Farzam Tajdari
email: farzam.tajdari@aalto.fi

Hasan Ramezanian
email: hramezanian@yahoo.com

Shadi Paydarfar
email: shadipaydarfar.91@gmail.com

Ali Lashgari
email: alilashgari@ksu.edu

Shakib Maghrebi

Flow metering and lane-changing optimal control with ramp-metering saturation

Optimal flow metering and lane-changing control with ramp-metering saturation

controller is anticipated in the work to guarantee that the total traffic flow entering the region where lane-changing control is utilized does not altogether surpass the bottleneck limit. This paper elaborates the results in [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF] considering the presence of partially autonomous and networked vehicles, proposing a unique way for combined lane-changing and saturated rampmetering control. The remainder of this paper includes, first definition of the used traffic flow model in Section II. Then, the proposed controller is introduced in Section III. Section IV explains the test-bed configurations, and in Section V the experiment outcomes are discussed. Section VI concludes the paper and the future research directions are highlighted as well.

II. LINEAR MULTI-LANE TRAFFIC FLOW MODEL

A N segmented multi-lane highway is used, indexed by i = 0, . . . , N , of length L i ,and each segment is composed of lanes, indexed by j = m i , . . . , M i , that m i and M i are the minimum and maximum indexes of lanes for segment i defines each element of the resulting network (shown in Fig. 1) entitled "cell", that is named by (i, j). To consider different network topology, containing lane decreases and lane increase on the right and left sides of the highway, it is assumed that j = 0 corresponds to the segment(s) containing the right-most lane. For instance, considering the motorway network showed in Fig. 1, m 0 = 0 and M 0 = 4, while m 3 = 1 and M 3 = 3. The model is structured in discrete time, including the time step T , indexed by k = 0, 1, . . ., that the time is t = kT . The total number of cells, according to this description, is H = N r=0 (M r -m r + 1) and the number of cells placed at the bottleneck area is S = M N -m N +1. Each motorway cell (i, j) is featured via the traffic density ρ i,j (k) (equals to the number of vehicles in the cell divided by L i ). Dynamically, density changes based on the conservation law equation as below, see, for example [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF],

Abstract-To accomplish a successful transportation framework, we present a novel methodology for incorporating lanechanging and ramp-metering control, which exploits the presence of connected and partially autonomous vehicles. The primary test tended to is coordinating the ramp-metering flow saturation boundaries in the optimal controller. Specifically, i t i s expected that a level of vehicles can get and carry out explicit control assignments (e.g., lane-changing control signals). Utilizing a Linear Quadratic Integral (LQI) controller and an anti-windup plan in view of a LTI model, another methodology is introduced for powerfully expanding the throughput at motorway bottlenecks. In terms of assessment, the methodology is implemented on the simulated model, performed on a first-order, multi-lane, macroscopic traffic fl ow mo del. Th e ca pacity dr op peculiarity is additionally executed, that addresses the effect of the created method to deal with be demonstrated, same as the improvements in rush hour traffic effectiveness.

Index Terms-Lane-changing, Ramp-metering, Saturation, Optimal control, Connected and automated vehicles.

I. INTRODUCTION

In recent decades, the automotive industry and enormous academic institutions around the world have devoted a significant and growing interdisciplinary effort to arranging, creating, testing, and sending new advancements that are relied upon to reform the highlights and capacities of individual vehicles later on [START_REF] Tajdari | Integrated ramp metering and lane-changing feedback control at motorway bottlenecks[END_REF]. Among the several available solutions, only a handful may have a direct influence o n t raffic flo w, while the vast majority are solely concerned with increasing driver safety or convenience [START_REF] Diakaki | Overview and analysis of vehicle automation and communication systems from a motorway traffic management perspective[END_REF]- [START_REF] Khodayari | A new model of car following behavior based on lane change effects using anticipation and evaluation idea[END_REF]. Only a few publications in the context of autonomous vehicle and connected vehicles have investigated optimizing lane distribution [START_REF] Amini | Optimizing operations at freeway weaves with connected and automated vehicles[END_REF]. Several previous works particularly addressed the challenge of determining effective vehicle lane-paths for a motorway under fully automated or semi-automatic driving [START_REF] Amini | Optimizing operations at freeway weaves with connected and automated vehicles[END_REF]. A feedback-based optimal controller [9]- [START_REF] Bahram | A novel online method for identifying motion artifact and photoplethysmography signal reconstruction using artificial neural networks and adaptive neuro-fuzzy inference system[END_REF] regarding lane-changing control is structured in [START_REF] Roncoli | Lane-changing feedback control for efficient lane assignment at motorway bottlenecks[END_REF] as a Linear Quadratic Regulator (LQR), reaches various traffic d ensity d istribution a t t he motorway bottleneck regarding the different lanes. However, another Fig. 1: A hypothetical motorway stretch.

ρ i,j (k + 1) = ρ i,j (k) + T L i q i-1,j (k) -q i,j (k) + T L i f i,j-1 (k)-f i,j (k) + T L i d i,j (k)+ T L i r i,j (k), (1)
where q i,j (k) represents the longitudinal flow exiting from cell (i, j) and penetrating to cell (i + 1, j) during the time interval (k, k + 1]; f i,j (k) is the net lateral flow going from cell (i, j) to cell (i, j + 1) during time interval (k, k + 1]; and d i,j (k) is any external flow entering the network in cell (i, j) during time period (k, k + 1], either from upstream of the considered stretch or from an on-ramp. Because a ramp is supposed to be controlled, we name r i,j (k) as the inflow able to enter the network from the ramp positioned in (i, j) during time interval (k, k + 1] (e.g., r 2,1 in Fig. 1). Some terms of (1) may not be considered depending on the network topology. The inflow q i-1,j (k) does not present for the first segment of the network, the outflow q i,j (k) does not present for the final segment prior a lane-drop, and the lateral flow terms f i,j (k) present just for m i ≤ j < M i . Based on the preceding assumptions, the whole number of lateral flow terms is F = H -N . Consider the familiar relationship q i,j (k) = ρ i,j (k) v i,j (k).

