[HamSat] AO-51 Operation Notes

[Timothy Pickett]

The 67 Hz PL tone must be transmitted continuously for a station to be
heard through the FM repeater.

Unlike previous similar satellites the Echo PL tone decoder both turns on
the downlink transmitter and gates the
audio path. The tone must be present on a signal for the audio to get
through. If a signal without a tone captures the receiver no audio will
get through the decoder chip and the satellite will be transmitting a
signal without modulation. If that condition persists for 10 seconds the
transmitter will go off until it again hears a signal with PL.

Multiple uplink signals can mix and cause no clear PL tone to be present
at the decoder. During this time no audio is being repeated. What has been
reported as a 'dead carrier' is usually a result of this condition.  It's
not a carrier with no modulation on the uplink, but no valid PL on the
uplink because of mixing, resulting in no audio passing through the
satellite.  May we again note this is just like a terrestrial FM PL
controlled repeater?

The PL decoder chip easily detects PL tone of 1 Khz deviation and is
quite forgiving. It is neither necessary or desirable to increase the
deviation of the PL tone you transmit above the nominal values. It will
not work any better.

It is possible to turn off the PL function. But there is no COR (carrier
operated relay) in the satellite. So without the PL function to control
when the transmitter is on it would have to be on all the time. In the
first few orbits in this mode, the B transmitter has been on about half
the time. If it was on all the time it would have to be operated at a lower
power level. There is only so much DC power available in the satellite
and it is a zero sum game one orbit at a time. One of the major advantages
of the PL control mode is it keeps the transmitter off when not in use and
therefore allows it to be at higher power when in use.  Of course another
is the satellite will not re-transmit signals not intended for it.  It is
not
possible for the PL tone to turn on the transmitter but not gate the audio.

We are watching the power usage closely and considering how to modify the
software to allow higher transmitter powers when adequate DC power is
available, and not pull the battery down to far for Echo to make it
through the next eclipse, and not go to deep into the battery capacity,
etc. This three week (aprox.) trial period is for the purpose of
measuring these performance parameters (and of course having some fun with
this
cool bird).

The trial move of the voice downlink frequency to 435.300 MHz does not
limit Echo in any way or change the number of options available for
various modes.  All three Echo transmitters are frequency agile in their
bands.  435.225 MHz was initially chosen and coordinated with the IARU on
the assumption GO-32 was not active.  The reactivation of that satellite
required Echo to move.  435.300 MHz was suggested by the IARU frequency
coordination group.  If feedback from users around the world indicates
this is a usable frequency we will complete the coordination process and
continue to use it.

The power level of the B transmitter on 435.300 MHz has been at about.5
Watts for a couple of days now. This is as high as we can go in the
current mode without over stressing the batteries near the end of eclipse.

There is no beep generator in Echo so it is not possible to send a
courtesy tone type beep when a valid PL goes away.  We put the idea of
sending a short 9k6 digital packet from time to time on the voice downlink
on our todo list.  It would have to interrupt any voice repeat going on at
the time for about half a second due to the switching that it would entail,
but at first glance seems doable.

There is nothing wrong with the sensitivity of the uplink receiver.  In
fact if anything it is too sensitive.  In any event it cannot be
changed.  There is nothing about the orientation that is causing a
reduction of sensitivity on the uplink.  The uplink antenna is for all
practical purposes omni-directional.

Jim White, WD0E