International Space Station 437.800 MHz cross band FM repeater activated | Hacker News Reader

International Space Station 437.800 MHz cross band FM repeater activated

amsat-uk.org

194 pointsmindcrime64 comments

Fun fact: There is an APRS repeater on the ISS. You can use the ISS as a relay to the APRS network. And APRS has a cellular gateway, which means you can send SMS messages to mobile phones via the ISS.

* Sending SMS Messages Through the ISS

https://www.kj7nzl.net/blog/sending-sms-messages-through-iss...

It’s off right now. Plans are to add a second radio in the Russian segment later so they have two IORS modules running at the same time. Then they can operate both the repeater and digipeater.

Apparently there is a non profit that relays some of those messages via satellite:

http://forums.othernet.is/t/aprs-outnet-on-outernet/2620

It’s pretty strong. You can listen to Amateur operators using it if you go to websdr.org, find one of the handful of receiving stations that cover 437.8 MHz, and time it right. One of the neat things is that you’ll easily see the Doppler shift in the spectrum waterfall.

I caught a little bit of traffic earlier tonight on the downlink frequency, but I wasn't able to contact anybody myself. Possibly if I'd realized the ISS was in the vicinity and gone outside earlier I would have had a shot (I only have an HT radio at the moment), but by the time I got out there they were somewhere over the North Atlantic. The ISS, if you didn't realize, moves pretty fast.

Oh well, there's always next time.

What sort of antenna would I need to listen in with a rtl-sdr? Would I have any chance with a simple home built 1/2 wave dipole?

Yes I've received it on a portable even. For a satellite it's easy probably because they're so close (LEO) and have plenty of power too burn.

Yeah, it's a lightly modified Kenwood mobile VHF/UHF set typically used in a car. Model D710GA. Stock it's a 50W radio, though can be run at 5W. Cube sats have around the hundreds of milliwatts. So yes, much higher power than a cube sat.

Some more info on the equipment being used:

https://www.jvckenwood.com/en/press/2020/03/press_200313_2.h...

I was able to hear some of the traffic using my Baofeng F8HP with a stock rubber ducky antenna, so you don't need much on the receive side. At least when the ISS is close to you. Not sure what the window is as far as how far removed it can be from directly overhead, and yet still be able to receive that signal with a small antenna like that.

What it takes to hit the repeater I'm less sure. I've heard it can be done with an HT like mine, but I'm guessing that's only possible when it's directly overhead, or very near so. That said, I'd be happy to find out that I'm wrong about that.

I believe you can hit it with an HT, although the Youtube videos I've seen of doing it have been an HT with a hand-held Yagi cabled in in place of the usual whip antenna. A resonant dipole would probably pick it up - as others have said, the signal's fairly powerful.

Satellite comms is something I want to play with, personally, so I welcome this news. Need to get my Foundation license (UK) before I can go two-way, though.

Go for it! It really isn't hard, it just requires a small amount of application.

Definitely can pick it up with a HT, just caught it for ~3 minutes in the Bay Area on a Baofeng HT with whip antenna, and even picked up the first part while I was still indoors. There was a lot of static although I could make out some of the sentences.

Wow, that's a stronger signal than I thought. Maybe I'll give it a go with a whip the next time the ISS passes over me.

For listening, a QFH antenna is ideal for LEO objects. It's typically made of copper pipe or the braid of stiff coax.

Absolutely you will do well with a dipole.

> The ISS, if you didn't realize, moves pretty fast.

I waited a few times to see it, and it's always just a few minutes it's visible. It orbits low and makes the full cycle around the globe in only 90 minutes.

For those curious when and where to look to see the ISS by eye only: https://james.darpinian.com/satellites/

The site is incredibly awesome and helped me see the ISS (and other satellites) quite a few times already!

Stellarium is great for this too.

I recommend the app ISS Spotter by Martin van Mierloo.

A 90 degree pass (overhead) is around 10 minutes. And obviously for radio, it doesn't need to be visible (reflecting sunlight), just ~above the horizon.

> A 90 degree pass (overhead) is around 10 minutes.

Yes, that's approximately what I remember. The path of ISS is created with the goal that ISS doesn't fly over the same pieces of land or sea. Instead, it takes a lot of orbits before I can see it crossing exactly over my location, giving me around 10 minutes when that cross is ideal, otherwise, it's less, if the current path is close enough at all. Seeing is of course limited more than obtaining a radio signal, as to see ISS it has to fly above you at the time that the sun can reflect from it, and to have clear skies. One doesn't need that to catch just a radio signal, but the ISS path should still be such that it's close to your location, and the time during the FM signal can be received will still be short.

