OBSERVATIONS FROM SCOTLAND                           09_September 2009                               GM1SXX

Getting started in Amateur Satellites.

Hi folks, this is meant as a very short introduction to amateur radio satellites. 

We were all beginners sometime, and like all things new, there is a learning curve. Amateur radio is a technical hobby so it's normal to be continually learning.

A satellite is basically a repeater in the sky. Unlike normal (Terrestrial) repeaters, this one is moving, which brings us very nicely to the small matter of Doppler shift.  Doppler effect 'shifts' the frequencies of moving objects, and its affected by frequency. Higher frequencies have greater Doppler shifts.  You can hear the Doppler effect even on audio signals (fire engine going past!)  The frequency starts out HIGHER than normal as it approaches and after it passes, drifts LOW in frequency. So it is with satellites, so the user has to compensate.

To compensate for Doppler shift , you can periodically alter frequency to keep the signal tuned. This is normally done by altering the higher of the two frequencies in use, and NOT changing the lower one.  Alternatively, instead of tuning manually, you could program the range of frequencies you expect to use into memories and step through the memories as the satellites flies over you. The choice is yours.

Unlike a normal repeater which transmits and receives in the *same* band, a satellite transmits and receives in *different* bands.  These sets of bands are called 'modes'. Mode VU is VHF up UHF down, while Mode UV is UHF up VHF down.  The satellite's 'repeater' is called a 'Transponder' and it essentially shifts any signal in the Uplink (your transmitted signal) by a fixed amount, and spits it out on the downlink.

There are two main types of satellite... Linear and FM.

Taking FM first, an FM satellite (such as AO51) can only handle a single QSO. FM 'capture effect' means that the strongest signal at the transponder will be 'transponded'... regenerated...  and any others rejected. This is *positively not* an excuse to run excessive power!

A linear satellite (such as VO52) takes a 'block of signals' , shifts them by a fixed frequency and retransmits the *whole block*. Since a CW signal is very narrow and an SSB signal is around 3KHz wide, such a satellite can carry many *concurrent* QSO's.  This of course, is a *huge* advantage over FM birds.  The main problem with linear birds is to keep doppler effect from causing signals to overlap.  DO a web search for the 'one true rule' for more information.

Here's a link to it... http://www.amsat.org/amsat/features/one_true_rule.html

Time to take a breather now, and watch a movie.  This is an MP4 movie made by Simon 2E0HTS, and I recommend you use VLC player VLC Player Download to play it.  Once you have downloaded and installed VLC, you can download the movie from 2E0HTS Operates via AO51.  This movie shows Simon 2E0HTS operating via AMSAT Oscar 51 satellite using simple equipment.  Thanks to Simon for allowing me to use his excellent short movie.

This movie shows Simon operating via The AO51 FM satellite. He's running low power with his radios attached to the boom of his dual-band yagi. This is hand held, and simply pointed at the correct area of the sky, moving over time to manually 'track' the satellite.  It's a simple and effective technique.   He has a compass taped to the antenna boom so he can quickly orient himself for tracking. This of course is done before the start of a pass!

Another technique (and a favourite of mine) is to place the antenna on a rotatable pole (non-metallic... wooden usually!) and angle it up 25 degrees. This can be rotated to track the satellite [AKA 'bird']  placing your radio(s) on a nearby table. A picnic table is ideal, especially if it has a central hole for a parasol. No more aching arms!

 Simon also has some information on the paper.. a printout of where the satellite will be at what time.  He compensates for the Doppler shift, as you can see from the movie.

Satellites are a lot of fun, and of course will extend your operating range as they fly overhead.  Amateur satellites are *not* like the commercial TV satellites.... 'nailed' to a spot in the sky, but instead are in low earth orbit incluned to the Equator so that everyone gets a chance of operating at least several times a day as they fly overhead.

So, now you know about how satellites are essentially cross-band repeaters in the sky and that they can be worked with simple gear and modest power. We have touched on Doppler shift and antenna pointing.  Next problem... how do you FIND the satellites?  There are several ways, but all of them centre around Keplerian Orbital Elements.

 'KEPS',  'Keplers' or 'orbital elements' are simply a set of numeric values that define accurately where your satellite of interest was 'at a known time in the past'..  They describe the satellite's orbit and it's position (orientation) against the stellar background at a specific time in the past.  Stop panicking! You DON'T need to know how Keplers work, only where to find recent ones for your satellite(s) of interest.  Fortunately you can download this data, free of charge from Celestrack.

See....http://celestrak.com/NORAD/elements/amateur.txt

Here you can download current KEPS or elements for all the amateur satellites.  It's just a plain text file,  You need to do this periodically (once a fortnight is good), to keep your information up to date.

Now that you have a set of elements,  you now need a tracking program.  Many free programs exist. I include a link to Orbitron as an example.

http://www.softpedia.com/progDownload/Orbitron-Download-71763.html

Once you have a tracking program, you need to set it up correctly. If you don't do this, I guarantee you will fail.

1) Current data and time (often set by the computer's clock)  Set this as accurately as you can and update it frequently. Computer clocks are generally pretty unreliable.

2) Local time offset... all satellite programs operate in UTC. Check that the time displayed  is the correct UTC time.

3) Your latitude, Longitude and Height above Sea level.

4) Get a set of up-to-date KEPS.

Armed with the KEPS and your tracking program, you should now be able to print out a timing chart, similar to that used by Simon 2E0HTS in his movie. 

The only thing left to consider now is the Doppler shift.  Simon handles this by programming the memories in one of his radios with frequencies below and above the published frequency for the bird. The frequency at the start of a pass will be on the high side, progressing through the nominal frequency and finishing the pass lower in frequency.  Like most things in life that are worth doing, 'practice makes perfect'... so 'go practice'!

If you liked the movie, please thank Simon 2E0HTS, both for making it, and for allowing me to use it.

See.... http://www.hamradiooperator.blogspot.com/

Simon 2E0HTS

Thanks mate!

73 Al.

GM1SXX

73 AL.
GM1SXX