7.1.1 Measurement requirements

Measurements of the absolute state, such as the attitude of the vehicle w.r.t. the local orbital frame F _lo and as position or orbital ephemerides of the vehicle in the Earth-centred equatorial frame F _eq,will be required during all phases of the rendezvous mission to determine spacecraft angles and position w.r.t. the Earth and the Sun for communication, power and illumination reasons. In addition, starting from the far range rendezvous, relative position and velocities need to be available in the target local orbital frame F _lo. Relative position and velocity information could of course be obtained by the differences of the absolute measurements made on chaser and target. However, as the subtraction of large values leads to large errors, this method can only be used at large distances. In the rendezvous phases proper, i.e. during far and close range rendezvous, an increasingly accurate knowledge of the relative position and velocities of the chaser w.r.t. the target is necessary. This requires direct measurements to be made between the two vehicles. Such measurements can in principle be performed on either vehicle. If performed on the target vehicle, the results would have to be transmitted to the chaser GNC system, which may cause additional noise and which includes the danger of link interruptions. If performed on the chaser, the vehicle's position or velocity cannot be measured directly in the target's F _lo frame. In this case, the attitude of the...