Appendix A — GPS Primer 311

Receivers generally use four or more satellites, however, to improve accuracy and provide precise altitude information.

In order to perform this simple calculation and determine where you are, the GPS receiver has to know two things:

The location of at least three satellites above you

The distance between you and each of those satellites

The more accurate these values are, the more accurate your position will be.

The GPS receiver determines both the location of the satellites and your distance from them by analyzing the high-frequency, low-power radio signals from the GPS satellites. Better units have multiple (or parallel) receivers, so they can pick up signals from several satellites simultaneously. If you buy a GPS receiver nowadays, it will be a multi-channel receiver (12 or even 14 channels simultaneously).

The radio waves from the GPS SVs, like all other radio waves, are electromagnetic energy, which means that they propagate at the speed of light (roughly 186,000 miles per second, or 300,000 km per second, in a vacuum). A GPS receiver can figure out how far a signal has traveled by timing how long it took the signal to arrive. The mechanism by which it does this is quite clever.

At a particular time (midnight, for example), the SVs begins transmitting a long digital pattern called a pseudo-random code. This code consists of a carrier wave that transmits the digital “chips” that make up the code. The receiver knows when this code starts (this information is transmitted to the GPS along with the signal) and begins running the same digital pattern at exactly the same time. When the satellite’s signal reaches the receiver, its transmission of the digital pattern will lag slightly behind the digital pattern that the receiver would expect. This lag between the two corresponds to the time delay from the satellite sending the signal and the receiver receiving it.

Speed × Time = Distance

Therefore, if it takes 0.08 of a second for a satellite’s signal to reach the GPS receiver, the distance between the two must be 14,880 miles (186,000 miles per second × 0.08 seconds = 14,880 miles). The GPS receiver must be located somewhere on an imaginary sphere that has a radius of 14,880 miles.

One thing that GPS relies on is accurate timing. To provide this, every GPS satellite carries four atomic clocks on board that provide an extremely accurate timing signal. The GPS receiver, however, doesn’t contain an atomic clock (this would make it preposterously expensive!). Instead, it contains an inexpensive quartz clock, and the internal timekeeping is updated when the receiver is capable of locking on and receiving a signal from four or more SVs.

GPS Signal Errors

The GPS system isn’t absolutely perfect, however, because the radio signal has to propagate through the atmosphere to the receiver. The receiver isn’t perfect either. The following table describes the sources of the possible errors present.

312 Appendix A — GPS Primer

Error	Amount of Error (feet/meters)
Ionosphere	13.1/4.0
Clock	6.9/2.1
Ephemeris	6.9/2.1
Troposphere	1.3/0.7
Receiver	1.6/0.5
Multipath	3.3/1.0
TOTAL	33.1/10.4

The errors resulting from the ionosphere and the troposphere are due to the slowing down of the signal (remember that the speed of light quoted is the speed of light in a vacuum). Clock errors are due to inaccuracies in timing. Even using atomic clocks, the speed of light is so fast and the distance that it has to travel is so short that small errors in timing add up. Ephemeris errors are due to the fact that it is impossible to know absolutely the orbits of the satellites.

Slight variations cause small errors.

Receiver error is also significant; this is the error in the antenna and the delays in processing caused by the internal circuitry (also due to temperature changes affecting the internal clock).

Finally, there are multipath errors. This error is caused by the signal being reflected or bounced off things (such as a building or the ground).

There are other issues with the signal. Because it is a microwave signal, it is absorbed by water, so a GPS receiver won’t work underwater. Fortunately, not many people need to use it underwater, so this isn’t a huge problem, but water can affect the system in other ways:

Leaves contain water and can absorb the GPS signal, dramatically reducing signal strength and quality. The denser the leaves, the more the signal is degraded. Worse than leaves are pine needles. These not only contain water, but are the right size and shape to act as miniantennas, catching the signal. This usually means that if you are traveling though dense forests (especially conifers), you will need to find a clearing to gain a good signal lock.

Humidity in the air, such as rain, snow, or fog, can also weaken the signal. In addition, areas of high mist (such as near waterfalls) can be problematic because the high water content of the air exacerbates the problem.

A layer of water on the antenna can dramatically reduce the signal’s quality.

