This 3D plot was generated from 40 consecutive spectra which were
acquired at the time that the Relay was transmitting in its mode 1
RC1 state. The Stanford receiver was configured at the time so that the Relay
beacon appeared at 4 kHz in our passband. The plot graphically illustrates
the transmit cycle in which the beacon is in the CW condition for 2 seconds
followed by modulated condition for 14 seconds. More than three cycles
are shown in the figure. The second harmonics of the RC1 subcarriers are
also clearly visible in the figure. Note that these subcarriers go off when
the beacon is transmitting CW. The time axis units are seconds. The
frequency axis units are kiloHertz. The power axis is linear and not
logarithmic (dB).
The following plot was generated from spectra which were acquired when
the Relay was configured to transmit in the mode 9 RC1/RC2 state. As in
mode 1, the plot illustrates the MR cycle in which the beacon is in the CW
state for 2 seconds followed by 14 seconds of modulated carrier. The modulation
in mode 9 is more complex than in mode 1, however. The RC1 subcarriers
are visible during one 16-second Relay cycle followed by RC2 subcarriers
in the subsequent cycle. There is no power in the beacon while one
set of subcarriers are visible but there is beacon power while the
other set of subcarriers are present.
The last 3D plot was generated from spectra which were acquired when
the Relay was switched from the mode 9 RC1/RC2 state to the mode 15 CW
state. The final RC1 and RC2 cycles are shown each with 2 seconds of
carrier followed by 14 seconds of modulated subcarrier. The mode was
then switched to CW and only the MR beacon was transmitted. Though not
evident in this figure, the amplitude of the beacon was modulated during
the 100-minute spin of the Mars Global Surveyor spacecraft and
the resulting change in the Mars Relay antenna pattern as viewed from
here on Earth. At the time these spectra were acquired, the Relay
signal strength was about 10 dB below maximum.
Data were generally acquired during the Test with a (complex) sample rate
of 15 kilosamples per second. Power spectra were computed
with a 2048-point complex Fast Fourier Transform. The results of
six FFT's were accumulated to produce each spectrum, further improving
our SNR. The long-term average spectra were computed by exponential
averaging of the accumulated spectra. The time constant for averaging
was approximately equal to the time for acquisition of 20 of the accumulated
spectra.
Nearly all of the spectra have been stored on disk. The archive looks to
contain about 200 Megabytes worth of spectra. In addition, a lower volume of
individual spectral values above a given threshold have also been stored.
Some representative spectra from the early part of the test are shown
below. At this time, the MR was transmitting in cycles consisting of
2 seconds of CW followed by 14 seconds of modulated subcarriers. The
subcarriers are clearly visible in many of the spectra below. The title
of each figure contains a time tag which indicates the time at
which the spectrum was acquired (in Universal Time). The frequency axis
units are kiloHertz and the magnitude axis units are dB. The center
frequency of the Stanford receiver at the time the data were acquired
is also indicated on each figure.
Note that the strong frequency components near DC are NOT
the Mars Relay beacon. They are due to a DC offset and 60 Hertz (plus odd
harmonics) noise. They will be removed from post-processed spectra. The
receiver was tuned during this portion of the Test so that the Relay
beacon appears near 4.7 kHz in the spectra.
Mode 1 RC1:
Mode 1 RC1:
Mode 9 RC1/RC2:
The following spectra were acquired while the beacon was transmitting in
its CW mode. This mode provided the strongest signal. The receiver was
tuned during this period so that the beacon appears near 4.5 kHz in our
spectra.
Figures showing the beacon power and frequency
during this portion of the test are available.
Last updated: 30 January 1997
Joe Twicken