Multiple Spacecraft Per Antenna (MSPA) techniques have been used for well over a decade to increase the efficient utilization of ground network assets while decreasing the antenna fees allocated to the missions. In the Deep Space Network’s traditional MSPA service, two missions that will be located within the same beam of a ground antenna (e.g., at Mars) can schedule to share the antenna and associated microwave electronics. The antenna, of course, must be equipped with two separate receivers — one for each spacecraft. Applying this MSPA service to more than two spacecraft at a time requires adding receivers. But adding lots of receiver and telemetry processor chains to each antenna to facilitate MSPA for multiple spacecraft within the same beam could prove prohibitively expensive for the Deep Space Network.
The Opportunistic MSPA solution is to employ recorders for each station capable of recording digital IF signals from every spacecraft in the antenna beam within the frequency bands of interest. The digital recorder operates 24/7 with disk overwrite every couple of days. Smallsats opportunistically transmit open loop when in a host spacecraft’s antenna beam. Via a secure Internet site, smallsat mission operators later retrieve relevant portions of the digital recording for subsequent demodulation and decoding, or subscribe to a service that does it for them.
In order for a smallsat mission to opportunistically downlink, the smallsat mission must design its trajectory to ensure that it is within the beam width of the other spacecraft’s ground receiving antenna. For registered smallsat missions, the ground antenna network could make the tracking schedule of each station available. The smallsat missions can then use that information to figure out whether or not they will be within the beam width of particular stations and for how long. The smallsat missions can then plan to downlink their data during these time periods.
Analysis shows that missions using Opportunistic MSPA at the Moon, lunar Lagrange point 2, the Sun-Earth Lagrange points, Earth Leading & Trailing Orbits, Mars, and Venus can, depending upon the spacecraft and ground receiving antenna’s capabilities, support average data return rates on the order of hundreds of bps to tens of Mbps while maintaining safe separation distances from the “host” spacecraft — distances on the order of thousands to millions of kilometers.
Opportunistic MSPA may also be applied to constellations of spacecraft operating in distant HEO orbits and beyond. This, of course, assumes that one of the spacecraft utilizes traditionally scheduled antenna passes and the other spacecraft at least periodically intercept its ground antenna beam.
To minimize radio-frequency interference during opportunistic MSPA, the smallsat missions need to be assigned to an appropriate sub-band that is not in use by the surrounding “host” spacecraft. If multiple smallsats will be operating with Opportunistic MSPA in the same vicinity, it may be desirable to have them share the allocated sub-band via Code Division Multiple Access (CDMA).
While Opportunistic MSPA does not allow missions to access their downlinked data in real time, it should enable them to avoid or reduce the allocated aperture fees and other charges associated with scheduling formal, traditional communication links on the antennas. The smallsat mission’s primary cost would be the labor to retrieve the relevant portion of the recording, pull the signal out of the “noise,” and reconstruct the downlink data. Hence, the smallsat’s use of more costly, formally scheduled antenna passes could be limited to a much smaller number essential for commanding, navigation-related radiometrics, and real-time critical event telemetry. (And, there is nothing that would preclude the use of one-way ranging during Opportunistic MSPA operations.) At the same time, the cost to the ground antenna network would essentially be limited to providing and maintaining the appropriate digital recorders and the secure Internet site through which the smallsat missions can access the needed portions of the recordings.
This work was done by Douglas S. Abraham and Bruce E. MacNeal of Caltech for NASA’s Jet Propulsion Laboratory.
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