4.8 Spacecraft unmodeled motion
Unmodeled motion of the spacecraft enters directly into the two-way Doppler time series (see Figure 7).
The lack of a time-domain signature makes it difficult to isolate spacecraft motion using the
Doppler data only. Such unmodeled motion can arise in principle from a variety of causes:
fluctuations in the solar wind hitting the spacecraft, fluctuations in solar radiation pressure,
physical articulation of spacecraft parts, leaking thrusters, sloshing of fuel in the spacecraft’s
tanks, etc. Solar wind and solar radiation pressure fluctuations are computed to be far too
small to affect observations at current levels of sensitivity [93
]. Articulation of instruments is
restricted during quiet spacecraft periods where good Doppler sensitivity is required. Leaking
thrusters and fuel sloshing are also thought to be small effects at current generation Doppler
sensitivity [93].
In one case the as-flown spacecraft motion noise was independently determined. Using telemetry from
Cassini’s reaction wheel assembly, Won, Hanover, Belenky, and Lee [122] inferred the time series of antenna
phase center motion projected onto the earth-spacecraft line (i.e. the sensitive axis for the Doppler system).
This test was done when Cassini was in semi-quiet cruise (thrusters off but with physical articulation of
elements of one science instrument, the Cassini Plasma Spectrometer, at 
0.0025 Hz) for 40 hours
during 2001 DOY 152-153. The resulting Allan deviation for unmodeled spacecraft motion at
= 1000 s was computed to be 2.3 
10–16. Figure 13 shows the spectrum of velocity noise
observed in that test. Unmodeled motion of the spacecraft – at least the Cassini spacecraft – is thus
negligible compared with other noises at the sensitivity of current-generation Doppler experiments (see
Figure 10 and Table 2).
).
The lack of a time-domain signature makes it difficult to isolate spacecraft motion using the
Doppler data only. Such unmodeled motion can arise in principle from a variety of causes:
fluctuations in the solar wind hitting the spacecraft, fluctuations in solar radiation pressure,
physical articulation of spacecraft parts, leaking thrusters, sloshing of fuel in the spacecraft’s
tanks, etc. Solar wind and solar radiation pressure fluctuations are computed to be far too
small to affect observations at current levels of sensitivity [93