(

) 2
Because the controller is intended to work in congested traffic conditions, we assume that the speed in all cells remains constant (e.g., the critical speed) v i,j (k) ≡ vi,j , ∀i, j, k. Despite the fact that this appears to be a strong assumption, it will be mentioned in simulation (Section V) where the controller reaches acceptable performance also when speed changes over time. As a result, we can define the resultant system as a Linear Time Invariant (LTI) system by replacing ( 2) with ( 1)

x(k + 1) = Āx(k) + Bu(k) + d(k) (3)
where (time index k is omitted to simplify notation)

x = [ρ 0,m0 . . . ρ 0,M0 ρ 1,m1 . . . ρ N,M N ] T ∈ R H , (4)
d = T L 0 d 0,m0 . . . T L 0 d 0,M0 T L 0 d 1,m1 . . . T L 0 d N,M N T ∈ R H , (5)
u = [f 0,m0 . . . f 0,M0 f 1,m1 . . . f N,M N , r i,j ] T ∈ R F +1 . (6)
Variables u and d are the controlled and disturbance inputs, respectively; u contains all the lateral flows f i,j and the ramp flow r i,j that is supposed to be controllable, on the other hand d contains the external flows that are not in u.

Matrix Ā ∈ R H×H , composed of elements a p,s , it shows the connections formed by a longitudinal flow between pairs of succeeding cells. Finally, matrix B, composed of elements b p,s it reflects the interconnections between cells via their lateral fluxes entering and exiting. Therefore, the matrices for the aforementioned system can be described as follows:

a p,s =          1 if p = s and (j < m i+1 or j > M i+1 ) 1 -T Li vi,j if p = s and (i = N or m i+1 ≤ j ≤ M i+1 ) T Li vi,j if p > H 0 and s = p -M i-1 + m i -1 0 otherwise (7) b p,s =          T Li if j > m i and s = p -i -T Li if j < M i and s = p -i + 1 T Li if j = j and i = ī 0 otherwise (8)
where ( ī, j) explains the location of on-ramp flow.

At the end, it is noted that the CFL condition [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF] is considered as

T L i v < 1 (9)
should be followed in order to create a realistic discrete time [START_REF] Tajdari | Discrete time delay feedback control of stewart platform with intelligent optimizer weight tuner[END_REF], [START_REF] Tajdari | Implementation and intelligent gain tuning feedback-based optimal torque control of a rotary parallel robot[END_REF] and discrete space traffic flow model.

III. INTEGRATED RAMP-METERING AND LATERAL FLOW CONTROLLER

A. Formulation

The linear system described in Section II is used here to design an optimal control problem, the solution of which leads to a MIMO (multi-input multi-output) feedback controller [START_REF] Tajdari | Robust control of a 3-dof parallel cable robot using an adaptive neuro-fuzzy inference system[END_REF]. Specifically, the controller should handle lateral flows as well as flow entering from an on-ramp located upstream of the bottleneck in order to avoid congestion and maximize bottleneck throughput.

To maximize bottleneck throughput, densities at bottleneck areas should be kept close to their critical values (ρ cr S×1 ), which are assumed to be known here. To avoid offset at the stationary state in the presence of disturbances (e.g., upstream mainstream demand or uncontrolled lateral flows), we apply an integral controller to reject constant disturbances [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF], [START_REF] Yang | Posture-invariant three dimensional human hand statistical shape model[END_REF], thus eliminating the requirement to measure the external inflows. We structure the problem by augmenting the original system (3) with S (i.e, equal to the bottleneck lanes) integral states, defined as z, where

z(k + 1) = z(k) + C x(k) -ρ cr . ( 10
)
The resulting augmented system will be as

x(k + 1) = Ax(k) + Bu(k) + d(k), (11)
where

x = x z , d = d -ρ cr S×1 , A = Ā 0 H×S C I S×S , (12)
B = B 0 S×(F +1) , C = 0 S×(H-S) I S×S . ( 13
)
Over an infinite time horizon, we define the quadratic cost function shown below, which provides for the penalization of integral states and control inputs:

J = ∞ k=0 x T (k)C Q C T x(k) + u T (k) R u(k) , (14)
where

Q = w Q I S×S , R = w R1 I F ×F 0 F ×1 0 1×F w R2 (15)
C = 0 S×H I S×S . (16)
Matrices Q and R are weighting matrices employed to magnify the integral states and control commands, respectively, explained via parameters w Q > 0, w R1 > 0, and w R2 > 0.

The optimal control problem addresses ( 14), ( 11) can be solved by a LQR, that gives a stabilizing feedback gain assuming that the original system is at least stable and noticeable [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF], [START_REF] Tajdari | Feature preserving nonrigid iterative weighted closest point and semi-curvature registration[END_REF], [START_REF] Tajdari | Image-based modelling for adolescent idiopathic scoliosis: mechanistic machine learning analysis and prediction[END_REF], that can be assessed employing, e.g, the Hautus-test [START_REF] Williams | Linear state-space control systems[END_REF]. Also, the stability of the system is comprehensively studied in [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF].

B. Controller design

The linear feedback control law is the answer to the proposed LQR problem [START_REF] Tajdari | Intelligent optimal feedback torque control of a 6dof surgical rotary robot[END_REF] u

(k) = -Kx(k), (17)
where

K = R + B T P B -1 B T P A (18)
P = C T QC + A T P A -A T P B R + B T P B -1 (19)
The optimal result (18) and the Algebraic Riccati Equation ( 19) are extensively discussed in [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF]. The feedback control law ( 17) is quite useful in practice, since the computation of offline manipulation of the feedback gain matrix K is possible. Practical-wise, the gain K may be properly split as

K = K P K I , (20)
that let us to reformulate the control law as

u(k) = -K P x(k) -K I z(k). (21)
In practice, it may not always be possible to obtain the intended density set-point at the bottleneck (for example, due to input saturation); hence an anti-windup technique must be included in our controller. We use the proposed method in [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF], that, in this case, modifies the dynamic controller's integral part [START_REF] Tajdari | Design, implementation and control of a two-link fully-actuated robot capable of online identification of unknown dynamical parameters using adaptive sliding mode controller[END_REF] as

z(k + 1) = (I + M K I )z(k) + C +M K P x(k) + M u sat (k). (22)
The saturated input u sat is explained as

u sat (k) = sat(u(k)) (23) sat(u m ) =      u min m if u m < u min m u max m if u m > u max m u m , otherwise, ( 24
)
where m is the index of the element inside vector u, and u min m and u max m are the lower and upper bound for the input u m . Matrix M should be selected in such a way that I + M K I has stable eigenvalues, for example, using classical pole placement. Stability of the closed-loop system (3), ( 21)-( 24) can be investigated through the results from [START_REF] Silva | Anti-windup design with guaranteed regions of stability for discrete-time linear systems[END_REF].

