ISS Ham Radio Repeater

There is a long history of spacecraft carrying ham radio gear, as the Space Shuttle, Mir, and the ISS have all had hams aboard with gear capable of talking to the Earth. However, this month, the ISS started operating an FM repeater that isn’t too dissimilar from a terrestrial repeater. You can see [TechMinds] video on the repeater, below.

The repeater has a 2 meter uplink and a 70 centimeter downlink. While you can use a garden variety dual-band ham transceiver to use the repeater, you’ll probably need a special antenna along with special operating techniques.

One of the problems you’ll find is that ISS moves fast enough that you will observe doppler shift in the frequencies. The video reproduces a table of frequencies you may have to move through to receive the shifting signal.

You can probably hear the ISS with a good pass with no special equipment, but [TechMinds] wasn’t able to close an actual contact in the video. But [K0LWC] got really close using a pretty standard radio setup, as you can see in the second video.

The ISS has been on the air with digital repeaters and conventional FM radio for some time. The antenna you need doesn’t have to be a huge disk. We’ve seen it done with a handheld beam antenna and a handheld radio.

28 thoughts on “ISS Ham Radio Repeater

  1. I’ve been easily able to receive the downlink, but my 5W through an Arrow antenna aren’t enough to “get in” over the many, many other dedicated hams. I’ll keep trying, and maybe a 2 am (local) pass may be better for that QSL. Cheers Craig KG5YVN

      1. Absolutely it is. Around here within 2 passes of any amateur satellite you’ll recognize all the same voices and call signs. I’ve made a few contacts to say I did it, and moved on to other less repetitive endeavors.

  2. The first video talks about adjusting the downlink frequency to account for doppler shift, but leaves the uplink frequency unchanged. Why would the doppler shift affect one frequency but not the other?

    1. The doppler shift on both uplink and downlink is the same percentage of the respective frequencies, but as the uplink frequency is 1/3rd of the downlink frequency, the absolute frequency error is also 1/3rd of that of the downlink error. Ie if the downlink as a 10KHz doppler then uplink shall have 3.3KHz. 3.3KHz error is so small that it is close enough to the wanted frequency for the link to work.

    2. The important factor here is that the uplink and downlink are in different frequency bands. The change in frequency caused by Doppler shift is directly proportional to the frequency of the original sequel. So basically the signal in the 70cm band experiences a much stronger shift than the 2m band. With the relative velocities involved in LEO satellite operations, the 70cm band requires retuning to compensate for Doppler shift while the 2m band does not.

    3. It doesn’t! It applies to BOTH freqs. One will go up, and the other will go down, This is what satellite radios do. The VFO’s track in opposite directions. If I move 5 khz down on the input, the output moves up 5 khz, There are people that can do this with two radios by hand. And talk at the same time. I’m just not one of them.

  3. Quote: “One of the problems you’ll find is that ISS moves fast enough that you will observe doppler shift in the frequencies. The video reproduces a table of frequencies you may have to move through to receive the shifting signal.”

    That continually tweaking for doppler takes much of the fun out of LEO communication, as does the brief window of opportunity to talk. The one-and-only ham radio satellite in geosynchronous orbit, the Qatar-OSCAR 100, doesn’t have either issue. It’s way up at 10 GHz and only in sight of Europe, Africa, and eastern Brazil, but you can listen to it via WebSDR here. Click on the “Click to Start Sound” to make it play, and then select the signal you want to hear on the frequency line in the waterfall display. For SSB, you will need to tune it carefully with the – and + buttons.

    It’s handling CW, digital and SSB at the same time and has ample room for them. It’ be great if we had several of these spread around the globe and perhaps even cross-linked.

  4. I am intrigued by the fact that doppler is less of a factor at lower frequencies and that a few satellites have had a 10-meter mode. Given the dismal state of the sunspot cycle and the deadness of the higher HF bands, I wonder if a 10 meter-15 meter repeater might make sense. If interfering with ionospheric communication is an issue, it could either be turned off when those bands are open or operate only at night.

  5. I’ve worked one the amateur sats (AO-52 as I remember) with a 5W handheld and an Arrow dual-band antenna. Juggling doppler shift and manually holding and tracking the antenna is a true juggling act but can work. I was in Massachusetts and worked a ham in Texas. It is incredibly exciting when the noise drops and you know the bird is within range. The problem is everyone wants to work at sat (especially the ISS), so make your QSO quick and let others get on. Want to build your own antenna tracker? Look up Elwood Downey at Clearsky Institute.

    1. @Stan. SWAN, The link you provided to the 2018 Doppler paper is behind a nasty Mendeley/Elsevier Email harvester pop-up. However I think the paper is free to read, the companion 2019 paper from the same principle investigator I link to in my previous reply certainly is anyway. I suggest if anyone else is interested in the 2018 Doppler paper, just stick the DOI into the Sci-Hub site and out it pops. Here’s the full title and DOI:

      Laboratory testing of LoRa modulation for CubeSat radio communications


  6. So much negativity in some of these posts. I use an Arrow dual band Yagi and a Yaesu FT60 to make contacts through SO-50. It’s a lot of fun, and yes, somewhat tricky, but that’s the essence of our hobby. If you think amateur radio is as simple as picking up your smartphone and making a call, you picked the wrong hobby.

    1. I think I heard myself through Oscar 6 in the winter of 1972 (as in I wasn’t completely sure it wasn’t the transmitter getting into the receiver directly). It was 2M up, 10M down. I used a Cushcraft beam kind of pointed in the right direction (no rotor) and a dipole on receive. An RCA Carfone transmitter converted to 2M and CW, and an SP-600 and nuvistor converter on receive. Half a racks worth of equipment

      It boggles my mind that one can use a handheld to reach the space station orsatellites, even if a better antenna is needed.

  7. Christian Doppler is rolling over in his grave reading some of these comments. As the source of radio waves moves toward a stationary observer, the frequency appears to increase in frequency. As it moves away from a stationary observer, it appears to decrease in frequency. Just like a car horn as it passes you. There is no “one increases as the other decreases”. VHF receivers are less-sensitive to this perceived change because the change in frequency is comparatively less than on UHF, because UHF is substantially higher frequency.

    1. “One increases as the other decreases” is actually a correct statement. On the ISS both the Rx and Tx frequencies are FIXED. Us folks on the ground are the ones that need to compensate for Christian Doppler’s effect. Our higher UHF Rx freq is the most sensitive to the effect and we definitely need to compensate for it.(approaching us higher freq, going away lower freq)

      Our lower VHF Tx freq is less sensitive to the effect but it is still there. Since the ISS freq is FIXED then we might want to adjust our uplink as it approaches by lowering our Tx freq to have it received on the correct freq at the ISS. Here’s an example scenario….We are on the edge of the ISS footprint as it approaches us and someone else is in the center of the footprint. When he transmits on the uplink he should use the 145.990 frequency (minimal doppler needed directly overhead) however at that same time the ISS is approaching us (max doppler) we might want to transmit on a lower frequency so it becomes Doppler corrected at the ISS receiver. (the ISS is approaching us).

      Here’s an interesting article about Doppler correction that explains the “One increases as the other decreases” statement.

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