MGS Uplink Arraying Experiment Description
The goal of this experiment was to illuminate MGS with unmodulated X-band uplink carriers simultaneously from two 34-m BWG antennas at Goldstone (DSS-24 and DSS-25), and determine the stability of the arrayed carrier power at the spacecraft.
Most of the differential Doppler was removed by the exciters at SPC-10 prior to transmission, in order to generate a stable far-field power distribution. With two antennas, the arrayed power distribution in the far-field is the product of the primary antenna pattern and the interference pattern generated by two point-sources located at the antenna phase-centers. Pointing the two antennas toward the spacecraft to a small fraction of the primary beamwidth is routinely achieved with the operational antennas at the Apollo complex, however this does not imply that the peak of the interference pattern will necessarily be illuminating the spacecraft: the transmitted phases also need to be aligned at the spacecraft to achieve power maximization. Initially, the spacecraft may be located near the peak or the null of the two-antenna interference pattern, but most likely at some intermediate point on the array gain profile.
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Figure 1. MGS Uplink Array Experiment conceptual block
diagram (click to enlarge) |
The far-field intensity pattern can be swept over the spacecraft electronically from the ground by varying one of
the transmitter phases over the entire range
(0, 2
).
This operation guarantees that the peak of the intensity pattern illuminates the spacecraft once during each sweep
provided the phases are stable. This power variation can be measured by the AGC to monitor the instantaneous carrier
power at the spacecraft. The results of these measurements are then relayed to the ground station as engineering
data, and evaluated to determine the optimal phase adjustment required to phase up the signals at the spacecraft. A
conceptual block diagram of the proposed experimental setup is shown in
Figure 1. Note that downlink reception during this
experiment was not carried out by a separate antenna, as shown in the concept diagram of Fig. 1, but instead the
X-band receiver of the DSS-25 antenna was used, tuned to the downlink frequency.
Two 34-m BWG antennas at Goldstone were scheduled to track MGS using frequency predicts generated by
mission operations. The DSS-25 transmitter was commanded to radiate a 10 KW X-band carrier, and the uplink
frequency swept to ensure that the spacecraft transponder could acquire the signal. After the MGS transponder locked
up, the DSS-25 ground station receiver acquired and locked onto the downlink carrier, and began recording
engineering telemetry from the spacecraft. The engineering telemetry contained readouts from the AGC, which
provided near real-time measurement of the received uplink carrier power. With only DSS-25 transmitting, the
spacecraft AGC power was recorded to serve as a baseline single-antenna power measurement: this power level was
-126 dBm, as expected.
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Figure 2. MOT2 RCV AGC Calibration Curve (click to enlarge) |
A few minutes later, the DSS-24 transmitter was configured to transmit a 3 KW carrier. The second antenna was
initially configured for a lower uplink power so that the spacecraft transponder could stay locked to the uplink carrier
even if the carrier phases were 180 degrees out of phase. When properly phased up at the spacecraft, arraying of 10
KW and 3 KW transmitters yields an equivalent power of approximately 25 KW at the spacecraft, yielding a gain of 4
dB over the 10 KW antenna. When both antennas transmit equal power phase-aligned carriers, the received power is
theoretically 6 dB greater than would be received from either antenna separately.
Figure 2 shows the calibration curve
for the 8-bit receiver AGC: note that the calibration curve is linear in the region of interest (-110 dBm to –130 dBm).
The AGC resolution in this region is about 0.3 dBm.
