GPS Satellites

How many Global Positioning Satellites are orbiting the earth? Were all of them put into orbit by the United States? Are any privately owned?

http://www.gps.gov/systems/gps/space/

The things I learn here. I had no idea that the dumbing-down of civilian GPS signals had been ended.

There are several Other Systems by Russia, the EU, China, Japan, and India that cover all or parts of the world.

Russia’s GLONASS is the only system with comparable coverage and resolution to GPS. All the others are either still in development or have very limited coverage areas.

Garmin (and probably others) makes GPS receivers that listen to both the American and Russian satellites, presumably for increased accuracy or redundant coverage.

Another neat thing about GPS is that it’s not just those satellites working alone. If you have a good enough receiver, they can be augmented by both additional corrections satellites (WAAS) and/or accurately-surveyed ground stations. See the wiki page or this PDF on “What method of differential correction works best?

Has there ever been any attempts, or is it even possible, restrict usage of U.S. satellites? For example, the restriction of usage from within Iranian borders?

http://en.wikipedia.org/wiki/Selective_availability#Selective_availability

No, it’s not really possible. Satellites don’t work this way. It’s like trying to block the sun in a specific country.

I suppose you can try to encrypt the signal to limit decryption to certain devices, but that defeats the purpose of a publicly available system, and Iran would simply smuggle them in anyway.

You can send in a blocking or deliberate signal, but that would affect a targeted area, rather than a whole country. I guess, technically, you can extend this to cover most of a country, but that involves lots (LOTS) of equipment dedicated to the purpose, and you’d have to be careful near borders. It’s like the sun example. You can block the sun in limited, small areas, but extended blocking over large areas is considerably more complex and expensive.

I would thing that if the USAF or Israel would be shooting at anything within Iranian borders, they would like for the GPS satellites to work. :slight_smile:

The military wasn’t affected by the selective availability. That was the whole point.

It’s absolutely possible and the US has done exactly that, which is what prompted countries like Russia and China to create their own systems. GPS was originally designed for military use, so selective availability was critical. See Machine Elf’s link right above your post.

And that’s why export controls exist. Even if Iran imported civilian GPS receivers, SA makes it so that ALL civilian users in that whole regional area will be subject to artificial error. You need special decrypting GPS units, normally available only to military users, to get the full accuracy. One would hope Iran would not be able to get their hands on large numbers of US military GPSes.

But as additional constellations come online, what the Americans do with our own satellites matters less and less, since they can simply get the signals from other nations’ satellites. That’s where GPS jamming, hijacking, etc. come into play.

Not really. It doesn’t actually limit access to satellites but introduces artificial error on the receiver end. Worse, the old selective availability could be engineered around with some basic engineering knowledge (we were “unofficially” shown how to do so in school - obviously this was before 2000 when the limited civilian availability went away). That was another reason the civilian limitations were removed. If smart engineers in other countries could get around limitations, there’s no longer a point in maintaining it.

The new selective availability system is more of an external system. It’s not built into receivers themselves. And something like it was coincidentally most likely used by Iran to bring down a US UAV. Actually, I think they used a GPS signal spoofer, but a denial of service system would be easier to build.

Maybe it’s just semantics now, but I think “limited accuracy” is functionally equivalent to “restricting usage” of satellites. Yes, it’s not the actual satellites that are blocked, but their signals are artificially degraded – the same end effect on the end-users. If the US were so willing, the entire unencrypted GPS broadcast over an area could be turned off so that only authorized users can use the encrypted data. I don’t believe they’ve done that so far, but it would only take a software update.

As for engineering workarounds, please do share. I’m curious. Does it have to do with the DGPS base stations outlined in the wiki article?

I thought selective availability referred to the artificial error introduced by the satellite software/signal that affects all non-decrypting receivers. Could you clarify what you mean between the “old” and “new” SA?

It’s been speculated that the drone was brought down by GPS jamming, which altogether denied GPS signals to the drone and made it commence a “fly home” routine, which then either flew along a predictable path or a spoofed path… so maybe a combination of both denial of service and spoofing. Presumably they overwhelmed the encrypted signal with strong noise, forced the drone to tune into the civilian unencrypted signal, and then took over that frequency with a local base station. But that’s not quite the same as SA sending both an encrypted and unencrypted signal, is it?

