Thomas Curt
email: thomas.curt@irstea.fr

Thibaut Frejaville

WILDFIRE POLICY IN MEDITERRANEAN FRANCE: HOW FAR IS IT EFFICIENT AND SUSTAINABLE?

Keywords: fire policy, sustainable risk management, fire prevention, fire suppression, global changes, fire paradox

A new fire policy reinforcing aggressive fire suppression was established in Mediterranean France in response to the devastating wildfires of 1990's, but to which extent this has changed fire activity yet remains poorly understood. For this purpose we compared the number and location of ignitions and of burned areas between two 20-year periods (1975-1994 vs 1995-2014), in parallel to the changes in fuel covering, human activity promoting ignitions, and fire weather. The number of fires decreased almost continuously since 1975, but sharply after 1994, suggesting an effect of better fire prevention due to the new policy. But the major change in fire activity is a considerable reduction in fire size and burned areas after 1994, especially during summer and in the most fire-prone places, in response to massive efforts put into fire suppression. These reductions have occurred while the covering by fuel biomass, the human pressure on ignition and the fire weather index increased, thus making the study area more hazardous. Our results suggest that a strategy of aggressive fire suppression has great potential for counterbalancing the effects of climate changes and human activities and for controlling fire activity in the short term. However, we discuss whether such suppression-oriented approach is sustainable in the context of global changes, which cast new fire challenges as demonstrated by the devastative fires of 2003 and 2016. We advocate for a more comprehensive fire policy to come.

INTRODUCTION

The pyrogeographic theory states that fire regime in a region depends upon three basic features: the availability of vegetation fuels [START_REF] Pausas | Fuel shapes the fire-climate relationship: evidence from Mediterranean ecosystems[END_REF][START_REF] Pausas | The global fire-productivity relationship[END_REF], the climate, and ignitions [START_REF] Parisien | Environmental controls on the distribution of wildfire at multiple spatial scales[END_REF][START_REF] Knorr | Demographic controls of future global fire risk[END_REF]. However, additional and often disregarded factors operating at local to regional scales such as fire policy may cause abrupt changes in fire activity [START_REF] Pausas | Abrupt Climate-Independent Fire Regime Changes[END_REF]. Indeed, policy can prevent ignition by humans, reduce the burned areas through suppression, modify fire behavior through fuel management [START_REF] Pyne | Introduction to wildland fire[END_REF] and thus mitigate the long-term risk of wildfires for humans and ecosystems. An efficient fire policy may thus act as a switching factor of fire activity in a similar magnitude to climate change [START_REF] Fréjaville | Spatiotemporal patterns of changes in fire regime and climate: defining the pyroclimates of south-eastern France (Mediterranean Basin)[END_REF]. As a consequence, adapting fire policies to global changes (i.e. climatic and socio-economic changes) can thus become crucial for controlling fire activity in the long-term and reduce fire impacts on assets [START_REF] Efimed | Wildfire Prevention in the Mediterranean. A key issue to reduce the increasing risks of Mediterranean wildfires in the context of Climate Changes[END_REF]. Indeed, Euro-Mediterranean regions are characterized by increasing density of humans, infrastructures and wildland-urban interfaces, which increase the potential for ignition [START_REF] Castellnou | Improving wildfire scenarios: Learning from experience. EFI Research Report 'Towards Integrated Fire Management -Outcomes of the European Project Fire Paradox[END_REF] and make suppression more complex [START_REF] Lahaye | Classification of large wildfires in South-Eastern France to adapt suppression strategies[END_REF]. Together with the increased frequency of exceptional weather conditions, these issues promote fire activity, and notably large and devastating fires [START_REF] San-Miguel-Ayanz | Analysis of large fires in European Mediterranean landscapes: Lessons learned and perspectives[END_REF].

Following the devastating wildfires of the period 1973 to 1990, France established a new fire policy in about 1994. This policy aims at better preventing but above all at suppressing wildland fires in its Mediterranean fire-prone areas [START_REF] Dsc | Protection de la forêt contre l'incendie guide de stratégie générale Ministère de l'intérieur et de l'aménagement du territoire[END_REF]. Indeed, the French authorities feared that fire activity could increase due to the intermingling of urbanized areas and wildland, and the increase of tourism and infrastructures that may generate more ignitions. One-third of the budget has been allocated to prevention and two-thirds to suppression [START_REF] Chatry | Rapport de la mission interministérielle "Changements climatiques et extension des zones sensibles aux feux de forêts" (National Report on Climate Change and the Extension of Fire Prone Areas in France[END_REF]. Fire prevention has been reinforced through public information and limiting frequentation into forests during the peak fire season. Fuel reduction has remained local, mostly beside roads or at rural-urban interfaces where most ignitions occur. However, the cornerstone of the tactic was to ensure a systematic, rapid, hard-hitting initial attack on all ignitions, regardless the fuel type and the weather conditions. The objective was to avoid any fire developing to a size and a level of intensity which would make it difficult and more expensive to suppress [START_REF] North | Reform forest fire management[END_REF], as it has been found that fire size and intensity grow exponentially with time during the buildup phase [START_REF] Pyne | Introduction to wildland fire[END_REF]. In practice, firemen tend to extinguish fires before they reach one ha in size and in less than ten minutes in order to follow a heuristic [START_REF] Chatry | Rapport de la mission interministérielle "Changements climatiques et extension des zones sensibles aux feux de forêts" (National Report on Climate Change and the Extension of Fire Prone Areas in France[END_REF].

Strong debates developed worldwide on the impact and the effectiveness of fire policies [START_REF] Efimed | Wildfire Prevention in the Mediterranean. A key issue to reduce the increasing risks of Mediterranean wildfires in the context of Climate Changes[END_REF][START_REF] Steelman | Is Wildfire Policy in the United States Sustainable?[END_REF][START_REF] Moritz | Learning to coexist with wildfire[END_REF][START_REF] Tedim | Wildfire risk management in Europe: the challenge of seeing the 'forest' not just the 'trees[END_REF]. Suppression-based policies have proved effective to reduce burned areas in boreal regions [START_REF] Cumming | Effective fire suppression in boreal forests[END_REF][START_REF] Schoennagel | Landscape heterogeneity following large fires: insights from Yellowstone National Park, USA[END_REF][START_REF] Stephens | Federal forest-fire policy in the United States[END_REF]. Likewise, increasing efforts of prevention and suppression in southern Europe may explain for a part the decrease of burned area since the mid-eighties in most countries, with the exception of Portugal [START_REF] Turco | Decreasing fires in a Mediterranean region (1970-2010, NE Spain)[END_REF][START_REF] Turco | Decreasing Fires in Mediterranean Europe[END_REF]. However, many of these countries (Portugal, Spain, Greece) have also suffered devastating megafires [START_REF] San-Miguel-Ayanz | Analysis of large fires in European Mediterranean landscapes: Lessons learned and perspectives[END_REF]. In California, some argue that suppression policy has allowed an unnatural accumulation of fuels (the 'fire paradox' effect sensu [START_REF]Towards Integrated Fire Management -Outcomes of the European Project Fire Paradox[END_REF]) and thus increased fire hazard and massive fires [START_REF] Minnich | Fire mosaics in Southern-California and Northern Baja California[END_REF] while others stated that fire suppression activities are not the cause of these fire events but rather exceptional weather conditions [START_REF] Keeley | Reexamining fire suppression impacts on brushland fire regimes[END_REF][START_REF] Keeley | Large, high-intensity fire events in southern California shrublands: debunking the fine-grain age patch model[END_REF][START_REF] Moritz | Spatiotemporal analysis of controls on shrubland fire regimes: Age dependency and fire hazard[END_REF]. In southern Europe, some authors argue that suppression may have caused more large fires and an increase in fire intensity [START_REF] Pinol | Modelling the effect of fire-exclusion and prescribed fire on wildfire size in Mediterranean ecosystems[END_REF][START_REF] Pinol | Conditioning uncertainty in ecological models: Assessing the impact of fire management strategies[END_REF] while others indicate that fire reduction is a complex response to fire suppression strategies but also to past fires and weather [START_REF] Brotons | How Fire History, Fire Suppression Practices and Climate Change Affect Wildfire Regimes in Mediterranean Landscapes[END_REF] and land cover changes [START_REF] Pausas | Fire regime changes in the Western Mediterranean Basin: from fuellimited to drought-driven fire regime[END_REF][START_REF] Salis | Analyzing spatiotemporal changes in wildfire regime and exposure across a Mediterranean fire-prone area[END_REF]. A recent study in the western part of Mediterranean France has shown that fire policy has altered the way weather conditions drive fire activity, and has produced a shift from a drought-driven system to a system driven by concomitant fuel dryness and strong winds [START_REF] Ruffault | How a new fire-suppression policy can abruptly reshape the fire-weather relationship[END_REF].

In order to test the extent to which fire regime has changed due to the new fire policy, it is important to analyze how fire weather, fuel covering and ignitions have also changed during the same period. Disentangling the part played by these different fire drivers is crucial for determining whether fire regime can be stabilized at the present level. This is especially important in the context of global change because ignitions (predominantly human) may increase with population growth and the development of road networks and wildland-urban interfaces [START_REF] Lloret | Wildfires and landscape patterns in the Eastern Iberian Peninsula[END_REF], fuel biomass and connectedness may increase due to the rural exodus syndrome [START_REF] Moreira | Landscape-wildfire interactions in southern Europe: implications for landscape management[END_REF], forest planting or spontaneous reforestation after land abandonment and the positive fuel-fire feedback in shrublands [START_REF] Curt | Wildfire frequency varies with the size and shape of fuel types in southeastern France: Implications for environmental management[END_REF]. In addition, fire weather may be more conducive to fires, and the fire season becomes longer [START_REF] San-Miguel-Ayanz | Analysis of large fires in European Mediterranean landscapes: Lessons learned and perspectives[END_REF]. However, no study has yet investigated the effect that this new fire policy has had on the extent, location, and timing of fire activity in southern France, in regards to fuel, weather, and ignitions. To this end, we compared the size distribution, location and seasonality of fires for two 20-year periods: 1975-1994 and 1995-2014. We aimed to determine the role of the new fire policy on fire activity, if it is sustainable in the context of global changes, and how it can be improved.

