GPS Buying Guide
Above your head, right now, at an altitude of about 20,200 km there is a system of navigation satellites. There’s 35 of them, 24 active at any given time. It cost millions and millions of dollars to put those satellites in the sky and link them appropriately and you know what they’re doing?
They’re helping you find that place your friend told you about that one time.
The GPS, or Global Positioning System, is accessible from almost everywhere on Earth and provides exact coordinates of your current location so that you can figure out where you are. Combine that information with a good map and there’s nothing you can’t find. But what if you want that uncanny sense of direction in, say, your pet robot? Good news, GPS modules are small, light weight and inexpensive. They’re also pretty easy to use.
There are a ton of GPS modules on the market these days and it can be hard to figure out what you need for your project, hopefully this guide will demystify GPS a little bit and get you on the right track.
Actually, tell you what: Go read about GPS on Wikipedia, it’s a great article to get you familiar with the technology. Our job is to teach you how to use it… Now how do we actually use it? It’s gloriously simple. Every GPS module works the same: power it, and within 30 seconds to a minute, it will output a string of ASCII characters like this:
What is all that? Those are NMEA sentences. You can view the text coming out of these GPS modules using any old terminal program. In the example above, can you pick out the 4003.8914 N latitude and 10512.5933 W longitude? That’s where SparkFun lives! We use GPS Visualizer to convert these numbers to something that google maps can understand.
Sounds Awesome, Where Do I Start?
There are a lot of options when it comes to GPS modules so it can be hard to just pick one and get hacking. The size, update rate, power requirements, these are all features that you’ll want to look into before you pick.
Size
This is something you need to consider if your project is supposed to be pocket-sized. GPS modules are getting ever-smaller (Your tiny, tiny cell phone has one in it!) but remember that in general, the antenna has to shrink to fit the module which will affect things like lock time and accuracy.
Update Rate
The update rate of a GPS module is basically how often it recalculates and reports its position. The standard for most devices is 1Hz (Only once per second). The fact is, unless you’re on an airplane or something, you’re probably not going fast enough to have changed position significantly in the past second. However, UAVs and other flying or fast vehicles may require faster update rates to stay on track. 5 and even 10Hz update rates are becoming more and more available for cheap. Keep in mind, though, that a fast update rate means that there’s more NMEA sentences flying out of the module, some microprocessors will be quickly overwhelmed trying to parse that much data. On the plus side, if you have a module that runs at 5 or 10Hz, it can usually be configured to run at an easier pace.
Power Requirements
If someone asked you to crunch a bunch of numbers that you had to get from satellites in orbit around the Earth and use that information to figure out where you were, you’d flat out refuse. It’s a lot of work, and yet that’s exactly what these tiny GPS units are doing (multiple times per second!) so they can use a lot of power. On average, around 30mA at 3.3V. Keep in mind, also, that GPS antennas usually enlist the help of an amplifier that draws extra power. If a unit appears to have super-groovy-low power consumption, make sure there’s an antenna attached.
Number of Channels
Even though there are only so many GPS satellites in view at any given time, the number of channels that your module runs will affect your time to first fix. Since the module doesn’t know which satellites are in view, the more frequencies that you can check at once, the faster you’ll find a fix. After you get a lock, some modules will shut down the extra blocks of channels to save power. If you don’t mind waiting a little longer for a lock, 12 or 14 channels will work just fine for tracking.
Antennas
Many modules come with this chunk of something on top of it. What is that? That is a precisely made chunk of ceramic. Each antenna is finely trimmed to pickup the GPS L1 frequency of 1.57542 GHz. Sound expensive? Well, they make a lot of them. There are some other GPS antenna technologies (chip, helical), but they are not as common, a bit more expensive, and require significantly more amplification and filtering.
Oh hey - as I mentioned, the satellites are in the sky like… 12,552 miles above you, so be sure and point the ceramic towards the sky, ok? GPS antennas are getting better, and you can certainly get GPS signal indoors, but it’s hit-or-miss. I hear there are reception problems in the urban canyons of places like New York City. If you can get near a window - it will help a lot.
Accuracy
How accurate is GPS? Well it varies a bit, but you can usually find out where you are, anywhere in the world, within 30 seconds, down to +/- 10m. Amazing! I say +/- because it can vary between modules, time of day, clarity of reception, etc. Most modules can get it down to +/-3m, but if you need sub meter or centimeter accuracy, it gets really expensive. I’ve heard stories of such fabled GPS receivers, but I have never gotten to touch one. Someone please prove us wrong.
| Module |
Antenna Type |
Dimensions | Start Time |
# of Channels |
Protocol | Update Rate |
Power Required |
Bonus Features |
|---|---|---|---|---|---|---|---|---|
|
|
External |
40.6mm x 33mm (1.6in x 1.3in) |
Cold: 29 sec Hot: 1 sec |
-- | PVT, RTK | 1Hz | 5V or 3.3V but all logic is 3.3V |
|
|
|
External |
19x19x2.54mm |
Cold: 38 sec Hot: 3 sec |
12 | NMEA 0183, TSIP and TAIP | 1Hz | 3.3V, 44mA |
|
|
|
External (SMA)
|
31.8x27.4x14mm |
Cold: 38 sec Hot: 3 sec |
12 | NMEA 0183 TSIP and TAIP | 1Hz | 3.3V, 44mA |
|
|
|
External | 10x10x1.3mm |
Cold: 29 sec Hot: 1 sec |
14 | NMEA-0183 V3.01, SkyTraq Binary |
20Hz max 1Hz default |
3.3V, 29mA |
|
|
|
Chip | 30x30x10.7mm |
Cold: 35 sec Hot: 1 sec |
48 | NMEA 0183 V3 and SiRF binary | 1Hz | 4.5-6.5V, 45-55mA |
|
|
|
Chip |
30x30x5mm |
Cold: 35 sec Hot: <2 sec |
66 | NMEA 0183 ver 3.01 | 5Hz | 3.3V, 41mA |
|
