GPS Buying Guide

Updated with SUP500F on April 20th, 2010.

There's lots of options out there for a GPS receiver. This guide will hopefully help you navigate (cough) through the forest of options.

Go read about GPS on Wikipedia. Now how do we actually use this technology? 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. Is the example above, can you pick out the 4003.8914 longitude and 10512.5933 latitude? That's where SparkFun lives! We use GPS Visualizer to convert these numbers to something that google maps can understand. See 203710.000? That's the current time, accurate down to a few miliseconds. See 250309? That's today's date - March 25th, 2009. There's a bunch more data there (altitude, speed, heading, satellites in view, etc), but the key is that you can easily record this data to a CSV file or write a program (even on a microcontroller) to capture and evaluate the data.

Criteria to think about when shopping:

Size:

These modules are getting smaller every day but figure out what your application is. As a general rule, the smaller you make the module, the more likely you are to have antenna problems (longer lock time, less accuracy, etc).

Update Rate:

How often do you want an update? 99% of consumer devices (think in-car navigation) operate with a 1Hz (or once per second) update rate. Your car simply doesn't move fast enough that we need to know where you are on the globe more than once per second. There are some applications, such as planes and UAVs, where you need greater update rates. 1Hz was the standard. 5Hz and even 10Hz are becoming available and cheap. You can always configure a GPS receive to slow down, and update less often (1Hz) if your microcontroller or application can't handle all the NMEA data.

Power:

GPS units are taking in large amounts of timing data from the satellites and crunching it down. Then they output 'you are here'. They do a lot of heavy lifting, and therefore use some juice. The current average is around 30mA at 3.3V. 30mA may not sound like a lot, but it's a lot. This power is coming down over time and we hope to see it in the ~1 to 10mA range in 2010.

Watch out: Antennas use power. They often use an amplifier which can account for 20-30mA of current. If a module has really low power specs, it's probably because the module doesn't have an antenna built in.

Cost:

The price of GPS modules are all over the place. A good, 1Hz receiver is well under $100 and always dropping. Be sure to shop around.

Number of Channels:

This is a great sales ploy in my opinion. You'll see GPS modules that have 50 channels of tracking. Why!? There's only 24 GPS satellites, and it's impossible to stand on the globe and be in view of more than 12 at a time. All modules designed since 2008 have more than ample channel tracking (the ability to track multiple satellites at the same time). I don't even consider this when shopping.

Antennas - it's ceramic, point it up!

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 - the satellites are in the sky, 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.

For more information on GPS accuracy, see the GPS Tracking Comparisons Tutorial.

Our Picks:

Below we list our favorites first. Remember, we are completely biased. We've used a lot of these modules, and we apologize to the companies that spent a lot of effort designing them. This is just what we think...

SUP500F - the new kid on the block. This GPS receiver has so many great features.

• Runs on a new Venus634 engine
• Wide VCC: 3V to 5.5V
• Uses a standard 0.1" connector (yay!)
• Great configuration software (but is Windows only)
• Fast locking
• Up to 10Hz update rate
• PPS output
• Lower 33mA active tracking current

Cons:

• Can take a long time to lock when it is first used (fresh out of the box)
• Doesn't have a backup battery, but with new firmware almanac data can be stored in flash (so a battery can be skipped)
• Doesn't have a activity LED
• TX and RX are 3.3V TTL and are not rated to tolerate 5V

Other:

• Output is 9600bps

EM406 - the workhorse. This GPS receiver works very well.
Pros:

• SiRF III means it's pretty new and accurate
• There have been lots of projects written up using this unit
• Very compact - includes antenna
• Status LED
• 5V Power

Cons:

• 1Hz Update rate
• 5V power (if you're running a system at 3.3V)
• Technology is a year or two old
• Uses a mildly annoying connector

Other:

• Output is 4800bps