UKC

I know that some recent iPhones have a in-built facility for two-way communication with satellites. And at the weekend I read about the 'Motorola Defy Satellite Link', which is a small bluetooth dongle intended to implement the same feature for any Android phone.

See for example https://www.theguardian.com/technology/2023/feb/24/new-mobile-puck-will-all... and https://arstechnica.com/gadgets/2023/02/motorola-brings-5-a-month-satellite...

Noting that the latter article states that the satellites being used for the Motorola dongle are geostationary ones - i.e. about 36,000 km away - my question is how do these things work. I can imagine these device being able to receive from such satellites - but to transmit to them seems almost incredible.

Does anybody have any technical insights?

Well I do know someone who managed to talk to the Mir on his back on the lawn with a handheld radio

I spent many years working on radio design (but so no much recently so some of the maths is a little hazy now)... As you say a radio signal decays the further you are from the source, however what determines whether is can be received or not at the far end is signal to noise ratio. Assuming your signal is not somehow hidden by an interfering noise source, you are limited by thermal noise at the receiver (a satellite will have an advantage here that the electronics will be very very cold). Its a line-of-sight link, so you wont get some of the other difficulties associated say with cellular transmission (signals bounding off buildings etc).

Given your signal will be much weaker than your typical phone to base station link, you need less noise, so you typically get rid of (thermal) noise by reducing the signal bandwidth, so if your phone can uplink to a satellite expect the data rate to be very slow - I just checked at the minimum data rate for satellite phones is 2.2kbps, so enough to send encoded speech, but would for example take 26 minutes to upload 1 photo from my Iphone! 

This incidentally is why your phone will still carry a 2G chipset going forwards (as well as 4G/5G), as you will always have more range with 2G due to its lower data rate, so if you are in the middle of nowhere its a get-out-jail backup to keep you connected (to the nearest base-station)

The signal is the transmit power minus the path loss.  So unless they use much higher power, they get the same signal from you.   But their receiver will be much better than yours.

The transmitter on the satellite will be vastly more powerful than the one on the dongle, I would have thought.

Question: does the iPhone satellite telephony also communicate with geostationary satellites? Or does it use low-earth-orbit ones?

The better receiver will probably improve their signal more than than added power does yours.

Apple use Globalstar which is low earth, T-mobile use Starlink, Huawai use Beidou which is geosynchronous.

Iridium (the original players) is also a geostationary network. To the OP: Iridium sat phones (voice and text) have been around since 1998. The tech isn't new, just becoming cheaper and more miniaturised. Having made way more than the average number of Iridium calls, my experience is that the quality isn't great, they drop out a lot (even being in a tent is enough to drop a call), and they're expensive (at least if my boss is to believed). For text comms I'd expect to see them become more common (for example some Garmin units already have this, and it's pretty cheap - 0.10$ a text IIRC), but unlikely for voice stuff.

The new facility is based on a huge array of micro SATs launched into low earth orbit. Whilst some use legacy geostationary satellites the new facility is with arrays of satellites in low earth orbit hurtling around the planet. Low earth orbit is between 120 miles and 600 miles, and phone to satellite is line of sight. The satellite might also have a antenna that can collect and focus the mobile rf. The challenge however would be connection handover between satellites as the satellite is moving rapidly which is something that is difficult on earth due to sporadic visibility of base station antenna as you hurtle along in a train through tunnels and between buildings but somewhat easier on motorways when there is a more straightforward line of sight. Satellites operate above obscuration so they can intercommunication much more reliably so handover can be achieved much more easily. They are grouped in orbit as well so handover is probably just handing over to the satellite coming along behind 

I suspect the handovers will be easier as it's known which satellite will be the next one to take up a call - with terrestrial mobile who knows which base station will be the best one to move onto until it happens.

The satellites talk to each other and ground stations which coordinate the handover, there aren't swarms though, there are 66 in service to cover the globe.

thanks for that ukc at its best .

I “ heart “ ukc 

As you say, stuff in space tends to be very cold.  

At some point with a good low noise receiver you could become limited instead by the shot noise (Poisson noise) in the flux onto the receiver.  I’d be interested to know how many photons per bit are received by the phone > GEO systems.  Shot noise is the limiting factor for most professional astronomical and biological fluorescence detectors, especially as the range of read noise free single photon detectors increases..  You canna beat the laws of physics…

Iridium satellites are in low Earth orbit at about 780km, to allow low power devices to be used. According to their website there are 66 of them.

Just an observation.

Today for the first time I noticed my iphone has a satellite icon appear where the 3G/4G/Wi-Fi icons normally display. I was in a known no mobile reception location but unlocked the phone to read a previously received WhatsApp message I had. The app was trying to communicate as it does when open with its server.

The satellite icon only displayed briefly but I was surprised to see it at all. 

It's an absolute disgrace, "the phone was switched on.... but there's no signal" excuse is being eroded. It's a quality of life matter and we should fight against this evil tech

It only uses the satellite network for emergency messages which you instigate. There's a demo mode in the settings you can play with to se how it works.

"Low power" in this context basically means something that runs off a battery - i.e. the Iridium handsets and Garmin devices I mentioned. The coverage is amazing, but the reliability in my experience (for voice calls) is far from what people expect with terrestrial networks. It's not a huge issue, just a thing which IMO will hinder widespread adoption without some tech gains I don't understand - I'm only a user of the system.

The primary technical 'insight" is: Very restricted bandwidth along with line of sight (same as probes like Voyager). Yes, receivers in satellites are a bit lower noise than for a handset or even a cellular basestation and yes they have better antenna gain (directivity) but the directivity is limited by the required coverage footprint and antenna size (again, like cellular basestations).

Thanks to all for the various comments on this.

On GrahamD's last post, he wrote, regarding receivers in satellites, that 'they have better antenna gain (directivity) but the directivity is limited by the required coverage footprint and antenna size.'

What about the aerials on the phones? Are they directional at all? For a conventional satellite phone, I had always imagined that the aerials might be designed so that the signal is directed upward when the phone is held conventionally to one's head. Is that so? Are the iPhone's satellite aerials also directional? What about that of the 'Motorola Defy Satellite Link' mentioned in the OP?

Some sources show the phones needing to be pointed in a certain direction towards the relevant satellite, guided by the display on the phone so they are likely to be somewhat directional. 

The antenna in the phone will be a massive compromise due to the tiny amount of space available and surrounding parts in the phone. 

As pointed out by others, the system will be transferring tiny volumes of data very slowly, hence why the service is limited to simple messages.

Handheld phones which can talk to geostationary satellites have been available for 20 years. They had reasonable external antennas. Those no longer meet people's usability expectations for a phone and so have been miniaturised at the cost of performance.

As a satellite RF designer, I think the innovation here is fitting an antenna with adequate performance into a tiny phone handset form factor. There will also be challenges matching user demand to available capacity as potentially the number of handsets could be very large.

Phone antennas aren't very directional.