|fahnen/11.jpg Working Your First Satellite|
Working your first Amateur Satellite!
Tony Langdon, VK3JED
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 isnt 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 is only one known orbiting FM repeater 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 dont 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 isnt 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 dont have any), as well as checking satellite home pages for transponder schedules and other information. Besides, the Internet is fun when the birds arent overhead!
Working your first satellite! This isnt 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, its 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, which satellite to work is an easy choice, as there is only one FM satellite available, namely the South African SUNSAT (OSCAR-35). This satellite is capable of transmitting 10 watts on either 2m or 70cm, and is usually configured to uplink on 70cm and downlink on 2m. Because of the high transponder power and relatively low orbit (650-850 km altitude during passes over VK), handheld transceivers are sufficient and will give good results. 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. [NOTE: As of February 2001, OSCAR-35 is no longer operational. See this page for more information.]
As an example of what SUNSAT is capable of, I have worked SUNSAT from within a moving tram, using a pair of handheld transceivers! However, replacing the standard rubber duck antennas with high performance whips is strongly recommended. 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 70cm, the extra power should largely compensate for the antennas radiation pattern. If you can use a ¼ wave or turnstile though, then youll 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 beams gain will be useful. And finally, dont 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, youll 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 SUNSAT page at http://esl.ee.sun.ac.za/projects/sunsat/ to find out when the transponder is scheduled to be active over Australia and the frequencies that will be used. (usually 436.291 MHz up and 145.825 MHz down) The times given on the SUNSAT page are the actual switch on and switch off times for the transponder. The satellite may not be visible in your area for all of this time. If you have tracking software, download the latest Keps while youre online and run a simulation of the pass. The software will allow you to know where the satellite will be at any given point in time, the maximum elevation of the pass and the exact times it will be visible, and often, the amount of Doppler shift that will be present. If youre using minimum equipment (e.g. a handheld with a rubber duck), are in difficult terrain or are going to be working the satellite from a difficult situation (e.g. while on a train or tram), this information can be crucial to your success.
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 436 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 uplink is on 70cm (usually the case for SUNSAT), tune 5-10 kHz below the nominal uplink frequency (the Doppler shift will make it arrive at the satellite on the correct frequency). If the downlink is on 70cm, youll have to tune the 70cm receiver 5-10 kHz above the nominal frequency. SUNSATs transponder usually sends over a minute of data or a voice preamble before its available for use. While this may be a waste of time, this minute is also useful for signal checks and fine tuning your position if youre portable. When the data ceases, youll hear FM receiver noise from the satellite. 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 cant hear the downlink at all, dont attempt to transmit, 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, youll be transmitting 5-10 kHz above (or receiving 5-10 kHz below, if 70cm is the downlink) the nominal frequency. From experience, the distortion caused by being off frequency isnt so noticeable, but it is much more difficult to access the transponder when more than 5 kHz off the correct frequency (SUNSAT is capable of correctly receiving signals up to 9 kHz off the uplink frequency, so the tolerance is pretty broad). 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 uplink 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 SUNSAT. I have only ever used handheld transceivers to work this 'bird'. The typical station is:
And in case you have got the satellite bug, here's a few more satellites you can work without too much difficulty.
Satellite related links
Project OSCAR http://www.projectoscar.net
Planet Emily http://www.planetemily.com
- South African satellite with an FM repeater on board.
StenSat - A "picosatellite", weighing only 220 grams. Launched January 27 2000. Payload is a crossband FM repeater. StenSat is now semi-operational.
ASUSat1 - Another amateur satellite launched on the Jan 27 2000 JAWSAT mission. Also features a crossband FM and packet repeater. ASUSat1 died shortly after launch, due to a failure in its power system.
And my own page describing my attempts to work SUNSAT from public transport, where you can download audio clips of SUNSAT operations, including the first ever amateur satellite contacts from trams.
Tony Langdon is an AMSAT member lives in Melbourne, Victoria, Australia and is self-described as working most of his time Public Transport Mobile. He is also the author of articles such as "Amateur Satellites on Rails" and "Satiquette" - a guide of good manners for working FM satellites.
You can contact Tony through his website at http://vkradio.com
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