WA8LMF Mirror of WB4APR Website - 21 July 2008 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.





These first two use an existing FM link on the satellite tied to a linear SSB transceiver on 10m to give proof of concept.




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





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





-          1.4 MHz separation possible in 10m band

-          Two tiny teathered CUBESATS could be used for isolation





WA8LMF Mirror of WB4APR Website - 21 July 2008