Testing Proba-3's Vision Based Sensor system, imaged here in infrared to show the lights in yellow. A wide-angle camera is used to track an LED pattern on the other satellite, providing relatively coarse information on the satellites’ distance from each other, as well as additional information on their attitude.
This is supplemented by a narrow-angle camera which locks onto a second, much smaller LED pattern, providing relative positioning information down to a scale of about a single centimetre.