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 useGPS 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.
Star
# of
Power
t
Updat
Bonus
Module
Channe Protocol
Require
Tim
e Rate
Features
ls
d
e
Cold
: 38
 Breakout
NMEA
sec
Externa
Board Ava
0183,
3.3V,
19x19x2.54mm
12
1Hz
l
ilable!
Hot:
TSIP and
44mA
3
TAIP
Copernicus II
sec
(12 Channel)
Antenn
a
Dimensions
Type
Cold
: 38
sec
31.8x27.4x14m
12
(SMA) m
Hot:
Copernicus II
3
DIP
sec
(12 Channel)
(Sale)
Externa
l
Cold
: 29
sec
Externa
10x10x1.3mm
14
l
Hot:
Venus638FLP
1
x-L 20Hz
sec
(14 Channel)
NMEA
0183 TSIP 1Hz
and TAIP
NMEA0183
V3.01,
SkyTraq
Binary

All pins
broken
out!

Multipath
detection
and
suppressio
n
Jamming
detection
and
mitigation
3.3V,
44mA
20Hz
max
3.3V,
1Hz
29mA
defaul
t

Module
Antenn
a
Dimensions
Type
Chip
Cold
: 35
sec
30x30x10.7mm
48
Hot:
1
sec
NMEA
0183 V3
1Hz
and SiRF
binary
Chip
Cold
: 42
sec
36.4x35.4x8.3
20
mm
Hot:
8
sec
NMEA
0183 and
1Hz
SiRF
binary
Chip
35x8x6.5mm
Cold
: 27
sec
50
Hot:
1
sec
21x6x6.2mm
Cold
: 35
sec
48
Hot:
1
sec
EM-506
Receiver
(48 Channel)
EM-408 SiRF
III (20
Channel)
GP-635T
Receiver
(50 Channel)
Chip
GP-2106
SiRF IV (48
Channel)
GS407
Helical
(50 Channel)
Star
# of
Power
t
Updat
Bonus
Channe Protocol
Require
Tim
e Rate
Features
ls
d
e
23x9x42mm
Helical
w/ antenna
Cold
: 29
sec
50
Hot:
<1
sec
NMEA
0183
NMEA
V3.01
4.56.5V,
4555mA

Extremely
high
sensitivity
: -163dBm

Extremely
high
sensitivity
: -159dBm

High
Sensitivity
: -161dBm

Breakout
Board Ava
ilable!
Tiny!
3.3V,
75mA
3.3-5V,
1-5Hz
56mA
1Hz
1.8V,
65mA


NMEA,
UBX
binary
4Hz
3.3V,
75mA
Based
on the
high
performan
ce 50channel ublox 6
Module
Antenn
a
Dimensions
Type
Chip
30x30x5mm
LS20031 5Hz
(66 Channel)
SiGe GN3S
Sampler v3
Externa
l
66x20x8mm
(MCX)
Star
# of
Power
t
Updat
Bonus
Channe Protocol
Require
Tim
e Rate
Features
ls
d
e
platform
Cold
: 35
sec
66
Hot:
<2
sec
NMEA
0183 ver 5Hz
3.01
raw
intermedia
te
frequency
samples

track up to
66
satellites at
a time

Fully
enclosed
RF Frontend
Calibrated
by
manufactur
er
designed
to directly
capture
low-level
signal data
3.3V,
41mA

USB
