Entirely aside from my tactical land navigation and GPS training efforts, I have been working on or with location based services for mobiles since around 1999. I have this pretty well baked into my brain, but not everyone does, and so it's a topic I had to expound about with some of the new guys lately.
Being complex enough (and changing over time) that I actually messed up part of it off the top of my head the other day, it's certainly too much for everyone to just listen to, so I decided it was a good thing to sit and write in some detail. I'll also be posting it to the D4M mobile design wiki when done, and will presumably remember to update it periodically.
Edit: This has been modified and posted to the wiki at this location. So, if you disagree (for example, because you are reading this years later) go there and edit the page to make it more accurate and up to date.
The most important thing is to never, ever, ever conflate "GPS" with "location." After that, it's best to dive straight into the individual technologies. I like to break down location services by (approximately) least to most precise:
Cell
Mobile telephony is based on communications and handoff between cells, each of which is (generally) controlled by a single tower with multiple antennas. The tower location is well-known, and its ID is attached to the CDR, so it can be detected.
This is a fallback, when other things are not available. Better technologies are generally not available when roaming, and the data is not or cannot (technology or politics) shared at more precision with your home network or because you are from another operator.
Precision will vary widely by the size of the cell, and there is no (good, universally-accepted) method to determine and communicate precision and accuracy. My rule of thumb – and something similar is generally used by devices – is a circle 3000 yds (m) across.
Sector
Recall that each tower has multiple antennas. Those three (or four, rarely anything else, depends on the carrier) antennas point different directions. Each of those is an independent mobile radio sector; just like signals are handed off between cells, they are handed off between sectors, and the data is generally known to the handset and can sometimes be shared with services or providers. However, since the direction the sector faces is really only known to the company that installed it, and the network operator, they are the only ones that can use the data well.
Precision will vary widely by the size of the cell, and by the antennas used, the number on the cell, the down-angle, and so on. There are some semi-useful ways of determining precision and accuracy for a single read, but they are not univerally implemented and there is no way to send the data to the information providers (e.g. the map program). My rule of thumb – and something similar is generally used by devices – is a circle 1200 yds (m) across, co-located with the radio centroid of the sector. Yes, sectors are not circles, but it's also hard to communicate shape and orientation, so this works.
Triangulation
Generally, triangulation is the practice of determining the location of a point by measuring the angle and/or distance from several known locations. More points give more precision. It can get more complex when you try to find points in 3D space, but all mobile triangulation systems I have known assume the earth is locally perfectly flat, and simply find the location on the geoid. Methods of triangulating get really complex and vary by network type and equipment available; multilateration and signal interpolation are some of those. The math can get pretty harrowing.
Precision varies mostly by the number of sectors being used, the distance between them and so on. Though I cannot recall what it is, I believe there is a method of communicating to services employing it an approximation of the precision available. In any remotely built-up area, where it most often has enough sectors to work, a circle 50 yds (m) across is typical. I have seen 12 yd precision, with good accuracy.
GPS Telemetry
The Global Positioning System consists of a ground based control system, a series of satellites and any number of anonymous receivers. So, reading that alone dismisses many misconceptions; its one-way, it's a system, and the signal is from satellites and has nothing to do with mobile networks.
Telemetry means data sent from a remote (usually mobile) site like a rocket, back to base. Here, it means the device figures out where it is by listening to the satellites, then tells the network or website or whatever where it is.
Ignoring SA GPS can give precision to... well, fractions of an inch at least. But those are big, car-expensive devices used for special surveying and so on. Portable, hand-held receivers like those in mobile handsets can generally be assumed to give precision of 20-50 ft.
There are other systems, like Galileo, GLONASS, and COMPASS but none of these are really fully-active, and there are few or no receivers in mobiles as yet. In theory, eventually, using multiple unrelated systems will give better accuracy yet, and give better coverage when in touchy situations like in dense cities.
WAAS
GPS uses radios, and the atmosphere and earth are imperfect and mess with them. WAAS is a satellite based augmentation system based in North America (others exist in the rest of the world, but are less well-established) that sends additional signals via another set of satellites to allow GPS receivers to adjust for those inaccuracies. I know of no mobile phones that have a WAAS receiver, but it's there.
AGPS
Assisted GPS requires the use of mobile networks, so only works when in data communication range. The cell tower has its own GPS (which many use anyway to get precise time codes), probably with a WAAS receiver. This provides a bit more precision, assures it of more accuracy, but mostly helps speed up cold or warm starts. Briefly, for a GPS to work, the receiver has to download data about all the satellites before it can calculate the position. AGPS caches this info for the phone, and sends over just the relevant bits, cutting minutes to a second or two. Which helps.
