p<0.05 for a difference with controls, b p<0.0001 for a difference with the pre-arousal period, and c p<0.05 for an interaction between the groups and periods; d p<0.05 for a difference with quiet arousals, e p<0.05 for an interaction between the types of arousal (quiet or parasomniac) and periods, Tukey-Kramer tests, and f p<0.05 for an interaction between quiet arousals (in controls vs parasomniac) and periods, Tukey-Kramer tests.

  arousal from N3 sleep, suggesting that an alarming event (possibly troubling sleep mentation or local, deep arousal) during sleep causes parasomniac (in SW/ST patients) or quiet N3 arousals (in healthy subjects and in SW/ST patients).Study fundingThis study was financed by the Hospital Clinical Research Program P130603 and sponsored by Assistance Publique -Hôpitaux de ParisAuthor contributionNL, EA, PCS and MCS performed the measurements, JLG performed the statistical plan and analysis, SR supervised the autonomic measures, and IA organized the project, recruited the patients, supervised the analysis and obtained the funding. All coauthors contributed to the final analysis of data and to the article.

Figure 1 -

 1 Figure 1 -A motor arousal from N3 is shown in a compact window (A) with

Figure 2 -

 2 Figure 2 -Changes in heart rate (upper panel) and pulse wave amplitude (a

Figure 3 -

 3 Figure 3 -Changes from baseline, before and after N3 arousal, in respiratory rate

Figure D

  Figure D

Table 1 -

 1 Demographic and clinical characteristics of the patients with sleep terrors and sleepwalking episodes vs. healthy controls

	Group	Patients	Healthy controls	P
	No	20	20
	Age, y	26.8 ± 6.6	28.2 ± 8.8	0.57
	Female sex, n (%)	10 (50)	10 (50)
	Body mass index, kg/m²	22.3 ± 3.6	23.5 5.4	0.44
	Epworth sleepiness score, 0-24	9.9 ± 5.4	10.4 ± 7.2	0.81
	Sleep measures
	Sleep onset latency	26.1 ± 18.0	25.6 ± 17.4	0.93
	REM sleep onset latency	118.4 ± 58.4	136.6 ± 73.5	0.41
	Sleep efficacy, %	88.3 ± 6.0	91.3 ± 5.7	0.11
	Wake after sleep onset, min	67.6 ± 38.7*	42.8 ± 32.5	0.04
	Total sleep time, min	501.7 ± 72.7*	435.6 ± 59.7	0.004
	Sleep stages (% of total sleep time)
	N1 sleep	3.6 ± 2.4	3.45 ± 2.4	0.83
	N2 sleep	50.3 ± 9	50.0 ± 5.9	0.91
	N3 sleep	24.8 ± 7.9	25.2 ± 6.9	0.87
	REM sleep	20.1 ± 3.9	18.9 ± 6	0.44
	Arousal index	8.2 ± 4.7*	14 ± 6.0	0.002
	Periodic leg movements index	0.5 ± 0.8	2.6 ± 4.9	0.07
	Apnea/hypopnea index	2.6 ± 6.0	2.5 ± 3	0.91
	Arousals from N3 sleep, n/night	5.6 ± 3.5*	1.8 ± 0.8	< 0.001
	* P < 0.05 for a difference between patients and healthy controls

Table 2 -

 2 Autonomic measures before and after N3 (quiet or parasomniac) arousals in patients with sleepwalking episodes and sleep terrors, and in healthy controls

		Controls	Patients	Parasomniac	Quiet
				arousals	arousals
				in patients	in patients
	N3 arousals, No	38	105	55	50
	Heart rate, beats per min
	Before arousals	64.9 ± 0.8	60.7 ± 0.5 a	57.2 ± 0.6 d	64.7 ± 0.7
	After arousals	79.3 ± 0.7 b	80.9 ± 0.4 b	81.4 ± 0.5 b	80.5 ± 0.6
	Before/after change	14.3 ± 1.1	20.3 ± 0.6 c	24.2 ± 0.8 e	15.8 ± 0.9
	Pulse wave amplitude, microvolts
	Before arousals	150.7 ± 3.7	140.5 ± 1.6 a	147.9 ± 2.2 d	131.8 ± 2.4
	After arousals	94.5 ± 3.3 b,d	60 ± 1.4 a,b	60.4 ± 1.9 b	59.6 ± 2 b
	Before/after change	-56.2 ± 4.2 d,f	-80.5 ± 2.3 c	-87.5 ± 2.9 e	-72.2 ± 3.1
	Respiratory rate, cycles/min
	Before arousals	17 ± 0.6 d	14.7 ± 0.3 a	14.4 ± 0.4	15.1 ± 0.5
	After arousals	17.7 ± 0.5	18.3 ± 0.3 b	19.0 ± 0.3 b	17.6 ± 0.4
	Before/after change	0.6 ± 0.7 d	3.6 ± 0.4 c	4.5 ± 0.6 c	2.5 ± 0.6
	Respiratory amplitude, mL
	Before arousals	0.79 ± 0.07	0.73 ± 0.04	0.75 ± 0.06	0.71 ± 0.07
	After arousals	1.35 ± 0.06 b	1.57 ± 0.04 a, b	1.63 ± 0.05 b	1.50 ± 0.05 b
	Before/after change	0.56 ± 0.1	0.83 ± 0.06 c	0.88 ± 0.08	0.79 ± 0.08

terrors episodes during parasomniac (solid blue line and circles) vs. quiet (orange dotted line with circles) awakenings from N3.

Supplemental


Jan Kowalski
email: jan.kowalski@rub.de

Martin Lauer

David Engelmann

Michele Cagna

Ronald Mailach

Francesca Di

O Tatin

Development of a Novel Test Rig to Investigate Explosion Safety in Gas Turbine Enclosures

Keywords:

To investigate explosion safety in gas turbine enclosures, development work was conducted to design a scaled test rig of a modern MAN MGT-6100 gas turbine package. Since the focus of research is on the ventilation flow, the turbine components were designed as inactive and mainly function as flow obstructions. Analytical as well as CFD investigations were carried out to determine the model scale and to evaluate operation condition restrictions. As a result the test rig was downscaled to 1:3 and the ventilation system was chosen to accomplish different operation conditions up to Re-similarity. e design, which was optimized for PIV measurements, as well as the experimental setup will be described in detail in this paper. During development the design of the actual package was optimized further, leading to differences in component arrangement. An additional CFD model was created to consider these changes not only in the test rig. is model was subjected to a grid sensitivity analysis, and used to identify additional measurement planes for PIV investigations.

