How handheld GPS units work
The Global Positioning System (GPS) provides precise time and location data
by using 24 Navigation Satellite Timing and Ranging (NAVSTAR) satellites.
Each satellite completes one Earth orbit every 12 hours, continuously transmitting
its position and a time signal, which a GPS receiver can pick up. A handheld
GPS unit must receive data from four or more satellites in order to calculate
a user's position, typically within 50 to 100 feet. Many factors can have
an impact on accuracy, including atmospheric conditions, buildings, tunnels
and heavy foliage. However, reviewers say that the latest high-sensitivity
GPS receiver chips all but eliminate those issues.
Most GPS units use the Wide Area Augmentation System (WAAS). This system
is based on a network of 25 ground-reference stations. Each station receives
GPS satellite signals, corrects any errors, and then forwards corrected signals
to a Wide Area Master Station, which makes some additional calculations and
uploads the newly corrected data to a satellite. The message is finally broadcast
on the same GPS frequency and picked up by GPS receivers capable of reading
WAAS signals that are within the broadcast coverage area of the WAAS. The
difference in accuracy can be significant: a WAAS-enabled GPS receiver can
fix your position to less than 10 feet. While that might not make a big difference
if you're driving in a car, it's a big deal if you're on the ground trying
to find the turn to your base camp.
Most handheld GPS units can store waypoints, which are coordinates for a
selected location. You can program waypoints while you're traveling, or you
can program them before you leave home or camp. These markers aid the unit
in plotting the routes that interest you -- sort of a virtual breadcrumb
Here's what experts say to look for when considering a handheld GPS unit.
Also see our companion report on auto GPS receivers and a separate
report on sports watches, which covers GPS-ready watches.
- Determine the map set and degree of
accuracy you will need. Different models come with different types of
maps, and some map sets are more detailed than others.
- Avoid unnecessary high-end
features. You can save some money if you do not need features such
as a barometric altimeter, electronic compass or heart-rate monitor, all
of which are found in costly, top-end GPS receivers.
- Look for a waterproof or water-resistant
handheld GPS receiver. As editors at GPSReview.net say, "The IPX-4 standard
means that the device will stand up to water splashed on it from any angle.
However, this does not mean you can drop it in a stream. For that you want
a device that is IPX-7 waterproof." A unit that meets IPX-7 standards
can withstand accidental dunks, but it isn't intended for swimming.
memory is standard, and some models can store as many as 50 routes. Make sure you get one with at least 20 routes. Keep in mind that GPS receivers
with a memory-card slot offer expandable route memory.
- Look for at least
500 user-entered waypoints. Some units can store many more.
- Check size and
weight. This is a very personal thing, but you should consider it if
you are thinking about hauling a unit around in the wilderness. Lighter
units come at the expense of screen size.
- Make sure the unit you are considering
has a feature that guides you from waypoint to waypoint. This feature
is essential for hiking use.
- Look for a unit that uses a 12-channel
parallel receiver system or better. This will give you the best reception
in wooded areas. Some GPS units can receive even more channels.