MGS Uplink Arraying - Experimental Results
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Figure 3. Comparison of predicted and measured phase difference
at MGS (click to enlarge) |
The AGC samples were continuously downlinked to DSS-25 when the spacecraft was visible, and observed one
light-time later at SPC-10 on a workstation monitor. The first AGC readings arrived on schedule, indicating received
power of -126 dBm from DSS-25. Soon thereafter the power reading was seen to increase as the second signal
transmitted from DSS-24 reached MGS. Because the station configuration was not fully optimized to support uplink
arraying, the frequency predicts used during this experiment contained some periods of large phase-shift (estimated to
be as large as ±65 deg) but also contained phase-stable regions. These differential phase errors, resulting
from frequency errors in the predicts, are believed to be a significant contribution to the total phase error between the
two X-band carriers. A comparison of the phase drift due to frequency predict errors and the actual phase difference
estimated from the AGC data during part of the second orbit is shown in
Fig. 3, indicating good correlation between the
smooth predicts and the noisier phase estimates.
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Figure 4. Example of phase-ramping sequence used to estimate
optimum phase. (click to enlarge) |
After phase-ramping to determine the optimum phase, 4 dB increase in arrayed power was noted during the
phase-ramp (-122 dBm AGC reading), and also during periods of stable phase. An example of the preamble,
fast-ramp, slow ramp sequence performed during the first orbit is shown in
Fig. 4 (preamble and end-markers are used to
identify the beginning and the end of each ramp). The fast ramp yielded a rough measurement of 60 degrees for the
optimum phase, while the finer slow ramp indicated 64 degrees, in good agreement. Note that the peaks of the ramps
are -122 dBm to -123 dBm in Fig. 4, in agreement
with predictions.
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Figure 5. Example of fast-ramp and stable differential phase near
power maximum. (click to enlarge) |
The entire second orbit was devoted to passive measurement of signal power and phase difference at the
spacecraft, in order to characterize phase fluctuations due to predict errors, thermal effects and equipment drift on the
ground. A fast ramp was performed at the beginning, at the middle, and near the end of the second orbit. The middle
measurement occurred near a region of good phase alignment and relatively stable phase, as can be seen in
Fig. 5: the maximum combined power observed
during this phase ramp was -123 dBm, close to the theoretical maximum, and indeed the phase ramp yielded a
required phase adjustment of approximately zero degrees, as would be expected for maximum combined power.
Following the phase ramp, the combined power remained near this maximum value for more than 10
minutes.
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Figure 6. Demonstration of 10 dB difference between max and min
combined power. (click to enlarge) |
An interesting test during the third orbit was the application of a 180 degree phase shift after power was
maximized, to verify the expected 10 dB drop in combined power: this power drop can be clearly seen in
Fig. 6. Finally, during the fourth orbit, the DSS-24
transmit power was slowly increased from 3 KW to 10 KW, resulting in 2 dB gain at the spacecraft. This is shown in
Fig. 7, where the maximum combined power
reached around 7:14:53 UTC is indeed close to the predicted maximum of -120 dBm.
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Figure 7. Demonstrating maximum combined power of -120 dBm
(with both antennas transmitting 10 KW). (click to enlarge) |
It should be emphasized that these experiments were not intended to develop a general technique for calibrating
uplink arrays in an operational setting, rather to characterize the combined carrier fields at the spacecraft, verify the
predicted gains due to coherent combining, and set the stage for near real-time tracking of phase-drift via local phase
measurements in future experiments. Follow-on experiments are planned after MRO Mars Orbit Insertion has been
completed, in September 2006. The focus of these experiments will be to extend the preliminary results reported here
by demonstrating that maximum power can be maintained for an entire orbit, based only on local measurements after
the combined carrier power has been maximized at the spacecraft.