MATERIALS AND METHODS

Study area

Our case-study region is the southeastern part of France, which covers 80500 km² (Fig. 1). This fireprone area encompasses wide bioclimatic, environmental and anthropogenic gradients. The study area includes Mediterranean lowlands propitious to fire activity [START_REF] Fréjaville | Spatiotemporal patterns of changes in fire regime and climate: defining the pyroclimates of south-eastern France (Mediterranean Basin)[END_REF] and heat-waves [START_REF] Ruffault | Objective identification of multiple large fire climatologies: an application to a Mediterranean ecosystem[END_REF], mid-elevation hinterlands and foothills with supra-Mediterranean climate, and mountain areas (western Alps, Corsica, and Pyrénées) (Fig. 1). Elevation ranges from 0 to 2000 meters a.s.l. (Fig. 1). This area also exhibits a range of land covers and land uses, from cultivated lands and orchards with very low flammability to highly flammable forests and shrublands. Mediterranean forests are dominated by Pinus halepensis Mill. and Quercus species, while mountain forests are dominated by Pinus sylvestris L., Pinus nigra Arn., Fagus sylvatica L. and Abies alba Mill. Flammable shrublands (so-called garrigues and maquis) are common in the southernmost part of the study area. They are dominated by Quercus coccifera L., Ulex parviflorus Pourr., Cistus spp., and Erica arborea L. Human presence and activity both vary from the Mediterranean coast with its high concentration of people, infrastructure and resorts to rural hinterland with very low human density.

Fire data

We used the fire data from the French Prométhée database, which includes all fires that affected forests or wildland since 1973 in the 15 departments of southeastern France [START_REF] Prométhée | La banque de données sur les incendies de forêts en région Méditerranéenne en France[END_REF]. Prométhée specifically provides information on the date and size of fires, and their location on a 2*2 km grid comprising 20,142 pixels. Like many fire databases in Europe (see [START_REF] Pereira | The history and characteristics of the 1980-2005 Portuguese rural fire database[END_REF]), Prométhée contains a number of data errors (e.g. false location) and incomplete data. In a previous work, we have made an extensive effort to correct the data in order to prevent spatial and temporal biases [START_REF] Fréjaville | Spatiotemporal patterns of changes in fire regime and climate: defining the pyroclimates of south-eastern France (Mediterranean Basin)[END_REF]. As in many fire databases worldwide [START_REF] Turco | Decreasing fires in a Mediterranean region (1970-2010, NE Spain)[END_REF], the minimum area recorded varied with time: it was of 0.1 ha before 1990 then 0.0001 ha. We tested this effect, but it was negligible on the cumulated burned because the smallest fires (< 1 ha) account for a very small proportion of the total. We have thus separated fires into five classes: < 1 ha, 1-10 ha, 10.1-50 ha, 50.1-100 ha, and > 100 ha. These classes corresponded approximately to the percentiles 70, 95, 97, 98 and 99 of the distribution of fire size in our database, and to thresholds commonly used in Euro-Mediterranean countries [START_REF] San-Miguel-Ayanz | Analysis of large fires in European Mediterranean landscapes: Lessons learned and perspectives[END_REF].

History of fire policies

In France, fire activity is known with accuracy since 1973. This corresponds to the establishment of the first fire policy of the modern era including notably the setting of the Prométhée database and the computation of the fire weather index for prediction of daily fire danger (see Fig. 9). Many fires raged from 1976 to the 1990s, including the largest fires ever recorded in southeastern France [START_REF] San-Miguel-Ayanz | Analysis of large fires in European Mediterranean landscapes: Lessons learned and perspectives[END_REF]580 ha in 1990 in Provence). A new fire policy and strategy called 'Vulcain' was progressively set up from 1992 to 1994 in response to these devastative fires [START_REF] Dsc | Protection de la forêt contre l'incendie guide de stratégie générale Ministère de l'intérieur et de l'aménagement du territoire[END_REF]. This strategy promoted the reinforcement of prevention to suppression, with particular emphasis on massive suppression of all ignitions [START_REF] Alexandrian | Public policies affecting forest fires in the Mediterranean Basin[END_REF]. Vulcain still remains an important milestone in the history of modern fire policies in France, even if gradual adjustments have been made to account for the 2003 fires [START_REF] Perriez | Rapport sur la protection contre les incendies de forêt après les feux de l'été 2003 (Report on the protection against forest fires afeter the 2003 summer wildfires). IGA, CGGREF, IGE[END_REF] or for the potential impacts of climate changes [START_REF] Chatry | Rapport de la mission interministérielle "Changements climatiques et extension des zones sensibles aux feux de forêts" (National Report on Climate Change and the Extension of Fire Prone Areas in France[END_REF]. We consider 1994 to be the date from which the new fire policy was fully established throughout the study area. We thus compared two 20-year periods: 1975-1994 and 1995-2014. We discarded the years 1973 and 1974 for which fire data was incomplete. In total, the data included 103,456 fires and a cumulated area of 818,342 hectares.

Fire size distribution

We compared the distribution of the number of fires and the burned areas among the two study periods using the qqplots R function [START_REF] Development | R: A language and environment for statistical computing[END_REF]. This indicates how much the two distribution differs, and computing the percentiles of burned areas.

Temporal and spatial distribution of fires

We tested whether the number of fires and the total burned area have changed after 1994 for each of the five classes of fire size (< 1 ha, 1-10 ha, 10.1-50 ha, 50.1-100 ha, and > 100 ha) in comparison to before 1994. Changes (%) were computed as 1995-2014 minus 1975-1994, relative to 1975-1994. For each size class, p-values and standard deviation were computed by bootstrapping (1000 runs with 70% randomly sampled data points with replacement). Results were considered significant if p < 0.001. We also tested whether the seasonal distribution of the mean annual fire density and the mean annual area burned differed among the five fire size classes and between the two periods (1975-1994 versus 1995-2014). For this purpose we computed the seasonal changes (%) in the number of fires and the total burned area between 1995-2014 and 1975-1994, relative to 1975-1994. For each season, p-values and standard deviation were computed by bootstrapping 1000 runs with 70% randomly sampled data points with replacement. Results were considered significant if p < 0.001.

Both the climate and the fire activity varied across our study area, thus defining pyroclimatic regions [START_REF] Fréjaville | Spatiotemporal patterns of changes in fire regime and climate: defining the pyroclimates of south-eastern France (Mediterranean Basin)[END_REF]. Two regions were especially fire-prone: the maritime mountains of Corsica and Maritime Alps, and the lowlands of Provence and Languedoc-Roussillon-Cévennes. The less fire-prone areas correspond to hinterland mountains of the Alps, Pyrénées and Massif Central (Fig. 1). It is thus crucial to determine if the establishment of the new policy has impacted all regions or not, because this can have implications for adapting specific measures of prevention and suppression. For this purpose we computed fire density and burned areas kernels maps for the two periods (1975-1994 and 1995-2014), and we used analyses of variance and HSD post-hoc tests to compare the number of fires and the burned area before and after 1994 in each of the three pyroclimatic regions.

In order to determine the location of the hotspots of number of fires and burned areas, we used a kernel density estimation under using the R package kedd [START_REF] Development | R: A language and environment for statistical computing[END_REF], which is a non-parametric method of estimating the probability density function of a continuous variable. This method is acknowledged as efficient among the spatial autocorrelation statistics that exist [START_REF] Koutsias | Fire occurrence patterns at landscape level: beyond positional accuracy of ignition points with kernel density estimation methods[END_REF][START_REF] De La Riva | Mapping wildfire occurrence at regional scale[END_REF][START_REF] Gonzalez-Olabarria | Using multiscale spatial analysis to assess fire ignition density in Catalonia, Spain[END_REF].

Computation of fire drivers

We investigated if the human and environmental drivers of fire (i.e. fire weather, ignitions, and fuels; (47) changed after 1994 and could have thus impacted fire activity along with the new policy. These data were calculated on the same 2*2 km grid than the fire data.

Author-produced version of the article published in Risk Analysis, 2018, 38, 3, 524-533. The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/risa.12855 DOI: 10.1111/risa.12855

The fire weather index (FWI) is a unitless index originally designed to forecast fire risk in Canada on the basis of past and current weather condition [START_REF] Van Wagner | Development and structure of the Canadian Forest Fire Weather Index System[END_REF]. We calculated FWI daily by combining five subindices which use daily values of air temperature, air relative humidity, surface wind speed, and rainfall in the past 24 hours for the whole study area (see [START_REF] Fréjaville | Spatiotemporal patterns of changes in fire regime and climate: defining the pyroclimates of south-eastern France (Mediterranean Basin)[END_REF] for details). The daily meteorological data were taken from the SAFRAN reanalysis system implemented by MeteoFrance, at a spatial resolution of 8*8 km [START_REF] Vidal | A 50-year high-resolution atmospheric reanalysis over France with the Safran system[END_REF]. They were interpolated to 1×1 km resolution with the help of regional statistical modelling using lapse rates (see [START_REF] Zimmermann | (Ed), Chapter 6 -Management Strategies to Adapt Alpine Space Forests to Climate Change Risks[END_REF], using regionalized linear regressions for each homogeneous climatic zone (called 'symposium', [START_REF] Vidal | A 50-year high-resolution atmospheric reanalysis over France with the Safran system[END_REF]). FWI provides a numeric method for rating fire danger and potential fire intensity through the probability and ease of ignition and propagation of a fire on the forest floor, taking into account the weather conditions. It is acknowledged as a good indicator of meteorological fire hazard worldwide, notably in Mediterranean areas [START_REF] Ruffault | Daily synoptic conditions associated with large fire occurrence in Mediterranean France: evidence for a wind-driven fire regime[END_REF].

