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A Repeater in Space?
Well, nearly. The uplink (equivalent to a repeater input, or
where you transmit) and downlink (equivalent to a repeater output, or
where you receive) are on different bands. The point of having the uplink and downlink
on different bands is that it allows you to monitor the downlink (normally
using headphones to stop feedback), and check that you are making it in. If the
uplink and downlink were on the same band, we’d need some expensive filters so
that we could listen to the downlink without our uplink de-sensing the
receiver. It’s possible to operate without listening to the downlink, but it’s
not recommended unless you’re pretty experienced. Being able to hear your
signals coming back from the satellite will save you a lot of frustration!
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How far can
I get?
The coverage area, or footprint, (see figure 1) is huge compared
to a terrestrial repeater, and is typically 3,000 miles in diameter. Depending
on the pass (how the satellite travels relative to the groundstation), most if not all of Europe is usually
available, together with Northern Africa. It’s
often possible to operate across the Atlantic.
This means that there are potentially hundreds of thousands of amateur stations
who could simultaneously operate the satellite. With only one channel, this
satellite is inevitably very busy! QSOs are generally
contest style, with an exchange of callsigns, QRA
locators, signal reports, and occasionally names are exchanged.
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Prediction
The
satellite is continually orbiting the Earth. This means that it is not always
visible to an observer. So how do you know when it will be visible? The answer
is to use prediction software. There are a number of software packages
available, including freeware. Check out http://www.amsat.org and
http://www.amsat-uk.org. Predictions are also online, at http://www.amsat.org/amsat-new/tools/predict/
and http://www.heavens-above.com/.
Having
a prediction listing together with a magnetic compass will allow you to plan
the pass in advance by knowing where to point the antennas. If you’re not
completely confident about the software you’re using yet, having a listing
printed out in advance will save a lot of ‘computer panic’ during the satellite
pass.
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Doppler
Because
the satellite is moving relative to the ground station, it’s necessary to be
aware of a phenomenon known as Doppler shift. This is similar to the
apparent drop in frequency you hear when a police car screams past with its
siren on. When the satellite is hurtling towards you, the frequency appears
higher than when it is going away from you.
o
o
As a rule of
thumb, at the beginning of each satellite pass (known as AOS, or
acquisition of signal) on the 435.300MHz downlink, the frequency on the
downlink will appear at the observer to be about 10kHz above the
published frequency. At the end of each satellite pass (known as LOS, or
loss of signal) the downlink will appear to be about 10kHz
below the published frequency. The frequency on the satellite in fact
remains constant.
o
o
On the other
side of the coin, on the uplink on 145.920MHz, in order for this frequency to
be correct at the satellite, you can uplink at about 3kHz below the
published frequency at AOS, and 3kHz above at LOS. In practice, it’s
generally not necessary to adjust the uplink on a 2m FM uplink, but you will
benefit from being able to tune the downlink. On some radios it’s possible to
set the uplink frequency together with the 67Hz PL tone into a memory, which will
help.
o
o
It’s possible
to automatically correct for Doppler using a suitable computer program and
transceiver.
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Equipment
A
commonly held misconception is that you need large Yagi
arrays and expensive rotators to operate satellites, not to mention expensive
radios in the £1k bracket. This is not true! To operate AO-51, you need a radio
that can receive FM on 70cm and transmit 5W FM on 2m with a 67Hz PL tone, i.e., a typical handheld
radio these days. You can also use two separate radios, one on each band, equally
well. If you use a single dual band radio, it’s extremely worthwhile using one 
which allows you to transmit on one band while simultaneously receiving
on another. Ideally the radio should be able to tune to within 5kHz so that you can correct for Doppler. Pre-programming
the radio’s memories will help here if you’re using an FM only radio. Be aware
that some ‘satellite-ready’ radios have a problem setting PL tones in satellite
mode.
The
old adage, “if you can’t hear them, you can’t work them” is especially true in
satellite operation. So before considering increasing your ERP, concentrate on
your downlink! This can either be with antenna mounted low noise preamps,
better quality coax and/or better antennas, depending on the configuration.
Operating
AO-51
AO-51 is a satellite in the Amateur Satellite
Service, and one of its functions operates very similar to a traditional terrestrial
FM repeater. It’s available for general amateur use, and described below are
the operating techniques and equipment used to access it. As with many areas of
technology, satellite operation includes its fair share of jargon. Although not
exhaustive, a number of these terms are used in this introduction, with
corresponding explanations. Good luck & welcome to the world of amateur
satellites!