Sounds like you're already on it, but anyone who is thinking of trying, (1) make sure you are correctly licensed, (2) make sure you only transmit while you can clearly hear the downlink, and (3) make sure you are listening to the downlink while transmitting (full duplex) to ensure you aren't stepping on other people.

> (3) make sure you are listening to the downlink while transmitting (full duplex) to ensure you aren't stepping on other people.

If possible. Not everyone has fancy-pants cross-band full duplex capability. I know a couple HTs will do it now, but this is still a bit exotic. This is the strongest space amateur FM repeater ever; surely those of us who are better equipped can use SO-50, etc, without issue, and we shouldn't get too mad if there's a little less-than-ideal operation on something as newbie-friendly as this.

My cheapest HT can do this. And my 20 year old radio can do it. So not sure about "exotic"

What radios are they?

Kenwood's TH-D72, TH-V71A, and TM-D710G can do it.

An incomplete list of full duplex vhf/uhf cross-band FM radios can be found in this reddit comment: https://www.reddit.com/r/amateurradio/comments/97jhik/gettin...

Sure they can, but I wondered what cheap radio the OP was talking about that runs full-duplex?

Yeah, there really aren't any. The cheapest option is two unwieldy independent Baofengs.

A second-hand Kenwood TH-D7 is an alright choice.

TM-V71A isn't a HT, nor is a TM-D710G. For a new ham who's scraped together some coins and gotten an HT, the only options that I'm aware that will work are the TH-D72.

AFAIK the only options are TH-D72 and Wouxun Dual.

While not an HT, the TM-V71A is very popular for LEO usage because you can easily ramp up to 25W when needed.

See this video of John Brier KG4AKV with his front-backpack TM-V71A mount. https://www.youtube.com/watch?v=w7Ukh_nT2Tg

I don't know of anything except Wouxun Dual and TH-D72 that can do it (that are HTs).

If a D72 is your cheapest HT, then well, congrats.

Dual watch, or full duplex?

My decades-old FT-470 can do true full duplex too, but it's less common in current radios. Just dual watch now.

Fair point, but FWIW, even a little Baofeng F8HP[1], which sells for around $70 can do cross-band like this. I'm not sure this feature really qualifies as "exotic" these days.

[1]: https://www.amazon.com/BaoFeng-BF-F8HP-Two-Way-136-174Mhz-40...

That baofeng isn’t full duplex, they’re using an incorrect definition. It can’t transmit and receive simultaneously on different bands.

Fair enough. I was just thinking in terms of the "not walking on people" thing... to achieve that you don't really need "true" full duplex, you just need the ability to configure a cross-band channel. Sure, there's a race condition if two people start transmitting at nearly the exact same instance, but in practice that doesn't seem to be a big problem.

Do you work satellites?

It's a really big problem-- people who can't hear whether and how well they're getting in, interfering with people who are.

There's often dozens of people contending for time on a little 7 minute pass. A QSO finishes and everyone keys at once to get in. Someone has bad doppler correction on the uplink and isn't making it in and you can't really tell who it is or tell them. Someone else with a PL tone mistake jams everyone. Someone else can't get through well enough to be intelligible, but still can cause big QRM and confusion for everyone else (is that someone answering? is that person successfully having a QSO and -I- am hearing badly?).

Just about every radio will do what you describe. You don't -want- a cross-band channel usually, because you want to tune separately for TX and RX. (I've also done it where you make many, many channels with pairs of uplink and downlink frequencies).

With real full duplex, satellite is, say, 5x easier -and- you stop being disruptive to other users of the sat.

Do you work satellites?

I'm just getting started in this world.

A QSO finishes and everyone keys at once to get in.

Aaah, interesting. I've never been around one of those situations and it never occurred to me that enough people would be doing something this niche at one time for that kind of jam-up to occur.

Thanks for the heads-up. I guess maybe I'll look at investing in a real full-duplex radio at some point. Probably a mobile though, not an HT. Getting a decent mobile rig for my truck is next on my ham wish-list anyway.

Satellites are a ton of fun! You should go for it. You -don't- need a full duplex radio to get started. It just helps and makes it easier for everyone else, too.

If you decide to... it's important to just plan to listen to a few passes and get to hearing the satellite really well before you try to transmit, though.

The experience operating sats is very different depending on where you are on the globe. Advice that's okay in one place may be bad advice in a different location.

