Pre-Phase A Report - Lisa - Nasa

6.4 Orbit configuration stability 139
Another alternative to reduce the noise caused by the Doppler shifts would be to modulate
the laser beams with a signal based on the spacecraft oscillators [120]. In this scheme
each arm would be essentially used as a delay line to stabilize the oscillators; the returned
oscillator signal would be compared with the local oscillator signal and the difference
would be used to measure fluctuations in the spacecraft oscillator. This scheme has been
adopted as the nominal plan for the LISA mission.
With this scheme it is still advantageous to have the arm rates-of-change small since this
reduces the dynamic range of the oscillator signal. For example, with arm rates-of-change
of 15 m/s and Doppler shifts of 30 MHz, it suffices to use a 200 MHz modulation derived
from the spacecraft oscillators on the laser signal [121]. The modulation can be imposed
using an electro-optical modulator already planned in the spacecraft payload for other
purposes. This is somewhat simpler than the two-laser scheme outlined in Hellings et
al. [120]. With these clock-noise reduction schemes there are a variety of possible choices
of nominal orbits that give acceptable ranges of Doppler shift over the period of the
mission.
The nominal operational orbits selected have initial orbits that could, if necessary, be
adjusted by small maneuvers each month to keep the rates-of-change of one pair of arms
nearly the same throughout the mission. However, no maneuvers are planned if performance
is nominal. Figure 6.1 is a plot of the arm rates of the rates-of-change of the
three arm lengths for the nominal orbit configuration. (The orbits will change slightly in
character depending on the chosen launch date.) The rate-of-change of arm length for two
rate-of-change (m/s)
15
10
5
0
-5
-10
-15
-20
0 1 2 3 4 5
time (yrs)
Figure 6.1 Nominal rates-of-change for the three arms of the LISA triangular
spacecraft formation, with the spacecraft orbits evolving under only gravitational
forces (i.e. with no maneuvers). The rates-of-change of the lengths of
arm 1 (solid line) and arm 2 (dotted line) (see Figure 3.1) are almost identical
for the first six months of the mission and could be kept nearly identical, if
necessary, through the use of occasional small maneuvers.
of the arms is almost identical for the first six months of the mission. The difference in
rate-of-change of these two arms could be kept small through the use of small maneuvers.
Corrected version 2.08 3-3-1999 9:33