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Re: APRS on AO27



On Tue, 7 Sep 1999, Assi Friedman wrote:

> The FM repeater on AO27 is a simple audio FM repeater. It does not
> have any digital regenerative capability in the mode. This means that
> the signal that goes up will hit the bird and bounce back. The
> downlink signal resulting will be=signal+noise_up. This does not take
> into acount the frequency response of the audio path on the bird. In

Since I was involved with AO-27, let me add to this.  These are my
opinions and recollections only.  I have no involvement with AO-27
operrations, so please, no requests to modify the schedule or anything, I
CAN'T DO IT!


1) Assi is correct, there is no digital regeneration capability in the
audio repeater - it is a simple bent-pipe.

2) We did try some tests sending 1200 baud AFSK through the bird - it
worked after a fashion, but it was extremly vulnerable to fades and noise
so as to make it effectively unusable.

> addition to this, the downlink antenna is mounted on the side of the

3) Not correct.  The downlink is a 1/4 wave whip on the bottom face of the
spacecraft, located towards the edge underneath the channel where the
cable bus is run.  There is no "spin" modulation as there is with AO-10,
however the signal is subject to fading as the orientation of the antenna
and the signal polarization of the downlink changes relative to a ground
station's antenna.


4) Now AO-27 does have a full set of manchester decoders in the digital
receiver module.  Their intended use was for the commercial payload (which
was a store and forward location and status reporting system I think was
rather analogous to APRS).  AFAIK (it's been a few years), the commercial
payload is not being used at all.

I DO NOT KNOW if ANY of these decoders can be connected to the output of
the amateur uplink or if they still work after almost 6 years in space.

5) AO-27 also has a 1200 baud AFSK modulator and a 1200/9600 FSK
modulator. If I recall correctly, the AFSK modulator can be connected to
the amateur downlink.  Performance, as I recall, was adequate, and subject
to all the problems a marginal AFSK downlink will have (signal to noise
and resulting error rates).  I do not know if or how well the FSK
modulator would work, other than that it has a less demanding
signal-to-noise requirement to achieve the same error rate vs. AFSK.

I DO NOT KNOW if decoded data can be sent out either the AFSK or FSK
modulators and through the amateur transmitter.

I DO NOT KNOW if the current software load on AO-27 can support any kind
of digipeating.  Or if any such digipeating can share the uplink with
current voice users.

These are questions the control station operators (Mike, Chuck?) may be in
a better position to answer.

6) As others have noted, the current digital birds are rather "brittle";
given the risks involved in reloading software, I would not want to mess
with something that is working.  The potential for disaster is too high.

7) Given it's limited operating schedule, AO-27 has limited usefulness as
a world-wide TRAKNET resource.  It *might* be useful to demonstrate the
concept in hopes it sparks development of a dedicated satellite or
payload.

8) Given that for the moment, AO-27 is the ONLY FM-repeater satellite we
have, there are obvious problems using it in a digital mode.


MY PERSONAL OPINION is that Mic-E type APRS beaconing could be attempted
with minimal impact on voice ops.  Subject to the limitations on sending
AFSK data round-trip without regeneration.


73, Steve  KA1LM


-------------------------------------------------------------------------
Stephan A. Greene                          sgreene@patriot.net    
Senior Systems Engineer, VA Linux Systems  sgreene@valinux.com
Mid-Atlantic Region                        703-980-3475
Amateur  KA1LM@amsat.org  Grid FM18hx      38 59'83.33"N 77 23'6.15"W
-------------------------------------------------------------------------

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