Hand Held GPS Receivers: How They Work and What They Do

One fine day long, long ago, my husband and I were driving around Mt. Shasta on Forest Service roads. Suddenly, we found ourselves high centered on what looked like a little patch of snow. All the digging and swearing did not do any good. We were high and dry and lost. We needed to walk back to town but where were we and where was the town? The only thing we could do was walk down and what we hoped was east. After several hours in the now bitter cold, we saw the lights of the town and limped our way to it, but not before several searchers were scouring all the nearby canyons.

Had we a handheld GPS, we still would have had a healthy walk but it wouldn’t have been so long and that awful feeling of not knowing where we were would be gone. But at that time the whole idea of GPS wasn’t even a gleam in someone’s brain. Now, most of us know that lots of cars come with them but there are also handheld GPS units used by hikers, boaters, campers, rock hounds, fisher people, and, as we will see, a new breed of gamers.

How did this all come about and what does GPS stand for? GPS stands for Global Positioning System. The military needed a better means of tracking all its ships, planes, and weapons systems and so it began to put satellites into low orbit. Each satellite was equipped with atomic clock. Ultimately, it had a constellation of 27 satellites, 24 operating and 3 extra as standby units.

The satellites transmit signals to GPS receivers on the ground, which passively receive them. All of this requires very accurate time which is provided by atomic clocks at the U.S. Naval Observatory. The GPS receiver acts like it, too, has an atomic clock when in reality; it just has an ordinary quartz clock. The receiver keeps such accurate time because it keeps resetting its time to that of the satellites.

Each satellite transmits its location and current time. All satellites transmit signals at the same time. These signals travel at the speed of light but arrive at the GPS at slightly different times because some of the satellites are father away. When the receiver estimates the distance to at least 4 satellites, it can calculate its position in three dimensions: longitude, latitude, and altitude. The satellite orbits are arranged so that at any time, any place on earth at least four satellites are “visible.” Hand held GPS units have about 10 to 20 meter accuracy. The more expensive units have more than one receiver in them and can track more satellites thus providing greater accuracy.

Two groups of people like hand held GPS: techno freaks and outdoors types of people. Ratings are slightly different from each group. You can use the maps stored in the handheld GPS memory. You can use your GPS with a paper map of your area that has longitude and latitude information. Some GPG receivers let you download detailed maps from your computer or supply detailed maps with plug in cartridges. You can input location data, such as where you want to go.

Which brings us to the games people play with the hand held GPS receivers. One game is called geocaching. You are given a set of co-ordinates, typically from an Internet site devoted to these things and you search for a treasure, or cache, filled with goodies. If you like something, you may keep it but you must leave something in its stead. Most also contain a logbook. Some are relatively easy and some require strenuous hiking. All require some brain work because there may be private property, a raging river, or a mountain between you and the cache. This hobby is growing by leaps and bounds and is especially popular with families.

There is also geography where a site is photographed and posted along with
its co-ordinates.

Now, what should a person buy?

Consumer research has amalgamated other ratings and come up with the following:

Garmin eTrex Legend $160. The Legend stores a database of basic American maps in 8 MB of memory. It allows you to down load maps from your computer.

Garmin GPSMAP765 $320. More toots and whistles plus more memory. It has an altimeter, electronic compass, and tidal data. It has a larger screen than the Legend and longer battery life.

In the middle of the pack are products from Magellan, Lowrance, and Navman.

At the bottom are the Cobras.

Cobra 500 $125. It was rated mediocre and took several minutes to obtain a fix. It also had a lower screen resolution than other receivers.

Cobra 100 $100. This unit was rated “poor.”

All units require open skies above them so that if you are lost in a forest, you will have to climb a tree to get a reading. In cities, not only does the GPS have trouble finding satellites; it can give off false readings as signals bounce off skyscrapers making satellites look farther away than they really are.