INTRODUCTION

Due to the still growing demand for more flexible power plant solutions, industry-type gas turbines of intermediate power class (up to 25 MW) have moved into the focus of research efforts over the past years, e.g. here [ ]. Since the dimensions of these machines o en allow being set up on a single frame they are usually designed as so-called gas turbine packages which include the electric generator on the same frame in case of a power generation setup. is specific design is commonly accompanied by the whole machine being engulfed in an acoustic enclosure. ese so-called compartments reduce the emi ed noise level of the gas turbine and protect it from environmental influences when operated as an outdoor installation. e enclosure separates the gas turbine from the surroundings, therefore the whole compartment has to be actively ventilated for two main reasons: Firstly, the ventilation flow removes heat losses generated from the Brayton cycle itself and thus keeps machine components at acceptable temperature levels. Secondly, the ventilation flow has to dilute and remove clouds of fuel from the compartment in case of gas leakage due to e.g. seal failure in the fuel system. ough the la er case is unlikely during engine operation it must still be considered. e British Health and Safety Executive (HSE) [ ] reported 134 gas leakages in the UK over a period of 13 years leading to a total of 19 incidents of fuel ignitions in gas turbine enclosures, thus underlining the importance of careful ventilation flow design to ensure explosion safety.

To investigate in detail the mixing of a fuel leakage jet within a state-of-the-art MGT-6100 gas turbine package from MAN Diesel & Turbo SE, Petrick [ ] conducted CFD investigations using ANSYS CFX with the k-ω SST turbulence model on the whole compartment, taking into account thermal convection as well as leakages at different positions.

Petrick concluded that the current package design meets modern explosion safety requirements but underlines the importance of applying appropriate boundary conditions for heat transfer. erefore, a research project was launched aimed at creating validation data for CFD investigations in a test rig of the aforementioned gas turbine package, which will be discussed in this paper. e focus, however, will be on the design process and numerical results.

. PROJECT GOALS AND APPROACH

Development work was carried out with three distinct longterm goals to accomplish: Firstly, validation data for CFD investigations shall be created for cold, heated, as well as leakage configuration. Secondly, a detailed experimental and numerical assessment of heat transfer and buoyancy effects shall be conducted in different operation conditions including cases of fan failure. Lastly, the optimization potential of reducing or redirecting the ventilation flow will be explored.

To fulfill the defined goals a scaled test rig of the gas turbine enclosure was designed, which can be seen in figure 1.

e test rig features all relevant components of the machine, including the gas turbine, gear, generator, as well as complex pipework of the oil and fuel system in reasonably simplified

Figure .

Overview of the flow phenomena inside the enclosure e regarded gas turbine system features an electric power generator on the base frame which is actively ventilated, leading to the aforementioned interaction of the IGCF with the MEVF at the generator in-and outlets. In the actual engine the primary air supply for the Brayton cycle process and the resulting exhaust flow are guided through separate assemblies and do not interact with the MEVF, apart from heat transfer effects: Waste heat from the process is transferred to the ventilation air overflowing the hot gas turbine components, leading to changes in fluid density and therefore causing buoyancy convection effects. Unlike the first three flow aspects, which occur during regular engine operation, fuel leakage is regarded as a case of system failure, which has to be considered, as already mentioned. Fuel leakage would most likely be caused by a seal or welding seam failure within the fuel system. In such cases the fuel gas release can be assumed to occur as a choked fuel jet interacting with the MEVF, according to Ivings et al. [ ].

Apparently the flow inside the compartment is highly complex and especially the separation flow interacts significantly with the other flow phenomena. Since not all similarity quantities can be matched during testing, an analytical and CFD-based design study was carried out to identify a suitable model scale and to derive requirements for the test rig and ventilation system design.

. Analytical Design Study

Vahidi et al. [ ] conducted similar research on a scaled gas turbine package with greatly simplified geometry featuring heated gas turbine surfaces as well as leakage with carbon dioxide (CO ) as model gas.

ey concluded that for hydraulic and thermal analogy a scaled test rig has to be operated with the same Reynolds (Re), Nusselt (Nu) and Grashof (Gr) number as the actual engine [ ]. ey are calculated as given in equations 1 -3 with c being the velocity at the package inlets, D the inlet duct diameter, ν the kinematic viscosity, h the heat transfer coefficient, L the characteristic length, k the fluid's thermal conductivity, g the gravitational acceleration, β the fluid's coefficient of thermal expansion, and ∆T the difference between the average temperature of the hot gas turbine walls and the ambient temperature T.

is given set of similarity quantities allows statements on heat transfer, natural convection as well as general flow structures. However, the ratio of buoyancy and ventilation flow inertia is yet le unregarded. For model testing of flue gas extraction systems in civil engineering the Archimedes (Ar) number is commonly regarded for that purpose according to the Verein Deutscher Ingenieure e.V. (VDI) [ ], as given in equation 4. e VDI furthermore recommends test rigs incorporating buoyancy effects to be set up as big as possible with scaling factors k smaller than 20 and a height of the ventilated volume greater than 300 mm [ ].

Re = cD ν ( ) Nu = h L k ( ) Gr = g β ∆T L 3 ν 2 ( ) Ar = g ∆T L T c 2 ( )
Due to safety regulations at the test facility the maximum permissible wall temperature of the heated gas turbine components was limited to 230 °C, which is close to the 232 -249 °C boiling temperature of the DEHS seeding particles used for PIV measurements. With a given limit for the wall temperature only the scaling factor k and the enclosure inlet velocity c remain as free design parameters. For a specific value of k the inlet velocity is given depending on the similarity quantity to satisfy during testing, therefore only one quantity can be matched exactly during testing.

For assessment of the degree of deviation for certain flow variables or dimensionless quantities φ, the ratio factor f φ is introduced and defined as given in equation 5, where the subscript M indicates the corresponding value for the scaled test rig model and the subscript O the value for the real-size machine.

f φ = φ M φ O ( )
Deviations in operation conditions between the real-scale machine and the test rig configuration, i.e. the influence of different fluid inlet temperature and pressure on fluid properties, were considered during the design process for determining the correct volume flow rates for Ma-and Re-similarity (see table 2). However, fluid properties and operation conditions are regarded as identical for both configurations during the following analysis:

f Pr = f g = f ν = f T = 1 ( )
To evaluate f N u a simplification approach is taken by approximating the heat transfer process around the hot gas turbine components with the heat transfer characteristic of a hot cylinder being cooled by a uniform flow under an angle of 40°to the cylinder axis. e cylinder dimensions were chosen to fit the length and surface area of the hot engine components. For the assessment of Nu free and forced convection with laminar and turbulent proportions have been taken into account, as specified in the VDI Heat Atlas [ ].

e results of the analytical design study are displayed in figure 3 showing f Ar , f Gr and f N u for different model scales and operation conditions. e different symbols hereby identify the operation conditions named a er the similarity quantity, which was set up by changing the inlet duct velocity of the enclosure ventilation flow. e suffix T232 indicates an operation condition in which the maximum wall temperature is reduced to 232 °C due to staff safety concerns, i.e. OP-Re would indicate an operation point, where Re-similarity is set up at the inlet duct with real-scale wall temperatures.

