How much power is needed to communicate with a geostationary satellite?

[Raswan]

Thinking just in terms of pinging the satellite for location, say, every five seconds. Is this prohibitive for cell phones in terms either of power or transmission strength? My understanding is that these babies reside 22k miles up, which seems way too far for anything inside a phone to ping. And yet (also my understanding) also doesn't the 911 emergency function work outside of all cell tower range precisely on this basis?

Thanks for setting a gent straight.

 

[serpretetsky]

If you have a cellphone and there are no cell towers anywhere near you or the signal is completely obstructed you are not going to be able to call 911.

You're probably confusing this with the ability to call 911 regardless of what kind of phone plan you have or what company you have service with. Cell phone towers must accept 911 calls no matter phone plan you have and no matter what company you are signed up with. But if there are no towers in the area there is no one to accept your call.

No, your cell phone does not communicate with satellites. However, there do exist satellite phones. Both geostationary ones and low orbit ones

 

[inachu]

You can make a call if it is a Thuraya satellite phone.

 

[Raswan]

If you have a cellphone and there are no cell towers anywhere near you or the signal is completely obstructed you are not going to be able to call 911.

You're probably confusing this with the ability to call 911 regardless of what kind of phone plan you have or what company you have service with. Cell phone towers must accept 911 calls no matter phone plan you have and no matter what company you are signed up with. But if there are no towers in the area there is no one to accept your call.

No, your cell phone does not communicate with satellites. However, there do exist satellite phones. Both geostationary ones and low orbit ones

Thanks for the clarification re: the 911 functionality.

My end-game wasn't necessarily about phone calls, though, but rather a quick location ping. I didn't know if the radio hardware in the phone was capable, technically, of hitting something that far away. Some quick checks suggest the dbW capability of a modern smartphone isn't enough, but I'm probably reading this wrong since my 6 year-old Garmin operates wholly independently of cell towers and just using those geostat sats, right? Is it just a hardware thing then, or is there some software/firmware action which could be done to tweak an old smartphone, say, into working as a standalone gps (assuming you could download and use a map program offline)?

 

[Jeff7]

A GPS module in a phone or navigation system doesn't transmit anything to the satellites. The thing you have is just a receiver.
GPS satellites contain extremely accurate clocks, and this is critical to the accuracy - now you're dealing with a speed-of-light signal, and if you want to be accurate to within a few feet.....well, light travels about 186,000 miles per second, so it won't take very long to traverse 10 feet. You're going to need a better stopwatch, and that's what the atomic clocks in the satellites permit.


Your receiver gets data signals from a few satellites and does some math based on the timestamps and location of the satellites to triangulate its position. The more satellites it can see, the better the position fix gets.

I've got an older Garmin module, and if I press and hold down on the signal indicator icon, it'll eventually open up a window that shows which satellites it can see, and their signal strength. With a few satellites locked, the position accuracy is 60-80 feet. As I get more satellites, the accuracy gets better, down to around 13 feet.

 

[gsaldivar]

I would think that the minimum power required for a terrestrial device to communicate with a satellite would largely depend on the sensitivity of the satellite receiver, and its ability to reliably detect a weak signal over large distance.

Standard cell phones (not satellite phones) communicate with cell phone towers, not satellites. If there is no cell tower nearby, there is no service, 911 or otherwise.

 

[serpretetsky]

Thanks for the clarification re: the 911 functionality.

My end-game wasn't necessarily about phone calls, though, but rather a quick location ping. I didn't know if the radio hardware in the phone was capable, technically, of hitting something that far away. Some quick checks suggest the dbW capability of a modern smartphone isn't enough, but I'm probably reading this wrong since my 6 year-old Garmin operates wholly independently of cell towers and just using those geostat sats, right? Is it just a hardware thing then, or is there some software/firmware action which could be done to tweak an old smartphone, say, into working as a standalone gps (assuming you could download and use a map program offline)?