C. Problem design with input saturation of bottleneck area

To design a controller for saturated input, we use a constant upper-bound (a) as a restriction for the on-ramp input and recast the Hamiltonian equation [START_REF] Tajdari | Switching controller design to swing-up a two-link underactuated robot[END_REF] as following:

r 2,1 ≤ a (25) f N +1 = r 2,1 -a (26) H = 1 2 x T Qx + 1 2 u T Ru + N i=1 λ i f i + λ N +1 f N +1 (27)
where f N +1 is a function that defines the constraint violation.

Then, we present an extra state x N +1 representing the integral of the difference between the on-ramp flow and its upperbound.

x

N +1 (k + 1) = x N +1 (k) + T ∂H ∂λ = x N +1 (k) + T f N +1 (k) (28)
We now determine a new augmented system considering

xn =   x x N +1 z   , ũn = ũ, d n =   d 0 -ρ cr S×1   (29)
To find the new A, B and C matrix of dynamic equation, r 2,1 must be defined based on either the states or the inputs, thus from the previous section we have:

r 2,1 = u(3, k) (30)
Ãn 1 =   A 0 H×1 0 H×S 0 1×H 1 0 1×S C 0 S×1 I S×S   , Bn 1 =   B 0 1×F T 0 S×(F +1)   (31) Cn 1 = 0 1×H 1 0 1×S C 0 S×1 I S×S (32)
We aim at designing an LQR/LQI for the new augmented system. In this case, the system is characterised by m + 1 marginally stable modes (λ = 1) and the stabilisability and detectability conditions as explained in the Hautus-test [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF] are again satisfied similarly to the previous case.

The resulting optimal gain is

Kn 1 = K 1 K C1 K I1 (33)
We implement our control logic by switching gains K and Kn 1 according to a threshold on the maximum on-ramp flow. Namely, we employ Kn 1 while u max m < u thres m and K otherwise.

IV. SETUP FOR EXPERIMENT A. Model of nonlinear multi-lane traffic flow

We provide simulation tests utilizing a model of first-order flow of traffic based on [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF] to test and assess the performance of the suggested control technique. The model is used to simulate traffic behavior on a multi-lane highway and includes: (i) non-linear capacities for lateral flows of physically determined vehicles (which may likewise go about as disturbances for the established regulator); (ii) a Cell Transmission Models (CTM) system used for longitudinal flow; and (iii) a nonlinear detailing to represent the capacity drop peculiarity. We momentarily survey the preservation regulation condition [START_REF] Tajdari | Integrated ramp metering and lane-changing feedback control at motorway bottlenecks[END_REF], that all variable are explained of in Section II. Lateral flow with respect to manual lane-changing, indicated as f M i,j (k) are assumed among contiguous lanes of a similar section, and matching guidelines are characterized to allot and bound their qualities really. They are determined as

f M i,j (k) = l i,j,j+1 (k) -l i,j+1,j (k), (34)
where

l i, j,j (k) = min 1, E i,j (k) D i,j-1,j (k) + D i,j+1,j (k) D i, j,j (k) (35) E i,j (k) = L i T ρ jam i,j -ρ i,j (k) (36) D i,j (k) = L i T ρ i,j (k)A i,j, j (k) (37) A i,j, j (k) = µ max 0, G i,j, j (k)ρ i,j (k) -ρ i, j (k) G i,j, j (k)ρ i,j (k) + ρ i, j (k) , (38)
and j = j ± 1. E defines the available space, in terms of flow acceptance, while D defines the lateral demand flow, calculated through the attractiveness rate's definition A. The equation ( 35) allows for the possibility of a limited space that is insufficient to allow lateral flow to enter from both sides of a cell. Regarding (38), the component G is generally equivalent to 1, suggesting that drivers mean to move to a quicker lane (prompting equivalent densities in lanes), However, it can also be tweaked to indicate specific area subordinate consequences, such as lateral flow toward a corridor from a lower to a greater density (for example upstream of on as well as off-ramps); while µ is a steady proportion in the scope of [0, 1] implying lane-changing "aggression".

Longitudinal flows are streams that move starting with one cell then onto the next downstream cell while remaining in a similar lane. To deliver a more reasonable way of behaving at low densities, the Godunov-discretised first-order model recommended in [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF] is utilized, For under-critical densities, however, a non-linear logarithmic demand model is used. A linearly falling demand formula for well over volumes and a linear decrease of the most extreme stream as a component of injected lateral inflows are also included in this model [START_REF] Yuan | Selection of the speed command distance for improved performance of a rule-based vsl and lane change control[END_REF]. The general definition for longitudinal flow is

q i,j (k) = min Q D i,j (k), Q E i+1,j (k) -d i,j (k) , (39)
where

Q D i,j (k)=    v max i,j exp -1 α ρi,j (k) ρ cr i,j α ρ i,j (k), if ρ i,j (k)< ρ cr i,j (1-γ)Q cap i,j ρ cr i,j -ρ jam i,j ρ i,j (k)-ρ jam i,j +Q B i,j (k), otherwise (40)
Q E i+1,j (k)= Q cap i+1,j , if ρ i+1,j (k) < ρ cr i+1,j w i+1 ρ jam i+1,j -ρ i+1,j (k) , otherwise. (41)
Q B i,j (k)=γ Q cap i,j -η [l i,j+1,j (k) + l i,j-1,j (k)] (42)
Parameter v max shows the maximum speed, Q cap defines the capacity flow, ρ cr denotes the critical density (i.e., the density in which the capacity flow happens), while α = ln Q cap v max ρ cr -1 [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF]. The parameter γ impacts the impact of capacity drop due to overcritical densities, whereas the parameter η affects capacity decline due to entering lateral flows. it is noted, by configuring γ = 1 and η = 0, we reach a conventional firstorder model, i.e. no capacity drop happens at the head of congestion.

B. Network definition and simulation parameters

To test and assess the efficacy of the suggested technique, we consider a hypothetical two-lane highway section, as depicted in Fig. 2. Particularly, we examine a network contains ten segments with the same length L i = 0.5 km, while a period step T = 10s is used. Various lanes highlight various boundaries, specifically an alternate FD, which might imply another traffic composition (e.g., countless heavy vehicles decreasing the capacity of a particular lane). Besides, the preowned traffic demand is portrayed in Fig. 3.

