Pre-Phase A Report - Lisa - Nasa

6.5 Orbit determination and tracking requirements 141
from the on-board data reduction algorithm for the interferometer data. For this data
reduction, the length of each arm needs to be known to better than 200 m at any time
(see Section 4.3.2).
Table 6.1 gives a summary of the orbit determination requirements.
Table 6.1 Required orbit determination accuracies.
Phase Accuracy (σrms) Requirements taken into account
Transfer position: 10 km classical interplanetary navigation,
velocity: 1 cm/s manoeuvre dispersions
Experiment position: ≤ 10 km laser acquisition,
velocity: 2 mm/s on-board laser phase processing
arm-length: ≤ 200 m
The navigation performance for the transfer phase is relatively standard for current interplanetary
missions that use X-band (8 GHz) radio systems for the acquisition of range
and Doppler measurements by tracking stations of NASA’s Deep Space Network (DSN).
The characteristics of the assumed ground tracking accuracy are :
• Station location uncertainties ≤ 3cm.
• Two-way range data (noise: < 2m; bias: < 10 m) plus two-way Doppler data (max.
error: < 0.1 mm/s for 60 s averaging).
• Ionosphere zenith delay after calibration by means of GPS signals: ≤ 3cm.
• Troposphere zenith delay after modeling: ≤ 4cm.
• Earth orbit orientation error: ≤ 5nrad.
These tracking assumptions are met by the DSN network, and could be met by the ESA
Multi Purpose Tracking System after a few enhancements and/or modifications (X-band,
GPS-calibration, highly stable frequency standards). With this type of tracking, and
assuming that tracking measurements are acquired for each spacecraft throughout 8-hour
tracking passes two to three times each week, the navigation requirements for the transfer
phase can be met. For the injection into the final orbits, one to two weeks of tracking
time may be needed between successive maneuvers until the expected error of the final
pre-separation maneuver is less than 1 cm/s.
During science operations, the requirement that the arm lengths be determined to ≤ 200 m
is not easily met using ground tracking only. However, it will be possible to augment the
ground tracking data with data acquired along the interferometer arms. Each interferometer
arm will include phase measurements taken every 0.1 s (see Section4.3.2). Differences
in the phase measurements give information on the rate of change of the arm length (i.e.
Doppler). The measurements along each arm will be noisy compared with the desired
gravitational-wave performance but are sufficient to aid in the determination of the arm
lengths. After the arm lengths are determined, the laser phases can be combined in a manner
to cancel most of the phase measurement error and result in the desired instrument
performance.
Corrected version 2.08 3-3-1999 9:33