17apr05 sband bistatic radar run has pointing trouble
A moon bistatic radar run was done between arecibo and
the gbt using the sband transmitter on 17apr05 (starting about 18:25 ast).
The telescope was pointed at the south pole of the moon. When the data
was reduced, the ranges near the south pole saw a large variation in the
signal to noise. After about 30 minutes (??) the signal to noise started
to drop. This continued until it was about 10db worse toward the
end of the observation. The observation was restarted at 18:45 ast.
Since no data was recorded at arecibo, there were
no headers containing the positions to compare with the ephemeris file.
The encoder values (that are independently archived once a second) were
used to compare with the ephemeris file. The computations done where:
The model removal was not perfect because the model was evaluated
at the encoder azT, zaT and not (az-modellaz, za-zamodelerr). This introduced
a small error.
Input the ascii version of the ephemeris file. The raE,decE (apparent)
and the time were used for the computations (E stands for ephemeris). This
has the ra to .01 secs of time and the dec to .1 secs of arc (the az,za
are only listed to ,.1 degrees).
Convert the ephemeris raE, decE to azE, zaE.
limit the raE,decE,azE,zaE to the time when the moon was in the beam.
Input the azT, zaT (T for telescope encoder). Remove the model and then
convert to raT, decT apparent.
Interpolate the telescope azT,zaT, raT,decT to the one minute time
stamps from the ephemeris file.
The plots show 17apr05 the
actual and requested positions (.ps) (.pdf):
Fig 1 : (requested position - Encoder Position). Black is azimuth,
red is za. The top plot is versus ast hour of day. The 2nd plot is versus
az while the bottom plot is versus za. The small bump at 18.75 (in
the top plot) is when the program was restarted. The ephemeris version
of this data was computed from the ra,dec fields.
Fig2 top: The ra versus dec. The black is the requested value from
the ephemeris file. The red is computed from the telescope encoder
Fig2 middle: this shows the one minute rates (computed from the ra,decs).
The units are arcminute/minute (great circle) Black is from the ephemeris
file while red is from the telescope encoder values. The dashed blue line
is when the program was restarted at 18:45 ast. The noise in the
encoder rates probably comes from the computations.
Fig2 Bot: This plots the 1 minute rates versus azimuth angle.
So what happened??
It looks like the rate was not getting updated for the
telescope. This could happen if
the sbposcycle program aborted or stopped working (after the restart at
The sbposcycle ran out of ephemeris file. Looking at the file, there
is plenty of the data so this does not seem likely.
The sbposcycle was not never started. This initial rate came from when
the datataking was started. This is probably what happened.
The next day a similar run was done. The plots show
actual and requested positions (.ps) (.pdf):.
The only problem on this day was when the zenith angle went below 2
degrees za (the keyhole).