When the test rig is operated in Ma-similarity condition Ar is decreased with deviations remaining within one order of magnitude for all regarded model scales, even under reduced wall temperatures, as seen in figure 3a. For Resimilarity the deviation of Ar increases, leading to discrepancies greater than one order of magnitude for scaling factors greater than two in both wall temperature configurations. Since Gr only accounts for the natural convection aspect of the flow, figure 3b shows no dependency of the inlet velocity. Similar to f Ar in OP-Re and OP-Re T232 it deviates for more than one order of magnitude for scaling factors greater than two in both wall temperature configurations. Regarding f N u as illustrated in figure 3c, both Ar-and Ma-similarity operation condition lead to decreased heat transfer characteristics of the test rig, as expected. For OP-Re T232 f N u is increased compared to OP-Re with higher wall temperatures because the heat transfer is dependent on the local Re number around the gas turbine with fluid properties at mean temperature levels. As already stated OP-Re only indicates Re-similarity at the package inlet. By decreasing the wall temperature the mean density of the fluid near the gas turbine increases, which in term results in be er cooling capabilities in relation to the given temperature gradient and therefore a higher Nu number. Although deviations of Gr appear to be rather large, they have li le influence on the heat transfer for OP-Re and OP-Re T232, since f N u is almost independent of k (figure 3c). is indicates that forced convection is dominating the heat transfer characteristic inside the enclosure. However, the calculation of Nu was carried out under several simplifications, e.g. uniform overflow of the gas turbine components. Since the real flow field also features stagnation zones, in which natural convection can play a dominant role, Gr as well as Ar have to be considered nonetheless. To achieve Nu-similarity while operating with reduced wall temperatures, the inlet velocity can be reduced to Re OP-N u = 0.735 • Re OP-Re T 232 while for investigations on stagnation regions and leakages operating the test rig in Ma-similarity condition is recommended, as will be outlined in the following CFD study.

. CFD Design Study

Due to the complexity of the flow phenomena, assumptions and simplifications had to be made, especially for the assessment of Nu, in order to carry out the analytical study. To account for the interaction of the different flow effects, the design process was accompanied by CFD investigations based on the work of Petrick [ ] using ANSYS CFX 17.0. e numerical grid used by Petrick was chosen based on an inten-sive grid resolution and grid type study comparing hexahedrons, tetrahedrons, and tetrahedrons combined with prism elements.

e chosen mesh consists of about 15.5 million tetrahedron elements including prism layers near bounding walls. is numerical model was modified and used as the design study model (DSM), with the surface mesh shown in figure 4 and the numerical setup given in figure 5 and in table 1. 5). Investigations on the influence of the IGCF were conducted, such as varying the exit angle as well as leaving the generator cooling flow (3,4) out of the CFD model completely. e results underline the influence of the IGCF on the overall pressure loss as well as on heat transfer effects (waste heat removed from the generator electrics), thus the generator cooling flow (3,4) was chosen to be included in the test rig assembly. To assess kinematic similarity during testing, the flow path of CH leakage (fuel system malfunction) was examined for the real-scale results as well as two operation conditions with k = 3 a ainable with the actual test rig, namely OP-Ma T232 and OP-Re T232, as seen in figure 6. In the investigated cases the CH jet was ejected (6) into the enclosure in opposing direction to the MEVF (figure 5,LP2). In all cases the MEVF mainly flows over the gear section before evading the inlet casing sideways but proportions of the MEVF also pass the gear side-on or flow downwards a er reaching the gear front surface. A erwards the MEVF enters the gas turbine section by a region of constriction where the flow is partly redirected towards the bo om because of the lesser blockage from the inlet casing. e flow separates at the edge of the inlet casing creating a wake space between the combustion chambers and the inlet casing. During real-scale as well as model-scale operation in Ma-similarity condition, the fuel jet reaches the constriction section around the gas turbine inlet casing before losing momentum and being transported towards the outlets of the enclosure, leaving via both available ducts (1). In both cases the outlet flow features a hemispheric region of outflow near both ducts. In case of Re-similarity operation the fuel jet is displaced almost immediately a er ejection. Since the jet can be assumed to be in choked condition, operating the rig in Re-similarity condition would result in an imbalance of momentum ratio between the fuel jet and the MEVF, thus explaining the deviant behavior illustrated in figure 6c. A erwards the air-fuel mixture is transported into a recirculation zone underneath the gas turbine diffuser before ascending spirally and being carried away by the MEVF, exiting through one outlet duct only. For this configuration the global outlet flow appears to be more directional as it changes towards a jet-type flow compared to the previously regarded cases, leading to lesser kinetic similarity to the real-scale machine as also illustrated in figure 3a.

A C E B D (A) Gas Turbine (B) Fuel System (C) Gear (D) Oil System (E) Generator
ese results indicate that Ma-similarity shall be preferred for kinematic similarity, especially when investigating leakage dilution. For separate investigation of buoyancy effects with correct ratio of thermal li and inertia, Ar-similarity would also represent a viable setup. Re-similar operation condition however is still necessary when measuring heat transfer, as mentioned in section 2.2, e.g. for applying realistic heat transfer boundary condition in CFD investigations.

erefore, based on the analytical and numerical design studies a model scale of 1:3 was chosen while determining that the ventilation system has to be able to accomplish Ar, Ma-as well as Re-similarity operation condition.

Hemispheric Outflow

. Test Rig Construction

To achieve Re-similarity during testing a total of four dualstage counter-rotating fans had to be installed. Like in the actual turbine package these blowers are set up in parallelpath air flow arrangement at the air inlet as well as at the air outlet side (see figure 1). ough all fan stages could have been arranged at the inlet side of the test rig leading to a less complex set up and reducing the construction height, testing of individual fan path failure would have been limited to three out of fi een possible cases. By utilizing the fan arrangement seen in figure 1, the rig ventilation flow also features the combined suction and pressurized operation condition in an open cycle as applied to the real-scale machine. Since available ventilation systems could not deliver the required performance for Re-similarity at the scaled model's resulting duct diameter, larger dual-stage counter-rotating fans were chosen. e required nozzles and diffusers were designed as a trade-off between construction length and pressure loss obtained by hydraulic correlations [ ]. Especially the usage of nozzles and additional piping for instrumentation (see figure 10a) at the package inlet can be expected to reduce the wake resulting from the fan motor and hub while the counterrotating configuration presumably will reduce the inlet swirl.

e impact of these deviations will be investigated in detail numerically a er the validation procedure.