Hypothetically, could you turn off the satellite’s broadcasting as it entered a certain part of its orbit, and turn it back on as it exited it, to deny GPS signal to a certain area? I guess that wouldn’t work because we’d want friendly GPS devices to work and enemy ones not to, but I’m curious if that’s a viable option.

I would think they were wired to cause a nasty accident if someone tried to take one apart.
Apparently not.

And if the non-encrypted GPS satellites in my Ratsnestistan airspace went dark, I’d be very worried. :slight_smile:

Think headlight. It produces a wide cone of light. You can’t turn it off to block light to one spot on the wall without turning it off for everybody else.

That’s one way of doing it.

But we were shown some basic electronics you could add to a receiver to correct for the artificial time delay added to the signal. It required occasional recalibration though.

There was also a more automated way, but it required having more than 3 GPS satellites in range - the more the better. You could start averaging out errors once you had 4 or more signals to work with.

Apparently, you could get a bit more fancy and add in some receivers for additional sideband information, but I didn’t follow that part very well. The GPS signal itself is pretty interesting. Parts of the bandwidth are reserved for encrypted military comms only. And there’s signal that nobody uses.

The military has a method that only affects a targeted area. It’s independent of receivers. I guess it’s not technically called selective availability, but that’s the end effect. It jams up enemy GPS without affecting military units. Basically a fancy form of jamming.

Wow, a lot of people really don’t know how satellites work. There is no such thing as an antenna that can cover a huge swath of the Earth. Well, at least not if you want accuracy. Each satellite typically has multiple transponders which cover a portion of the satellite foot print. So, in theory, you could calculate which satellite(s) would be covering a specific area and turn off those corresponding transponder(s). You could, then, deny GPS to that area. With extreme accuracy? No. You’d probably be blocking out huge portions of a country but it would work.

Also, the military has ITS OWN BAND so denying GPS to civilian GPS receivers would in fact be totally possible and have no impact on any US military operation.

But, would it actually do anything? Probably not. There are many DGPS stations around the US (Coast Guard has bunches), Africa, the Med, EU, asia, etc. So many to list. a DGPS station is extremely accurate as they don’t move. The deviation of the satellite are has to be accounted for, plus, the atmosphere and other factors can make it difficult to get an accurate position, even with military GPS units.

The only real limitation that I know of when it comes to GPS is the speed at which the receiver can process the signal. This actually imposes a limit as to how fast the GPS receiver can be moving, basically, you could not built a cruise missile with a off-the-shelf GPS unit. A cruise-glider, yes. :slight_smile: I think the limit is about 200 MPH and commercial aircraft are an exception.

Geosynchronous satellites have a range of directional antennae tailored to the needs of the customers, and have definable footprints. Because they are fixed in position in the sky overhead this is viable.

GPS satellites orbit in a mid range, and are not geosynchronous, so move overhead, but are not low earth orbit. This means that at any time any satellite can be essentially anywhere in the sky from your viewpoint (including under your feet.) For best accuracy you want the satellites to be well distributed around you - you get pathological accuracy problems if they are all clustered together, or form a line in the sky. Swichable antennae might be useful to limit the footprint, but it would need to be done dynamically and would be very broad brush. Especially when the satellite is low in the sky relative to the receiver. (GPS satellites don’t have transponders, they simply transmit.) A highly coordinated system of switching areas, where care it taken to keep enough satellites in the sky visible to friendly countries, and limit the number visible - or usefully visible - to less friendly ones might be viable. With a full enough constellation this may be reasonable, but if the constellation is sparse, there will not be the flexibility to do this.

The way the military GPS works is neat, and for full precision requires that the civilian channel remain in operation. Denial of operation doesn’t need to be done by turning off the C/A channel, rather they could change the keys.

The limit on civilian receivers is dual - max speed of 1000 knots and max altitude of 18,000 metres. There is some argument about whether this is any one, or both conditions together, but civilian receivers will stop providing results when they decide the limit is exceeded. This is both supersonic and much higher than ordinary aircraft fly, and is directed at missiles. High altitude unmanned balloon enthusiasts seek out the limited set of receivers that regard the limit as both together.

A cruise missile is however very viable with civilian GPS, and the prospect probably keeps some security professionals awake at night.

I thought this was due to artificial export restrictions placed on US-sold receivers to prevent them from being used in long-distance guided munitions, rather than any inherent trait of the receiver.