We present parameter Φ crt (k) to define whether or not the controller is used at time k (Φ crt (k) = 1) or not (Φ crt (k) = 0), reflecting the scenario of using the designed lateral flows in our simulation runs. Furthermore, when we use our controller, 50 % of the vehicles are supposed to be connected and automated, so a portion of the controlled lanechanging flow is assumed as extra noise. Variety percentages of controlled vehicles are being examined, and it is expected that more percentage of autonomous vehicles leads in a less TTS. As a result, the used lateral flow for the simulations run is configures as

fi,j (k) = f M i,j (k), if Φ crt (k) = 0, sat (f i,j (k)) + 0.5 f M i,j (k), if Φ crt (k) = 1. (43)
Because ramp-metering activities may cause a backlog to form outside of the highway network, we propose the following dynamics for the queue length w(k) (in veh).

w(k + 1) = w(k) + T (g 10,1 (k) -r 10,1 (k)) , (44)
where g 10,1 (k) is the demand for on-ramps during time intervals (k, k + 1]. In our simulations, we use the following ramp flow

r10,1 (k) = g 10,1 (k), if Φ crt (k) = 0, sat (r 10,1 ) (k), if Φ crt (k) = 1. (45)
It should be noticed that the influenced the outcome are bound by the following constraints.:

sat(f i,j ) =      f min i,j = -Li T ρ i,j if f i,j ≤ f min i,j f max i,j = Li T ρ i,j if f i,j ≥ f max i,j f i,j , otherwise;
(46)

V. EXPERIMENT RESULTS

A. No-control case

The nonlinear traffic model's implementation (34)-( 42) in this specified network defines the no-control case. Figs. 4(top) and 5(top) shows a significant congestion starts at the bottleneck area located at segment 10 and moves back to the segment 1. The congestion happens because of two reasons as a) the significant volume of traffic approaching from the ramp, because the whole demand is approximately 4600 veh/h during the peak period, while entire capacity is 4200 veh/h; same as b) the inefficient "natural" flow of lane-changing. Capacity drops also occur at the locations of bottleneck units in the highway network, causing congestion to worsen.

B. Controller for lateral flow and ramp metering

We use Tajdari's method [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF], employs the linear dynamic compensator ( 21), [START_REF] Tajdari | Intelligent optimal feedback torque control of a 6dof surgical rotary robot[END_REF] to the model of nonlinear traffic (34)-(42). A set of experiments have been performed to assess the controller's sensitivity to the parameters selection as w Q , w R1 and w R2 . As a result, for a wide range of parameter values, the controller exhibits satisfactory performance in terms of Total Time Spent (TTS), as measured by [START_REF] Tajdari | Feedback-based ramp metering and lane-changing control with connected and automated vehicles[END_REF]. Here are the findings obtained with w Q = 1, w R1 = 1 and w R2 = 0.001. Congestion completely avoided and the the bottleneck area's densities maintained at their critical values, according to Figs. 4(middle) and 5(middle). The outputs of control are determined and proposed throughout the duration of the simulation experiment to ensure that critical density is not exceeded. In addition, during the peak period, a big line is formed at the on-ramp, which in our research is not upper-bounded (see Fig. 7). The improvement of TTS is approximately 26% (see Table II).

C. Ramp-metering flow saturation control

To compare the effect of considering the ramp-metering saturation, we integrate the control gain in (33) to the controller in the previous section. As explained in Section III-C, II). Furthermore, we showed in Fig. 8 that how the on-ramp flow is controlled considering the saturation.

VI. CONCLUSIONS

The paper proposes an integrated approach for changing lanes and metering ramps on highways experiencing bottlenecks. In the used model for assessing the controller the existence of connected vehicles and partially autonomous vehicles are considered. Simulation tests using the first order, the multi lanes, and the flow of macroscopic traffic models that include capacity drop phenomena are used to demonstrate the effectiveness of the proposed approach. A future publication will investigate the controller's stability features, and robustness of the controller for variety of parameter choices.

Fig. 2 :

 2 Fig. 2: The motorway segmentation used in the simulation experiments.

Fig. 3 : 1 r

 31 Fig. 3: The employed traffic demand in the simulation experiments.

Fig. 4 :

 4 Fig. 4: Bottlenecks' density. (top) The no-control case. (middle) The controlled case. (bottom) The constraint case.

Fig. 5 :

 5 Fig. 5: Contour plots of densities. (top) The no-control case. (middle) The controlled case. (bottom) The constraint case.

Fig. 6 :

 6 Fig. 6: Contour plots of lateral flows in the controlled case; without considering the ramp saturation (left) and with considering the ramp saturation (right).

Fig. 7 :

 7 Fig. 7: The queue of ramp (left) and flow of ramp (right) in the controlled case.

Fig. 8 :

 8 Fig. 8: Controlled on-ramp input in the constrained case.

TABLE I :

 I The employed parameters in the nonlinear multilane traffic flow model.

		v max	Q cap	ρ cr	ρ jam	γ	η	G µ
		[km/h] [veh/h] [veh/km] [veh/km]
	j = 1	100	1800	22	120	0.6 0.8 1 0.6
	j = 2	100	2400	26	160	0.6 0.8 1 0.6

TABLE II :

 II TTS report.

	Case	No	Considering no satura-	Considering
		control	tion (Tajdari's method	saturation
			[14])	(our method)
	TTS [veh.hr]	1060	783	665
	Improvement (%)		26	37


Valentin Degrange

Active Debris Removal: a joint task and obligation to cooperate for the benefit of Mankind

This analysis will aim at proving that the cooperation of all spacefaring nations for the removal of space debris is not only a necessity in terms of the sustainable development of a limited natural resource, but could also constitute an obligation under international law. Furthermore, the establishment and observance by the international community of rules concerning space debris removal, as well as the creation of a dedicated international organization charged with dealing with these matters, could be one of the next steps in the development of mankind's space capabilities.

The importance of securing humanity's doorstep to outer space cannot be overstated and should indeed be a priority for all nations. But will the international community take such a step? 1

Introduction

The Cold war Space Race, which arguably began in 1957 with the launch of the first artificial satellite Sputnik 1, was an ideological race and involved only two real actors: the ex-USSR and the United States of America. At the time, the USA and the USSR both tried to assert their technical and scientific superiority over the other, which led to great advances from a technological as well as a legal point of view. The present situation is notably different.

First of all, the objectives have changed and the conceded goal of contemporary space exploration is mostly of a commercial nature. The preeminence of telecommunications in our modern society, and the increasing development of private activities in the space sector have highlighted the intrinsic value of the "spectrum-orbit resource" and saw the development of a real commercialization of outer space by States. The exploitation of outer space for both scientific and economic purposes in the future is therefore more likely to rely on the private sector.

