One of my main interests in amateur radio involves picking up radio signals from satellites and (one day) talking to astronauts on the space station. Getting a contact with the ISS may be difficult, but there is an easier thing to have a go at first. The space station is fitted with an AX.25 APRS (Automated Packet Reporting System) repeater which transmits on VHF. The repeater is nearly always activated, and sends out packets roughly every couple of seconds.

Hardware Setup

There are several types of hardware setups you can use to pick up the space station’s packet repeater. Handheld yagi antennas are a common choice, and omnidirectional antennas can also be used. Personally I use the following setup:

• RTL-SDR Dongle (wideband receiver)
• Crossed Dipole Satellite Antenna

This is the antenna I have been using. It is a 2-element crossed yagi tuned to 137mhz. Originally it was designed to pick up signals from weather satellites, but it receives a good signal from the ISS packet repeater (145.825mhz).

I believe it is also possible to receiving packets from the space station using an omnidirectional antenna. My Kenwood VHF radio was also picking up data packets on a good pass, using a 2m Slim Jim.

The antenna is fed with about 10m of RG-58 coax into the radio shack window. The cable is then plugged into my Realtek Software Defined Radio (discussed in my earlier post) using a MCX to SO259 pigtail adapter.

Software Used

• SDR# – The first piece of software that I used is called SDR# (SDR-Sharp). It is used to control my SDR dongle, and tune into the frequency transmitted by the ISS’ APRS repeater (145.825mhz). A full guide to installing and setting up the software can be found in my previous post. Download Link

• VB-Cable – This is a virtual audio cable, used to send the audio from SDR# to the packet decoding software. Download Link

• QTMM APRS Decoder – This is the software that decodes the APRS packets. Download Link

Software Setup

First you need to set up VB-Cable to send the audio output of SDR# to the AX.25 Decoder.

• Download VB-Cable from the above link
• Extract the .zip file
• Run “VBCABLE_Setup_x64.exe” as an administrator (don’t use x64 version if you are using 32bit windows)
• Click on “Install Driver”
• Accept the unverified driver warning
• Restart when prompted
• Go into the sound settings (right click on volume icon, click “sound”)
• Click on “CABLE Input”, and click on “Set Default”

Now that the virtual audio cable is setup, you will need to is set up SDR#. Tune the radio to 145.825mhz, and set the mode to NFM. You can adjust the squelch control but that is not required. Set the bandwidth to around 10khz.

Remember, due to the Doppler Effect, the signal will move as the satellite moves across the sky. You will need to adjust the frequency regularly to compensate for this.

Now, your SDR# window should look something like this:

Finally, extract and open qtmm, which decodes the APRS packets. The file that runs the program is called “afsk1200dec.exe”

You will need to do two things:

• Select the appropriate audio input (“Virtual Audio Cable” or “CABLE Input“)
• Hit the play button

The program should be displaying “Decoder Running” in the bottom corner, like this:

That’s it, all you need to do now is wait for the space station to fly past.

Orbit Tracking

In order to receive data from the ISS, you need to wait until it flies overhead. Enter your locator into Amsat’s Pass Predictor to get a list of passes. You’re looking for something with a maximum elevation as close to 90° possible. At my QTH I get a couple of good passes a day, and have had as close as 89° of elevation.

There is also orbit prediction software available, such as Orbitron, which I have found useful.

I also recommend keeping an eye on an ISS tracker, so you know the exact position of the ISS. The space station also has a high definition camera streaming live, which you  can view from here.

What To Expect

When the ISS flies overhead, you should expect up to about 10 minutes of data reception. On my best attempt, I’ve received data between Cork, Ireland and Frankfurt, Germany. The signals will appear as bursts of digital data, repeating every few seconds or so.

The decoded data is the raw form of the APRS packets. Some will contain GPS co-ordinates, whereas others will just be CQ calls or messages. Every now and again the ISS will transmit it’s ID, which will appear as “ARISS – International Space Station.”

Here an example of some decoded messages from the ISS repeater:

18:42:27$ fm RS0ISS-0 to CQ-0 UIv PID=F0
>ARISS – International Space Station

18:42:44$ fm EI8ETB-0 to CQ-0 via RS0ISS-0 UI PID=F0
=5151.51N/00826.55W-Thomas IO51SU Cork {UISS54}

Make sure to also listen out on 145.800mhz, the voice downlink used by the ISS. I haven’t heard anything yet, but if the astronauts do use their amateur radio set, it will be on this frequency (145.200mhz uplink / 145.800mhz downlink).

Conclusion

I hope that you have as much luck as myself when trying to receive data from the ISS. If you have any questions then please leave a comment below, or catch me on a club night. I always have my laptop and SDR with me, if you want to have a look.

-Peter Barnes (M6KVA)