This is a reason you may not see WAAS, and might see some benefits from other GNSS (say, GLONASS) before they bother getting receivers into the handsets.
These generally give precision, even in dense areas and on the move of 10 ft or better. I have seen 2 ft indicated precision and have no reason to doubt it.
AGPS is not generally selectable as a separate service; when the user chooses to turn on or off "GPS," and has a network connection (and an AGPS compliant phone, etc.) they get the advantages of AGPS.
WLANs & PANs
Local networks like WiFi and Bluetooth are not exactly more precise, but I stuck them at the bottom of this list because they are local, and can add another layer of location, so could increase precision. They can also be used instead of other technologies when there is no GPS signal, or a bad mobile signal so triangulation cannot be used.
Both of these currently use the "cell" (or network identifier) and triangulation methods described above. There are no sectors, and no universally-adopted AGPS system that I am aware of. Naturally, a handset getting good GPS data can send that telemetry to anyone over any network, including WiFi, but that's not what we're talking about here.
They are both relatively more rare, and often are supported by third party software or individual services, and are not universally implemented on the handset. I have no reliable numbers on the real-world precision, but tests of WiFi triangulation alone have gone down to inches. On the other hand, poor network identification has led to placing users on the wrong continent. There's room to improve here.
Ask
People often know where they are. If you cannot get any location, or there's a reason they might want to over-ride it (even as simple as the data is bad) let them enter a location. If you do this, allow lots of methods. Only accepting ZIP or postal code is not useful for e.g. travellers, who do not generally know the ZIP where they are.
All the above is pretty well known by network engineers, and chip-makers and various low-level handset software guys. But it is often missed a bit by various software implementations (and certainly by marketing). All too often GPS is navigation, such that turning it off kills all location services.
Happily, Android seems pretty good about this, and just gives poorer precision when you turn the GPS off, but it does allow location services to work.
Which brings up a good point. Precision vs. accuracy is often also messed up, whether its confusing the two terms or just not communicating well. A quick primer: precision is the number of decimal places you measure something to; accuracy is how correct it is. The less accurate you think your measurement is, the less precisely you should report it.
Following straight on from precision and accuracy is appropriateness of information. I believe that the biggest problem with turn-by-turn directions is that they assume perfect location precision, and say "you have arrived" instead of "it's the big, white building on the north side of the street," because maybe it's 30 yards behind you, or 50 in front.
The other problem with driving directions is bad data. If your map might be wrong, or imprecise or inaccurate (and they all might be) then think about this when planning the interaction. Don't assume your data is perfect, and give contextually-helpful information. Let users note problems, and do something with the problem reports.
Speaking of data (though it rambles, this thing just writes itself), where does that map data come from? Well, it depends. On my handheld GPS unit, most in-car units and some handsets now, it's in the device. But too many people (including designers) are so used to mobile phone maps they think GPS is AGPS, and only works when you can get network location, network maps, etc. Wrong. And a bad assumption. Why don't people want to know where they are when coverage is spotty or non-existent? Well, if your service or app has that use case, carry through on it.
And one last hint is to not confuse location with navigation. Turn-by-turn directions is not the end-all be-all of LBS. I don't know what is, and neither do you. Location is an enabling technology (or set of them, I guess) and if there even is a killer app, we haven't seen it yet. More likely, it's going to be like simply being on the internet, and will become so baked into our mobile experiences that it will become invisible to everyday people.
While I am sure the commoditization of location service annoys operators and handset makers, it's going to be a a good thing for the end user.
So, to wrap up with a simple guide to design and development of location services, remember that:
- Every phone is location enabled. GPS or not, working or not, there's some location available. And if you are giving something like a weather report or local news, a few thousand yards is practically pinpoint precision.
- All the available location technologies must be addressed when designing your application or service. Don't shut off the service because the GPS is off.
- Precision and accuracy must be understood by designers, and correctly exploited by the product. When the GPS is off, or doing badly, show that circle so it's clear that we don't know /exactly/ where you are. Say "turn right in about 1/4 mile" and so on.
- If you work for a carrier, exploiting the network like this should be a snap. If not, your devices or software may or may not be able to be talk to the phone enough, and you might need to negotiate with the carrier or find a third party provider.
- Consider what to store locally. Does the handset really need to be on the network, or should you give some functionality when the connection is bad or unavailable?
- Though I didn't mention it above, look useful without acting creepy. And don't just follow the letter of the law. Be careful with your user's data, share only what is needed, and store only what you really have to. Often, these last two are answered as "nothing." If that is the decision, make sure it's implemented as such, and no one gets lazy with the development or doesn't get the memo.
