The subject of this chapter is the discussion of the measurement principles of sensors for relative navigation, required in the far and close range rendezvous phases to measure the relative state between the chaser and target vehicles. In the rendezvous phases proper (see figure 2.1), the accuracy of absolute navigation will no longer be sufficient. With one exception, sensor principles for absolute navigation will not be discussed here, since the measurement principles for absolute attitude and absolute position for spacecraft applications can be considered well-known. Measurement and control of absolute attitude is a feature of practically every spacecraft. Onboard measurement of absolute position is required, e.g., in Earth observation missions, where receivers for satellite navigation and for ground-based radio-positioning systems, e.g. DORIS (Carrou 1995), are accommodated on the spacecraft. In most other missions, absolute orbit and position determination is usually done by observations from ground, since, in the majority of cases, mission requirements do not justify the accommodation of an absolute position sensor aboard the spacecraft.

The above-mentioned exception, to be described in this chapter, comprises the basic functional principles of absolute position measurement by satellite navigation. At the time of writing, GPS and GLONASS are the satellite navigation services used, and, for the purpose of rendezvous navigation, the navigation results of, e.g., GPS receivers w.r.t. an Earth-fixed coordinate frame, are termed absolute GPS.