We also calculated a 'human pressure index' (HPI) which estimates the potential amount of ignitions caused by humans, which contribute for 90% of all ignitions in the study area [START_REF] Curt | Modelling the spatial patterns of ignition causes and fire regime features in southern France: implications for fire prevention policy[END_REF]. HPI sums the logtransformed density of population, houses and tourists based on data at the municipality level (source: Insee). For each data we computed values 1975-1994 and 1995-2014 at the 2*2 km grid. Natural ignitions caused by lightning strikes were not considered because they are a minority.

In order to estimate the availability of flammable wildland fuels in our study area, we computed the coverage by flammable fuels within each 2*2 km pixel. We used CORINE Land Cover (CLC) data in 1990 for the pre-1994 period, and in 2006 and 2012 for the post-1994 period. Flammable fuels comprised all forests (CLC 3.1), shrublands-grasslands-heathlands and herbaceous vegetation (CLC 3.2) and pastures (CLC 2.3) which are the most likely to burn and to propagate fire [START_REF] Curt | Wildfire frequency varies with the size and shape of fuel types in southeastern France: Implications for environmental management[END_REF]. We compared the total area of flammable fuels before and after 1994 using an analysis of variance with a Kruskal-Wallis test.

As we hypothesized that fire suppression may have a strong impact on fire activity, we also calculated an index of 'fire suppression effort' which combines the quantity of fire crews in each fire brigade, the ground fire suppression means such as trucks and fire patrols, and the financial investments in suppression in each of the 15 departments of the study area. We did not consider the aircraft detection and the water bomber aircrafts, which can intervene throughout the study area and thus can be considered as spatially uniform. These data are incomplete and unreliable for the pre-1994 period, but we had data of good quality for 2012 at the scale of fire brigades and municipalities (source: DGSGC 2012). For each 2*2 km pixel, this index was compared to the temporal trends (tau statistic of modified Mann-Kendall trend tests; R package 'fume') in fire number and burned area, which were computed by (7) over 1973-2009. This indicates to what extent the spatial variation in fire suppression capacity may affect fire activity based on the current system established in 1994.

RESULTS

Overall, our results reveal that several important features of fire activity decreased throughout southeastern France after the establishment of the new policy in 1994, while at the same time the environmental and human drivers promoting fire activity (i.e. fire weather hazard, human pressure on ignitions and the fuel coverage) increased.

Changes in fire activity 1975-1994 versus 1995-2014

The total number of fires decreased of ca. 25% and the total area burned of ca. 60% after 1994 (Table 1), thus making the fire cycle jumping from 276 years to 684 years. Despite large annual fluctuations, the number of fires has decreased almost linearly since 1975 (Fig. 2), whereas the burned area changed more abruptly. Indeed, the mean annual area burned was more than 30,000 ha before 1994 with large interannual fluctuations: every 2-3 years the area burned peaked up to 50,000 ha (Fig. 2). After 1994, the mean annual burned area peaked dramatically at 12,650 ha, and interannual fluctuations were reduced except in 2003 (Fig. 2). If we discard the exceptional year 2003 from the second period, the mean annual burned area is only ca. 8000 ha, that is to say 0.001% of the study area. In 2013 and 2014, France has recorded the lowest number of fires and the lowest annual burned since 1975.

Clear changes also occurred after 1994 for all fire size classes: the number of fires (Fig. 3A) and the total burned area (Fig. 3B) followed the same tendency than the annual amounts for all size classes, with a clear decrease since 1994. The only exception is the number of very small fires (< 1 ha) which decreased weakly. Changes were also visible in the allocation into fire size classes. The annual number of very small fires (< 1 ha) decreased weekly but all fires larger than 1 ha were significantly less abundant after 1994 (Table 2; Fig. S1A). The total burned areas greatly decreased for all fire size classes including small (1 ha) fires (Table 2; Fig. S1B). The fire size distribution curves show that the number of fires and the area burned were clearly lower after 1994 (Fig. 4). The gap between the curves increases with the fire size classes, especially for the burned area (Fig. S2A) than for the number of fires (Fig. S2B). The comparison of percentiles indicates that 95% of wildfires were suppressed before reaching 8 ha in size after 1994 instead of 15 ha before 1994.

The seasonal distribution of fire activity was also modified. The number of fires was dramatically reduced in summer (July to the end of September) but less during the 'winter' season from October to June (Table 3; Fig. 5A). The burned area decreased sharply during summer and moderately from autumn to spring (Table 3; Fig. 5B). Overall, the daily peaks of burned area (i.e. maximal area burned on a certain day) were clearly reduced after 1994 (Fig. 5B).

The kernel density maps show hotspots of fire activity in Corsica, along the Mediterranean coast and in the Cévennes-Ardèche hinterland (Fig. 6). The comparison of maps before and after 1994 indicated that the hotspots remained rather similar but that their size and their values decreased. The fire density decreased throughout the study area after 1994 (Fig. 6A and6B) even if local variations exist (Fig. S3). The burned area sharply decreased, especially in the most fire-prone areas of Corsica and the Maritime Alps, and in the coastal lowlands (Fig. 6C and 6D; Fig. S3). The tests of changes at the scale of pyroclimatic regions indicated that the number of fires and the burned area decreased is statistically significant in all regions (Table 4) although there was local variability at the scale of the 2*2 km pixels.

Changes in environmental and human drivers of fire

The analysis of the pre-and post-1994 periods indicates that the main environmental drivers of fires (fuels, weather and anthropogenic activities promoting ignitions) increased. The fire weather hazard increased almost linearly since 1975, and this tendency continued after 1994 (Fig. 2). Indeed, the FWI index which estimates the wildfire potential was 22 ± 2.9 (mean ± SD) before 1994 then 25 ± 3.9 after 1994 (F = 7.86; P = 0.0079) (Fig. 7 right panel). Likewise, the human pressure index (HPI) which estimates the amount of ignitions linked to human activities statistically significantly increased after 1994 (F = 3.92; P = 0.047). The fuel coverage in the whole study has increased of about 3% after 1994. The shrublands, the forested shrublands and the forests are the main fuel types that have increased after 1994 (Fig. 8). These fuels have expanded to the detriment of agricultural lands, pastures and sparse vegetation. A large part of pastures (and a small part of agricultural lands) were progressively transformed into shrublands and sparse vegetation. A part of sparse vegetation also transferred into shrublands. Transfers among the major fuel types (shrublands and forests) were also important, with many shrublands evolving towards wooded shrublands or forests, and wooded shrublands evolving towards forests. This suggests that the fuel biomass has also increased, although specific data of biomass are missing. An interesting finding is that the number of fires and the burned area clearly decreased the most where fire suppression effort is high and strong (very high), especially during the summer season (Fig. 9).

DISCUSSION

The efficiency and impacts of the new fire policy since 1994

The new fire policy adopted in 1994 marked an important milestone [START_REF] Dsc | Protection de la forêt contre l'incendie guide de stratégie générale Ministère de l'intérieur et de l'aménagement du territoire[END_REF][START_REF] Alexandrian | Public policies affecting forest fires in the Mediterranean Basin[END_REF]. The effectiveness of fire surveillance, fast detection and massive suppression has undoubtedly reduced fire activity, especially in the historical fire hotspots. It can thus be seen as effective because fire activity has considerably decreased during the two following decades while many factors promoting fires have increased during the same time. A series of variables clearly indicate a decline in fire activity. The number of wildfires was halved in comparison to the pre-1994 situation, likely resulting from the reinforced prevention and surveillance of forests, and from higher communication on daily fire danger [START_REF] Alexandrian | Public policies affecting forest fires in the Mediterranean Basin[END_REF]. Fire frequency tended to diminish slowly since 1975 from the establishment of the first modern policy (Fig. 10) but the decrease was sharply accentuated after 1994. However, the major successes of the new fire policy were registered on burned areas, which were reduced to an unprecedented level. The total burned area annually peaked to one fifth of the initial (1970's) due to the strategy of massive attack with all the means available [START_REF] Lahaye | Classification of large wildfires in South-Eastern France to adapt suppression strategies[END_REF], coupled with the pre-positioning of fire crews and the reinforced surveillance in wildland. This strategy aimed at preventing fire propagation and reducing the size of fire at the moment of attack. This reduction of burned area affected all fire size classes. The total area burned by very large fires thus became low except in 2003, and 95% of wildfires were suppressed before reaching 8 ha in size before 1994 instead of 15 ha before 1994 (Fig. 4). The burned area was reduced throughout the study area but mostly in densely-populated and fire-prone areas with many values at risk, and especially during the hazardous summer season. All these results reflect the fact that the policy was mostly oriented toward the suppression of hotspots and focused on the major human stakes. This argument is corroborated by fact that the fire suppression effort strongly controlled the fire density and the burned area, notably during summer (Fig. 9).

Meanwhile, the south-eastern France became a more fire-hazardous area on average. Indeed, the potential for human-caused ignitions increased in parallel to the expansion of population and tourism [START_REF] Feranec | Map presentation of changes in Europe's artificial surfaces for the periods 1990-2000 and 2000-2006[END_REF], infrastructures [START_REF] Curt | Modelling the spatial patterns of ignition causes and fire regime features in southern France: implications for fire prevention policy[END_REF] and wildland-urban interfaces (54) which generate most of fire ignitions. Likewise, the fuel coverage increased thanks to shrub encroachment into agricultural or pastoral areas and to the spontaneous afforestation with pines and oaks into many shrublands. These land cover changes due to the 'rural abandonment syndrome' are widespread in France as in the southern Europe, leading to more fire-hazardous landscapes [START_REF] Moreira | Landscape-wildfire interactions in southern Europe: implications for landscape management[END_REF]. The fire weather hazard (FWI value) also increased significantly in southeastern France due to climate changes and global warming [START_REF] Fréjaville | Spatiotemporal patterns of changes in fire regime and climate: defining the pyroclimates of south-eastern France (Mediterranean Basin)[END_REF], and the occurrence of synoptic conditions propitious to large fires driven by wind [START_REF] Ruffault | Daily synoptic conditions associated with large fire occurrence in Mediterranean France: evidence for a wind-driven fire regime[END_REF] or by heat [START_REF] Ruffault | Objective identification of multiple large fire climatologies: an application to a Mediterranean ecosystem[END_REF] increased.