To avoid transmission of vibrations from the main ventilation fans into the package enclosure the connection pipes at the in-and outlet are decoupled by using flexible hose piping. e test rig additionally features eight small-scale high-performance fans operating at up to 11,000 RPM to include and to control the IGCF within the generator casing (see figure 2). e overall test rig properties and performance data are given in table 2. In order to keep the test rig weight and cost at acceptable levels, most of the gas turbine components were constructed from thin sheet metal with standard parts being used for providing the structural integrity of the package base frame and the fan support. e enclosure housing was built from modular profiles incorporating easily removable window segments for convenient window maintenance and test rig access. Pipework of the fuel and the oil system was constructed from standard tubes. Since the velocity field within the enclosure is of main interest, optical measurements were chosen as the primary measurement technique. erefore, all side and roof walls except the ones which are flange-mounted to the package fans were constructed from transparent material (see figure 1). Due to the addition of heating elements in the following phase of the project, Perspex was discarded and borosilicate glass was chosen for its higher temperature resistance as well as for its high optical quality. High surface temperatures at the gas turbine model components can be obtained by a aching silicone rubber heaters to their inner surfaces without changing the outer geometry. However, the real temperature levels cannot be realized due to the aforementioned safety aspects and limitations of the silicone mats, leading to a wall temperature limit of approximately 220 °C. A numerical model of the gas turbine as a 1-D thermal network was developed as seen in figure 7.

All relevant components of the gas turbine were modeled as thermal capacitances (C i ) and were coupled through heat conduction (G λ,i ) to each adjacent component as well as to the MEVF ( m, Q conv ) via convective heat transfer (G h,i ).

e applied heat transfer coefficients were determined from CFD results using the DSM. e estimated heating powers for the different sections ( Q el,i ) indicate that with the given geometry and materials the desired temperature distribution can be obtained well. During heated operation condition the surface temperatures will be controlled by a closed-loop circuit and measured via thermal imaging.

      𝐶 1 , 𝑄

Figure . ermal network model of the gas turbine components for heating element design

To allow leakage investigations for explosion safety assessment in the test rig, the pipework corresponding to the fuel system was designed for pressurized operation with gas supply access below the main frame. Carbon dioxide, selected as the model leakage gas, can exit the fuel piping at specific locations, e.g. flanges or seals.

ese parts can be made permeable by loosening fixing screws, seal removal, or installing perforated disks in order to simulate worst-case failure scenarios like fuel jet ejection into zones of recirculation. A gas concentration measurement system with sensors at several representative positions inside the enclosure and at the outlet will provide validation data for numerical results of leakage gas distribution and dilution. A er validation the CFD model can then be used to investigate leakage cases of real engine fuel and will allow more reliable explosion safety investigations.

. CFD STUDY OF THE ACTUAL TEST RIG

During the course of the test rig development the design of the actual gas turbine package was optimized further, leading to slight differences in component arrangements. To accomplish maximum accuracy of the project results, a new numerical flow model of the test rig (TRM) was created to be used for validation purposes. is model was generated using ANSYS ICEM and ANSYS CFX 17.0. A grid sensitivity analysis was carried out as well as investigations on the flow field in cold configuration without leakage for a Re-similarity volume flow and adiabatic wall temperatures (OP-Re AWT). Re-similarity was chosen to allow adjustment of y + at a velocity level suitable for future heat transfer investigations.

Similar to the work of Petrick [ ] the simulations were carried out with a steady-state approach, which is commonly used for investigations of enclosure ventilation [ ]. Best practice guidelines on conducting CFD investigations in gas turbine enclosures are available from Ivings et al. [ ] and in a more general form from Casey et al. [ ] and were taken into account during the course of model generation. e investigations of Petrick [ ] proved the validity of using a combined tetrahedron and prism meshing approach, therefore it was incorporated during the investigations of the TRM. At first the grid study was carried out utilizing only tetrahedron-type elements while varying the element size globally without resolving the boundary layer (no BLR) to ensure sufficient resolution within the core flow region. Subsequently prismtype elements were included in the mesh to separately add boundary layer effects to the core flow phenomena.

Since heat transfer investigations shall be conducted in the future, these mesh configurations resolve the boundary layer (BLR) with y + values around 1 on the gas turbine surfaces and y + values under 11 for the remaining domain walls to avoid transition through the buffer region of the near wall flow. e boundary layer is resolved with 13 elements perpendicular to the surface wall at an expansion ratio of 1.5, chosen based on a trade-off between element count, cell volume change within the boundary layer as well as between the outer prism element and the adjacent tetrahedron core flow mesh. During the refinement process of the BLR meshes, the total height and therefore spacing of the boundary layer elements was kept constant and only the element count along the bounding walls was varied. e influence of the mesh resolution on the dimensionless pressure loss coefficient ζ is given in figure 8 for a model setup without and with BLR.

Element count [-] ζ [-]

0 2E+07 4E+07 6E+07 8E+07 0.8 0.9 1.0 1.1 1.2 1.3

TRM no BLR TRM BLR A B A# B# C# D# Figure .
Grid sensitivity study of the TRM ζ is calculated as defined in equation 7 with ∆p tot being area-averaged total pressure difference between package inlet and outlet ducts and p dyn,in the area-averaged dynamic pressure at the inlet ducts. With the first investigated mesh without BLR and about 3 million elements, ζ was obtained to be just below unity. During the course of refinement ζ increases and converges to a value of about 1.11 for a cell count of about 29 million elements. Increasing the element numbers by more than a factor of two to a total of about 77.8 million elements leads only to an insignificant change of ζ. Based on the results without BLR mesh A could have been chosen as being sufficiently resolved. To prove this assumption is also valid for models with BLR, additional meshes A# and B# were created incorporating the previously described prism element distribution near bounding walls as well as the same global element size as mesh A and B, respectively. When resolving the boundary layer the overall level of ζ is increased and it decreases over mesh refinement. In contrast to the non-BLR configurations neither A# nor B# can be regarded as sufficiently resolved and therefore further refinement was investigated utilizing meshes C# and D# which resulted in significantly lower values compared to B# but insignificant differences to each other. is indicates that pressure losses are resolved sufficiently in the core flow as well as in the near wall regions leading to mesh C# being chosen as the grid study's resulting mesh with a value of ζ around 1.18 and 31.6 million elements. To identify these regions of interest (ROI), which are located especially around the hot gas turbine components, the TRM with the chosen mesh C# was analyzed as illustrated in fig-

ure 9. Generally the MEVF is well designed, with only some stagnation regions remaining. Due to the blockage of the inlet casing a wake space occurs between the combustion chambers and the inlet casing (ROI 1 and ROI 2), similar to the flow observed in figure 6c. Since the flow is redirected downwards at the inlet casing, only smaller stagnation regions remain underneath the combustion chambers. ROI 3 is located underneath the gas turbine diffuser. ough the ventilation system would have diluted any possible leakage being transported to ROI 3 way below its lower explosion limit, long term accumulation could still lead to explosive air-fuel mixtures. erefore, ROI 3 should also be considered.

