GPS Buying Guide

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GPS Buying GuideAbove your head, right now, at an altitude of about 20,200 km there is asystem of navigation satellites. There's 35 of them, 24 active at any giventime. It cost millions and millions of dollars to put those satellites in thesky 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 exactcoordinates of your current location so that you can figure out where you are. Combine that information with a goodmap 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 yourproject, 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 moduleworks 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 oldterminal program. Is the example above, can you pick out the 4003.8914 longitude and 10512.5933 latitude? That'swhere SparkFun lives! We use GPS Visualizer to convert these numbers to something that google maps canunderstand.

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 themodule 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 mostdevices is 1Hz (Only once per second). The fact is, unless you're on an airplane or something, you're probably notgoing fast enough to have changed position significantly in the past second. However, UAVs and other flying or fastvehicles may require faster update rates to stay on track. 5 and even 10Hz update rates are becoming more and moreavailable for cheap. Keep in mind, though, that a fast update rate means that there's more NMEA sentences flying outof the module, some microprocessors will be quickly overwhelmed trying to parse that much data. On the plus side, ifyou 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 anduse that information to figure out where you were, you'd flat out refuse. It's a lot of work, and yet that's exactly whatthese tiny GPS units are doing (multiple times per second!) so they can use a lot of power. On average, around 30mAat 3.3V. Keep in mind, also, that GPS antennas usually enlist the help of an amplifier that draws extra power. If a unitappears 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 yourmodule runs will affect your time to first fix. Since the module doesn't know which satellites are in view, the morefrequencies that you can check at once, the faster you'll find a fix. After you get a lock, some modules will shut downthe extra blocks of channels to save power. If you don't mind waiting a little longer for a lock, 12 or 14 channels willwork 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 ofceramic. 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 ceramictowards 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 awindow - 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, within30 seconds, down to +/- 10m. Amazing! I say +/- because it can vary between modules, time of day, clarity ofreception, etc. Most modules can get it down to +/-3m, but if you need sub meter or centimeter accuracy, it gets reallyexpensive. I've heard stories of such fabled GPS receivers, but I have never gotten to touch one. Someone pleaseprove us wrong.

Module AntennaType Dimensions Start

Time# of

Channels Protocol UpdateRate

PowerRequired

BonusFeatures



12 ChannelCopernicus IIGPS Receiver

External 19x19x2.54mm

Cold:38sec

Hot:3 sec

12NMEA0183, TSIPand TAIP

1Hz 3.3V,44mA

BreakoutBoardAvailable!

12 ChannelLassen IQ GPSReceiver withDGPS

External

(H.FL-RF)

26x26x6mm

Cold:50sec

Hot:10sec

12NMEAStandard at2400bps to19200bps

1Hz 3.3V,26mA

Very smalland LowPower

14 Channel10Hz GPSReceiver -Venus634FLPx


Cold:29sec

Hot:1 sec

14

NMEA-0183 orSkyTraqBinarysentences at9600bps to115200bps

10Hzmax

1Hzdefault

3.3V,28mA

BreakoutBoardAvailable!

14 Channel20Hz GPSReceiver -Venus638FLPx-L


Cold:29sec

Hot:1 sec

14

NMEA-0183V3.01,SkyTraqBinary

20Hzmax

1Hzdefault

3.3V,29mA

MultipathdetectionandsuppressionJammingdetectionandmitigation



20 Channel EM-406A SiRF IIIReceiver withAntenna

Chip 30x30x10.5mm

Cold:42sec

Hot:1 sec

20NMEA0183 andSiRF binary

1Hz 4.5-6.5V,70mA

Extremelyhighsensitivity :-159dBm

20 Channel EM-408 SiRF IIIReceiver withAntenna/MMCX

Chip 36.4x35.4x8.3mm

Cold:42sec

Hot:8 sec


1Hz 3.3V,75mA

Extremelyhighsensitivity :-159dBm

20 ChannelLS20126 GPSReceiver

Chip 29.4x12.4x10.6mm

Cold:36sec

Hot:1 sec


1Hz 3.3V,31mA

GPS +magneticsensor + 3-axialaccelerationsensor

32 Channel SanJose NavigationGPS 5HzReceiver withAntenna

Chip 30x30x8.5mm

Cold:41sec

Hot:1 sec

32 NMEAV3.01

5Hzmax

1Hzdefault

3.3-5V,59mA

Verysensitive, wewere able totrack over 9satellitesindoors!



48 Channel GP-2106 SiRF IVGPS Receiver

Chip 21x6x6.2mm

Cold:35sec

Hot:1 sec

48 NMEAV3.01 1Hz

1.8V,65mA

BreakoutBoardAvailable!Tiny!

50 ChannelD2523T HelicalGPS Receiver

Helical 22.9x8x50mmw/ antenna

Cold:29sec

Hot:<1sec

50NMEA,UBXbinary

4Hzmax

1Hzdefault

3.3V,43mA

Based on the highperformance50-channelu-blox 5platform

50 ChannelGS407 HelicalGPS Receiver

Helical 23x9x42mmw/ antenna

Cold:29sec

Hot:<1sec

50NMEA,UBXbinary

2Hz 3.3V,75mA

Based on the highperformance50-channelu-blox 5platform

66 ChannelLS20031 GPS5Hz Receiver

Chip 30x30x5mm

Cold:35sec

Hot:<2sec

66NMEA0183 ver3.01

5Hz 3.3V,41mA

track up to66 satellitesat a time

66 Channel UP-501 GPSReceiver

Chip 22x22x8mm "veryfast" 66 NMEA

protocols

10Hzmax

1Hzdefault

3.3V,30mA

Configurablefix rate, upto 10Hz



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SparkFun is an online retailstore that sells the bits andpieces to make yourelectronics projects possible.Whether it's a robot that cancook your breakfast or aGPS cat tracking device, ourproducts and resources aredesigned to make the worldof electronics moreaccessible to the averageperson. In addition toproducts, SparkFun alsooffers classes and a numberof online tutorials designedto help educate individuals inthe wonderful world ofembedded electronics.

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SiGe GN3SSampler v3

External(MCX)

66x20x8mm rawintermediatefrequencysamples

USB

Fullyenclosed RFFront-endCalibratedbymanufacturerdesigned todirectlycapture low-level signaldata

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GPS Buying Guide