Friday, 4 March 2016

Working the International Space Station with 2m APRS

The proof - G0KYA works RS0ISS as shown
at, along with other EU stations.
Yippee! It works! Yesterday I wrote about how I had managed to get 2m APRS working on my Icom IC-7400 using Soundmodem, UISS and a program by G8BPQ to connect a virtual comm port to the Icom CI-V interface to key the radio.

It all seemed to work on terrestrial APRS on 144.800MHz, but the acid test was whether it would connect with the APRS system on the International Space Station (ISS) on 145.825MHz.

I already knew that it could decode packets from the ISS, but would the ISS be able to decode me?

So this morning it was time for a test. Using a 2m Slim Jim in the loft, I wound the Icom IC-7400 up to half power and waited for the ISS pass between 10:11-10:20hrs. This was a maximum 73 degree pass here in East Anglia.

Actually – small confession. I thought the IC-7400 was a maximum 50W out on 2m, when it is actually 100W (when I looked at the manual). So I was actually using 50W not 25W.

The 50W power might be a little excessive, but my coax is only RG58 (although it is only about 12m) and the antenna is a compromise in the attic too, so the ERP was probably more like about 20-25W. I figured that if I could connect with 50W I could reduce it on other passes and see what QRP power would do.

This attempt was just to prove that the system (with a software modem and not a packet TNC) could work.

Anyway, using the UISS program I put in my locator square (JO02NN) and details and waited for the pass.

The ISS became audible at about 10:13hrs and I adjusted the rig's frequency to about 145.828MHz to allow for Doppler.

The system decoded quite a few packets and I keyed the UISS APRS TX button.

The Doppler shift fell away as the ISS got closer and at 10:16:09hrs I was successful – RS0ISS heard G0KYA. Yippee! So now I can apply for the ultimate QSL card.

You can check whether you have been heard at and this will also plot your location on a map. You can get the details of how you apply for a QSL card at:

Some learning points though:

1. Spend some time getting your latitude and longitude absolutely spot on. Just using the locator square of JO02NN put me about half a mile away from actual location. The easiest way to do this is with APRIS32 and zooming in on the map to get the exact degrees, minutes and seconds of your lat. and long.

2. Keep an eye on the Doppler shift. This can vary from about +3.5kHz to -3.5kHz over the whole pass. Orbitron will calculate this for you. It does mean that at AOS your TX frequency will have to be lower than 145.825MHz and you you will listen higher. At the time of closest approach they should be both be about 145.825Mhz, and that might be the best time to attempt the contact.

3. Use a small beam rather than an omni-directional antenna. I have a tiny three-element delta loop beam that I may fix pointing due south at about 35 degrees elevation to see how that fares. It might be better for the ISS, PSAT and the other satellites.

4. Use better coax. RG58 isn't ideal for VHF – the only reason I used it was because I normally use that run for HF. I had three lengths put in between the shack and the loft when I first moved in. As it is only about 10-12m long the losses are about 2dB on 2m. Low enough for the limited VHF work I do, which normally amounts to local 2m FM work only.

But as an experiment the ISS APRS contact via Soundmodem worked, showing you don't need to have a hardware packet TNC to work the ISS on APRS - or complex antennas.

Update 5th March 2016

I reconfigured the station for my daughters, who are both licensed. At 07:48hrs Ellie M6ELE got through with 10W. Her digipeated signal was captured from the ISS by PD0SBH-10 in the Netherlands.

On the next pass Sarah M6PUP also got through with 10W and the return signal was picked up by ON7EQ-10, so my thanks to both stations.

Jean-Jacques ON7EQ has a stonking ground station - see

1 comment:

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