Second, new players have emerged alongside the traditional space powers: China, India and Japan, for example, have considerably developed their space capabilities in recent years. They join the USA, Russia and the Member States of the European Space Agency in the fairly closed circle of spacefaring nations. Taking Europe as one "launching State", for calculation purposes, the number of states that actually engage in/license private companies to perform launches from their territory amounts to a total of 12 launching states 1 .

As space activities are undergoing a rapid development, the emergence of new prospects for the utilization and exploitation of outer space are making it increasingly "contested, congested and competitive" 2 . Many aspects of our daily lives have already come to depend on our occupation of Low Earth Orbits (LEO) and Geostationary Earth Orbits (GEO). As the number of space-related activities increases exponentially, most notably for commercial purposes, so does the number of orbiting space debris threatening to provoke potentially irreparable damages. The "big sky" theory which protected airborne travelers for almost two decades before becoming obsolete will soon suffer the same fate in regard to space travel.

The future of space activities partially depends on the answer from the international community to this problem. As the number of space activities and debris grows exponentially, not only is it necessary to guarantee the sustainable development of Outer Space and Earth's orbits, a limited natural resource, but it could also constitute an obligation under both international space 3 law and international law for spacefaring nations to cooperate in order to accomplish this. of materials and human lives, but cut the access to the space domain as well. Furthermore, the number of dangerous space debris present in Earth's orbit has drastically increased since the beginnings of space exploration, reaching the hundreds of thousands. It may not only cause malfunctions, sometimes beyond repair, on our satellite constellations, but also pose the threat of a potential implementation of the Kessler syndrome. Also called the Kessler effect, collisional cascading or ablation cascade, this theory was proposed by the NASA scientist Donald J. Kessler in 1978. It is a scenario in which the density of objects in LEO is high enough that collisions between objects could cause a cascade where each collision generates space debris that increases the likelihood of further collisions 10 . One implication is that the distribution of debris in orbit could render space activities and the use of satellites in specific orbital ranges infeasible for many generations. Its resultant effects on world economies, information systems, and national security systems, are now fully acknowledged by spacefaring nations.

Space traffic management and space debris removal are not dealt with in the present body of space law and are thus left to either separate international agreements or to national laws and policies for two reasons. First, there is neither a dedicated international organization, nor a trade association responsible for the creation, and perhaps implementation, of an international space code of conduct. Second, as national space launches mostly do not involve entry into foreign airspace, spacefaring nations usually apply their own national safety regulations to their governmental and/or private launch activities. This phenomenon can be assimilated to the "tragedy of the commons" theory 11 , as benefits of individual space missions accrue primarily to the entities conducting these activities while the detrimental impact of space exploitation can have negative consequences for all those involved in the sector and even others. As a consequence, regulations adopted by a single or multiple states to fight off the harmful environmental effects of space activities would be ineffective. Space-faring nations adhering to debris mitigation requirements may find themselves at a competitive disadvantage against those who do not observe similar measures.

On the international scale, the U.N. space treaties do not address the issue of space debris, mostly because there was no issue at the time the treaties were adopted. Actual international dialogue concerning the regulation of space debris started in the early 1980s, and the Inter-Agency Space Debris Coordination Committee (IADC) was established in 1993 upon the initiative of the world's major space agencies. After several years, the IADC developed space debris mitigation guidelines in 2002, which served as the basis for the space debris mitigation guidelines developed with a revised draft released in September 2010, is an instrument meant to be applicable to the space activities of the subscribing states and nongovernmental entities under their jurisdiction. It also addresses the issue of space debris and contains several measures aiming to prevent, reduce and mitigate the creation of space debris. While these various national regulations have helped to improve the mitigation of space debris, they might not constitute an optimal solution in the long run.

The need for the coordination of these activities, the harmonization of national rules and policies, as well as the development of a space debris cleanup plan by the international community has recently been emphasized by the aforementioned incidents as well as several scientific studies [START_REF]Scientific and Technical subcommittee of the UNCOPUOS[END_REF] .

This might represent the only viable solution to ensure that all actors of the space sector, governments and private entities alike, act in such a way so as to ensure the protection of the space environment as well as the safety of space objects. But it would be needed to demonstrate the existence of an international obligation to cooperate, in order to force nations of the world to protect a limited natural resource and Mankind's access point to outer space.

An international obligation to cooperate for the benefit of Mankind

The importance of cooperation has often been pointed out, especially after the cold war, as a necessity to ensure the rule of law, including relevant norms of space law that play a prominent role in international cooperation for the exploration and use of outer space for peaceful purposes. In 1994, the UNGA emphasized the general interest of this principle in inviting States that were not yet parties to the Outer Space Treaty to consider ratifying or adhering to them 18 . It is only in 1996 that a more general statement 19 was specifically dedicated to international cooperation for space activities, repeating the provisions of the previous texts, but emphasizing the need to take into account the needs of developing countries. If this resolution had a commendable objective, it is regrettable that its provisions are, for the majority, non-binding, and are in fact only suggestions, leaving States free to follow them or not. There are however several principles in the corpus spatialis, as well as other branches of international law, that could be used as a basis for the existence of an international obligation to cooperate for the benefit of Mankind. These various dispositions stem not only from international space law, but from telecommunications and environment policies as well.

Firstly, it is necessary to address the principles of space law that could constitute the basis for an international obligation to cooperate. As such, the Common Interest principle is particularly interesting. This principle appeared very early during the first Space Race through three resolutions of the UN's General Assembly 20 , and was then consecrated in the Outer Space Treaty of 1967 by both articles I et IX 21 . Mainly two rules result from these different texts: first, the exploration and use of outer space should be carried out for the benefit and in the interest of all countries, and second, that it should be carried out in due regard of the corresponding interests of all States Parties to the Treaty 22 . However, the scope of this Common Interest principle is still the object of controversies, especially concerning the word "countries" which has no legal consistency. The United States government, in particular, objected that while article I "would serve as a guide for space powers in developing their programs and conducting their activities in space […] it does not undertake to set any terms and conditions on which international cooperation would take place" [START_REF][END_REF] .

