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Air Force Gives 10-Year-Old Orbiting Satellite To Ham Radio Operators (arrl.org)
Longtime Slashdot reader Bruce Perens writes: The U.S. Air Force has transferred control of a 10-year-old orbiting satellite to AMSAT, a ham radio organization, which has enabled it for any licensed ham to use on the air, as the satellite's Air Force missions have ended. Falconsat 3's first mission was science: measuring gravity gradient, spectrometry of the plasmasphere, electronic noise in the plasmasphere, and testing three-axis attitude control using microthrusters. Secondarily it was used to train Air Force Institute of Technology students in space operations, with close to 700 cadets obtaining ham licenses in order to operate a number of Air Force satellites using ham frequencies.
Now in its third mission, control of the satellite has been transferred to AMSAT, the Radio Amateur Satellite Corporation, and all government frequencies have been disabled with only ham ones remaining. The satellite will relay APRS (position and status reporting) signals, it will operate a BBS in the sky, and will broadcast telemetry.
Now in its third mission, control of the satellite has been transferred to AMSAT, the Radio Amateur Satellite Corporation, and all government frequencies have been disabled with only ham ones remaining. The satellite will relay APRS (position and status reporting) signals, it will operate a BBS in the sky, and will broadcast telemetry.
The "FalconSAT" name certainly suggests a link to SpaceX's Falcon 9 rocket, but it is actually unrelated, predating SpaceX's founding. The satellite series has used a number of different lift vehicles - FalconSAT-3 used an Atlas V 401 rocket, as part of a multi-satellite launch.
The closest the two Falcons came was the launch of FalconSAT-2, which got bumped from the Space Shuttle's manifest after Columbia. It got re-used as the payload on SpaceX's first-ever launch, the first Falcon 1 flight. Which failed catastrophically a half-minute in. The satellite apparently survived with "minor" damage, falling back onto the island, but it was never re-launched to my knowledge.
The Air Force probably doesn't need FalconSAT-3 anymore because they have FalconSAT-5, which presumably can fill a similar purpose.
Receivers are de-sensitized by close-by transmitters on the same band. Having the uplink and downlink be in a separate band avoids this. Terrestrial repeaters use a big piece of RF plumbing to avoid this, which would increase the weight of the satellite.
Bruce Perens.
Having separate frequencies makes sense, but what purpose is there to having them this far apart?
This is standard for satellite operations.
There is one overriding technical goal in creating a working repeater. The receiver must not be swamped by the transmitter, which has a much stronger signal than anything it will hear from Earth. Earth-based repeaters (which this basically is) have physical isolation that is based on the wavelengths of the signals. A VHF duplexer, as it is called, is about 3 feet tall and 6 to 8 inches in diameter, and there are usually four or six used. The physical cavity allows for very sharp notches and passbands that are applied to both transmit and receive. The receive duplexers selectively pass the receive signal and notch the transmit. The transmit duplexers selectively pass the transmit signal and notch the receive. The antenna end of the duplexer chain (e.g. three in series from the transmitter, three in series to the receiver) is simply tee'd together. This setup works when the signals are 600kHz apart.
You can build smaller duplexers, such as those used for ancient mobile telephone systems, or for some current repeater systems, but these require a minimum of 5MHz separation between transmit and receive, and support lower power transmitters. There is only 4 MHz in the entire amateur 2M band (2MHz in some countries), so this separation is not possible within that band.
It is VERY easy to build an LC (coil/capacitor) duplexer for considerable amounts of power when the frequencies are 300 MHz apart. Like VHF (146MHz) and UHF (440MHZ). This can fit in a package smaller than a pack of cigarettes. And it is much lighter (pun intended).
That's why amateur satellite operations use widely-split duplex. If it is UHF uplink and VHF down it is referred to as U/V mode, opposite is V/U mode (or vice versa. I don't do satellite ops.)