.

TEST RIG INSTRUMENTATION

Having a large test rig with multiple regions of interest, Particle Image Velocimetry (PIV) was selected as the main technique for measuring the flow field inside the enclosure, since it allows the acquisition of complete 2-D (or with Stereo-PIV even 3-D) vector fields over large image planes at one instant. e required double-shu er camera and the laser sheet optics are mounted perpendicular to each other using profile frame constructions adaptable to the model enclosure.

ereby, PIV images can be recorded under various orientations from several positions around the enclosure, since optical access is given by the glass windows with only few shadowed or hidden regions remaining due to geometrical restrictions.

Further standard measuring equipment, consisting of several pressure and temperature probes, mainly serves for monitoring the test rig's operation point at the package inlets, outlets and the generator volume flow rate during PIV imaging in order to create definite, comparable flow behavior under varying environmental conditions (see figure 10). Total pressure probes combined with Type-T thermocouples as well as static wall pressure taps are installed at each inlet and outlet duct for measuring the flow rates and the overall pressure loss of the package (setup 1 figure 10a). e angular adjustable probes can be traversed along two perpendicular axes in order to determine the flow velocity profiles and the swirl of the inflow resulting from the main ventilation fans as well as of the outflow for every operation condition. At the generator inlet opening a traversable rake with 10 pitot tubes provides the inflow total pressure and velocity field in combination with wall pressure taps inside the generator casing (setup 2 figure 10b). Additional pressure probes or static wall pressure taps can be deployed around the enclosure as required by utilizing the modular test rig design, e.g. replacing the glass plates within the window modules with instrumented metal sheets, thus allowing additional measurements for CFD validation purpose at several regions of interest. Pressure signals are transferred to two pressure transducer arrays (Scanivalve ZOC17IP/8PX) and recorded by a data acquisition system (NI PXI), which are located in a temperature-controlled room.

At the package inlet time-resolved velocity measurements will be carried out within the region of the free jet using 1-D Constant Temperature Anemometry (CTA) probes, as the .

CONCLUSION AND OUTLOOK

A scaled test rig of a state-of-the-art MGT-6100 gas turbine package from MAN Diesel & Turbo SE was developed with focus on the ventilation flow within the enclosure to be used for experimental and numerical explosion safety investigations. erefore, the gas turbine components were designed mainly as flow obstacles and do not feature any combustion system or power output. During the course of development, analytical as well as CFD-based studies were carried out to identify a model scale which is both feasible and allows the best possible results in terms of transferability to the realscale machine, even under operation condition restrictions in the test facility. Due to the complex nature of the flow within the enclosure, several aspects were taken into account like heat transfer, natural convection, the interaction of the MEVF and IGCF as well as the momentum ratio of the MEVF and possible fuel leakage flows for cases of engine failure. ese studies showed that a model scale of 1:3 represents the best trade-off between similarity, construction effort, and cost and that the ventilation system has to be able to accomplish Re-similarity operation condition. Allowing all possible cases of fan failure to be investigated experimentally, the test rig features fans at the inlet and outlet ducts of the enclosure, as seen in the real-scale machine. Changes of component arrangement in the actual package led to the creation of an additional CFD model, which was used to obtain regions of interest for PIV measurements.

Future work will include the commissioning of the enclosure test rig as well as PIV measurements in cold configu-ration. ese results will then be used to comprehensively validate the TRM, which shall be used to carry out detailed investigations on the flow structure within the enclosure. e test rig will then be modified to investigate the optimization potential of the MEVF inlet orientation being changed from horizontal to vertical as well as to feature heated gas turbine components and gas supply for leakage investigations. ese results can be used to validate the TRM even further and will allow more comprehensive explosion safety investigations in the future.

  Figure . Final construction of the downscaled (1:3) package test rig

Figure .

 . Figure . Influence of model scale factor and test rig operation condition on selected similarity quantities

Figure .

 . Figure . Surface Mesh of the numerical base model [ ] modified and used as the design study model (DSM)

( 1 )Figure .

 1. Figure . Boundary setup of the numerical base model [ ] modified and used as the design study model (DSM)

Figure .

 . Figure . Flow path of the diluted CH leakage jet (red) being released from leakage position 2 (figure 5, LP2) in opposing flow direction to the MEVF (green) near the hot gas turbine components using the DSM

  high complexity of the flow problem, most flow quantities do not converge asymptotically towards a constant value over the course of simulation iteration but rather fluctuate around a mean value. When evaluating only the last iteration step of such steady-state simulations, the results can vary depending on the state of the local fluctuations. erefore, all calculations were carried out over 2000 iteration steps with the results being averaged over the last 1000 steps. e averaged value of ζ is then displayed along with its corresponding standard deviation during the averaging interval, thus minimizing errors due to fluctuations in the flow field while maintaining information about the degree of fluctuation quantitatively.

Figure .

 . Figure . Distribution of stagnation regions within the enclosure near the gas turbine for the TRM using mesh C# in OP-Re AWT

Figure .

 . Figure . Test rig instrumentation for operation point monitoring and flow profile measurements

Table .

 . Numerical setup of the DSM and TRM

	Parameter		Description
	Medium		Ideal gas mixture
			(air and CH )
	Turbulence model	k-ω SST, automatic
			wall function
	Heat transfer model	Total energy
	Advection scheme	High resolution
	Simulation type	Steady-state
	Boundary Type	Physical property
	1	Outlet	Static pressure
		(air & CH )
	2	Wall	No slip wall
			Adiabatic
	3	Inlet (air)	Velocity
			Static temperature
	4	Outlet (air) Velocity (DSM)
			Mass flow (TRM)
	5	Inlet (air)	Velocity
			Static temperature
	6	Inlet (CH ) Velocity
			Static temperature
	7	Wall	No slip wall
			Temperature profile

Table .

 . Test rig properties and performance data

	Dimensions		Value Unit
	Model scale		1:3 -
	Enclosure Volume		3.63 m 3
	Performance	OP-Ma OP-Re Unit
	Flow rate MEVF	5,584 16,707 m 3 /h
	Flow rate IGCF	3,111	9,308 m 3 /h
	Room changes	25.6	76.7 1/min

ACKNOWLEDGMENTS e development work was conducted as a part of the joint research program COORETEC-Turbo. e work was supported by the Bundesministerium für Wirtscha und Technologie (BMWi), as per resolution of the German Federal Parliament under grant number 03ET7030B. e authors gratefully acknowledge COORETEC-Turbo and MAN Diesel & Turbo SE for their support and permission to publish this paper. Additionally, this publication was supported by the Ruhr University Research School PLUS, funded by Germany's Excellence Initiative [DFG GSC 98/3]. e responsibility for the content lies solely with its authors.


