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Section 5.4 of Descanso I, Radiometric Tracking Techniques for Deep-Space Navigation by Catherine L. Thornton and James S. Border titled Future directions in Radiometric Tracking; Same Beam Interferometry includes the text and figures below.

Question: Has Same Beam Interference (SBI) been used or at least tested at Mars? Are there plans to use it in the next few years? Considering that there's already some orbiters and landers and rovers there, and there are several more on the way this seems like an opportunity to pursue the spirit of tracking and although the technique has been known for a while, perseverance and curiosity may prevail.

Note that the error budget in the second figure is in units of picoradians!

As more spacecraft begin operating at Mars, SBI could be used to improve orbit determination while requiring fewer Earth-based tracking resources. All spacecraft within Mars stationary orbit would be visible within the 1-mrad beamwidth of a 34-m antenna at X-band. All signals would be acquired simultaneously. SBI data acquired during ground-station overlaps could provide enough geometric data strength to offset the loss of long arcs of ground-based two-way Doppler measurements. One-way Doppler combined with SBI may meet navigation requirements, eliminating the need for multiple uplinks.

5-1. Same-beam interferometry measurement geometry

Fig. 5-1. Same-beam interferometry measurement geometry.

Fig. 5-2. Error budget for same-beam  interferometry measurements for a lander and rover on the surface of Mars and for two spacecraft in orbit about Mars

Fig. 5-2. Error budget for same-beam interferometry measurements for a lander and rover on the surface of Mars and for two spacecraft in orbit about Mars. X-band radio links and a Sun-Earth probe angle of 20 deg are assumed.

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