Working your first Amateur Satellite!
Updated for AO-51, October 2004
Original SO-35 version here!
It is a common perception that it requires sophisticated equipment and large circularly polarised antenna arrays to work amateur satellites. While this may be true for using some of the high altitude ‘birds’ or on the higher bands such as 23cm, it isn’t the case for all satellites. There are several low Earth orbiting satellites which can be worked with relatively simple transceivers and antennas. This article will concentrate on voice operation, as I have no experience at all with digital satellite operation.
Amateur voice satellites can be divided broadly into two groups. Firstly, there are the traditional "linear transponder" satellites. These satellites receive a specific range of frequencies (typically 40 - 100 kHz) in one band, convert them to another band using a mixing process similar to that used in a superheterodyne receiver and amplify the converted signal for transmission back to Earth. Linear transponders are capable of relaying several different signals simultaneously. More recently, some satellites have been carrying crossband FM repeaters instead of linear transponders. These repeaters are similar to their familiar terrestrial cousins in that they receive an FM signal on a specific channel, demodulate the signal and retransmit the signal on a new frequency. Unlike linear transponders, but like conventional FM repeaters, these satellites can only carry one QSO at a time. Most amateur voice satellites use linear transponders (there are only 3 known orbiting FM repeaters accessible in VK at the time of writing).
To successfully work an amateur satellite, you need to have transceivers suitable for the satellites you wish to work. For linear transponders, SSB and CW transceivers on the bands of interest are required. For the FM repeaters, either a dual band FM transceiver with crossband transmit/receive capabilities or separate 2m and 70cm FM transceivers are suitable. A related issue is which bands to use. FM users don’t have much choice. All of the FM satellites (operational or proposed) use 2m and 70cm, with one of these bands being used for the uplink, the other for the downlink. There are a wider variety of frequencies in use by linear transponder satellites. The suggested bands to try for a first attempt are 2 metres uplink and 10 metres downlink. If you have 2 HF transceivers, it might be worth trying the 10m/15m satellites as well.
For antennas, an existing HF dipole and VHF/UHF omnidirectional antennas will work in a pinch. The typical VHF/UHF collinears typically have a low angle of radiation, and better results may be obtained with a simple ¼ wave groundplane, or for the more serious, a turnstile antenna. If you have crossed Yagis and AZ/EL rotators, all the better (but then this article isn’t aimed at you in this case! :-) ). Finally, though not essential, it is very strongly recommended to have a computer, satellite tracking software and an Internet connection available. The Internet connection is for downloading the latest Keplerian elements for the tracking software (and the software itself if you don’t have any), as well as checking satellite home pages for transponder schedules and other information. Besides, the Internet is fun when the birds aren’t overhead!
Working your first satellite! This isn’t anywhere near as daunting as it sounds. The first thing is to have a look around your shack and see what equipment you have. If, like many amateurs, you have FM only radios on VHF/UHF, then you are limited to the FM satellites. Those lucky ones with all mode transceivers can also try their hand at the linear transponders. The rest of this article will concentrate on FM operation as nearly everyone has FM gear for 2m and 70cm, and the operating techniques are easier to master. If SSB or CW satellite operation interests you, it’s a natural progression to move on from FM. For those interested in exploring SSB/CW operation via linear transponders on satellites, there is are several excellent introductory articles on AMSAT's web site.
First, time for an inventory, as the gear you have available will partially determine the satellite to use. As the satellite bands are outside the Novice voice segments, Novice operators will need to upgrade to a Limited, Intermediate or Full call, if they aspire to working satellites. At the time of writing, there are 3 satellites to choose from - SO-50, AO-51 and the ISS. While the ISS is the easiest to work, it is not always in FM repeater mode, so we'll concentrate on the next easiest satellite, AO-51. This satellite is capable of transmitting 7 watts on 70cm, and there are other bands available. Normal downlink power is around 2W. The uplink is on 2m, and is usually configured to uplink on 70cm and downlink on 2m. Because of the high transponder power and relatively low orbit (800 km altitude), handheld transceivers are sufficient and will give good results, but a directional antenna is best for the downlink. Regardless of the rig you use, it has to be capable of tuning in 5 kHz or smaller steps, to enable you to follow the Doppler shift as the satellite passes overhead. I have worked AO-51 with rubber ducks, but the downlink is tough to hear at times. A handheld beam antenna will work wonders, and such antennas can be built for a few dollars. Home operators will most likely use their existing omnidirectional or beam antennas. Modern omnis tend to have a very low angle of radiation and therefore may not give good results when used to work satellites. However, as most modern rigs put out 35-50 watts on 2cm, the extra power should largely compensate for the antenna’s radiation pattern. If you can use a ¼ wave or turnstile though, then you’ll enjoy better satellite performance. If you have a beam, you will need to track the satellite as it passes, especially at low angles, where the beam’s gain will be useful. And finally, don’t forget an earpiece or headphones. You will be operating full duplex (i.e. being able to transmit and receive simultaneously) and without headphones, feedback can be a problem. With them, you’ll be able to hear what you sound like while you transmit, which will be helpful for correcting for Doppler shift.