Fire activity has thus become low in southeastern France (9.7% of the total number of fires and 5.5% of the burned areas in Europe; see [START_REF] Schmuck | Forest Fires in Europe, Middle East and North Africa in 2013[END_REF]) in comparison to Spain, Greece and Portugal, although their situation has improved recently [START_REF] Turco | Decreasing Fires in Mediterranean Europe[END_REF][START_REF] Rodrigues | An Insight into Spatial-Temporal Trends of Fire Ignitions and Burned Areas in the European Mediterranean Countries[END_REF]. These successes are only possible because France benefits from a high ratio of fire suppression means in relation to the annual burned area [START_REF] Efimed | Wildfire Prevention in the Mediterranean. A key issue to reduce the increasing risks of Mediterranean wildfires in the context of Climate Changes[END_REF]. Investment in equipment and preparedness has been considerable since 1994, including air trackers, water bomber aircrafts, and fire suppression crews. The suppression capacity has captured two-thirds of national funding for wildfires while only one third goes to prevention [START_REF] Chatry | Rapport de la mission interministérielle "Changements climatiques et extension des zones sensibles aux feux de forêts" (National Report on Climate Change and the Extension of Fire Prone Areas in France[END_REF][START_REF] Muñoz | Europe: Development and Fire[END_REF]. In addition, the density of road networks generally allows rapid access to most fires. However, this costly system could be called into question in the future due to budget cuts [START_REF] Chatry | Rapport de la mission interministérielle "Changements climatiques et extension des zones sensibles aux feux de forêts" (National Report on Climate Change and the Extension of Fire Prone Areas in France[END_REF][START_REF] Perriez | Rapport sur la protection contre les incendies de forêt après les feux de l'été 2003 (Report on the protection against forest fires afeter the 2003 summer wildfires). IGA, CGGREF, IGE[END_REF][START_REF] Makala | Mission d'évaluation relative à la défense de la forêt contre l'incendie (Mission of protection of forests against fire)[END_REF].

What could limit the sustainability of the new fire policy?

This study confirms that an aggressive suppression-based fire policy can counterbalance in the short run environmental conditions and human activity that become more hazardous [START_REF] Pausas | Abrupt Climate-Independent Fire Regime Changes[END_REF][START_REF] Ruffault | How a new fire-suppression policy can abruptly reshape the fire-weather relationship[END_REF]. In 'normal' weather years the current system of fire suppression is well organized, directed, equipped, and remarkably efficient. However, increasing evidences suggest that these policies are challenged in the long-term by several facts. Indeed, large and devastative fires occurred in 2003 and in 2016, burning thousands of hectares and destroying many houses of infrastructures. These dramatic events are indicative of structural factors becoming more conducive to fires in the long term, and of the difficulty to manage prevention and suppression under exceptional circumstances. However, the current policy does not fully address the future fire challenges: it does not account for the increase of structural factors promoting fire risk, for climate changes, and it is unbalanced (Fig. 10).

Fire risk would increase due to several factors. Fuel coverage and biomass still grow due to encroachment and spontaneous afforestation [START_REF] Moreira | Landscape-wildfire interactions in southern Europe: implications for landscape management[END_REF], and low practice of fuel management, in particular around houses [START_REF] Chatry | Rapport de la mission interministérielle "Changements climatiques et extension des zones sensibles aux feux de forêts" (National Report on Climate Change and the Extension of Fire Prone Areas in France[END_REF]. This is exacerbated by the fire paradox: active suppression practiced since 1994 increased the fuel biomass and the likelihood of intense and large fires [START_REF]Towards Integrated Fire Management -Outcomes of the European Project Fire Paradox[END_REF]. Exurban sprawl into wildland promotes ignitions and increases the vulnerability of houses [START_REF] Lampin-Maillet | Land cover analysis in wildland-urban interfaces according to wildfire risk: A case study in the South of France[END_REF], and makes suppression more challenging [START_REF] Lahaye | Classification of large wildfires in South-Eastern France to adapt suppression strategies[END_REF]. More generally, the expansion of infrastructures and tourism promotes ignitions and hazardous areas where humans and assets are exposed to high fire hazard. In the same time, there may be concerns more specific to southern France: as fire use for agricultural purpose is banned from many areas, the culture of firewise practices decreased among population. Likewise, the reduction of fire activity and fatalities gave credence increases in the media that the current system cannot be overwhelmed; this may demobilize the public decision-makers [START_REF] Makala | Mission d'évaluation relative à la défense de la forêt contre l'incendie (Mission of protection of forests against fire)[END_REF] and decrease the memory of fire the general public (see [START_REF] Mcintosh | The Way the Wind Blows: Climate, History and Human Action[END_REF]). These structural changes are exacerbated by the ongoing climate changes. Higher temperatures increase fuel dryness [START_REF] Lindner | Climate change impacts, adaptive capacity, and vulnerability of European forest ecosystems[END_REF] and produce more necromass [START_REF] Doblas-Miranda | Reassessing global change research priorities in mediterranean terrestrial ecosystems: how far have we come and where do we go from here?[END_REF] thus promoting intense fires [START_REF] Moriondo | Potential impact of climate change on fire risk in the Mediterranean area[END_REF]. Exceptional meteorological events (heat droughts, heat waves) like 2003 and 2016 promote concomitant fires of high intensity [START_REF] Ruffault | Objective identification of multiple large fire climatologies: an application to a Mediterranean ecosystem[END_REF] which are likely to overwhelm the suppression capacity of fire crews [START_REF] Giannakopoulos | Comparison of fire danger indices in the Mediterranean for present day conditions[END_REF]. This may foreshadow the near-normal fire activity in the longterm [START_REF] Chatry | Rapport de la mission interministérielle "Changements climatiques et extension des zones sensibles aux feux de forêts" (National Report on Climate Change and the Extension of Fire Prone Areas in France[END_REF]. The present suppression-oriented policy is unbalanced because it places too little emphasis on prevention [START_REF] Collins | Forest fire management to avoid unintended consequences: A case study of Portugal using system dynamics[END_REF], and on hinterland and rural areas where prevention and regulation could be effective to control future fires. The unprecedented combination of increasing structural fire hazard, climate changes and unbalanced policy seems to be a syndrome common to many Euro-Mediterranean countries [START_REF] Fume | FUME Project: Forest fires under climate, social and economic changes in Europe, the Mediterranean and other fire-affected areas of the world. Lessons learned and outlook[END_REF] and would be the origin of a new generation of wildfires intense and difficult to suppress [START_REF] Castellnou | The great fire changes in the Mediterranean -the example of Catalonia, Spain[END_REF][START_REF] Valese | Fire, humans and landscape in the European Alpine Region during the Holocene[END_REF].

Facing new challenges: towards sustainable fire policy and adaptive land management

Since 1973 the French fire policy has recurrently tried to adapt to fire challenges after the fact ( Fig. 10). Following the 2003 fires when the fire suppression framework collapsed [START_REF] Perriez | Rapport sur la protection contre les incendies de forêt après les feux de l'été 2003 (Report on the protection against forest fires afeter the 2003 summer wildfires). IGA, CGGREF, IGE[END_REF], the firemen now adopt a 'fire analysis' [START_REF] Lahaye | Classification of large wildfires in South-Eastern France to adapt suppression strategies[END_REF] approach for better detecting and fighting large fires, based on the Catalonian approach [START_REF] Castellnou | Improving wildfire scenarios: Learning from experience. EFI Research Report 'Towards Integrated Fire Management -Outcomes of the European Project Fire Paradox[END_REF]. In 2010, the potential effects of climate change on fires were investigated by [START_REF] Chatry | Rapport de la mission interministérielle "Changements climatiques et extension des zones sensibles aux feux de forêts" (National Report on Climate Change and the Extension of Fire Prone Areas in France[END_REF], and in 2016, policy makers have questioned the sustainability of the current policy (59) facing possible budget costs while suppressing fires tends to be more dangerous and expensive [START_REF] North | Reform forest fire management[END_REF]. Such reflections about sustainable fire policies have recently gained momentum worldwide (see [START_REF] North | Reform forest fire management[END_REF][START_REF] Steelman | Is Wildfire Policy in the United States Sustainable?[END_REF][START_REF] Moritz | Learning to coexist with wildfire[END_REF]. A new perspective is to better cope and coexist with fire in a context of global changes based on the assumption that fire cannot be fully extirpated from our territories (see [START_REF] Steelman | Is Wildfire Policy in the United States Sustainable?[END_REF]). A consensus also emerged on the fact that sustainable fire policies are those which favor prevention, participation of people to the culture and memory of fire, and long-term management of fuels and areas at risk [START_REF] Efimed | Wildfire Prevention in the Mediterranean. A key issue to reduce the increasing risks of Mediterranean wildfires in the context of Climate Changes[END_REF][START_REF] Tedim | The largest forest fires in Portugal: the constraints of burned area size on the comprehension of fire severity[END_REF]. This would limit unwanted ignitions, tolerate them where and when this is possible, while protecting the human assets from destructive fires and decreasing their vulnerability. This includes self-protection of people against fire at a local scale, because fire cannot be the exclusive issue of firemen. A comprehensive vision of this future is encapsulated into the 'fire-smart territory' approach (70) which promotes firewise communities and fire-smart landscapes (Fig. 10).

