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發表於 26-2-2010 19:04:48
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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, you’ll have to tune the 70cm receiver 5-10 kHz above the nominal frequency. SUNSAT’s transponder usually sends over a minute of data or a voice preamble before it’s 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 you’re portable. When the data ceases, you’ll 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 can’t hear the downlink at all, don’t 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, you’ll 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 isn’t 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:
Uplink - Icom IC-T81A handheld running 3.5 watts into a 70cm 1/2 wave ground independent handheld whip.
Downlink - Alinco DJ-500T handheld or Standard C58 all mode portable with a "ScanDucky" scanning antenna (roughly equivalent to a 1/4 wave on 2m).
Station monitor (for recording the passes) - Icom IC-R1 handheld scanner or Standard C58 with a Diamond V2000 triband vertical or a 2m 1/2 wave aligned to the satellite pass. This sits in the shack and feeds audio to a PC running audio recording software.
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 SUNSAT. With the above gear, I am able to work SUNSAT from a good outdoor location at up to 3000 km range (which is practically on the horizon). The above station is also capable of working SUNSAT from a train at up to 2000 km range or up to 1500km from a tram (the tram range is limited by downlink noise, not uplink power). Due to the flexibility of antenna alignment possible with a portable station, this setup often equals the performance of base or mobile stations running up to 10 times the power into a vertical antenna. 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!) :-) |
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樓主: VR2GY