Nathalie Rouché

Jean-François Boyer

SIEREM : Information system of long-term historical data recorded in Africa

Data and information are often scattered, heterogeneous or incomplete ; they are rarely comparable and suited to needs. Numerous public, semi-public and private organizations produce and manage data, but often they do not have the resources to exchange assemble standardize summarize and capitalize on the data

General inventory of Environnemental Information System SIEREM

The Congo river is the world's second largest river both for its mean Number of Series for CONGO watershed exchange, assemble, standardize, summarize and capitalize on the data that they possess. Over and above these difficulties, there is also the more general problem of a natural and widespread reluctance to share information, particularly when it is considered strategic because it can be used for paid services or to provide access to power. second largest river, both for its mean flow and for its catchment area 3,7 millions of km2. Discharge 40,000 m3/s at Kinshasa Source : L'hydrologie tropicale: géoscience et outil pour le développement IAHS Publ. no. 238,1996. While most countries and basins (national or transboundary) clearly need to make an effort to alleviate current data deficiencies, it is also vital that they develop links between data producers and users no matter what the theme or level of intervention (local, basin, national and international) and reinforce capacities for accessing, processing and using existing data It is in this context and aware of these stakes that using existing data. It is in this context and aware of these stakes that HydroSciences Montpellier Laboratory has developed an information system known as SIEREM, which contains several types of environmental variables for the whole of Africa.

Hydrometric data recorded in Sierem database

Metadata of discharges stations

Different Type of data can be managed : discharges, water level, rainfall, temperature, humidity The SIEREM site provides free access to all information except raw measurement data, which are the property of the national services of African countries.

Catchment basins -Photos -Book Map Library

  daily discharges in SIEREM With 13,720 measurement stations and 33,740 chronological series (i.e. more than 120 million recordings) for 1837-2015, this is the million recordings) for 1837 2015, this is the largest environmental information system in Africa.

  data is combined with spatial data : 201 contours of catchment basins and 2,962 rivers. SIEREM has also been enriched with data recovered from historical hydrological archives. More than 1,342 photos have been brought together in 391 geo-referenced albums. http://www.hydrosciences.fr/sierem/produits/index.asp?frame=datasig Annual rainfall map of Africa Duration in years of daily discharges in Western and Central Africa SIEREM produces kml files that place the station on Google Earth. The data become more accessible, time series plotted and metadada can be seen on the context of the landscape. Number of daily discharges Western and Central Africa Map of the density stations of the SIEREM data base Number of daily discharges Western and Central Africa Gabon : Ogooue at Ayem Lat -0 1037 long 11 4151 GIS layers of Nyong watershed, hydrometric stations Cameroon Historical dossier file Mali : Bani at Douna Lat. 13.21, long. -5.90 G. MAHÉ 02/2001 Lat. -0.1037, long 11.4151 JP. BRICQUET 01/2017 RÉFÉRENCES -BOYER JF, DIEULIN C, ROUCHÉ N, et al. / SIEREM an environmental information system for water resources. 5th World FRIEND Conference, La Havana -Cuba, Nov. 2006 in Climate Variability and Change -Hydrological Impacts. IAHS Publ. 308, p.19-25, 2006. -LOUVET S, PATUREL JE, MAHÉ G, ROUCHÉ N, KOITE M / Comparison of the spatiotemporal variability of rainfall from four differents interpolation methods on the result of GR2M hydrological modeling -case of Bani River in Mali, West Africa. Theoretical and Applied Climatology, 2016, 123 (1-2), p. 303-319. -MAHÉ G, ROUCHÉ N, DIEULIN C, BOYER JF, IBRAHIM B, CRÈS A, SERVAT E, et al. /. Carte annuelle en Afrique = Annual rainfall map of Africa. Montpellier (FRA) : IRD, 2012 -SIDIBE, M. DIEPOIS, B. ROUCHÉ, N. et al. Trend and variability in western and central Africa streamflow, and their relation to climate variability between 1950 and 2010. Geophysical Research Abstracts .Vol. 19, EGU2017-8302, 2017 .EGU General Assembly 2017


E G Piracci
email: piracci@disp.uniroma2.itpetro@ieee.orgg.galati@ieee.org

N Petrochilos

G Galati

Single-antenna projection algorithm for Mode S based airport traffic surveillance

Keywords: Secondary Surveillance Radar, Mode S, Multilateration system, Source separation, Projection algorithm

Within the Surveillance function in the A-SMGCS frame, Multilateration (MLAT) systems are being more and more used for location and identification of aircraft and of cooperating vehicles [1] on the airport surface. This situation, with increasing traffic in the downlink SSR channel, may lead to a too high number of superimposed, i.e. garbled, signals. The aim of this work is the need for reliable, simple effective algorithms to separate overlapped SSR signals. We proposed earlier some solutions [2,3], as well others can be found in the literature [5,6]. All of them require an array antenna at the receiver stations, while today's receiver stations are equipped with only one omni-directional antenna. In this paper, the sources separation algorithm based on array processing (PA: Projection Algorithm) is adapted to a single-antenna configuration, where the variation of carrier frequencies of different vehicular transponders [8] is exploited. Simulations with signals synthesized from real measurements give results that demonstrate the effectiveness of the algorithm in a frequency agility environment.

I. INTRODUCTION

Location and identification of cooperating aircraft in the airport area (and beyond) may be implemented by multilateration (MLAT) systems. These systems are based on a network of receiving stations and use the Secondary Surveillance Radar (SSR) Mode S signals. The SSR Mode S signals are pulse position modulated finite-length at a carrier frequency of 1090 MHz (see Sect. II). The Multilateration systems rely on the estimation of time of arrival of the emitted signal from the target, to perform intersection of hyperbolic surfaces. Using non-avionic requirements transponders (i.e. "non transponder devices") it is possible to extend the surveillance to cooperating vehicles [START_REF] Galati | Multilateration applied to airport vehicles management systems: the Agile Trasponder[END_REF]. Long range distributed receive stations allows to perform a wide area multilateration (WAM) extending the surveillance area. The two enhancements, i.e. vehicular applications and WAM, increase the number of received replies per unit time. Most of these signals, spontaneously emitted from on-board Mode S transponders (and called "squitter"), arrive randomly at the receiving stations, with an arrival density that is expected to increase in the future. The use of omni-directional antennae at the receive stations increases the probability of overlapped Mode S signals in the time domain. When replies overlap, very often the reply message is corrupted and cannot be recovered, nor the aircraft can be located and identified.