Do some research on satellite phones.
Here's a link to bluecosmo FAQ for satellite phones
http://www.bluecosmo.com/satellite-phone-faqs/
you'll notice they have a couple of links to the actual phones and some battery life estimates. They don't seem that much larger/heavier/power hungry than regular cell phones. However, I have no experience with satellite phones and am not really knowledgable about them.

edit: oh, and like Jeff7 pointed out, GPS devices only receive signals, they don't send out signals.

 

[Nograts]

Thinking just in terms of pinging the satellite for location, say, every five seconds. Is this prohibitive for cell phones in terms either of power or transmission strength? My understanding is that these babies reside 22k miles up, which seems way too far for anything inside a phone to ping. And yet (also my understanding) also doesn't the 911 emergency function work outside of all cell tower range precisely on this basis?

Thanks for setting a gent straight.

I do SATCOM in the Army, a common phone we use in Iraq/Afghan was the Iridium phone (can't remember if that was the name of the company or what).

But when I am pumping user data through my dish we usually operate anywhere between -20 to -60 decibels RF. Geostationaries are 22.5k roundabout.

 

[Aluvus]

Thanks for the clarification re: the 911 functionality.

My end-game wasn't necessarily about phone calls, though, but rather a quick location ping. I didn't know if the radio hardware in the phone was capable, technically, of hitting something that far away. Some quick checks suggest the dbW capability of a modern smartphone isn't enough, but I'm probably reading this wrong since my 6 year-old Garmin operates wholly independently of cell towers and just using those geostat sats, right? Is it just a hardware thing then, or is there some software/firmware action which could be done to tweak an old smartphone, say, into working as a standalone gps (assuming you could download and use a map program offline)?

It's not clear what you are trying to accomplish. To answer your direct question, no, you cannot reprogram an arbitrary smartphone to function as a GPS receiver.

Some relevant information:

Geostationary satellites orbit at an altitude of 35,786 kilometres (22,236 mi) (they are in ~24 hour orbits).

GPS satellites orbit at an altitude of 20,200 km (12,550 miles) (they are in ~12 hour orbits).

Communicating with a receiver is not just about your broadcast power; the receiver's sensitivity, the directionality and gain of your antenna, the directionality and gain of the receiver's antenna, distance, interfering sources, motion of the devices, and atmospheric effects are all very important (google "link budget"). If you want 2-way communication, then the other device's transmitter and your receiver are also important. The good news when dealing with satellites is that, as long as they are at a high elevation angle, you don't have to go through a ton of the atmosphere.

Receiving and decoding GPS signals is not uniquely hard, but you do need receiver hardware for the appropriate band (primarily 1575.42 MHz) and the ability to configure the RF front-end and demodulation system appropriately (which a random phone is unlikely to be capable of).

You also need to interpret the message format (which is readily available) and you need some patience; GPS signals transmit information veeeeery slowly. And if you are starting from scratch you will need to receive the entire almanac of satellites before you can do much, which will take several minutes.

 

[Aluvus]

I do SATCOM in the Army, a common phone we use in Iraq/Afghan was the Iridium phone (can't remember if that was the name of the company or what).

But when I am pumping user data through my dish we usually operate anywhere between -20 to -60 decibels RF. Geostationaries are 22.5k roundabout.

Iridium is the name of both a satellite constellation and the company that manages it. Iridium satellites orbit at 781 km (485 mi), which is low Earth orbit (LEO).

LEO has the advantage of cutting down distance from transmitter to receiver, and (in their case at least) the disadvantage that you need a lot of satellites to achieve consistent coverage. The name Iridium comes from the original plan of fielding 77 satellites (atomic number of Iridium). They actually launched 66 (minimum for full coverage with their configuration) at a cost of $5-6 billion (depending on who you ask), then went bankrupt. A few years later some investors bought the remains for $25 million (one hell of a deal) and turned it into a profitable company. Little wonder; they got the existing constellation basically for free and have only launched a handful of satellites since.

If you're wondering, 66 is the atomic number for dysprosium, one of the lanthanides.

 

[Raswan]

This is exactly what I was looking for. Thanks for the extra information. I didn't know if decoding a GPS signal was mostly a function of software at this point. Apparently not. Thanks!