Furthermore, the US Department of State added that considering the vague formulation of article I, it carries no legal obligation and is therefore not self-executing, claim that has been supported by several authors [START_REF] Gorove | Interpretations of international space law for private enterprise[END_REF] . Others however, consider that the use of the word "shall" in the wording of article I reveal the intention the Parties had to give this disposition a binding value [START_REF] Peyrefitte | Droit de l'espace[END_REF] . In any case, the Common Interest principle entails significant practical measures, such as mutual assistance, and some [START_REF] Couston | [END_REF] consider that it could constitute the basis for an international public service, as defined by Charles Chaumont [START_REF] Chaumont | Perspectives d'une théorie du service public à l'usage du droit international contemporain[END_REF] . All in all, the Common Interest principle does not seem to be self-executing in and of itself. However, it can be conceived that it could form the basis of an international obligation to cooperate for active debris removal, just as it was used to found the creation of UNCOPUOS in 1958 [START_REF] Unga | Question of the peaceful use of outer space[END_REF] .

Secondly, it should be noted that the Earth's orbits are subjects to specific rules, not only thanks to the current international body of space law, but to the rest of international law as well.

Article III of the Outer Space Treaty indeed stipulates that State Parties shall carry on their space activities in accordance with international law 29 . This dispositions, while seemingly obvious, objectively determines the outlines of certain concepts and other principles. It also means that while State Parties must abide by space law, they are not exempt from other principles of international law simply because their activities happen in outer space. As a matter of fact, several points of international law should be mentioned for their potential applicability to Active Debris Removal.

For example, article 33 §2 of the ITU convention of Malaga Torremolinos -which most closely concern the management of Earth's orbits -highlights the fact that "geostationary satellite orbit are limited natural resources" and that they must be used "efficiently and economically" 30 . This article is the basis for the ITU's Orbit/Spectrum Allocation Procedure, ensuring a equitable repartition of limited natural resources to all nations. But considering the essential nature of geostationary orbits for all kinds of telecommunications in contemporary society, it seems crucial not only to use them in an efficient manner but to ensure a sustainable development of these orbits for the future.

Arguably, mitigating the effects of the tragedy of commons [START_REF] Viikari | Environmental aspects of space activities[END_REF] and preventing a catastrophic implementation of the Kessler syndrome fits well within such a goal. Thus it seems logical to consider than Member States of the ITU should be legally bound to not only reduce to a minimum their production of space debris, but to partake in active debris removal as well.

Finally, the UN space treaties have very little to say about pollution issues and even the Moon Treaty, which is arguably the most advanced of these on an environmental standpoint, has gained no more than 15 states parties, which renders the instrument practically meaningless. It could be argued however that principles taken directly from international environment law might be applied to the Earth's orbits in order to found a joint task, and most precisely the principle of Though it had no binding effect on its own, the principle of sustainable development has since then attained widespread endorsement and has been incorporated in various other international instruments as well as legally binding treaties. It could also be argued that obligations of international environmental law may have an erga omnes value, especially those pertaining to the protection of the global environment, as it was addressed by the ICJ in the 1974 Nuclear Tests cases [START_REF] Viikari | The environmental element in Space Law : assessing the present and charting the future[END_REF] . The principle of sustainable development was even directly referred to by the ICJ in the Gabcikovo-Nagymaros case of 1997 [START_REF]Gabcikovo-Nagymaros Project[END_REF] . In this last case, Hungary tried to have an erga omnes value recognised to the precautionary principle in order to invalidate a 1977 Treaty concluded with Slovakia, argument that was later rejected by the court. It can be concluded that at the moment there is no international obligation that development be sustainable, especially in space law, although it has been been argued that protection of the space environment is an obligation erga omnes 37 . It could however easily acquire a binding value if integrated in the corpus spatialis, thus founding an international obligation to cooperate for active debris removal.

While none of the aforementioned principles drawn from the different bodies of international law could offer on its own the legal basis to set up a joint task of Active Debris

Removal with the objective of ensuring the safety of Earth's orbits and the sustainable development of Mankind's space activities -or at the very least to justify the adoption of a legally binding document -a combination of these might be able to. As a matter of fact, it seems today that the adoption of an international agreement is the only viable option to ensure safety for future space activities. Such a document would not only need to increase the level of cooperation between

States, but devise a way to efficiently manage space debris removal as well. The adoption of a legally binding treaty thus seems unavoidable.

The implementation of a specialized international treaty

The issues of contemporary space activities mainly focus on the establishment of the sustainable development of human activities into a precious environment. As previously said, a few key items have already been the subject of several attempts to formulate rules of different shapes, including a variety of recommendations, charters, guidelines, and codes of conduct but might benefit from being integrated into a more compelling international regime. As such, it would seem relevant to make all the aforementioned soft law into a comprehensive and binding treaty. This regime, adopted on the basis of the existence of an international obligation to cooperate in order to ensure the sustainable development of outer space activities, would need to deal with the various following issues.

First of all, it seems imperative to resolve the matter of the absence of clear definition of "space debris". Existing international law states that "[t]he term of 'space object' includes component parts of a space object as well as its launch vehicle and parts thereof" 38 vehicles (including rovers), obviously fall into the aforementioned definition, space debris are far more ambiguous. The fact that they are neither registered nor registrable, and that they are out of any sort of control, makes it that much harder for a State to apply the Liability regime in order to receive damages. Lucien RAPP identifies several major difficulties with the application of the standard liability regime to space debris [START_REF] Rapp | Repenser le contentieux relatif aux débris spatiaux[END_REF] . First, assimilating space debris to space object would seem to be a far too extensive interpretation of existing space law, especially considering both the nature and number of these debris. Additionally, RAPP points out the difficulty of proving a causal link between a damage suffered by a standard space object and a debris in orbit, as well as the question of the identification of a liable State. And lastly, the absence of any mandatory international standard, from which it would be possible to assess the behavior of State concerned and determine whether or not this behavior is at fault, makes it nearly impossible to deal with space debris the same way as a regular space object. This further highlights the need for an update of international space law. met with two answers 47 . The first possibility would be that these disputes, based on national policies, would be dealt with by national jurisdictions, which eliminates the need for an intervention by the international community. The second possibility, inspired by three Japanese authors [START_REF] Kozuka | The international regime for space debris remediation in light of commercialized of space activities[END_REF] , would be to substitute to a liability for fault a liability of each operator for defective product. The failure of the operator to fulfill its obligation of removal would be then assimilated to a defect of the product's security.

Finally, a dedicated international treaty would need to address the gaps in the State responsibility and liability regime for damages caused by space activities. The current liability system of international space law establishes fault-based liability for in-orbit damages and absolute liability for ground damages [START_REF]Convention on International Liability for Damage Caused by Space Objects[END_REF] [START_REF] Takeuchi | [END_REF] . As for in-orbit damages, as the case of Cosmos-Iridium pointed out, identifying the liable state is relatively difficult because of the laboriousness of collecting objective data on the circumstances of the collision. The implementation of a SSA data sharing regime, part of a future treaty, could help determine liability more easily.

