Report 1999 DOY 004 from the desk of CAPS PI: Dave Young
The first day out of 3 days of CAPS checkout was completed successfully
this morning. The data quality throughout the pass was generally very good
with minimal to no time lags. Commanding went smoothly, including the
creation and radiation of several contingency commands. Parallel operations
with MIMI and INMS were accommodated without problems.
The IBS was brought up first. The 3 channeltron detectors operated
flawlessly as indicated by the expected level of the noise count rate
(~1/sec) once detector high voltages were on. The electrostatic analyzer
voltages were checked out successfully, after which IBS was allowed to
operate in its solar wind survey mode for 20 minutes. The accompanying data
were taken several hours later, following ELS checkout. In the IBS plots,
ion energy/charge increases logrithmically from right to left, varying from
about 250 eV to 10 keV. IBS has 3 detectors (coded by color) which view in
roughly the same direction. The figure shows a weak solar wind-like
spectrum of ions taken with the IBS aperture facing roughly in the
direction of the Huygens Probe (along the s/c -X axis). Since the
fields-of-view of all 3 CAPS sensors are pointed roughly perpendicular to
the direction of solar wind flow throughout most of ICO, the origin of
these ions is not yet understood but may be caused by scattering off of s/c
or Probe surfaces.
After its checkout IBS was commanded off and ELS was brought on. The ELS
checkout went smoothly, culminating in observations of photoelectrons
(generated by the Cassini s/c) and solar wind electrons. The accompanying
figures show superimposed energy spectra from eight different ELS
look-directions covering electron angles of arrival from directions near
the high gain antenna (EL channels 6-8) to near the s/c +Z (thrust)
direction (EL channels 1-3). The prominent dip in the electron spectra near
energy channel 54 is thought to divide lower energy photoelectrons from
solar wind electrons. Electron energy increases from right to left in the
figure and covers the range from ~1 eV through 27 keV. The s/c potential
may be inferred from this plot to be about 6.5 volts positive at this time.
Some difficulties were encountered with initial tests of the IMS data
processing system (CPU2). Only 50% of the data is received at one time in
alternating 32 sec periods. The problem is thought to be caused by a
software timing conflict that has been observed and fixed before. The
problem is under study in the lab using the CAPS engineering model. It is
believed to be correctable with a modification to the flight software.
Difficulties were also encountered with the actuator/motor that turns the
CAPS field-of-view by up to +/- 104 degrees about the s/c Z-axis. Some
temporary sticking of the actuator occurred at the ~ -100 degree point of
motion with the motor continuing to drive towards the -104 degree limit.
Sticking is apparently caused by the stiffness of the flexible cable that
carries CAPS signals from the s/c through the actuator to the DPU. Before a
command could be uplinked to turn off the actuator, the system moved freely
through its range of motion before being commanded to stop at +90 degrees.
The actuator was then commanded back to its neutral position (FOV aligned
with the imager boresights)