[eagle] 2 meter digital downlink for geostationary satellite

John B. Stephensen kd6ozh at comcast.net
Thu Feb 14 20:01:08 PST 2008

I've been thinking about how the use of mode UV might change with a
geostationary satellite. It could allow for the use of very simple antennas
for mobile operation as was done with AO-16. Since the satellite is
stabilized and fairly large, it may be possible for AMSAT to procure
space for a broadside array with at least 2 elements on 2 meters with
vertical polarization. This would give 8 or 9 dBi of gain and be
compatible with 1/4-wavelength whip antennas mounted on automobiles
in mid-latitude locations. It would also allow the speed of the currently
planned digital uplinks and downlink to be increased.

This digital downlink could allow multi-user digital voice operation with 
fairly low power given that 2400 and 1200 bps MELP vocoders are now 
available. 50 kHz could allocated for SSB and 50 kHz for a TDM digital 
downlink. A 24 kBaud QPSK downlink could handle 10-20 users using a rate 1/2 
error correcting code. If we assume that the man-made noise level is 6 dB 
above the thermal noise level, a 1 dB NF LNA, 6 dB SNR and a 32 kHz 
bandwidth for the downlink signal, -116 dBm is required at the receiver. 
Path loss is 168 dB, so at least 52 dBm EIRP is required from the satellite 
which means 20-25 W of transmitter output. This seems feasible for the 
HELAPS transmiter or there could be a separate transmitter and a high-level 
power combiner.

Both 70-cm analog and digital uplinks could be handled by a common receiver 
using a short (1 m) yagi antenna or a broadside array.



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