Finally, concerning ground damages, the Liability Convention of 1971 doesn't address the potential risk of damaging daily activities on the ground as -sometimes critical-systems malfunction might occur due to in-orbit collision. Should ground damages be considered fault liability, or should inorbit damages considered absolute liability? The lack of major accidents with a third party's casualties on ground has kept this issue from being resolved by the international community, but the question calls for an answer. An international treaty could be able to clarify the situation as well as the responsibility and liability regime that weighs or does not weigh on States for debris-related damages.

It seems that most issues contemporary space activities suffer from could be resolved by the adoption of an international regime focused on the management of orbit pollution and space debris.

Not only would it help to develop international cooperation regarding the exchange of information and data on Space Situational Awareness, but concerning space debris mitigation and removal as well, allowing for safer orbits for all space activities and actors. Moreover, developing the liability system of the UN space treaties to incorporate Space Traffic Management would enable States to conduct their space operations without fear of not being compensated for potential damages. Such a treaty would make outer space safe enough to allow further development of space activities.

The creation of a dedicated international organization

But, is an international treaty sufficient to ensure that these requirements are met with?

Could a dedicated international organization be created with the purpose of implementing the principles of such a treaty? Should it be? The idea here would be to establish an Inter-governmental organization based on the early INTELSAT model to foster the development of the technology(ies)

for active debris removal and on-orbit satellite servicing, and subsequently to perform removal and servicing operations on a commercial basis. Such an organization could not only act as a clearinghouse charged with collecting and redistributing SSA information and data, but could actively coordinate the launches of space objects around the world, as well as the management of space debris in orbit, in order to avoid collisions and maintain control over the re-orbiting or deorbiting of said debris. If appropriate, it could also settle disputes between States according to new rules of responsibility and liability and, ultimately, enact regulations relevant to its domain of competence.

The first issue to address is the need for a comprehensive Space Situational Awareness would not be realistic, especially since the concept implies the mutual recognition of every State having "complete and exclusive sovereignty over the airspace above its territory" 53 , which does not exist in international space law [START_REF]Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies[END_REF] . Therefore, it seems more efficient to designate a clearinghouse as the central data provider for STM, instead of requiring each State to directly provide information for their own space operations 55 . This Data Sharing Center 56 would be charged with gathering observation and operation data from SSA entities and operators in order to then provide the information necessary for space activities to the operators concerned with space traffic. This clearinghouse could also delete, or anonymize sensitive information and data (for example relative to matters of national security) upon request from data providers as well. For now, only the respective militaries of the US and Russia can provide comprehensive information and data for SSA, but the limited ability of other states (such as Australia, China, France, Germany or Japan) could still be used to increase their accuracy. The data provision from concerned States could be on a voluntary basis at first but it would have to become a legal obligation eventually in order to achieve constant data gathering with equal basis among all spacefaring nations. A new international treaty would be needed to impose such obligations to States.

Secondly, after setting up a regime imposing obligations on States Parties, it would be needed to create a dispute settlement organism charged with dealing with matters of responsibility and liability according to the updated Corpus spatialis. The action of a supranational judicial body on the basis of a legal corpus, which can be very limited (such as the international body of space law), is likely to produce an efficient legal system 57 . It is no longer possible to ignore the fact that new sources of law have emerged 58 , beyond the usual procedure of international negotiation between governments. In the specific context of space law, it can also be noted that no international jurisdiction except the International Court of Justice (ICJ) is competent to settle space-related dispute. The creation of a specialized supranational judicial body could then be a sufficiently flexible method of creating standards to accommodate the evolutions of space technologies and activities. Such a judicial body could be chaired by specialized judges but also allow non-states parties, such as private firms, to make their cases before the court. In any case, the creation of such a judicial body might become a necessity considering that the increase in space activities will most certainly be accompanied by an increase in the number of disputes. However, the ICJ is not equipped to deal with a potentially huge number of cases. The creation of a space-related judicial body, if not only a space debris-related court, would allow for a more efficient management of litigations and for the increase of the body of space law through jurisprudence.

Conclusion

To conclude this paper, it should be reminded that the Corpus spatialis established by the UN space treaties holds some lacunae of law. While various new issues did not exist at the time of negotiation of these treaties, the recent development of telecommunications and the promise of new prospects for the exploration and exploitation of outer space (space tourism, deep space mining, etc.) reveal the gaps left by States during the first Space Race and the need for an update of international space law. The subsequent increase of both space traffic and the number of space debris therefore has to be dealt with in an efficient manner, preferably with the participation of all space-faring nations as the existence of an international obligation to cooperate has previously been proven. The development of national regulations hold out until now and will probably still hold for at least a few years on specific issues, but an international treaty would be ideal to deal with other issues that can't be settled by individual States such as the need to ensure the sustainability of Earth's orbits and to make outer space in general a safer environment to conduct operations.

  Information and Data Sharing legal framework, in order to settle political challenges and assure the effective and transparent provision of unified SSA information and data to spacecraft or space object operators on a global scale. To accomplish that, it seems crucial to consider the establishment of an international clearinghouse for data and information sharing, based on article X of the Outer Space Treaty of 1967 which promotes international cooperation by asking States Parties to consider requests to observe space objects launched by other States Parties. The Space Data Association Limited (SDA), founded by the three major worldwide satellite operators (Inmarsat, Intelsat and SES) could be used as a model for such a clearinghouse, insomuch as the necessity of this type of structure is widely recognized by both commercial and civil governmental operators. While in Air Traffic Management the provision of services from States is achieved by dividing airspace into multiple flight information regions (FIR), the nature of space activities does not allow for the same system. Indeed, satellites in LEO can go around the Earth in about ninety minutes, passing over a country merely in a few seconds. Using a similar technique as flight information regions for STM