One final heads-up, the vertical antennas typically fitted to vehicles make really poor satellite antennas because you can easily emit enough power to make it into the sat, but there isn't enough directional gain to hear the downlink, so you're back in the same situation. A dual band yagi antenna really is the way to go. You also don't want to use a mobile radio with a yagi unless it's turned down to low power, because of RF exposure limits at VHF/UHF. A typical portable station is a dual band yagi (e.g. Arrow II) with one FD or two HD handheld radios, a headset, and a voice recorder, and a typical fixed setup is az-el rotator with computer control, dual circularly polarised X-quad yagis, masthead preamplifiers, and Doppler control of uplink and downlink frequencies.

Yes, you absolutely do need true full duplex to (a) know you're getting in, (b) not step on anyone else, (c) know when you're being stepped on, all of which are really required to operate effectively and courteously.

Let's not excessively gatekeep things. Someone who is curious and wants to try a satellite once shouldn't need to buy new equipment. A dumb HT with a simple whip is enough; just keep your transmit duty cycle low and try hard to not step on people.

If I followed this advice a couple decades ago I'd never have gotten into operating sats. Yes, now that I'm equipped for FD I'd never operate without it, but..

Not full duplex. Only "cheap" option is Wouxun KG-UV8D; only OTHER HT option I know is Kenwood TH-D72A.

Alinco DJ-G7T has real full duplex.

Kenwood TH-D7

Fancy-pants cross-band full duplex can be achieved with two £15 Baofeng UV-5Rs and a DIY duplexer with about 2 components on it (or just two antennas on one boom).

That's my setup - I'm not better equipped than that.

I tuned into 437.800 MHz here in South Africa - the ISS is probably a couple of thousand km's south of here now. As I tuned in, I got morse code for 5-6 seconds. Didn't record or pick up what it was. I wonder if it was the ISS or something else.

Could be station identification. Many repeaters identify their call with morse code periodically.

To elaborate: It's a requirement that amateur radio stations periodically identify themselves by the operator callsign during transmission. For unmanned stations like repeaters this is usually accomplished with an automatic morse-code transmission of its callsign.

The transmission occurs if the repeater has been activated, usually every 10 mins or so after a moment of silence, or after a longer period of silence, whichever comes first.

(The repeater users still have to identify themselves with their own callsign during use of a repeater.)

I am personally quite excited about this and look forward to giving it a try. A simple homemade antenna is all that's needed, and they're not expensive to make:

http://ve2zaz.net/Arrow_Ant/Arrow_Style_Ant.htm

A sat tracking program I have shows ISS passing almost directly overhead in about 40 minutes. How would the downlink frequency vary due to doppler shift as it transits from local horizon to local horizon?

Good question. Some info on tuning for Doppler shift:

https://www.qsl.net/ah6rh/am-radio/spacecomm/doppler-and-the...

Thank you, that was very informative. Seems like the calculations are pretty straight forward. I need to see if I can create my own charts like that.

The open source software gpredict can do this live for you.

Short answer - by one or two of the smallest steps on a typical handheld, starting above the nominal frequency and ending below it. A bit more than that for UHF downlinks.

Nice to see. The smallsat phenomenon has done very little for hams so it is nice to see the ISS facilities upgraded, also i think it is funny they can afford to build ten copies of the repeater.

What is the use case of such repeater?

Radio amateur experimentation and communication.

That it is cross-band is just a detail that simplifies the repeater (single-band repeaters generally use humongous filters).

Is the highly non-stationary nature of the repeater an interesting part of the experimentation?

http://images.spaceref.com/news/2003/speedlimit.jpg

Of course. You'll have to adjust for the Doppler shift (about 7kHz swing during a pass IIRC), and depending on your antenna design you might have to do precise tracking during the pass.

There are also sub-sports such as QRP that tries to perform communication using very low power levels, which makes everything much more challenging, including the already slightly tricky contact with the ISS. I believe the current QRP record is 1.6 billion miles-per-watt, with 1 microwatt making it from Oregon to Alaska (1650 miles).

But if the ISS passes over in just a few minutes, is that useable?

You have a ~10 minute window every ~90 minutes IIRC.

The HAM sport is mostly about establishing contacts of increasing difficulty, not about making boring, rock-solid reliable communication links. 10 minutes is plenty for that, and helps stop people trying to hog the thing as only one person can be received at a time.

Outside experimentation (e.g. trying an antenna, transmitter or receiver design), the HAM radios on the ISS are also used for school-astronaut contacts.

These radios are built by a retired Qualcomm engineer in La Mesa California

https://www.cbs8.com/article/news/local/zevely-zone/la-mesa-...

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