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

Thursday 3 March 2016

2m ISS APRS on the Icom IC-7400 with Soundmodem

The UISS software for sending packets to/via the ISS.
After my recent work on the Tim Peake ISS contact project I was quite taken with the idea of being able to contact the ISS using 2m APRS data on 145.825MHz.

I haven’t really played with packet radio for years and sold my TNC in the dim and distant past. But I read that you can use a software-based modem now instead of a TNC.

I already had a data interface that would work with my Icom for RTTY and PSK31.

So it couldn’t be that hard to get APRS running, could it? I never learn!

Firstly, we have got two APRS experts in my local club (Norfolk Amateur Radio Club – James M0UKS and Kevin M0UJD who could write a book on the subject), but even a chat with them left me none the wiser. But I eventually figured it out, so thought I would share the details.

The problem is that Soundmodem (the software modem that you can use for APRS) expects to see a signal on a serial port to transmit or key the PTT. Icom, on the other hand, uses its CI-V interface to send a command.

I messed around with this for days, but eventually came up with a solution, thanks to John G8BPQ. He has written a small program that creates a virtual serial port. He then has another program called CAT7200 that translates the rig’s CAT control command and sends it to the virtual serial port. The result is that the rig then transmits.

So what do you need to set up APRS on an Icom IC-7400 or IC-746 Pro? I would imagine that this would work with the IC-706, IC-7000 and IC-7410 too.

Firstly, I’ll presume you have a data interface for RTTY/PSK connected up and working with your PC. Secondly, I’ll presume you have a CI-V interface connected between the radio and the computer too – the type that sends the frequency and other information to your logging program, for example.

Now you need to do the following:

1. Download and install Soundmodem from

2. Download and install UISS from

You should be able to configure the two programs and receive packet data on 144.800MHz (if in the UK) or from the ISS or PSAT on 145.825MHz.

Quick tip – make sure Soundmodem is set to look for tones on 1200/2200Hz – I must have changed mine by mistake and although I could see the packet tones on the waterfall it wouldn’t decode.

Orbitron let's you work out when the ISS is overhead,
as well as other packet-enabled satellites like PSAT.
I won’t go into any more detail on UISS or Soundmodem as there are better reference documents out there, but if you’ve got this far you are half way there.

Now to enable TX.

Go to and download the file Unzip it into a suitable folder.

Now go to back to the CAT7200 web page and follow John's instructions for creating a virtual serial port using the files you have just downloaded – there are instructions for XP and Windows 7.

Once you have created the virtual serial port it should appear in your Control Panel. You now set up Soundmodem to trigger this Com port in devices >> PTT Port.

Now the clever bit. Run CAT7200 and set it up to watch the virtual serial port you have created (on the left of the screen) and to trigger the CI-V CAT port you use (on the right of the screen).

Now, if you send a packet from UISS it should put the radio in TX first. Just make sure you get your audio levels right.

As for how you send a data packet to the ISS please see