  The idea of World Space Organization was proposed for the first time in 1968 in Vienna at the UNISPACE-I, then finally postponed in 1982 (UNISPACE-II). The subject appeared again in 1999 during UNISPACE-III, when the representative of the Greek delegation, M. Vasily Cassapoglou, raised the fact that space activities and space exploration was escaping more and more to the effective control of international institutions and that one could almost called this "new global neo-colonialism" 59 . Ms. Simone COURTEIX was partisan of the creation of a WSO in the creation of which States would participate and under which they would continue to work together to establish long-term goals. The CERDE has already identified several areas and themes that might require the creation of such an organization, including the implementation of programs of launch services that could benefit developing countries (transfer of technology, creation of spatial data bank or specialized staff training), as well as the coordination and control of the growing and sometimes alarming number of objects launched into space 60 . Furthermore, as part of the development of the exploitation of resources in outer space, project OASIS 61 has developed the idea of a spaceport network infrastructure allowing, in the long term, for the exploitation of resources on planet Mars. The first step of the project would be to build a "node" in LEO, otherwise capable of offering in-orbit services (refueling, reparation, etc.), that would work under the authority of an 58 For exampe, the movement of private international law production at the initiative of enterprises, or the intensification of production of public international law following the establishment of the World Trade Organisation's (WTO) Dispute Settlement Body (DSB). 59 Juan Manuel de FARIMIÑAN GILBERT et Claudio ZANGHI, « L'organisation mondial de l'Espace, un défi oublié ? », in Armel KERREST, L'adaptation du droit de l'espace à ses nouveaux défis, A. PEDONE, 2007, 318 p.. 60 Ibidem. 61 CLEGG, et. al, ISU Team OASIS (September 2012) Operations and Service Infrastructure for Space. Team Project: Spaceports, Final Report Edition, SSPI2, Strasbourg, France, International Space University (ISU), 105 pages.

  Sustainable Development. The concept was first introduced in 1987 in the report "Our Common Future" of the Brundtland Commission 32 . According to the definition given in this report, sustainable development is development that "meets the needs of the present generation without compromising the ability of future generations to meet their own needs". The Stockholm Declaration of 1972 33 frequently referred to sustainable development, and two decades later the 1992 Rio Declaration on Environment and Development 34 set out the concept in more detail.

  . It should be noted that these space objects are the only subjects to the Liability Convention and Registration Convention, as well as Article VII (State liability of damage) and VIII (registration of space objects) of the Outer Space Treaty, while Article VI of the Outer Space Treaty only regulates the "activities in outer space" 39 . Therefore, it is reasonable to assume that the subject matter designated as "space object" is a dedicated description for the Liability and Registration Conventions, that is to say only a part of the body of international space law 40 . While satellites, launch vehicles, and trans-orbital

38 Convention on International Liability for Damage Caused by Space Objects, 29 March 1972, 961 UNTS 187; 24 UST 2389; 10 ILM 965 (1971) (entered into force 1 September 1972), art 1(d), URL: http://www.unoosa.org/pdf/gares/ARES_26_2777E.pdf ; Convention on Registration of Objects Launched into Outer Space, 6 June 1975, 28 UST 695, 1023 UNTS 15 (entered into force 15 September 1976), art 1(b), URL: http://www.unoosa.org/pdf/gares/ARES_29_3235E.pdf 39 Op. cit., note 3, article VI, VII & VIII. 40 Yu Takeuchi, Legal aspects of international regime for space traffic management, Institute of Air and Space Law, Faculty of Law, McGill University, 2014, 100p., p. 59.

  . But both proposition present gaps in regard to contemporary space activities. First of all, the gap between the State exercising jurisdiction and control over a space activity (or State of registry), as per article VIII of the Outer Space Treaty, and the State authorizing and supervising the activity, as per article VI of said Treaty. For example, if a private company in

State A were to contract with a launch operator from State B to launch a space object, State A would normally consult with State B regarding the object's registration in accordance with Article II.2 of the Registration Convention 50 . However, a problem could occur if State B were to transfer the satellite's operation to a third State (State C) after having operated it for a certain amount of time. Since the State of registry is assumed to be the launching State 51 , how can State C exercise its jurisdiction and control over the satellite? The problem with the concept of "launching State" is that the liability regime of the UN space treaties does not seem to consider that the operation of a spacecraft could be conducted by a different State. Its discrepancy with the role of "operating State", as well as generally insufficient coordination among launching States, highlight the need to entrust certain responsibilities to the appropriate States

UNGA, International cooperation in the peaceful uses of outer space, A/RES/48/39, 10 december

19 UNGA, Declaration on International Cooperation in the Exploration and Use of Outer Space for the Benefit and in the Interest of All States, Taking into Particular Account the Needs of Developing Countries, A/RES/51/122, 4 february 1997. 20 UNGA res 1348 (XIII) 15 december 1958 ; UNGA res 1472 (XIV) 12 december 1959 ; UNGA res 1721 (XVI) 20 december 1961. 21 Op. cit., note 3, articles I & IX. 22 M. COUSTON, Droit spatial, Ellipses, Paris, 2014, p. 103.

Some authors 42 have suggested that an ADR regime should be modeled on the 2007 Nairobi International Convention on the Removal of Wrecks 43 . This specific regime is based on three principles: the liability of the wreck's owner for the costs of locating, signaling and removing said wreck; the requirement of an insurance to compensate for a possible failing of the owner, with verification by the State of registration; the right for the State to take direct action against the insurer or any other person from whom the financial guarantee covers the liability of the owner.

This regime would certainly have to be adapted to the specificity of active debris removal, mainly for two reasons 44 : wrecks -contrary to most space debris -may still have commercial value, and they are notably easier to remove. According to M. RAPP, the solution might lie in the fostering of commercial interests and the advent of space debris removal as a lucrative activity. It has also been suggested by some 45 to simply create a commercial activity equivalent to that of the removal of wrecks. RAPP adds that such an activity could be financed by an international fund alimented by launching States and operators, in a "Pay or Play" fashion akin to electronic communications 46 . He then suggests that the future multiplication of space debris-related disputes would most probably International Spaceport Authority (ISPA) and offer services via a Spaceport Company (SCP). The ISPA would be created through a public-private partnership (PPP), taking example on Arianespace, and the SCP would have member States as shareholders 62 . Such an Authority could be able to fulfill the role of the aforementioned WSO. However, the idea of creating this ISPA through a PPP could be detrimental: while the example of Arianespace offers some hope, it should be reminded that the European Union failed to finance the GALILEO GNSS program through private-public partnership, and was even slowed down by the subsequent disagreements that sprung from the discussions 63 .

The creation of such an organization would encounter some difficulties of course as the fact that such programs are generally fulfilled by individual States, or that such activities can concern important economic and industrial interest, but also national security, etc. All these considerations explain the reluctance so far of States to participate in a large-scale cooperation in this area and the preference for international agreements of limited scope. For all these reasons, it now seems important to focus on contemporary and future forms of cooperation in space and their influence on Space Law, both domestic and international.