WA8LMF Mirror of WB4APR Website - 21 July 2008
PSK31 Xponder Aux Payload
Bob Bruninga, WB4APR
My other paper <A HREF=”psksat.html”>Other PSK31 Satellite ideas</A>
Described several ideas in how to take advantage of the tremendous weak signal capabilities of PSK-31 in a variety of satellite transponders. Included were ideas on how to fit them into the small 4” cube size of the Stanford CUBESAT program.
This paper focuses on trying to add a simple PSK-31 Parrot transponder to our PCsat mission for launch in only 6 months. The block diagram is as follows:
The advantage of the Parrot Transponder is that all users need only a single 10m all-mode radio to operate both the uplink and downlink. Due to the parrot’s delayed repeat of all uplinks, users can operate half Duplex, yet still hear their own signal to allow for Doppler adjustments and finding other users in the passband. The usable passband is 1000 to 1800 Hz which can support 14 continuous channels of PSK-31.
Since there is a maximum of 1200 Hz two-way Doppler and we want the users transceivers to remain fixed tuned throughout the pass, this limits the usable bandwith to only the central 1 KHz. Thus users will transmit from 400-1400 Hz at the beginning of the pass and 1400 to 2400 Hz toward the end and still be within the fixed tuned bandwidth.
To fit this 10m parrot transponder on our existing PCsat, we will need to add a deployable 10m whip (about 8’). The following sketch shows how this can be accomplished:
Although the size of the satellite is very small relative to the 29.4 MHz wavelengh, our research suggests that a match can be made to a monopole slightly longer than ¼ wave as follows:
Link budget calculations suggest that only a few milliwatts per carrier is needed for successful downlinks. Thus a 100 mw parrot transponder can probably be accommodated full time without impact to all other PCsat mission requirements. The PCsat Telemetry Command and Control system already has reserved two auxiliary on/off functions for just such an auxiliary payload.
OTHER IDEAS THAT WERE CONSIDERED:
FM Uplink => 10m LINEAR Downlink
- Proposed as add-on proof of concept to existing FM satellite
that has existing FM receiver and can accommodate a low power
SSB 10m downlink which would not interfere with existing mission
- Downlink would only need milliwatts
- Only ONE combined uplink could be accommodated. Upbound user
streams would be bundled by an internet linked uplink site.
- Only has downlink Doppler, the same for all signals
10m Linear Uplink => FM Downlink (suggested by G3PLX)
- No downlink Doppler
- All users remain in same relative position in passband
- Stations accommodate Uplink Doppler to fit in common Passband
- Downlink FM transmitter can be on 2m band with no Doppler issue
- No requirement for Linearity on Downlink Transmitter
- Downside is higher power budget and full power even if 1 user.
- Each user can adjust Uplink power for equal audio level in downlink
- Easy reception with any FM scanner
10m LINEAR UP => 12m LINEAR down
- Dopplers are less than 600 Hz each way
- Uses 25W uplink from low cost 10m All Mode Rigs
- Uses inexpensive XTAL 12m Receive kits for downlink
- Uses low power (milliwatts) SSB downlink XMTR
- Self power saving. No users, no RF power on downlink
10m INBAND UP/DOWN
- 1.4 MHz separation possible in 10m band
- Two tiny teathered CUBESATS could be used for isolation
ISSUES with adding PSK-31 experiment to PCsat:
If we can put together a simple 1W linear 10m SSB transceiver soon, we might be able to fly it on PCsat. The UHF FM receiver and 2m FM transmitter already exist and the on/off command system exists. The only unknown is adding the 10m diplexer to the existing 90 degree phased 2m whips. Due to the much longer wavelength at 29 Mhz, ay use of existing satellite whip antennas would be inefficient.
Further, any transponder that required sharing our 2m downlink would impact our primary mission and would not be able to be operated except on an occasional basis. Past experience has shown these kinds of experiments to be frustrating for users who cannot plan or count on reliable modes.
This experiment would demonstrate the advantages of PSK-31 in satellites taking advantage of the weak signal performance as well as the low Doppler on 10 meters and Zero baseband Doppler on FM. Such low data rate PSK signals are typical of what is used for weak links from interplanetary explorers such as Sojourner on Mars, etc.