During your preparation, log onto the Internet and check the pass times for your location. http://www.heavens-above.com is a good site to get quick pass predictions for anywhere in the world, though I prefer to run my own tracking software, which can give more detail.
Also important to know is the amount of Doppler shift that will be present on the uplink and downlink frequency. Doppler shift is a phenomenon that all of us will recognise in a different situation. Imagine you're waiting at a railway crossing. A train passes at high speed, blowing its horn. As the train passes you, the pitch of the horn appears lower than when it was approaching. That apparent shift in frequency is Doppler shifting caused by the relative speed of the train to you shortening, then later lengthening the wavelength of the sound as seen by the observer. On board the train, the pitch of the horn does not alter, but the pitch of the bells at the crossing does. When a satellite passes overhead, the transmitted and received signals are affected in a similar way. With the satellite passing at 27,000 km/h or more, a signal at 435 MHz can be shifted by up to 10 kHz from its actual transmitted frequency. Some satellites are designed with this in mind, and have AFC (Automatic Frequency Control) circuits to partially compensate for Doppler shift. Doppler shift is only significant for FM satellites on 70cm or higher bands. On 2m, the 3 kHz Doppler shift can usually be accommodated by an ordinary FM receiver, provided it's on the correct frequency. Here's some more information on Doppler shift for those interested.
As the satellite approaches, you should be listening to the downlink frequency, with the uplink ready to transmit when needed. Remember to allow for any Doppler shift (for FM, it will only be significant on 70cm – around 5-10 kHz). If the downlink is on 70cm, you’ll have to tune the 70cm receiver 5-10 kHz above the nominal frequency. Also note that AO-51 requires a 67 Hz CTCSS tone to open the receiver. You will need to ensure that your uplink transmitter can generate this tone and that it's enabled, or you won't get through. Because the transmitter is CTCSS controlled, you won't hear anything until someone keys the system. AO-51 is literally an orbiting tone activated repeater. If you live in a busy area (e.g. Europe, North America), there is most likely someone who will be on the satellite when it comes within range. Once you can hear the downlink clearly, you're ready to go. However, in quieter parts of the world (e.g. VK/ZL), you may be the first on the bird and not know it. In this instance, you may have to key the transmitter and announce your callsign once. If you hear yourself come back and the tail of the satellite, you're in business. If not, DON'T transmit any more. Wait a little and try briefly later. At this time, the satellite is ready for use, and you can put out a call. While calling, pay attention to your signal as heard on the downlink. Too much noise may indicate a need to move the uplink antenna, increase power or adjust frequency to compensate for Doppler shift. If you can’t hear the downlink at all, don’t attempt to transmit any more than the time it takes to say your callsign ONLY, as you may interfere with someone else. Also, keep things short while using the satellite. Only one person can use the transponder at a time and the satellite is usually only accessible for about 10 minutes. Others will appreciate your efficiency and courtesy. Most FM satellite contacts are usually an exchange of callsigns, signal reports and occasionally a comment about the weather.
As the satellite passes, you will need to make occasional adjustments to the 70cm frequency as the Doppler shift changes, so that by the end of the pass, you’ll be receiving 5-10 kHz below the nominal frequency. Some tracking software is capable of telling you the exact amount of Doppler shift present at any given time as the satellite passes, provided you tell the software the downlink frequency (read the manual on how to do this). Once the satellite has passed, you can relax and plan your next attempt.
A quick run down on the results that are possible with AO-51. I have only ever used handheld transceivers to work this 'bird'. The typical station is:
As you can see, this isn't a particularly
sophisticated setup and is very portable. However, it is also
capable of very good results with AO-51. With the above gear, I
am able to work AO-51 from a good outdoor location at up to 3000
km range (which is practically on the horizon).
The most exciting part of satellite operation is the anticipation of the pass as the time approaches and the fast pace of operation, not unlike during a contest but with more order. It’s a bit like a brief band opening on VHF/UHF, except that unlike ducting or sporadic E, satellite openings can be predicted to the second. With the advent of orbiting FM repeaters, it is now possible to enjoy the excitement of satellite operation without paying the earth in hardware (Almost any VHF/UHF operator already owns the necessary gear). However, a word of warning: For some people, the thrill of satellite operation can be addictive! You may find yourself trying unusual situations, or decide to invest in multimode gear and work some of the linear 'birds' that are up there. You have been warned! (and I have the audio clips and 2m all mode box to prove this theory!) :-)
Satellite related links