WA8LMF Mirror of WB4APR Website - 21 July 2008
OTHER PSKsat DESIGN NOTES
Bob Bruninga, WB4APR
With the sweeping popularity of PSK-31 as a very weak signal digital communications mechanism requiring only a PC sound card and radio, the potential is great to use this as a low cost easy-entry satellite application. In only 3 KHz of bandwidth, as many as 20 or more simultaneous users can be accommodated. Using less than 1 watt, the design should easily fit within a 4” CUBESAT or as an add-on payload to other small amateur satellites.
Unfortunately,
the same advantage of the 60 Hz narrow bandwidth that makes this possible also
tends to make normal operation via satellites impossible due to very high
Doppler shifts. But several ideas have
come to mind that can mitigate this.
The objective is to keep all of the Doppler contained within a 3 Khz
bandwidth so that NO hardware Radio interface is required. Doppler on 10 meters is only 600 Hz. All
Doppler compensation can be done manually or in the PSK-31 sound card software.
This
paper suggests several ideas leading up to a full bi-directional linear
transponder. The first one is a simplex
“parrot” repeater that uses 10 meters both up and down. It has many advantages. It is fully described in <A
HREF=”PSKsatP.html”>PSKsat Parrot Transponder </A>.
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 fo 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.
MORE
IDEAS:
These
first two use an existing FM link on the satellite tied to a linear SSB
transceiver on 10m to give proof of concept.
-
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
PCsat PSK-31 Experiment
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. See the sketches above and below:
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.
The remainder of this page addresses some other possible PSK-31 linear
transponders which use SSB up and down…
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