CONCLUSION

In this study we showed that the fire policy established after the devastating wildfires of 1990 has been very effective for reducing fire activity in southeastern France. The strong financial, technological and human investments made to implement this policy of aggressive suppression, pre-positioning and prevention have offset the effects of increasing human activities promoting ignitions, of increased fire weather hazard, and of the expansion of flammable fuels. France is thus often referred to as the good pupil in the matter in Europe because its fire suppression framework is effective under normal weather conditions. However, we strongly question the sustainability of this policy and its ability to mitigate fire risk in the long-term in a context of increasing fuels and fire weather hazard. The hazardous fires of 2003 and 2016 have recalled policy-makers to order and reminded that a suppression-driven policy can collapse when facing exceptional weather conditions and new generations of fires that challenge firemen. We advocate for a comprehensive and sustainable fire policy allowing coexistence with wildfires. It should include reinforced prevention, involve the participation public and private, and promote fire-smart landscapes and fire-resistant/resilient ecosystems.

ACKNOWLEDGMENTS

We greatly acknowledge the Prométhée fire database (www.promethee.com) for providing fire data.

Figure captions Figure 1 .

 captions1 Figure captions Figure 1 . Location and elevation map of the study area. The upper right box displays the three main pyroclimates (modified from Fréjaville and Curt 2015)

Figure 2 .Figure 3 .

 23 Figure 2 . Mean annual number of fires and total burned area in southeastern France (1975-2014). The dotted grey bar indicates the setting of the new fire policy Figure 3 . Number of fires (A) and total burned area (B) along the 1975-2014 period for different fire size classes. The vertical grey dotted line indicate the transition between the two periods 1975-1994 and 1995-2014

Figure 4 .Figure 5 .Figure 6 .

 456 Figure 4. Fire size distribution (A) and burned area distribution (B) in southeastern France (1975-1994 versus 1995-2014). The vertical and horizontal scales are log-transformed Figure 5. Comparison of the cumulated daily number of fires (A) and the mean daily burned area (B) in southeastern France (1975-1994 versus 1995-2014) Figure 6. Kernel distribution of the number of the mean fire density (A: 1975-1994; B: 1995-2014) and of the mean burned area (C: 1975-1994; D: 1995-2014) in southeastern France

Figure 7 .Figure 8 .Figure 9 .Figure 10 .

 78910 Figure 7. Changes in fire activity (left panel) and in the drivers of fire activity (right panel) for the period 1975-1994 versus the period 1995-2014. The difference between the two periods has been computed using F tests on 20,142 pixels. NS: non-significant at a level of 0.05; * P < 0.05 ; ** P < 0.01 ; *** P< 0.0001 Figure 8. Sketch of the main changes in fuel coverage before and after the new fire policy (1994) using the CORINE Land Cover (1990 and 2012). For the fuel types having increased the most in coverage between 1990 and 2012, the percentage of increase in indicated. Conversions from one fuel type to another are indicated in %. These fuel types contributed for 95% of the increase in fuel biomass from 1990 to 2012. AGRIC: agricultural lands; SPARSE: sparse vegetation; PAST: pastures; SHRUBL: shrublands; FORSHRUB: woody shrublands; FOREST: forests of all types Figure 9. Contrasting seasonal patterns (left, winter: December to February; right, summer: June to August) of temporal fire trends (top, fire density; bottom, burned area) with spatial variation in fire suppression effort (x-axis, increasing suppression from left to right). Temporal trends were computed by the tau' statistic of Mann-Kendall modified trend tests in each pixel from log-transformed fire variables over 1973-2009. For each class (quantiles 0-25, 25-50, 50-75, 75-100) of the fire suppression index, distribution of tau values was compared to a zero-centered random distribution using Student tests within a bootstrap re-sampling procedure. Significant positive or negative trends by class of fire suppression effort are indicated by asterisks (* p < 0.05, ** p < 0.01, *** p < 0.001) Figure 10. Temporal sketch of fire policy in Mediterranean France since 1975. The lower graph indicates the variations of annual burned area, and periods of high fire activity in red. The central text indicates the main fire policy options according to time. The upper arrow indicates the structural causes and drivers that likely increase fire hazard and fire risk along time

Fig. 7 Fig. 8

 78 Fig. 7

Fig. 9 Fig. 10

 910 Fig. 9

  Author-produced version of the article published in Risk Analysis, 2018, 38, 3, 524-533. The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/risa.12855 DOI: 10.1111/risa.12855

  Author-produced version of the article published in Risk Analysis, 2018, 38, 3, 524-533. The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/risa.12855 DOI: 10.1111/risa.12855

  Author-produced version of the article published in Risk Analysis, 2018, 38, 3, 524-533. The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/risa.12855 DOI: 10.1111/risa.12855

  Author-produced version of the article published in Risk Analysis, 2018, 38, 3, 524-533. The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/risa.12855 DOI: 10.1111/risa.12855

Table 1 .

 1 This study was funded by The FUME research project (Forest fire under climate, social and economic changes in Europe, the Mediterranean and other fire-affected areas of the World) funded by the European Community's Seventh Framework Program (2010-2013) under grant agreement n°243888. Characteristics of fire activity and environmental-human drivers (mean ±SD) in southeastern France(1975-1994 versus 1995-2014)

	Characteristics of fire activity	1975-1994	1995-2014	Total	F ratio	P-value
	Total number of fires (n)	58813	44642	103455
	Mean annual fire density (n. yr -	2940 ± 680	2232 ± 556	2586 ± 710
	1 , mean ± SE)
	Annual fire density (n.yr -1 .km -² ,	0.037	0.028	0.032	12.44	0.0011
	mean)
	Total burned area (km²)	5829	2355	8183
	Annual burned area (ha.yr -1 .km -	0.36 ± 0.02	0.15 ± 0.02	0.25 ± 0.02	13.91	0.0006
	² , mean ± SE)
	Fire cycle (yrs.)	276	684	393
	Environmental-Human Drivers
	Fire Weather Index	22 ± 2.9	24.9 ± 3.6	23.4 ± 3.5	7.86	0.0079
	Human Pressure Index	1665 ± 154	2115 ± 167	1890 ± 180	3.92	0.0047
	Fuel Coverage	6625 ± 75	6814 ± 50	6720 ± 67	6.50	0.0090

Table 2 .

 2 Changes (%) in the number of fires and the total burned area by fire size classes between1995-2014 and 1975-1994 (relative to 1975-1994). For each size class, p-values and SD were computed by bootstrapping (1000 runs with 70% randomly sampled data points with replacement. Mean values (±SD) are reported, all are significant at p < 0.001.

		< 1 ha	1-10 ha	10-50 ha	50-100 ha	>100 ha
	Number of	-16 ± 0.4
	fires	***	-48.8 ± 0.5 ***	-52.1 ± 1 *** -49.1 ± 2.7 *** -59.2 ± 1.9 ***
		-44 ± 0.4
	Burned area	***	-52.5 ± 0.5 *** -55.1 ± 1.1 *** -52 ± 2.5 *** -62.3 ± 3.2 ***

Table 3 .

 3 Seasonal changes (%) in the number of fires and the total burned area between1995-2014 and 1975-1994 (relative to 1975-1994). Mean values (±SD) are reported, all are significant at p < 0.001. For each season, p-values and SD were computed by bootstrapping (1000 runs with 70% randomly sampled data points with replacement)Author-produced version of the article published in Risk Analysis, 2018, 38, 3, 524-533. The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/risa.12855 DOI: 10.1111/risa.12855

		Winter	Spring	Summer	Autumn
	Number of fires	-20.5 ± 0.8 ***	-12 ± 0.9 ***	-32.7 ± 0.4 ***	-28.2 ± 0.9 ***
	Burned area	-50.9 ± 2.8 *** -58.2 ± 2.9 *** -60.4 ± 3.2 ***	-62.3 ± 4.6 ***

Table 4 .

 4 Comparison of the number of fires and the burned area(1975-1994 versus 1995-2014) for each of the three pyroclimatic regions (PC). Comparisons were made using an analysis of variance with a post hoc Bonferroni test. P-values lower that 0.05 are statistically significant Author-produced version of the article published in Risk Analysis, 2018, 38, 3, 524-533. The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/risa.12855 DOI: 10.1111/risa.12855

	Number of fires	1975-1994	1995-2014	Total	F ratio	P-value
	PC1	24226	18762	42988	7.35	0.0033
	PC2	32355	22899	55254	10.21	0.0014
	PC3	2981	2232	5213	3.83	0.0042
	Total	59562	43893	103455
	Burned area (km²)	1975-1994	1995-2014	Total	F ratio	P-value
	PC1	2462	1025	3487	21.53	<0.0001
	PC2	3111	1159	4270	16.66	<0.0001
	PC3	255	171	426	12.62	0.0004
	Total	5829	2355	8183

Author-produced version of the article published in Risk Analysis, 2018, 38, 3, 524-


Steve Chappell

Alistair Macarthur

Dan Preston

Dave Olmstead

Bob Flint

Chris Sullivan

Exploiting real-time FPGA based adaptive systems technology for real-time Sensor Fusion in next generation automotive safety systems

We present a system for the boresighting of sensors using inertial measurement devices as the basis for developing a range of dynamic real-time sensor fusion applications. The proof of concept utilizes a COTS FPGA platform for sensor fusion and real-time correction of a misaligned video sensor. We exploit a custom-designed 32-bit soft processor core and C-based design & synthesis for rapid, platform-neutral development. Kalman filter and sensor fusion techniques established in advanced aviation systems are applied to automotive vehicles with results exceeding typical industry requirements for sensor alignment. Results of the static and the dynamic tests demonstrate that using inexpensive accelerometers mounted on (or during assembly of) a sensor and an Inertial Measurement Unit (IMU) fixed to a vehicle can be used to compute the misalignment of the sensor to the IMU and thus vehicle. In some cases the model predications and test results exceeded the requirements by an order of magnitude with a 3-sigma or 99% confidence.

Automotive Sensor Fusion

Next generation automotive systems and Advanced drive assistance systems (ADAS) such as Lane Departure Warning, Collision Avoidance, Blind Spot Detection or Adaptive Cruise Control require the overlay or "fusion" of data from sensors such as video, radar, laser, global positioning systems and inertial measurement devices. Techniques for sensor fusion, namely the incorporation of disparate and complementary sensor data in order to enhance accuracy, are well established in advanced aviation systems i . Indeed, as automotive systems incorporate ever more advanced sensor electronics, manufacturers will look to new approaches to system design incorporating reconfigurable devices for rapid development and deployment. As with the application example discussed here, sensor fusion in defense and aerospace systems may provide some solutions.

