Howard Long, G6LVB
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!
· 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!
· 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.
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.
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 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 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 It’s possible to automatically correct for Doppler using a suitable computer program and transceiver.
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.
Because the signals on the downlink are not always very strong, and because the downlink will have fading due to polarisation changes, it’s recommended that the receiver’s squelch is opened permanently during the pass.
It can be tricky to receive AO-51 using just the rubber duck antenna supplied with most hand held radios. Using a small hand-held directional antenna will help enormously. Even an HB9CV, or alternatively a three element yagi will make quite a difference on the downlink. For AO-51, you shouldn’t need more than about 10W ERP to make contacts, but this is assuming there’s no other stations running QRO and have insensitive receivers. There’s nothing more frustrating than knowing that you’re making it in (by listening to yourself on the downlink), only to be trampled on by a station who clearly cannot hear the satellite at all. These operators are known as alligators – all mouth and no ears.
One antenna, now available in the UK from AMSAT-UK is the Arrow Antenna (Fig. 3) (http://www.arrowantennas.com/146-437.html) which is infamous in the amateur satellite community. Alternatives include homebrewing your own antenna. Two WA5VJB antennas (http://www.clarc.org/Articles/uhf.htm), one for 70cm and the other for 2m mounted orthogonally on the same boom (http://www.g6lvb.com/HomebrewArrow.htm), or commercially, the Cushcraft A270-6S.
The real benefit of these hand held Yagis over a combined azimuth and elevation rotator system is that the operator has feedback and can very rapidly correct for polarisation mismatch by turning the boom along its axis.
· Operator conveniences
AO-51 passes are short (normally only 15 minutes or less), so preparation will pay off. Also keep in mind that Wednesdays are Experimenters’ days when the voice transponder may be switched off – check http://www.amsat.org/amsat-new/echo for the schedule.
Consider all the tasks you’ll be doing:
o Retune due to Doppler: on FM-only radios, pre-program five memories in 5kHz steps into the memories for the downlink: 435.310, 435.305, 435.300, 435.295, 435.290. The start of the pass will be at 435.310 and will decrease in frequency.
o Key the PTT: you might want to consider a foot switch. Beware of VOX: the VOX tail delay can hamper the rapid QSO style on FM satellites. There is a short delay for the uplink PL tone to be recognised.
o Hold the microphone: a headset/microphone combination is a good idea, giving you a free hand. Use headphones to avoid feedback in full duplex operation!
o Point and orient the antenna: although the antennas are light, your arm will start to ache during the pass!
o Check the predicted heading and elevation with the prediction listing and a compass: it’s worth preparing for the pass by tracking the predicted pass of the satellite, ensuring that there are no obstructions.
o Log the QSOs: a cassette recorder or dictation device helps a lot here.
o Have a QSO in a very busy channel: patience is a virtue here. It is likely that you won’t succeed on your first attempt, and this is where preparation and the learning experience come in.
· Good luck!
Satellite operation is a challenge, both technically and from an operating perspective. But you don’t need large or expensive equipment to operate them. Remember to support AMSAT-UK who represent amateur satellite operators in the United Kingdom. Finally, welcome to the space age and the world of amateur satellites! 73, Howard G6LVB