Source separation by signals acquisition from an array antenna is well investigated [START_REF] Petrochilos | Projection techniques for separation of multiple secondary surveillance radar sources in a real environment[END_REF][START_REF] Petrochilos | Separation of multiple Secondary Surveillance Radar sources in a real environment by a novel projection algorithm[END_REF]. However today MLAT receive stations are equipped with a single omni-directional antenna/receive channel. In the following, we describe our main contribution, i.e. the adaptation of the PA (projection algorithm) [START_REF] Petrochilos | Projection techniques for separation of multiple secondary surveillance radar sources in a real environment[END_REF][START_REF] Petrochilos | SSR receiver: A real implementation[END_REF] for the case of a single antenna receiver. The PA allows us to separate superimposed Mode S replies by the use of spatial filtering, as the overlapped signals originate from different angles of arrival. We remind that for SSR signals there is a tolerance of ±1 MHz centred at 1090 MHz [START_REF] Stevens | Secondary Surveillance Radar[END_REF]10], and that propose to use frequency shifted signals for vehicles [START_REF] Galati | High-capacity location and identification system for cooperating mobiles with frequency agile and time division transponder device on board" Assignee: Università degli Studi di Roma "Tor Vergata[END_REF]. One antenna receiver cannot exploit the spatial diversity, therefore our modified algorithm uses the residual carrier frequency of the signals, and so the phase accumulation sample by sample as a "time diversity". In short, the novel idea is based on the re-shaping of the acquired data, and on the use of the frequency diversity of the overlapped signals to perform the source separation.

This paper deals with the problem of only two equal-length Mode S replies overlapping in time. We first present (Sect. II and III) the used data model and the PA algorithm for an array antenna system. In sect. IV we show the data model introduced to adapt the PA algorithm to the mono-antenna case. Finally (Sect. IV) we present a preliminary analysis on the algorithm performances and some study cases to show the source separation capability.

II. DATA MODEL

We use a model for Mode S reply. A mode S reply contains 56 (short) or 112 (long) binary symbols [ ] n b . The symbol period is 1 µs, and each symbol is made up by two 0,5 µs chips. The bit are encoded as the Manchester scheme, that is the b n =0 is coded as [0,1], and the b n =1 is coded as [START_REF] Galati | Multilateration applied to airport vehicles management systems: the Agile Trasponder[END_REF]0]. A reply is formed by a preamble, [ ] 1, 0,1, 0, 0, 0, 0,1, 0,1 = p , followed by the encoded data,

[ ] 1 2
, 0,0,0,0, 0,0, , , where p(t) is a rectangular pulse of width T= 0,5 µs. For transmission the signal is up-converted to the nominal frequency f n =1090 MHz, with a ±1 MHz of tolerance permitted to the airborne transponder. At the ground receiver, after the downconversion to base band, a residual frequency f r remains, adding a phase rotation to the transmitted symbols. The received base band signal is

: [ ] [ ] ( ) [ ] exp 2 n r s n b n j n f T b n π φ = =
, where:

( )

exp 2 r j f T φ π = (2.1)
is the phase shift due to the residual carrier frequency over a sampling period.

We consider an m elements array antenna and d independent source signals.

= + X MS N, where [ ] [ ] 1 , , N ⎡ ⎤ = ⎣ ⎦ X x x … is the m N × received signal matrix. [ ] [ ] 1 , , N ⎡ ⎤ = ⎣ ⎦ S s s … is the d N × source matrix, where [ ] [ ] [ ] 1 , , T d n s n s n ⎡ ⎤ = ⎣ ⎦ s … is a stacking of the d source signals.
N is the m N × noise matrix, whose elements are temporally and spatially white. M is the m d × mixing matrix that contains the array signatures and the complex gains of the sources. We assume that the replies are independent (so uncorrelated) and M is full column rank. Finally, we assume d m < .

III. PA PROJECTION ALGORITHM

In this work we consider the case of two sources, i.e. two overlapping Mode S signals of equal length. (see Fig. 1).

If the two Mode S replies have significantly different times of arrival, t 1 and t 2 , the time support of the two sources is partly overlapping. Then at the beginning of the data record (t 1 till t 2 ) there is only one source present; and at the end of the data (t 3 till t 4 ), only the second source is present.

The first step of the algorithm consists in the detection of the t i 's. The data resulting from signal sampling at f s =50 Ms/s is By doing so, we aim at recovering our previous model: = + X MS N . Meanwhile, there are two major differences: first, as shown in example from Fig. 3, it is obvious that this signal will need two components to be described: the one from slot 1 and the one from slot 2, slot 3 being only noise, and slot 4 being equivalent to slot 1. It appears then that a source is not anymore described by a rank-one matrix. In [START_REF] Petrochilos | SSR receiver: A real implementation[END_REF], we have seen that the rank of this matrix for a single reply is equal to m (the number of lines), with a notable exception: when m is set equal to number of samples in a pulse (i.e. in half a symbol), then a special case arises and two components only are needed. As in the example from Figure 3, the rank of matrix M is 2 (in the noiseless case) instead of m. We define the rows of the matrix M as the signature vectors (in conventional array processing, they are called array signature vectors). By extension it appears that for m= 25 × p (corresponding to time slices integer multiple of 0.5 µs.), the number of component needed is 2p.

Second, if we look more carefully at the matrix M, we note that a column is not originated from space diversity anymore, but by frequency diversity, Indeed, if the initial sampling would be synchronized with a pulse, a vector m would in fact contains the accumulated phase shift φ, described in eq. 2.1: m = [1 φ 2 … φ m-1 ] T . Given that the frequency shift with respect to the nominal value is a kind of signature for each reply, and frequency shifts are variable from one reply to the other one, our technique generates, with several sources, a matrix M which is full column rank. Nevertheless the conditioning number directly depends on the choice of m and, of course, on the difference between the frequency shift. It is easily understood that small values of m causes high conditioning numbers. The consequence of a high conditioning number is that the pseudo-inverse is not robust, i.e. the noise contribution increases dramatically. Indeed, we observe that to separate two overlapped pulses, the needed conditioning number of 10, when the two frequencies have a difference of 150 kHz, the length m has to be over 35 [START_REF] Petrochilos | SSR receiver: A real implementation[END_REF].

Next to the generation of the model, out technique propose a projection over the subspace spanned by the matrix M, thus reducing the noise by removing the noise subspace and separating the contributions of the two sources into two matrices X 1 and X 2 , the last step is to apply the reverse rearrangement, in order to produce the time series s 1 and s 2 .

