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.