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Re: July 2001 President's Letter



The satellite can provide timing information for the digital downlink. One
frame could hold telemetry and that could be the reference. What the ground
station has to compensate for is the delay between it and the satellite. The
current tracking software provides the necessary range information.

TDM requires high power for a short period of time. The big question is how
much uplink power are we going to assume? This determines the bandwidth of
the receiver on the satellite and how many users can share the uplink. My
own opinion is that the satellite should be designed so that existing
antennas and transverters can be used. I would assume a 16 watt amplifier, 1
dB transmission line loss and a 20 dBic antenna for an uplink power of 1260
W EIRP. On the downlink side I would assume a 23 dBic antenna (0.9-meter
dish) and receiver with a 1 dB system noise figure. Both antennas easily fit
on a Yaesu G-500 rotator (8 sq. ft. max. windload).

Since I'm an engineer, here is some speculation on what I think may be
possible with this constraint. This assumes a simple "bent pipe"
hard-limiting transponder. Limiting is used to increase SNR on the downlink
and allow the use of class C power amplifiers for efficiency. I haven't seen
anything published on this concept but it has been discussed in the past on
the AMSAT BB.

Path loss in a GTO orbit will be 187 dB maximum at 1.27 GHz and the
spacecraft receiving antenna could have 14 dB of gain so the receiver would
see a -112 dBm signal. 10 dB of SNR would guarantee hard limiting so the
receiver noise level should be -122 dBm or less. If the receiver sees a warm
earth in most of the antenna pattern and has a 1 dB NF, this amount of noise
would exist in a 126 kHz bandwidth. The raw uplink could then be 75 KBPS FSK
but would need forward error correction so the user data rate would be
somewhat lower.

If the downlink bandwidth is also 125 kHz wide and the receiving antenna is
looking at cold sky, it will have a noise level at the feedpoint of -129
dBm. With a 23 dB gain antenna, 13 dB SNR and 192 dB of path loss the
satellite needs 53 dBm (200 W) EIRP. If it also has a 14 dB gain antenna and
1 dB of power combiner/feedline loss that is 10 watts of transmitter power.

There is also the possibility for stations with more uplink power and better
receiving systems to use MFSK in their frames to increase the bit rate.
Multiple transponders should be present to eliminate single point failures
and increase available bandwidth.

At these data rates, a PC could control everything. The transmiitter could
be a FSK-modulated VCO at 2 meters feeding an existing transverter. The
receiver to attach to a downconverter would be similar to an FM broadcast
receiver but with a high-speed A/D converter on the output.

73,

John
KD6OZH

> >(snip...)
> >What is the estimated cost of the peripheral equipment
> >that will be required to synchronize everyone's uplink
> >signals? Is it expected to be able to use the shack PC
> >for the necessary timing and, if so, what will be the
> >master clock source?
>
> The key is accurate timing. Why not use GPS? Many appli-
> cations use GPS for distributed sub-microsecond timing.
> So could we. We'd have to fiddle a bit to compensate for
> transit-time delays, but that's not difficult.


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