Summary

This appendix provided a brief look at the GPS network and what makes it work. GPS is a vastly complex multi-billion-dollar system, and certain key parts of it are classified. Nonetheless, this appendix should give you a good working knowledge of the system and what can affect its accuracy.

Websites/Internet

Resources

Here is a list of useful GPS-related websites, categorized according to type.

GPS Manufacturers

Site	Description
www.garmin.com	Garmin website
www.magellangps.com	Magellan website
www.delorme.com	DeLorme website
www.lowrance.com	Lowrance website
www.laipac.com	Laipac website
www.haicom.com.tw	Haicom website
www.navman.com	Navman website
www.miogps.com	Mio Technology website
www.emtac.com.tw	Emtac website
www.trimble.com	Trimble website

Digital Map Makers

Site	Description
www.memory-map.com	Memory-Map (U.S.)
www.memory-map.co.uk	Memory-Map (U.K.)
www.maptech.com	Maptech website
www.fugawi.com	Fugawi website

Continued

appendix

in this appendix

˛Useful GPS Websites

314 Appendix B — Websites/Internet Resources

Digital Map Makers (continued)

Site	Description
www.destinator1.com	Destinator website
www.tomtom.com	TomTom website
www.garmin.com/cartography	Garmin cartography
www.magellangps.com/en/products	Magellan mapping

Geocaching

Site	Description
www.geocaching.com	Premier geocaching site
www.navicache.com	Geocache listing site
shop.groundspeak.com	Geocaching store
www.brillig.com/geocaching	Geocaching site

Software

Site	Description
www.robogeo.com	RoboGEO enables you to insert GPS coordinate
	information into digital photographs.
www.teletype.com	Navigating software for PDAs
www.oziexplorer.com	Excellent mapping software
www.expertgps.com	Excellent mapping software
javagps.sourceforge.net	Java application that enables communication with
	attached GPS receivers
www.gartrip.de	GARTrip enables you to take control over waypoints
	and routes. Works with Garmin and Magellan
	receivers.
www.gpsassist.net	Helper application for TomTom Navigator software
www.sping.com/seaclear	SeaClear provides a chart plotter and navigation
	software for the PC.

Appendix B — Websites/Internet Resources 315

Site	Description
www.wherify.com	Personal GPS locator
www.topofusion.com	GPS mapping for Windows
www.compegps.com	Tracking and routing software
www.gpsu.co.uk	GPS utility created to enable easy management,
	manipulation, and mapping of GPS data
www.easygps.com	Import/export waypoints easily
www.geopainting.com	This is the home of GPSMapEdit, which enables you
	to create your own maps.

Hardware

Site	Description
www.pfranc.com	Cable connectors for GPS receivers
www.lynks.co.uk	Cables, connectors, and GPS accessories
www.ram-mount.com	Mounts for GPS receivers for all occasions
www.stormcase.com	Stormcase — the ultimate rugged outdoor case for all
your electronics
www.otterbox.com	Rugged, waterproof, dustproof, crushproof boxes for
	your GPS and other electronics
www.smartsolar.com	Portable solar power solutions
www.aquapac.net	Waterproof bags for GPS

Information

Site	Description
www.gpsinformation.net/	Extensive GPS information
www.ultimategps.com	GPS news and information
www.gpsworld.com	GPS news and technology website
www.mobilegpsonline.com	A site concentrating on GPS and mobile computers
www.gpspassion.com	Information and discussion forum
	Continued

316 Appendix B — Websites/Internet Resources

Information (continued)

Site	Description
www.travelbygps.com	Information for the traveler using GPS
www.gpsnuts.com	GPS information and reviews
www.trailregistry.com	Plan your hiking and backpacking trips here!
www.confluence.org	Interesting GPS game
gpsgames.org	The name says it all — GPS games!
www.nomad.ee/micros/etrex.shtml	Extensive information on Garmin GPS
www.keenpeople.com	Large, very active GPS community
www.gps-practice-and-fun.com	A lot of good GPS information
www.pocketgps.co.uk	Good information and a great selection of
	international points of interest
groups.yahoo.com/group/	Group-specific discussion on the Magellan Meridian
	Magellan_Meridian/
groups.yahoo.com/group/GarminF	Group-specific discussion on the Garmin GPS units
finance.groups.yahoo.com/group/etrex/	Group-specific discussion on the Garmin eTrex units