Can I text on my cell phone from the ISS to Earth?

We know from Can we detect the cell phone, satellite phone, or walkie talkie of someone walking on Mars from Earth? that cell phone signals on Mars are potentially observable on Earth.

I am not an expert, and assume that the ISS is moving to fast for a cell phone to establish a connection to cell tower(s) that would allow for a phone call. But what about a text message?

Would it be possible to send a text to someone on Earth from the ISS using a standard cell phone?

• It's an easy 2-step process! Step 1, go to the ISS... :) But seriously, interesting question. – Don Branson Dec 28 '18 at 17:00
• I assume you'd just be too far from any towers, unless there's some specially-built repeater on board. – Don Branson Dec 28 '18 at 17:01
• Bandwidth and data rate of a cell phone on Mars would be too high and transmitter power too small for the huge distance from Mars to Earth, But a cell phone without a cell tower nearby would not transmitt anyway. If the cell phone in the ISS uses the normal data rate for a small text message, it would be too fast for the distance. The antenna of a cell tower just below the ISS would be optimized for phone on the surface of Earth but not vertically above the tower. The delay from tower to phone at the ISS and back would be too long for the transmission protocol. – Uwe Dec 28 '18 at 17:11
• Are you really wanting to ask "can a cellphone send a text from the ISS", or "can a cellphone on the ISS connect to a ground-based cellular tower long enough to send a text"? The former question is a superset of the latter. For instance, the ISS does have WiFi, which most smartphones could use to send a text or make a phone call, and the ISS could contain a cell of it's own, which is routed through other communication paths after the communication from the cellphone to a hypothetical cell base station on the ISS. – Makyen Dec 28 '18 at 20:53
• @MikeWaters On the other hand you have pure line-of-sight propagation working for you. The loss in cell phone signals through buildings and terrain is huge. I recall that in urban environments the signal falls off between the third to fourth power of distance. – user71659 Dec 29 '18 at 2:40

No.

• "200 km" is the shortest distance (actual ISS altitude varies but is closer to 400 than 200 km), but at that distance you're far outside the main lobe of the ground station antenna. Cellular traffic is usually at an elevation below the antenna (which sits on a tower), so the antenna main lobe is aimed in the horizontal direction or slightly below. When you're overhead, you're at 90º relative to the main lobe, and the antenna is much less sensitive in this direction.

• So you can't communicate with the cell tower directly below, you have to use one on the horizon. How far is that? The ISS appears above the horizon maybe 5 minutes before it's at the zenith. It's 2300 km away at that point. (28000 km/h/12) At this point, it's in the main lobe of your ground station, but 30 times too far away to be heard.

• Cell towers in populated areas are designed to have a limited range: you don't want a tower 5 km away to interfere with a tower only 1 km away. The only places where you'll find long-range cell towers is in sparsely populated areas.

• depending on the network you're using, there may be physical limitations (GSM has a limit of 35 km)

• your standard cell phone on the ISS is inside a metal can, so its signal will be attenuated a lot. Putting an antenna outside isn't enough to get coverage though.

• a typical cell is 1-5 km across. If we use 5 km, the ISS spends 0.6 seconds in a cell. Handovers (to the next tower) will be a problem. Setup/teardown of the SMS transmission takes time too.

• nobody is going to set up a cell tower with 150 km range: only 30 km is usable on the ground (signals are line of sight, and 30 km is the horizon), so in all cases the range of the tower is far short of what you need. In mountainous terrain with a ground station at high altitude, range can be longer but you have something close to free space between transmitter and receiver, so you're already at an optimum case, I don't expect such a station to have 400 km of free space range.

• Transmission power falls off with the square of the distance. When you transmit a signal across 10x the design distance, the signal at the receiver is now 100x weaker.

