Operating AO-51



AO-51
Uplink : 145.920MHz 67 Hz PL tone
Downlink : 435.300MHz

AO-51 is a satellite include in Amateur Satellite Service, and its function very similar to a traditional terrestrial FM repeater. It’s available only for general amateur radio use. Let us see what is Amateur Satellites is about. Good luck & welcome to the world of amateur satellites!

Satellites communication sometimes the jargon is slightly different that what we used to use. Well let see the common one.
UPLINK, equivalant to repeater input or where we transmits.
DOWNLINK is the output from the repeater and where we receive.
Usually but not all the time amateur satellites operates in different band U/V(Mode J) or V/U(Mode B) for example.
Footprint or coverage area, is large compared to a terrestrial repeater, and is more than 4000 kms in diameter. Depending on the pass (how the satellite travels relative to the ground station), all places is usually available. This means that there are potentially hundreds of thousands of amateur stations who could simultaneously operate the satellite. With only one channel, this satellite will be very busy. QSOs are generally contest style, with an exchange of callsigns, QRA locator's, signal reports, and occasionally names are exchanged. Look at the below AO-51 footprint example.



Prediction. The satellite is continually orbiting the Earth. This means that it is not always visible to an observer. To know when it is visible to an observer. The answer is to use prediction software. There are a number of software packages available, including freeware out there. 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 time trying to refer to the computer screen all the time during the satellite pass.

Doppler. 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 moving towards you, the frequency appears higher than when it is going away from you.
- 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.
-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 really help.
- It’s possible to automatically correct for Doppler using a suitable computer program and transceiver.


Equipment. A common misconception is that you need large Yagi arrays and expensive rotators to operate satellites, not to mention expensive radios. 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 is worthwhile while you transmit in one band and simultaneously received on the downlink band in the same time.
Because the signals on the downlink are not always very strong, and because the downlink will have fading due to polarisation changes during the pass, 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 greatly. 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.

Click here to look at the status of amateur satellites that orbiting the earth.

Good luck hunting. From V17 Lahad Datu editor. Info provided by 9M6WST.

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