Boresighting

It is critical for these next generation applications that the sensors used are accurately aligned and continuously realigned to each other and the vehicle. Without alignment the information from one sensor cannot be effectively combined with that from a different sensor. The current state of the art is to use costly mechanical and optical methods, such as autocollimators and laser boresight tools during vehicle production. Moreover, these alignments must be repeated if a sensor is disturbed (e.g. through typical 'car park' bumps) or subsequently repaired/ replaced after production in order to ensure that safety critical systems are reporting accurately. It is therefore highly desirable to align or "boresight" sensors dynamically through digital computation rather than mechanical means. We present the development of a system for this purpose below.

Application overview

In order to boresight a sensor we use a combination of two other sensor systems:

• A 6 degree-of-freedom (6-DOF) inertial measurement unit (IMU), such as that from BAE Systems ii , consisting of 3 gyroscopes and accelerometers, fixed to the vehicle and; • A two-axis accelerometer (ACC) fixed to the sensor to be boresighted.

Fixed to the vehicle the IMU defines the moving platform reference frame. It can sense angular rate changes and linear accelerations relative to the principal axes of the vehicle (x,y,z). The ACC is fixed to the sensor to be aligned and defines the sensor axes (x',y',z'). As the vehicle accelerates, the common acceleration vector will be sensed by both the IMU and the ACC (Figure 1). Any differences in acceleration components along the sensor axes are a result of the misalignment between the two and individual instrument errors. We exploit these differences to computationally boresight the sensor. We calculate sensor misalignment as roll, pitch, and yaw values using a "Sensor Fusion Algorithm" iii incorporating a Kalman Filter iv . The Kalman Filter also generates a statistical confidence level in the misalignment values. For the purpose of visualization we chose to boresight a video camera as the misaligned sensor, however the method extends to the alignment of any meaningful directional sensor type (e.g. radar, lidar). The misalignment angles are input to an "Affine Transform" v to calculate and display a realigned version of the video input in real-time.

Sensors

The system incorporates a 6-DOF IMU (DMU) from BAE SYSTEMS vi and dual-axis ACC from Analog Devices (ADXL202EB-232A) vii . These sensors use Micro-Electrical-Mechanical Systems (MEMS) technology.

The DMU incorporates vibrating gyroscopes viii . The principle of operation is based on the Coriolis effect whereby, on rotation, secondary vibrations are induced in a vibrating element orthogonal to the original direction of vibration. The rate of angular turn can be detected by measuring these vibrations. The vibrating element in the IMU used consists of a ring resonator micro-machined from silicon. This creates a robust gyroscope able to withstand extreme shock and vibration. The accelerometers in both IMU and ACC determine acceleration through changes in the capacitance between independent fixed plates and central plates attached to a moving mass

Sensor Fusion Algorithm

After data reconstruction and subsequent data fusion, the data is passed through a Kalman Filter that tracks the sampled data and provides a confidence level of the tracking quality.

The resultant values from this combined sensor fusion algorithm are roll, pitch and yaw of the boresighted sensor with respect to the IMU axes, with associated covariance values, that give an indication of the error in predicted output.

Affine Transformations

In order to boresight and stabilize the image from the video sensor we take the roll, pitch and yaw values directly from the sensor fusion algorithm. These movement values are then used in co-ordinate transforms for correcting the video picture. These transforms preserve parallel lines and are known as Affine transformations:

r' = Ar + B ,
where A is the coordinate rotation matrix for angle θ about the z axis

A = (cosθ -sinθ) (sinθ cosθ)
and B is the linear transformation vector for corrections b x and b y in x and y respectively

B = (b x 0) (0 b y )

System Architecture

We use a COTS (Commercial off the Shelf) platform to demonstrate rapid integration and development of the proof of concept prototype. The system is illustrated in Figure 2 and consists of the IMU and ACC sensors plus video camera to be aligned. The IMU interfaces to CAN. The ACC interfaces to Serial. By using a CAN to Serial converter we limit any customisation of the COTS hardware to incorporating a second serial interface onto the chosen platform. The Celoxica RC200E ix was used as the base platform for the prototype. It incorporates a Virtex2 FPGA (XC2V1000 ) x , two banks of 2 Mbyte ZBT RAM, Video I/O, serial interfaces and a TFT display.

FPGA System

We exploit the flexibility of FPGAs to rapidly develop and integrate the various system components using the Celoxica DK Design Suite xi for C based design and synthesis. The FPGA system contains the sensor interface drivers, the sensor fusion algorithm, video manipulation and user display code. The main architectural components are shown as a schematic in Figure 3 and as code in Figure 4. A platform support library xii (PSL) provides ready-coded components for implementation of controllers and data transfer to the board peripherals such as RS232/ video. For implementation of the main Kalman Filter computation and to control program flow including parts of the Graphical User Interface (GUI) we use the "Sabre" soft-core processor. All other components are implemented directly in the FPGA. In particular the real-time video transformation has intensive processing requirements beyond the capabilities of typical embedded micro and DSP devices.

Video Transformation

Video processing consists of two routines described using Handel-C xiii , an ANSI-C superset optimized for hardware design in Programmable Logic. VideoInProcess() makes use of the RC200 library routines to take data from the relevant video input device and write successive frames of data to RAM. VideoOutProcess() computes the Affine transformation of coordinates on the RAM framebuffer, copying the relevant pixels to output to the video display. The video processing makes use of both RC200 RAMS in a double-buffering scheme. The routine for the rotation transform is given in Figure 5. This is a five-stage pipeline which, once loaded, computes the rotated output location (OutX,OutY), of each input pixel (InX,InY) on each clock cycle. The transforms operate on 16-bit precision fixed point values with sine and cosine angles stored in a 1024-element lookup table.

Sabre Processor

Sabre is a 32-bit RISC, designed in Handel-C, and programmed into the FPGA as a soft-core. It has a Harvard architecture xiv , with expandable data and program memories, limited only by the availability of FPGA embedded block ram. On the VirtexII 1000, there are 80 BlockRams, giving us up to 8kbyte program memory, for instructions and stack, and 64kbyte of data memory for constants. The Sabre is connected via 32bit buses to each of these memories. Peripherals are simply connected via another 32-bit bus into the processor memory space (see Figures 6 and7) where the Sabre acts as the bus master. The peripherals are designed to be as "smart" as possible, reducing the workload for the processor and making best use of the parallel processing capabilities of the FPGA. For example, the SabreBusControlRun peripheral simply consists of a set of twelve memory-mapped registers including roll, pitch and yaw values and status flags that are used directly by the FPGA video transformation block.

Similarly the SabreBusExpansionPort peripherals connect to the IMU and ACC devices via serial communications blocks. In this application the serial communications blocks, Sabre processor core, peripherals and board interface support were compiled to a device optimized EDIF netlist using the DK Design Suite. Xilinx place and route tools generated the FPGA configuration file. The Sabre program code was written in C and compiled to the Sabre Instruction Set Architecture. Since the Sabre machine code resides entirely within BlockRam memory of the FPGA, it is a simple process to merge the BlockRam initialisation into the FPGA configuration file. This technique eliminated the need for full hardware recompilation following changes to the Sabre software during development. We could thus rapidly prototype functionality in C software and later partition and re-partition functionality into C for hardware to improve performance and reduce processor load. Note that, as a result of the dynamic range of the Kalman filter, it was necessary to use floating-point values for all intermediate stages. The version of Sabre used here has no floating-point co-processor. We therefore emulated IEEE floating point operations using the "Softfloat" library xv . Softfloat is widely used amongst RISC processors with no native floating point support, such as low power ARMs and PowerPCs.

Testing and Results

Sets of static and dynamic tests were performed to calculate the response and accuracy of the system. In these tests, the system was calibrated first and then misalignments of a few degrees were introduced in roll, pitch and yaw to the boresighted sensor. The correction system was then started and data was collected for 300 seconds. The residuals, the difference between the true accelerometer measurement and the expected acceleration measurement, were used to help tune the Kalman Filter by selecting a good measurement noise value. For the static tests the value could be set very low, about .003 to .01 m/s, since the only noise was the noise of the instruments. This value had to be increased to .015 or higher when the vehicle was moving because of the addition of the vehicle vibration. Figure 8 shows the X-axes residuals and it's 3-sigma value plotted together for a static run and a moving run. The static run shows the residuals well within the 3-sigma values while the moving tests show that the residuals do exceed the 3-sigma values. Since the residuals should only exceed the 3-sigma value about once every 100 samples, the Filter noise was increased.

1. Static Testing

The instruments were calibrated using a level test platform. The absolute misalignments were measured directly using a laser attached to the boresighted sensor.

Note that static roll and yaw tests are more difficult to perform than the pitch tests since the platform must be oriented and use gravity to generate components of acceleration in the ACC and DMU accelerometers. Table 1 demonstrates that the resulting alignment estimates were very accurate in all three axes.

Dynamic Testing

The dynamic tests consisted of placing the test equipment in a standard private passenger vehicle, calibrating the ACC and IMU, misaligning the ACC-Camera system and then running during car motion. Sample results are given in Figure 9. Note that it is difficult to run precisely the same test profile using a moving vehicle, however the results for two driving tests are shown in Table 1. It can be seen that there is very close agreement between the tests with a high confidence level result.

Conclusion

Results of the static and the dynamic tests demonstrate that using inexpensive accelerometers mounted on (or during assembly of) a sensor and an Inertial Measurement Unit (IMU) fixed to the vehicle can be used to compute the misalignment of the sensor to the IMU and thus vehicle.