• How long would an antenna outside the station have to be in order for it to pick up a decent signal at its shortest distance? A 170km antenna? – TylerH Dec 28 '18 at 20:06
• There are some good points here, but also some mistaken ones. Just holding it in front of an earh observation window would solve the shielding issue; in fact this is what SAREX did for years. Range in terms of what a ground station is designed for isn't directly transferrable since you have an exceptional line of sight, even if off the antenna design direction you have none of the obstructions commonly seen on the ground. I'd expect signals to be seen at times, but the link management layers to generally dislike them. With different software on both ends, short messages may be possible. – Chris Stratton Dec 28 '18 at 20:24
• @TylerH A very long antenna is useless at cell phone frequencies. A good antenna should fit to the used wavelength.. – Uwe Dec 28 '18 at 22:33
• In regards to your last point, the horizon distance depends on the terrain. If you stick your cell tower up on a hill, you get a fair bit of range increase, which is useful if you're trying to cover a large rural area. Back in the analog days, I got a cell-phone connection 60+ km from the tower once. – Mark Dec 28 '18 at 23:33
• @TylerH: the antenna doesn't need to be longer, it needs more gain. You can increase the gain by using a dish antenna, or by adding an amplifier. In both cases, you aren't using a standard cell phone any more. – Hobbes Dec 29 '18 at 12:18

Yes, but only with heavily modified equipment and modified protocols.

If you think about it, LEO satellite phone networks are the same principle in reverse. You use a cell phone-like device on Earth and you transmit up to LEO. If you swap the handheld and satellites, then you get a cell phone you can use at ISS altitudes.

In fact, first-gen Globalstar uses a modified CDMA protocol, and Iridium uses a TDMA protocol which bears some resemblance to GSM. If you just want to use text messages, you'd end up with something that looks like the Garmin inReach which runs on Iridium.

If it was possible to use unmodified cell phones, they would have built them instead of the special units Iridium and Globalstar use.

(Caveat: LEO networks are constrained by launch costs and capabilities, so you could build something with much larger ground antennas. On the other hand, siting of the ground stations is constrained by things like oceans.)

Yes and no :) It's complicated: it is very improbable, but I wouldn't rule it out completely, given very carefully chosen conditions.

Communicating with the ISS by text message (APRS packet radio) actually is quite common with radio amateurs (see How to work the ISS using APRS Packet Radio). You don't need much equipment to do that, low power will do if you can use an external antenna. Even these short "rubber antennae" on handheld devices may work, if you get a ISS "pass" directly overhead. - There are a lot of people doing just that on a regular basis. As it's a broadcast, you may see the packets received by the ISS here in the log file.

But due to the relative motion of ISS and earth station, there will be Doppler shift - a lot. This will limit the usable time window (unless you are willing to modify either cell phone or earth station to re-adjust working frequency in the range of +/-1.5MHz, depending on time and orbit). Here are some graphs and tables. It's easier to do this with APRS packets that with SMS messages, as they are just unidirectional short burst transmissions with about 1s duration... when using a cell phone and SMS, you would have to "book into the network" first (i.e. handshake, authenticate), and maintain a connection (with repeatedly exchanging packets), which is much more demanding, and would have to be done in a very short timeframe.

Another problem at least with GSM network would be the signal propagation delay (due to the distance). The assignment of "protocol timeslots" (introduced to serve many devices on a common frequency) requires a tight synchronization. If I remember correctly (unfortunately I cannot find any references on the quick), this does not work well with distances greater than about 60km due to the signal delay... but this might be a limitation to GSM only; and there are a lot of different protocols.

In parts of Northwest Wales it was/is quite common to pick up Manx Telecom on your mobile (Cell) instead of mainland providers. The Isle of Man sits roughly equidistant between the mainlands of Ireland & Britain at about 50 miles (90km). Nowhere near the 200-400km distance of the ISS but way outside the 5km mark. During the development of radio much use was made of "skip" whereby radio transmissions would bounce off the ionosphere to enable communication over the horizon and beyond line of sight, I experienced this at times with CB radio, often talking with the US from the UK. I'm not a telecommunications expert by any means but could this be possible with cellular?

• Skip (bouncing a signal off of atmospheric layers) happens at low frequencies, lower than most cellular networks use. When a signal is reflected by the atmosphere, you can't really use this frequency to send signals to space. – Hobbes Dec 29 '18 at 9:34

Would it be possible to send a text to someone on Earth from the ISS using a standard cell phone?

Yes, and no. Let's examine the no part first, then I'll explain the yes.

