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Forget GPS, Hello WPS
No France writes "A company known as skyhook wireless has announced the commercial availability of its Wi-Fi Positioning System, or WPS. The company has compiled a database of every wireless access point it can find in a given city. When a mobile user running th Skyhook client is in a recorded area, their position is calculated by selecting the surrounding signals and comparing them to the reference database. Currently there are 25 US cities mapped, including New York City, Boston, Chicago, Los Angeles and San Francisco. Apparently this device is accurate to within 20-40 meters, though one has to wonder how well it deals with people moving their wireless access points."
PlaceLab has been doing this for a while, and it's free.
Martin May
Road naming is non-existant outside major thouroughfares; it works more by an irregular grid numbering system of blocks, not the roads in between them. House numbering is similarly vague, with no guarantee that house number 2 will be beside number 3 or 4. Block nameplates are usually pretty small and not in easy-to-predict places very often; GPS, even for pedestrians, is very useful in Japan. Even the taxi drivers haven't a clue where most places are!
Basically, the military figured out how to easily jam GPS in an area. But before then, there were GPS field units available that averaged out the error and got better than 2-3 meters so that it didn't really matter that much...
I'm not sure what kind of system you're working with (or how old), but the system I linked to above is *very* roughly $3500. The software would add another $1500. So for about $5000, you have submeter accuracy. Multipath is still a problem, yes, but there have been great advances in solving that, too.
GPS has come down in price incredibly in the last few years. You don't even need a subscription to a DGPS service anymore.
"Equal bytes for women!"
Access point MACs are (mostly) unique.
Sub-5-meter accuracy: As another poster pointed out, SA was turned off a while ago. Pretty much any GPS unit will be accurate to within 5 meters if it has a decent signal. With tricks like code smoothing, most errors are probably less than 3-4 meters even for a moving receiver.
Sub-meter accuracy: A little bit of position averaging + basic DGPS makes this easy for a stationary receiver, even when SA was on DGPS could cure the intentionally added errors. Very difficult to use with a moving receiver unless combined with an inertial navigation system. (Rare except in modern airplane navigational systems)
Millimeter accuracy: Also possible before SA was turned off, but required the receiver to be stationary for a long period of time, and required significant postprocessing of the data using a variant of DGPS. It still requires stationary receivers for nonmilitary systems.
About the only thing that can't be done without a method for decrypting the P code is sub-centimeter positioning of a moving object. Even with the P code available it can't be done without combining a high-grade inertial navigation system with the GPS system.
retrorocket.o not found, launch anyway?
But vanilla GPS just isn't accurate in urban canyons. In London's docklands area (high buildings) GPS gives accuracy in the 200 to 400m region due to multipath effects. As well as obscuring satellites, tall buildings cause reflections of GPS signals which can cause large errors in the pseudo-range calculations making accurate position reporting very difficult. Places like NYC or Chicago are useless for GPS.
I would imagine that supplementing GPS with other position determining mechanisms (like WiFi) could be beneficial in these circumstances.
The client software running on the tracked device measures the signal strength of the access points, forwards the data to the server which calculates the position. The big-brother scenario is avoided as long as you still have to install the client yourself.
The major drawback of the system is that it needs extensive calibration, since they are using not only the available access points, but also the signal strength of these. Normally they suggest calibration in a 5x5m (15x15ft) grid. More calibration points yield a more accurate result.
And now the piece of information you have all been waiting for: accuracy. With a good calibration this can yield accuracies of arround 1m. In my tests (indoor) the accuracies fluctuate a bit, but is at least better than 3m 95% of the time.
Just as the system described in the original post, Ekahau requires no extra hardware (we already have 2-300 APs on campus).
Since a large number of people would check "Do not broadcast the SSID" following their manufacturer's manual on security,
What manual says to do this? Turning off SSID broadcast is *not* a security measure in any sense, at all.
For one thing, the SSID is included in every single packet that the access point sends out. Period. So getting it is easy with or without the SSID broadcast.
For another thing, turning off SSID doesn't prevent anybody from connecting to the network. It will prevent stupider displays like Windows's wireless page from showing that access point as available, assuming no other AP has the same SSID being broadcast, but if they select that SSID from another AP or if they put it in manually, then they'll connect to your network just fine.
If you want security, enable WPA. Turning off SSID, filtering by MAC address, these are not security related adjustments, and add precisely nothing to your overall security strategy. They might be a way to keep your idiot neighbor from connecting to your network by accident, but they won't keep anybody from connecting to your network on purpose.
- Give a man a fire and he's warm for a day, but set him on fire and he's warm for the rest of his life.
> Let's say I'm within range of 50 access points all called 'Netgear'.
> Where am I?
You do know that access points have MAC addresses, don't you? Every single MAC address is globally unique. They have a database of those, _not_ of the names.
Note that this database is going to get stale quickly, as people turn on new APs, move existing ones, or upgrade broken ones. Still, I think it's a great thing to keep in your arsenal of positioning tools.
Even better... the more sophisticated systems use some other data to improve accuracy: signal strength, and known mobility patterns. You may be able to get a better indication of where you are by examining the relative signal strength of nearby APs compared to the last known survey; the known mobility patterns will adjust if the algorithm places you somewhere you're not likely to be. e.g. if you're moving faster than 5 m/s you are probably not in a building (unless you've seriously taken a wrong turn).