What it takes to listen to the ISS repeater

[dosw]

There are a few websites on this topic, but I just had a fun experience and thought I'd mention here what was involved.

Background: The ISS operates an amateur repeater. The downlink for that repeater is at 437.8000 MHz. What you will hear on that repeater is mostly people trying to make DX contacts through the repeater. As an example, if I can hit the repeater 400 miles away, and someone else can respond back, from 400 miles away, we could have 800 miles between us (not counting the 260 mile trip to space and back). An 800 mile contact over VHF through a single repeater is kind of cool, so amateurs are working this repeater constantly as it passes near enough to reach.

 

Today's pass, the one I heard, was never any closer than Calgary, Canada (I'm in Salt Lake City). The measuring tool on Google Earth put the land distance at about 850 miles. And if you calculate the hypotenuse of 850 miles base plus 260 miles elevation that's about 888 miles. Accounting for the curvature of the earth changes the angle of that 260 mile leg, changing the shape of the triangle, and making the hypotenuse (the distance from me to the ISS) closer to 950 miles.

 

I wasn't trying for contact; I assumed it was too far away, which it probably was for my setup. But I started hearing it before it hit the west coast of British Columbia, Canada, and kept hearing it all the way to New Brunswick. That's well over 2400 miles maximum distance.

 

How do you know you're hearing the ISS repeater? First, you'll hear it starting to show up at the approximate fly-by time here: https://spotthestation.nasa.gov/tracking_map.cfm

 

Next, you'll notice that the transmission is coming in off-center from the downlink frequency. This is due to the Doppler effect of the ISS moving 17,000+ miles per hour. Today when I first started hearing it, it was coming in at 437.8065 MHz, and by the time it faded out, I was listening on 437.7935 MHz. That's a 13kHz Doppler shift (+6500Hz to -6500Hz). Sensing that Doppler shift pass from above the center frequency to below the published center frequency is a dead giveaway you're dealing with the ISS moving 17500MPH.

But here's what's cool, in my mind. My setup wasn't extravagant:

• RTL-SDRv4 (<$40) with SDR++ software (free).
• Cheap no-name (<$20) inline linear amplifier powered by the Bias-T setting of the SDR.
• And the big massive antenna? Hold onto your britches: Comet CA2X4SR-NMO mobile antenna ($80) mounted on a Midland magnetic mount ($30) stuck to an old 16" square steel griddle.
• Laptop computer to run the SDR and its software.
• Total cost (excluding laptop): $170, all of which I have cobbled together already.

My technique was just to look up the fly-over time, and set the SDR++ software to just a few kHz high, then start watching the waterfall and listening. When I started seeing the FM signal in the waterfall I centered on it and listened. And by watching the waterfall I was able to continually adjust the frequency center through the fly-over until it was out of range. It was amazing to me that I was able to follow it all the way from the west coast of Canada to the east coast, and that I did it with a simple mobile antenna just sitting on the window sill -- not even a yagi. I wasn't expecting to hear anything at this time since it never got closer than about 950 miles.

The pile-up on the frequency was pretty heavy, so I imagine the people getting through are using good, directional antennas to transmit with. I don't expect to be able to get a contact out of it without a directional antenna. But it was impressive to me today to be able to hear it over such a great distance.

[dosw]

I should also mention: What YOU need to be able to do this...

Although an SDR is really, really convenient for listening, because it allows you to observe the waterfall, I have also listened to the ISS with a Baofeng amateur radio (AR-5RM), the same antenna, a few adapters, and the same tiny linear amplifier, this time powered with a micro-USB cord. You still have to set your frequency step as narrow as possible in VFO mode, start high, and step down as the Doppler shift sweeps down. But it's not hard to do. Turn off squelch. Parts list: AR-5RM (<$30), LNA (<$20), antenna ($80), mount ($30), a couple of adapters ($10): still about $170.

(And you'll need your Amateur Technician license -- ham)

[WRYZ926]

I will add that if you want to talk to the ISS, you will need a dual receive dual band radio. And it doesn't take much power either. People talk to the ISS all the time using a 5 watt HT and a hand held dual band Yagi antenna. 

And the ISS also does slow scan TV at times too. A fried is setup for slow scan TV and was getting pictures from the ISS to show his grandson a few weeks ago.

[marcspaz]

I have talked to 2 people on the ISS and used the ISS repeater several times.  It was surprisingly easy and only required a few watts.  I didn't use any fancy hardware or software, just a zero-gain dual-band vertical antenna and a dual-band radio.  I adjusted the frequency for Doppler manually, fairly easily, too.

 

It's a lot of fun, but I got bored waiting for comms windows, and gave up on it after awhile.

[dosw]

44 minutes ago, marcspaz said:

I have talked to 2 people on the ISS and used the ISS repeater several times.  It was surprisingly easy and only required a few watts.  I didn't use any fancy hardware or software, just a zero-gain dual-band vertical antenna and a dual-band radio.  I adjusted the frequency for Doppler manually, fairly easily, too.

 

It's a lot of fun, but I got bored waiting for comms windows, and gave up on it after awhile.

Yeah, that's the hard part. We had two windows today, but it will be a few days before the next good one, and probably in the middle of the night. A few weeks ago I heard Sunita Williams taking questions from school kids and answering them. I guess some ham operators were volunteering at various schools to facilitate. This wasn't on the repeater, it was on their 2m/70cm amateur frequency pair. Pretty cool.

 

From what I've read, the ISS repeater and amateur voice downlinks are transmitting at 25w. Pretty amazing that a 25w FM voice transmission over UHF can reach me over 2000 miles away and be readable, when I'm using nothing more than a mobile antenna and a small LnA.

[WRXB215]

It flew directly over me today and I was going to try to get through the pileup but I got busy forgot.

[WRQC527]

While not as noteworthy as working the ISS or working through the ISS repeater, I've made a bunch of contacts through the SO-50 AMSAT repeater. The first few were using a $10 DIY dual-band Yagi and a UV5R, and after that using my Arrow dual-band Yagi and a Yaesu FT-60. The process is the same with the ISS. Pretty fascinating.

[Lscott]

5 hours ago, dosw said:

From what I've read, the ISS repeater and amateur voice downlinks are transmitting at 25w. Pretty amazing that a 25w FM voice transmission over UHF can reach me over 2000 miles away and be readable, when I'm using nothing more than a mobile antenna and a small LnA.

They can use the phone on board.

https://weather.com/news/news/2019-01-07-astronaut-accidentally-dials-911-space-station

https://youtu.be/3L82DHQfcF8

 

[AdmiralCochrane]

I have listened to the ISS repeater many times with a $30 Baofeng.  Antenna polarization helps.