ESA GNC Conference Papers Repository

One challenge in guidance, navigation and control is the orbital maneuvering to an uncooperative object, in particular if a completely autonomous rendezvous and docking process is intended. A correct approach maneuver to an uncooperative target requires i.a. precise knowledge about the actual distance and orientation between the two objects. Reliable and accurate sensor systems are one key element for successful rendezvous and docking techniques. We are investigating degradation mechanisms of several commercial off-the-shelf (COTS) near distance measurement sensors that work with different measurement principles and develop methods to increase the reliability of the measurement. Applying an elaborated combination of COTS parts to improve the overall system reliability could furthermore open up a possibility to lower system costs, to shorten the development time and to enable the use of state of the art technology in space applications. Those reliable and cost effective near range distance measurements could then be used for many small satellite missions such as servicing or docking to non-cooperative objects for e.g. active space debris removal, formation flying in small satellite constellations and rover missions. Based on a market research of state of the art COTS distance sensors with low power consumption and a small form factor the following partly automotive qualified sensors were selected: the EPC610 PMD (photonic mixer device) sensor, the VL6180x TOF (time of flight) sensor, the IVS-4005 radar sensor and two AVT-Mako-G cameras combined to a stereo camera system. We have already published [1] our results on how a fusion of the PMD, TOF and radar can help to counteract the measurement degradation under space environmental conditions such as temperature variation, different lighting conditions and different target materials with changing orientations. To maintain the accuracy of the distance measurement we described a cross calibration approach. In this work we present measurement results of the four considered distance sensors behavior under ionizing radiation and analyze the consequences for our sensor fusion concept. Therefore, we conducted a 3 MeV proton test with the PMD and TOF as well as a total ionizing dose (TID) test with all four sensors. We applied a total dose of 100 krad(Si) with a flux of 1.3*109 p/cm²/s respectively a dose rate of 1.4 krad(Si)/h and 4.3 krad(Si)/h. Various degradation effects of the sensors were observed. Except for the camera which suffered a breakdown at 45 krad(Si) all other sensors were operating up to the dose of 100 krad(Si). The test results show a distance measurement drift of the PMD sensors of around 0.6 m, the radar sensors distance measurement drifted about ± 2 cm. The TOF sensors remained accurate up to 100 krad(Si). All observed drifts turned out to be a constant offset under varying distances. For our COTS sensor combination we determined two main indicators which will limit the usability in a radiation environment. First, a drop of the internal measured PMD temperature which occurred at around 70 krad(Si) prevents the usage of this sensor due to the necessary temperature-distance calibration. A similar effect was also detected in the earlier performed 3 MeV proton test at almost the same dose. Second, the radars transmitted power drops by about 6 dB at around 70 krad(Si). This leads to a shorter measurement range. The voltage drop of the radars LDO can be referred to this. These test results consolidate our confidence about our sensor fusion cross calibration approach to use the rarely successful radar measurements to calibrate the PMD and stereo camera system. Furthermore, the temperature drop of the PMD sensor is well recognizable, so it can be used as a malfunction indicator of this sensor.