AMSAT Space Symposium
[This was drafted for the Palomar Amateur Radio Club newsletter, the Scope.]
Here are a few interesting tidbits from the AMSAT Space Symposium held near San Franciso in October. The big news seems to be that “software-defined transponders” (SDX) are coming soon. A traditional satellite transponder connects the analog IF output of a receiver to the analog IF input of a transmitter, repeating every signal on the uplink band, amplified, on a downlink band. An SDX modifies this by sampling the received signal, passing it through a DSP for processing, and sending the output samples directly into a digital transmitter. The digital transmitter is more efficient, and has a much higher dynamic range (80+ dB) than HELAPS (26 dB), the high-efficiency analog technique we’ve been using on transponders. The SDX demonstration at Dayton was a hit; everybody said it “sounds sweet” and it was quite robust to interference. As a nice side effect, the DSP can also make every input signal the same strength, so that “alligator” stations using too much uplink power won’t cause a problem anymore. Beyond that, the DSP can insert signals of its own (such as telemetry beacons) and even receive and act on uplink signals, all with no additional hardware. This technique is scheduled to be used not only in AMSAT’s big Eagle project, but also in AMSAT-DL’s Phase 3E satellite and much smaller satellite projects such as AMSAT-UK’s ESA SSETI ESEO.
Many small satellite projects are in the works. The US Naval Academy is working on ANDE, RAFT, and MARScom missions. South Africa AMSAT is working on Sumbandila (roughly, “Pathfinder”). AMSAT-ZL (New Zealand) is working on KiwiSAT. And that’s not to mention all the tiny “CubeSAT” spacecraft from educational institutions world-wide. AMSAT’s policy of concentrating on larger, high-orbit spacecraft and leaving the small, low-orbit spacecraft to the rest of the community seems to be working out well.
AMSAT’s Eagle project has been subjected to some hard-eyed engineering scrutiny. There’s a new mechanical design and a definite, feasible set of payloads now. An analog transponder will cater to the traditionalists with primary uplink on 70cm (U band) and downlink on 2m (V band), plus secondary uplink on 23cm (L band) and downlink on 2.4 GHz (S1 band). The U/V transponder should be usable over 75% of the orbit, but the L/S1 transponder will have a shorter access window due to antenna constraints. All these transponders will be implemented digitally with SDX, but ground stations will still use normal SSB/CW equipment.
Piggybacked on Eagle’s U/V transponder will be a digital short message service intended to be workable from a hand-holdable device. Envision a standard PDA clipped into a transceiver cradle, with a smallish omnidirectional antenna on top. AMSAT will have to develop the cradle and make it available to all at a reasonable price. Exact details of what kind of services will be offered are still to be determined, but expect something like internet Instant Messaging with some exciting twists. A hand-held satellite station could be just the thing in some emergency communications scenarios.
The primary service on Eagle will be a digital voice and data system called the Advanced Communication Payload or ACP, with uplinks on 3.4 GHz (S2 band) and downlinks on 5.8 GHz (C band). Steerable phased array antennas on the satellite will make these links available for 75% of the orbit. Two classes of user stations are envisioned. A user with a single 60-cm (about 2 feet) diameter dish will be able to operate digital voice or about 4800 bits/second of any kind of streaming data. A user with a big dish (6 feet) will be able to do the same, and also handle higher-rate data streams, fast enough for compressed digital video. Here too, AMSAT will make available ground station equipment kits to make it possible for anyone to get on the air without being a microwave guru or independently wealthy. The small dish service is specifically designed to be usable by apartment dwellers and those constrained by antenna restrictions: the small dish isn’t so different from a satellite TV dish. It will have to be pointed at the satellite, but only in one axis if the preferred orbit is attained.
The ACP is designed to take advantage of advanced digital techniques to maximize performance. It can handle about 20 simultaneous digital voice (or low rate data) channels, and all of the voice channels are transmitted as a single unified data stream. That means that a user station can, if desired, listen to any or all of these channels simultaneously. Each user’s uplink can be addressed to a logical channel, which might be a one-on-one QSO or a conference room. It’s all done in software. This opens some very interesting possibilities. As part of the campaign to get the ground station hardware into the field, I am hoping that AMSAT will make this system available for terrestrial applications, well before Eagle’s launch.
For those interested in antenna design, Tony AA2TX came up with two novel variants on the Lindenblad antenna. A traditional Lindenblad is four folded-dipole driven elements in phase, arranged in a square, each tilted 30 degrees from horizontal. It generates a nice omni sky-coverage pattern, circularly polarized, just the thing for low-orbit satellites with relatively strong signals. Tony’s design for 2m replaces the folded dipoles with regular dipoles, and uses a tuned coax phasing harness and slightly overlength elements to achieve low SWR without the mechanical complexity of folded dipoles and their twinlead feedlines. Tony’s design for 70cm eliminates the feedlines altogether, driving a Lindenblad-like array of parasitic elements with a vertical dipole at the center. Both designs are easy and inexpensive to build using hardware-store plumbing parts and aluminum tubing.
More information on Eagle and other upcoming satellites is available on www.amsat.org. The printed Proceedings of the Space Symposium will be available from the AMSAT store — www.amsat.org and click on “Store”.