Next, in this simulated example two overlapping Mode S replies, with the standard length of 64 µs, an amplitude ratio equal to 0.6 (i.e. the leading reply has an amplitude 60 % of the trailing one) and a time delay of 33 µs are received on a single antenna. The SNR is 85 dB, and the residual frequency shifts are equal to -100 and +50 kHz respectively. Figure 4 shows the absolute values of the matrix X displayed column-wise.

We observe that we need only 2 or 3 signature vectors to represent the pulses of each reply (and not m). In this example, 5 signature vectors only are needed for both sources (3+2) to create a projection matrix that reach a very acceptable separation, (see Fig. 5). The conditioning number of the estimated M was 36.

V. EXPERIMENTAL PERFORMANCES

In this section we present the results of performance evaluation tests with semi-synthesized signals. The analysis is done using real-world SSR signals, received and recorded (at 50 Mega samples per second) by means of an ad-hoc system implemented by the Technical University of Delft (CAS/IRCTR). This system is made up by a four-elements receive array connected to a wide band acquisition system, but of course in this study we used one channel only. We mean by the term "semi-synthesized" a real signal modified after the recording, in order to change the carrier frequency and the SNR of each source. Choosing a time delay we apply a coherent sum of the two sources to obtain an overlapping case very close to a real case. The carrier frequency variation becomes a residual frequency shift after the base-band translation.

We generated the overlapped signals in order to appreciate the algorithm behavior versus the frequencies shifts and the value of m, equal to {25, 50, 75}. The evaluations were performed in the following conditions. First, we selected replies allowing to obtain a signal-to-noise ratio (SNR) of at least 20 dB. Second, the delay of the trailing reply with respect to the leading reply was set to 30+ε µs where ε is a random variable uniformly distributed in [-0.5, +0.5] µs. Third, the frequency shift between both replies was set between 0.1 and 1 MHz with 10 steps by 0.1 MHz each. Overall, we got about 1560 cases.

A. Analysis of the conditioning numbers

Figure 6 shows the measured probability density of the conditioning number's of the M matrix for each semi-synthesized signal as a function of m. As we did expect, there is an improvement for the conditioning of M when m increases. The "rule of the thumb" for the conditioning number says that to obtain a good separation the conditioning number must be less than 10 for SNR equal or above 20 dB. Figure 6 shows that over 1560 superimposed signals, only approximately 200 signals present a conditioning number for the matrix M less than 10 (using m=75). Table 1 presents, for each m, the statistical properties of the conditioning number's (mean and median) and the percentage of conditioning number's less than 10.

B. Effect of frequency shift

We compare the PA with the conventional decoding algorithm presently used in SSR receivers. Fig. 7 shows the failure rate (i.e. the fraction of cases when reply decoding failed, [START_REF] Petrochilos | Separation of multiple Secondary Surveillance Radar sources in a real environment by a novel projection algorithm[END_REF]) as a function of the frequency shift between the overlapped replies: at the left, shows the failure rate for detection of the leading reply, at the right shows the failure rate for detection of the trailing reply. In the case of the trailing reply, the failure rate is equal to one for the conventional decoder which, in absence of a sources separation function upstream, cannot work because of the garble of the preamble due to the leading reply. We evaluated the effect of the m value on the source separation and decoding. Figure 7 shows that the failure rate decrease as m increases. The results with m equal to 50 or 75 are quite similar but there is a lot of difference using m equal to 25. In any case we found a much better performance of the PA with respect to the conventional decoder. The error bit number per reply decreases as the frequency shift increases. As for the failure rate analysis, setting m equal to 50 or 75 gives quite similar results, but there is a significant performance degradation using m equal to 25.

VI. CONCLUSION

In this article, we presented an adaptation of the PA for mono-antenna receiver, by the application to the data of a "reshaping" into a matrix form. In which case, the frequency diversity between the replies is naturally exploited, and we advocate to force this diversity to the cooperating vehicles found on the airport grounds. With our preliminary study, we demonstrate the improvement to overcome the inevitable garbling by using mono-antenna-PA. The results evidence that to obtain a satisfactory separation it is necessary set m at least equal to 50, while it is not clear if the improvement by m equal to 75 is worth the computational cost. Future studies will allow to evaluate the performances of our algorithm under various operating conditions.

  S reply emitted by the transponder has the form:

Fig. 1 :Fig. 3 :

 13 Fig. 1: A record of two Mode S overlapped replies (case w5)

Fig. 4 :Fig. 5 :

 45 Fig. 4: Plot of the columns of submatrix of X, left: before n= 60 (i.e. 30 µs), right: after n= 140 (i.e. 70 µs)Where n is the column number, and m= 25

Fig. 6 :

 6 Fig. 6: Conditioning number density of M for m = {25, 50, 75}

Fig. 7 :

 7 Fig. 7: Failure rate as a function of the frequency shift

Fig. 8 :

 8 Fig. 8: Average error bit number as a function of the frequency shift

Table 1 : Conditioning number of M for m = {25, 50, 75}: statistics

 1

		m=25	m=50	m=75
	mean	26	18	17
	median	24	17	16
	% of cond(M)<10	4 %	8 %	11.7 %

ACKNOWLEDGMENT

The contribution by the Radar Research Centre (CRR: Centro Ricerche Radar) is gratefully acknowledged.

sliced by time slots of 200 samples (4 µs), on each time slot is performed a whiteness test based on the Singular Value Decomposition (SVD), see Fig. 2, of the matrix X.

This allows us to estimate the presence of the sources as a function of time, and to isolate the two time support when each source is single.

By the notation ( ) ( )

. we indicate the matrix collecting the subset of the columns related to the time interval t 1 till t 2 (selection of the columns). Similarly, we define the notation ( ) ( )

for the subset of the columns related to t 3 till t 4 . Hence, we have the following relationship:

where the matrix ( )

where the i m 's are the columns of M, and the i s 's the rows of S.

Note that ( ) 1 X and ( )

2

X are rank-one matrices in the noiseless case. By SVD on ( ) i X , we can estimate the main vector ˆi m , which is the vector corresponding to the highest singular value (i=1, 2).

Once the space signatures 1 m and 2 m have been identified, the matrix M is thus estimated. We finally multiply X by the Moore-Penrose pseudo-inverse of M ( ( )

, and recover the estimated sources: † ˆ= S M X.

IV. MONO ANTENNA ADAPTATION

The adaptation of the PA to the single-antenna case is achieved by a rearrangement of the received data: from a time series originated by a single antenna/receiver consisting of N samples, we construct a matrix of size m × l, where the first column of this matrix contains the first m data samples, the second column the next m samples, and so forth, l being to the rounded part of N/m. An example can be seen in Fig. 3, where the principle is shown for a single reply.