A regular cellphone, the one most people think of when you talk of a cell phone, use cell towers that point sideways and slightly downward. The distance to the ISS, over 300 km, is too far for both the transmission power and the transmission protocol; if the tower could receive a signal it would try to disregard it as interference and it isn't powerful enough to transmit that far to a standard internal cellphone antenna.

The International Space Station orbits in a low Earth orbit between 330 and 410 km.

The Burj Khalifa is by far the world's tallest structure at 828 metres (2,717 ft); while it is said to have "communications equipment" on the spire it's not clear that there are cellular antennae installed. It's less than one kilometer tall in any event.

Quora has a Q&A: "What's the highest altitude you can still get cellphone service from a cell tower?", which is answered: 'using a GSM phone, 35.406 km (116,160 ft)' and "... in New York, people have issues making cellular telephone calls from the top of a skyscraper. (because the tower's antennae don't point upward)".

A cell phone booster will only get you 32.187 km (20 miles).

There is 4G coverage on Mount Everest, but it's at 5,200 metres above sea level.

All of the above means no.

But what is to follow means yes.

IF your definition of "a standard cell phone" includes a "satellite phone" then you are in luck, afterall the ISS orbits at satellite altitude; what would you expect.

There are basically two means of using satellites for communication by phone, some satellite phone satellites orbit at low Earth orbit (same as the ISS) so the "distance" is relatively short. Other satellites orbit geosynchronously, which places their downward pointing antennae above the ISS. Either way you should expect them to work, with the geosynchronous orbiting satellites providing longer (probably constant) reception.

Here is what a satellite phone looks like:

Not much different than a 4G phone, except for the tiny external antenna, and the U\$1300 price tag; though used phones cost ~U\$200 and up. The Iridium Extreme 9575 is their top of the line model so the price isn't too bad, what you need to know (don't want to know) is how much per minute it costs to use.

But you want to use a regular phone you say, how about a phone like this:

Yes. How about a laptop, a fax machine, ISDN, yes: The Hughes 9202 BGAN Terminal is 21.6 x 21.6 x 4.6 cm (8.5″ x 8.5″ x 1.8″ inches) and as a hotspot supports up to 10 concurrent wireless connections. That means 10 standard cellphones can communicate simultaneously with the ISS using this device:

You wouldn't need quite so fancy a device aboard the ISS since you'd be much closer than a Terran.

There was no problem receiving 300 km far 3G cells in airplane 10 km above the ground as horizon distance at this flight level is 357 km so line of sight range is far enough and no obstacles were in the way. Received signal strength (RSRP) was around -90 dBm quite often but Ec/Io was bad (-14 to -18 dB) as I received tens of interfering cells at one time. Imagine being somewhere above Faroe Islands and have all Iceland, UK, Faroese and Norway networks in list.
Receiving 2G and 4G was possible but searching for cells took very long time, probably because of doppler compensation.

Transmission to ground cells was hard but occasionally successful, always at full TX power. No 2G or 4G transmission was successful.
So maybe receiving 3G ground cell networks could be possible from ISS (above areas where not many cells are visible so interference between cells could be lower) but I guess that transmit would be close to impossible.

• Then it wasn't GSM, because apparently due to latency issues (rather than signal strength) there seems to have been a hard cutoff at 35 km according to this answer. Can you cite a source that documents "no problem receiving 300 km for 3G cells in airplane 10 km above the ground"? – uhoh Jul 1 '20 at 15:12
• Own experience - reception of 3G ground networks was perfectly possible most time of the flight (when some inhabited land was <200km far). 2G is of course hard limited by Timing Advance at 35 km (unmodified GSM - but some Extended Range features can extend this to 219 bits of TA = 121 km), 4G has similar limits depending on Preamble Format - absolute maximum in unmodified LTE network with PF=3 is 100 km. 3G has no hard limit so two-way communication is possible if link budget and Ec/Io is sufficient. – dj-bobr Jul 10 '20 at 13:02
• That's fascinating! I'll have to figure out how to ask a proper question for which that is the answer, so that you have more space to post it. Thanks for the info! – uhoh Jul 10 '20 at 13:23