A Lateral sensor for the alignment of two formation-flying satellites

The lateral sensor is a system able to measure the lateral position between two satellites. It bridges the gap between the alignment accuracy achieved with the radio frequency metrology, and the alignment accuracy required to start the high-precision optical metrology (fine-lateral and longitudinal sensor).
This project concerns the demonstration of formation flying technologies for future European scientific and application missions of the EUROPEAN SPACE AGENCY.
Technological developments enabling formation flying have already been initiated and some precursor missions already cover part of the related technologies, for example SMART-2.
The lateral sensor developed at Centre Spatial de Liège (CSL) is based on a camera (CMOS detector) a telecentric lens designed at CSL, a fibre-coupled laser-diode bar and a corner cube.
The fibre-coupled laser-diode bar emits a diverging beam, from the master spacecraft to a slave spacecraft. A corner cube located on the slave spacecraft sends the light back. This light is captured by the telecentric lens and camera (build by Deltatec Liège-B). A lateral shift “d” of the corner cube is seen on the camera as an image displacement. Real time centroidisation algorithms will allow tracking the image position and feed the on-board computer with this information via a RS422 link, allowing further position stabilisation. The imaging system needs to operate in a depth of field from 25 m to 250 m.
Because of the large depth of operation, all lateral system performances are angular ones.
The system is build to meet:
• Tracking capability for a spot moving @ 0.5 arcdeg/sec
• Spot detection with the sun in the field of view
• Centroidisation accuracy: 0.1 camera pixels (3.5 arcsec)
• Absolute calibration accuracy: 0.14 camera pixels (5 arcsec)
In order to comply with these requirements, the following features were implemented :
• The camera allows quick read-out of sub-windows of interest (in 1 ms).
• Optimize radiometric budget and laser diode power consumption, and its related heat dissipation.
• An interference filter blocks all unwanted light centred on the laser wavelength (980 nm).
• The detector captures the images. An electronic unit records the images and localizes the bright spot (return image from the corner cube).
• Implementation of a “slow detection mode”, which allows to discriminate sun from moving image
The system is a standalone unit. Once connected to an unregulated 28V power-supply, it delivers, after spot detection, pixel coordinates to the RS422 link at a 10 Hz rate. Its average power consumption is 8W.
The system can be put in a programming mode, to allow uploading settings in the FPGA.
Tracking, absolute calibration, resolution and sun avoidance have been measured in a field of 10 arcdeg and the range of depth of 5 m to 80 m.
The coarse lateral sensor has successfully undergone thermal qualification (40°C and -30 °C), and vibration test (high-level sinus, random and shock test in the 3–axis). Its metrology performances (centroidisation accuracy (<0.1 pixel RMS) and tracking) remained unchanged after these qualification tests. The absolute calibration is 0.3% in the interval of ±5arcdeg (11 arcsec in ±1arcdeg).