The overall accuracy is dependent on the accuracy of the inertial instruments, mounting accuracy of the instruments, noise present at the sensors and time allowed for the filter to compute the misalignment angles. It is clear that a software based sensor fusion approach to sensor alignment for vehicle applications meets typical automotive industry requirements. In some cases the model predications and test results exceeded the requirements by an order of magnitude with a 3-sigma or 99% confidence. The use of a COTS FPGA board and C based design and synthesis greatly simplified and accelerated the task of creating and verifying a real-time proof of concept system. By abstracting the core design from the details of the platform, the system is highly portable across different platforms and lends itself well to product migration.

We note that optimization of the performance (clock-speed, program size etc) was not a design goal in this exercise and there are many obvious enhancements. For example, a full fixed-point analysis and conversion of the Sensor Fusion Algorithm from float to fixed-point calculations is possible. The boresighted sensor system presented here provides the basis for developing a range of dynamic real-time sensor fusion systems.

Future implementations will demonstrate self-aligning and self-referencing methods for dynamic alignment of multiple sensors; the fusion of data from the vehicle into the system for additional improvements; and alignment for other sensor features such as headlights. The fusion engine presented here provides self-boresighting functionality for individual sensors, but it can readily be extended to fuse data from multiple sensors together (eg. lidar and video) to provide low-cost situational awareness systems for automotive use. It is proposed these implementations and tests be seriously considered by the automotive industry, as we believe these will aide in the reduction of future system costs, shorten development time to market, facilitate and consolidate the improvement of the next generation of features for the automotive industry.

Figure 1 :

 1 Figure 1: Sensor and vehicle reference frames

Figure

  Figure 2: System Architecture.

Figure

  Figure 3: FPGA system

Figure 4 :

 4 Figure 4: FPGA system code

Figure 5 :

 5 Figure 5: Affine Transformation C code

FigureFigure 7 :

 7 Figure 6: Sabre Processor System Architecture

Figure 8 :

 8 Figure 8: X Axis residuals from Static (Top) and Dynamic (Bottom) Tests.

Figure 9 :

 9 Figure 9: Sample results from dynamic test

Table 1 :

 1 Results from Static (Top) & Dynamic (Bottom) Tests

Proceedings of the Design, Automation and Test in Europe Conference and Exhibition (DATE'05) 1530-1591/05 $ 20.00 IEEE

Proceedings of the Design, Automation and Test in Europe Conference and Exhibition (DATE'05)


L Fanucci
email: l.fanucci@iet.unipi.it

A Giambastiani

F Iozzi

C Marino

A Rocchi

Platform Based Design for Automotive Sensor Conditioning

In this paper a general architecture suitable to interface several kinds of sensors for automotive applications is presented. A platform based design approach is pursued to improve system performance while minimizing time-to-market.. The platform is composed by an analog front-end and a digital section. The latter is based on a microcontroller core (8051 IP by Oregano) plus a set of dedicated hardware dedicated to the complex signal processing required for sensor conditioning. The microcontroller handles also the communication with external devices (as a PC) for data output and fast prototyping. A case study is presented concerning the conditioning of a Gyro yaw rate sensor for automotive applications. Measured performance results outperform current state-of-the-art commercial devices.

Introduction

The increase of reliability and performance brought by electronic circuits in automotive industry is leading to the progressive replacement of predecessor mechanical and electromagnetic devices. Furthermore, electronics made possible several new applications in car's safety (e.g. airbag, ABS, TCS, ESP, proximity sensor) and/or comfort (e.g. driving assistance, air conditioning): all these advanced facilities are based on a continuously growing set of sensors, spread all over the car. As they get more sophisticated, according to market trends and requirements, they also need more accurate and reliable sensing elements; as well the high number of sensors (more than 100 at present time) will inevitably drive future choices on the cheaper and less-consuming ones. This rapid growth has recently lead to the necessity of developing new design methodologies, allowing the designers to keep up with the increasing complexity of design and decreasing time-to-market requirements [START_REF] Michael | A Top-Down Microsystems Design Methodology and Associated Challenges[END_REF]. In order to deal with these issues, several solutions have been proposed, among which the so called Generic (or Universal) Sensor Interface [START_REF] Zhang | A Universal Micro-Sensor Interface chip with network communication bus and highly programmable sensor readout Circuits and Systems[END_REF][START_REF] Li; Meijer | A high-performance Universal Sensor Interface Sensor for Industry[END_REF][START_REF] Yazdi | A smart sensing microsystem with a capacitive sensor interface Circuits and Systems[END_REF][START_REF] Zhang | A highly programmable sensor network interface with multiple sensor readout circuits Sensors[END_REF]. This kind of interface approach integrates on chip all the analog/digital resources that are needed to perform conditioning for a wide range of sensors (capacitive, resistive, inductive, etc.). A cost and time-to-market reduction is apparently achieved, as one chip can interface several sensors (often simultaneously), providing only the essential circuitry, while non-standard customized functions are left to an external microcontroller. Indeed, these sub-optimal architectures lead, for a given sensor, to an increase in overall area and power consumption, and a reduction of overall performance and safety level (automotive sensors can be very different from each other), if compared to chips dedicated to a specific class of sensors.

In this paper a platform-based methodology for a generic automotive sensor interface is presented, in order to overcome the Universal Sensor Interface limitations while reducing time-to-market. Platform-based approach defines the electronic system design as sequence of several abstraction layers (each one can be considered as a platform) [START_REF] Chang | Surviving the SOC Revolution: A Guide to Platform-Based Design[END_REF]. Concerning an embedded system, a platform can be defined as a set of modules, interfaces, services and software that should be as much as possible configurable. They are built up taking into account the wide-ranging signal conditioning electronics for different sensors of modern automobiles, in a way that from such generic platform, the optimum interface for a specific sensor can be easily derived in a short time by means of system simulation, verification and possibly prototyping. This entails that only the required analog/digital components are integrated onto silicon, resulting in minimized area and power overheads.

Platform based design flow

Our approach is initially based on the realization of a MATLAB TM model for the system at the highest abstraction level, which is made of a set of functional blocks with no distinction between analog/digital sections and software. The sensor itself can be modeled with MATLAB™, and thus co-simulated with the conditioning circuitry, helping the designer to find the most appropriate elaboration chain for the given sensor. Through simulations, design iterations and functional blocks refinements a project space exploration can be performed, together with the definition of more detailed descriptions of each block's required functionalities. Such investigation allows a first partitioning of the system in analog, hardwired and programmable (software) digital building blocks: even if this subdivision cannot be taken as frozen, the accurate MATLAB™ modeling and simulations guarantee the reasonability of these choices. Then each block is modeled with the most appropriate description language: VHDL for digital hardware, VHDL-AMS for analog circuitry and C/C++ for software routines. The top-down design flow is depicted in Fig. 1: from the initial behavioral model we get to lower levels of abstraction via synthesis steps. The result of a synthesis step is then validated with the previous one through a verification phase. For what concerns the digital section, through a RTL (Register Transfer Level) description and a synthesis tool we produce a Gate-Level VHDL for the selected technology. Software development goes along with digital Intellectual Property (IP) macrocells progress: low level drivers are provided just after the first stable VHDL (behavioral), so that general high level software can be developed almost independently from the IP implementation On the analog side, from VHDL-AMS (modeling no more than the specifications) we generate a transistorlevel description, from which a more accurate VHDL-AMS model can be obtained and employed in a mixed simulation, together with standard VHDL and C software running on the programmable hardware (microcontroller or general purpose processor). After the implementation of each single block, the relevant behavioral model is updated, so that it behaves closer to the practical circuit. This feedback process increases mixed simulations efficiency, allowing a more comprehensive design space exploration and reducing the probability of system architecture re-design [START_REF] Michael | A Top-Down Microsystems Design Methodology and Associated Challenges[END_REF]. VHDL-AMS becomes therefore crucial to let designers simulate the whole system during its own development, compare results with those given by MATLAB™ and thus be able to fix as soon as possible unexpected behaviors, by trimming the architecture and updating blocks functionalities or specifications. The above mentioned design flow ends up with the prototyping phase, through which we can quickly test the whole system (or parts of it) in real operating conditions. Analog hardware can be implemented at first with discrete components mounted on a board (as our aim is to keep the analog section as simple as possible, this emulation should not differ too much from the final ASIC implementation, giving reliable results). As well it can be later mapped onto a dedicated analog chip, which can be used as a reference for digital hardware and software prototyping. The digital section is tested by means of a proper board, whose core is a large FPGA, provided with connectors to link an external PC and the analog front end. This system prototype allows the designers to verify the correct interaction between analog and digital parts, to develop firmware on a real platform implementation (out of the slow simulation environment) and, above all, to have a first, reliable estimate of system performance, from which architectural improvements can be further derived. Such prototyping phase is essential to get to the ASIC implementation with good confidence and high chance of success at first attempt: FPGA and analog front end not only have to satisfy functional specification for the targeted sensor, but also have to pass strict self-checking tests concerning full hardware read-back capability.

Generic platform for sensor interface

The generic platform shown in Fig. 2 is intended to address the design of the sensor interface for a wide range of automotive applications. It is based on three main parts: analog, hardwired digital and programmable digital (software). All non-trivial signal processing required for sensor conditioning is performed by the digital section, which also monitors system activity and handles communication with external devices (such as a PC during the prototyping phase). The monitoring/control functions are particularly useful for platform development and customization on the given sensor. Two kinds of computing resources are available: dedicated IPs for digital signal processing (DSP block in Fig. 2), such as FIR/IIR filters, modulator, demodulator, etc., and a general purpose processor or a microcontroller (an 8051 CPU core has proved to be sufficient up to the current development) provided with memories, buses and peripherals for communication. The CPU is mainly in charge of monitoring the signal processing chain and providing output status data through standard interfaces such as UART and SPI. This partitioning between processing and monitoring functionalities is important to have maximum performances without hardware overhead (only the required blocks for both processing and monitoring will be finally implemented on silicon) and at the same time guarantee fast and accurate platform optimization for the given sensor.

