2.1

Sensor function

The optical sensor is mainly devoted to detection and classification of small space debris and the mission duration is about two years. The classification of the object is made in estimating the albedo surface product. A rough information on debris speed is also needed.

2.2

Integration constraints

The optical sensor on-board micro-satellite will present the following characteristics :

2.3

Debris characteristics

Debris albedos in visible band were measured for large objects (ref. 1,2).

Fig 1.

Fig 2.

In thermal band, debris signature is depending on the material emissivity and the surface temperature. The temperature is varying periodically during solar illumination and eclipse cycle.

3.1

Flux versus debris size distribution

The fig.3 shows the theoretical debris flux per year per squared meter versus the debris diameter. For performance calculations the debris repartition is supposed homogenous in space. Of course the goal of the sensor is to complete this curve assumption with experimental measurements.

Fig 3.

3.2

Detectable debris number

The angular coverage of the sensor is represented below. Fov represents the sensor field of view and the range represents the maximum distance of detection.

Fig 4.

For discontinuous observation sensors (ex: pulsed source) a figure of merit was defined to represent the number of detectable debris for diameter classes defined by [Φ, Φ+ΔΦ].

For active sensors we obtain Eq. 1.

The term dFlux is derived from the theoretical debris flux (fig.3), Pro_illum is the debris probability to be illuminated when crossing the field of view of the sensor at a range r, F_laser is the laser repetition rate, τ_pulse is the temporal length of the pulse.

For passive sensors with a quasi-continuous temporal observation the number of detectable debris is given by Eq. 2 after simplification of Eq. 1.

where k_eclipse is the eclipse ratio.

4.1

Active concept

The principle of detection with active sensors is based on debris illumination with a source and measurement of the back scattered light with a receiver. Various optical sources are available (incandescent light, flash, laser diode, pulsed laser) to realize the illumination. To respect the energy budget and to reach the best performances in terms of spatio temporal coverage we have selected laser diode and pumped laser.

The best temporal coverage is reached with Laser diode (Continuous Wave) but the range performances are limited by the available averaged power. The best spatial instantaneous coverage is obtained with pulsed pumped laser but unfortunately with a low temporal coverage.

Performances are illustrated for a pulsed micro laser Nd:Yag operating at 1.064 μm. We have calculated the probability of illumination (fig. 5), the maximum range (fig. 6) and the number of detection per class and per year (fig. 7) for debris with 10 % albedo and 8 km/s relative transverse speed. A diameter class is defined by the following interval [Φ, 1.6*Φ].

Fig. 5

Fig. 6

Fig. 7

The sensor feature is defined by the following parameters.

Following this criteria we conclude that the active concept is better suited for small space debris at short range. The return signal is penalized at long range because the return energy is inversely proportional to the range to the fourth power.

4.2

Passive concept

4.2.2

visible sensors

Solar illuminated debris can be detected with focal plan array CCD or APS camera. Images are processed to detect aligned pixels corresponding to debris displacement during the integration time of the camera (fig. 8). Detection will be performed after signal integration in the Hough domain to get high detection sensitivity for speedy debris or long traces.

Fig. 8

Range measurement is performed by stereoscopic observations with at least two consecutive detections. The measurement of the satellite displacement between two frames and the angular position of the debris allows range estimation.

The class of “albedo-surface” product is calculated from the debris range and the debris signal.

The speed vector is estimated after range and angular position differentiation or by exploitation of the debris trace length in the focal plan.

The synoptic of the sensor is given fig 9.

Fig. 9

The instrument includes:

• – optical head including the optics, the focal plan array detector (CCD or APS) and related electronics,

• – signal processing for image correction, bright star rejection, calculation of image differences, Hough transform, detection,

• – high level information processing for debris localization, albedo-surface product classification and kinematics estimation.

Detection performances were evaluated for passive sensors with the following features.

Optics	Pupil diameter	100 mm
Aperture	F/1 or F/2
Transmission	70 %
Detector (backthinned CCD)	Number of pixels	512*512 or 1024*1024
Pixel size	13 μm*13 μm
Fill factor	100 %
Wave length (QE=10%)	[300 ; 1000] nm
Readout noise	10 e
Integration time	50 ms (512*512) 100 ms (1024*1024)
processing	Detection	Integration in Hough domain
SNR threshold	3

Figure 10 shows the number of detectable debris per class and per year for 10 % albedo debris, three fields of view, two pupil diameters and 8 km/s transverse relative speed.

Fig. 10

Figure 11 shows the number of detectable and localizable debris per class and per year in the same conditions.

Fig. 11

Integration of the signal corresponding to debris traces enables small debris detection at short range. At long range in passive mode the signal is less penalized than in active mode because the received flux is inversely proportional to the range to the square power.

The range localization domain is lower than the detectable domain.

A preliminary design analysis leads to the following physical features:

• – mass : 3 kg

• – electrical consumption : 35 watts

• – optics volume : Φ = 110 mm * 200 mm

• – 2 electronic cards