Multi-Spacecraft Coherent Doppler and Ranging for Interplanetary Navigation

Future interplanetary mission concepts are increasingly focusing on multi-spacecraft missions and on small sample return missions which may involve the rendezvous between a spacecraft which brings a sample from the surface of a solar system body and a spacecraft which will return the sample to the Earth. These types of missions place tight requirements on the knowledge of the relative positions of the spacecraft. Historically spacecraft positions have been determined by the use of radio metric data (Doppler and range) between the spacecraft and terrestrial receiving stations. However, the various systems available place tight requirements on the stability of onboard frequency standards on each spacecraft and on the precision of the data extraction hardware on the spacecraft. An alternative is to extend the coherent ground to spacecraft link through one spacecraft to the other and then to the ground or back through the first spacecraft and to the ground receiver. Although this might appear initially to complicate the separation of the dynamics of the two spacecraft, this paper will show that in actual application, judicious selection of spacecraft transponder frequency ratios and the use of coherent Doppler and ranging and the derived observable, DRVID (Differenced Range vs. Integrated Doppler) can allow for the generation of observable equations which are dominated by the spacecraft to spacecraft link.