A case study

In this section we present a case study for the above mentioned platform based design flow, consisting in a customization of the architecture described in section 3 for a gyroscopic angular rate sensor, as implemented by SensorDynamics AG for a commercial chip.

Gyro sensor

Vibrating ring gyroscopes consist of a circular ring provided with drive, sense and control electrodes [START_REF] Ayazi | Design and Fabrication of A High-Performance Polysilicon Vibrating Ring Gyroscope Eleventh[END_REF].

Even though they are also available as discrete components, we focused on MEM implementation [START_REF] Geiger | The silicon angular rate sensor system DAVED© Sensors and Actuators A: Physical[END_REF], being our aim to carry out a stand-alone device able to provide accurate yaw rate measurement.

Gyro sensors base their working on the Coriolis force acting on a vibrating mass: while the driving electrodes keep the ring vibrating along the primary direction (with fixed amplitude, around the z-axis in Fig.

3 [START_REF] Geiger | The silicon angular rate sensor system DAVED© Sensors and Actuators A: Physical[END_REF]), the rotation of the device ( x ) causes an energy transfer to the second vibrating mode, which is located at 90° from the primary mode (on the y-axis). The amplitude of this vibration is proportional to the angular rate, and can be capacitively detected through the sense electrodes placed on the rectangular structure (open loop mode). A closed loop configuration exploits the control electrodes, by means of which the secondary vibration can be compensated, in order to let the sensor work around its rest point, thus achieving more linear and accurate measures. Such sensors basically require a PLL (for primary drive), which has to keep the ring in resonance (at a frequency of approximately 15 KHz), an AGC (to control the amplitude of this vibration) and a chain including demodulators, filters, temperature/offset compensation and modulators for secondary drive and rate sensing.

System architecture

The generic platform architecture depicted in Fig. 2 has been customized to the gyro sensor conditioning requirements, in order to achieve an almost optimal implementation for interfacing this kind of sensor, together with several communication resources for easier debugging, development and performance improvement.

For what concerns the analog front end, it only absolves functions of driving sensor's electrodes (through couples of DACs for each loop) and performing signal acquisition (by means of SAR ADCs, amplifiers and basic filters). It also provides stable power supply and clock to the digital section.

Each analog cell in the front end is digitally controlled, and this programmability can be of paramount importance for the whole system functioning. Particular effort has been placed in building a reliable and efficient interface between the analog and digital world. A JTAG standard interface has been selected for the following main reasons: (i) the standard protocol has largely been studied and tested for several applications, so it guarantees high reliability since first implementation; (ii) JTAG bases on asynchronous communication, which limits clock skew issues that may lead to undetectable faults; (iii) it employs a short number of wires (only 4 per chain), thus resulting easy to route also on very complex chips; (iv) it allows for full read-back capability (for fast verification or debugging).

The digital section performs almost all the signal processing (within the DSP block), and manages the communication resources (through the CPU core). The DSP contains a chain of IPs for signal elaboration, featuring the blocks schematically depicted in Fig. 4.: its architecture and sub-blocks dimensioning are derived from the MATLAB™ model set up during the design space exploration phase. Though all the required processing can be performed independently by analog front end and DSP block, a CPU core is also present to fulfil control, monitoring and communication tasks. These functionalities naturally find their best implementation via software routines, as they may vary through system updates and new system requirements. Control and monitoring are performed real-time by the processor on both DSP and analog front end: a routine constantly checks the system status by accessing the several readable registers spread along the processing chain (for example makes sure that the PLL is locked). Meanwhile other routines handle communication services, providing status and output data to the user: during prototyping phase, the system can be linked to a PC and through a graphical interface manual trimming can be performed and all intermediate data of the chain can be accessed. The same communication resources are used during the chip normal working conditions to provide external devices (typically the car's electronic control system) the required angular rate measure and status information.

As shown in Fig. 4, CPU core architecture comprises the Oregano 8051 processor [9] (freely distributed under LGPL licence), which provides a good compromise between performance and area occupation, and fits well the mentioned microcontroller applications; it is provided with ROM/RAM memories and cache controller, all configurable (both with hardware generics and at runtime) in order to get the maximum flexibility for software download, development and update. Just to give a few examples, an 'ASIC' version could include a big ROM (16 Kb) with all the needed software (the latest available at the moment) and through the cache (which is conceived to access big external RAM with a custom 2wire protocol) newer software versions could be downloaded and tested; in a 'prototype' version, a big RAM would be instantiated and used as Program Storage (while the boot placed in a small 1 Kb ROM would perform software download via UART) and cache would not be instantiated. Software download is also possible by means of RS485 (in place of simple RS232 protocol implemented by the UART) and SPI (at start-up all the communication devices look for a response on their channel, in a way that the connected peripheral is automatically detected); moreover it's possible to store the downloaded software into an external SPI EEPROM, and so reboot directly from EEPROM instead of downloading each time after reset. This high configurability gives the designers the chance of developing software with maximum simplicity and efficiency, through the whole system prototyping phase and even after the first ASIC releases, in order to achieve the utmost confidence on the final product success. Cache controller and UART are located on the 8051 Special Function Register (SFR) Bus (8-bit), while the other peripherals (SPI, timer, watchdog, and SRAM controller) are accessed via a custom bridge by means of a 16-bit bus. SRAM controller is used during the prototyping phase, to store at real-time (into a 512 Kb SRAM) digital data coming from any node of the DSP chain, with chance of later read-back for analysis purposes.

Results

The platform based design flow has been carried out step by step as described in section 2: simulation of the entire system (in particular sensor locking) has been first performed with MATLAB™ model (see Fig. 5 for main PLL signals), and then the same result has been reached in a HDL simulation environment, thanks to the full VHDL-AMS modelling of the analog section and the sensor itself. After this important achievement, designers have been able to get to the final mixed-signal platform implementation in a short time, being already defined block partitioning, dimensioning and connections.

The prototyping phase has then proved the validity of this approach, and an emulation environment has brought real sensors to locking (see Fig. 6 for measured PLL data) and output yaw rate data. The digital part of roughly 200 Kgates complexity has been implemented in a Xilinx X2S600E running a 20 MHz clock frequency. Performance of higher level (as reported in Table 1, in comparison with other commercial gyro sensors of tables 2 and 3) have been achieved with the integration of the analog front end into a 12 mm 2 custom chip implemented in a 0.35 µm CMOS technology.

  Fig. 1. Design flow

Fig. 2 .

 2 Fig. 2. Platform architectureFor the analog section, it can be critical in automotive applications to meet the required noise, area and power consumption constraints within a wide temperature range (-40°C÷125°C). This is particularly true if compared with the digital counterpart: for this reason we try to use as little analog signal processing as possible and to perform most of the conditioning in the digital domain. Thus the analog front-end only consists of ADCs, DACs, amplifiers and voltage/current sources, which are essential building blocks for automotive sensors conditioning. All the other platform functions and services are implemented either through digital hardware or software, keeping as much as possible fixed, simple and thus reliable the analog section. The front-end can be customized for different classes of sensors (inductive, capacitive, etc.) by choosing the most suitable analog cells from a well-stocked IP portfolio. Moreover programming main components parameters (such as amplifier gains and bandwidth, number of ADC bits, etc.) through the digital part allows a more accurate adaptation of the front end circuitry to the requirements of different sensors, both at design stage and during real working conditions (with the chance of on-line trimming of such parameters). All non-trivial signal processing required for sensor conditioning is performed by the digital section, which also monitors system activity and handles communication with external devices (such as a PC during the prototyping phase). The monitoring/control functions are particularly useful for platform development and customization on the given sensor. Two kinds of computing resources are available: dedicated IPs for digital signal processing (DSP block in Fig.2), such as FIR/IIR filters, modulator, demodulator, etc., and a general purpose processor or a microcontroller (an 8051 CPU core has proved to be sufficient up to the current development) provided with memories, buses and peripherals for communication. The CPU is mainly in

Fig. 3 .Fig. 4 .

 34 Fig. 3. Gyro sensor. (a) Cross-section. (b) Top view.

Fig. 5 .

 5 Fig. 5. Waveforms of PLL locking (MATLAB).

Fig. 6 .

 6 Fig. 6. Measured waveforms (AC probe).

Table 1 . Performance of SensorDynamics implementation.

 1

	Parameter	SensorDynamics	Units
		Min.	Typ.	Max.
	Sensitivity
	Dynamic Range	+/-75		+/-300	°/s
	Initial	4.85	5.00	5.15	mV/°/s
	Over Temperature	4.80	5.00	5.20	mV/°/s
	Non Linearity	0.07	0.10	0.20	% of FS
	Null
	Initial	2.70	2.50	2.53	V
	Over Temperature	2.70		2.53	V
	Turn On Time		500.00		ms
	Noise				__
	Rate Noise Dens.	0.04	0.09	0.13	°/s / Hz
	Freq. Response
	3 dB Bandwidth	25.00		75.00	Hz
	Temp. Ranges
	Operating Temp.	-40		+ 85	°C

Proceedings of the Design, Automation and Test in Europe Conference and Exhibition (DATE'05) 1530-1591/05 $ 20.00 IEEE

Proceedings of the Design, Automation and Test in Europe Conference and Exhibition (DATE'05)

Conclusions

In this paper a mixed signal platform based design methodology for automotive sensor interface has been presented. This approach aims to fast identify and verify suitable analog/digital interface architecture, for a given sensor, before integration onto silicon. An outline of such architecture has been obtained from the platform based methodology, and its customization for gyro sensors conditioning (implemented by SensorDynamics) has been described in detail. Results achieved by this implementation (outperforming competitors' chips, in spite of the incomplete development stage) prove the validity of the proposed approach, not only in terms of performance (practically no area overhead and best fit circuitry), but also related to short time-to-market, due to the high reconfigurability of the general platform architecture. As well power consumption and final product cost are likely to be positively affected by optimal hardware selection and high level on-chip integration. The whole system including the analog and digital circuits is being integrated into a single chip in a 0.35 µm CMOS technology.