Thursday, July 29, 2010

Satellite communciations for the beginers


It is a common perception that tracking the satellite 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. Our focus shall be to track the LEO (Low Earth Orbit) Satellites.

But before we being lets understand some of the terminologies commonly used in Satellite communication.

OSCAR – Orbiting Satellite Carrying Amateur Radio.

LEO – Low Earth Orbit ( 500km to 1500 km from earth)

Uplink Frequency- The frequencies on which you can transmit

Downlink frequency - The frequencies on which you shall receive the signal

To successfully work an amateur satellite, you need to have transceivers suitable for the satellites you wish to work. All of the FM satellites 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.

For antennas, 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.

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.

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.

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 HAMSAT), 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.

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, grid position, signal reports and occasionally a comment about the weather.

IMPORTANT:- For the beginners , start copying the HAMS AT (VO-52) satellite signals through your current VHF setup.

I shall upload the steps for receiving HAMSAT signals in the next article.

Best Wishes


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