Welcome and congratulations on taking the initiative to increase your knowledge of GPS navigation. Our goal is to provide the relevant and practical information necessary to get you there and back, making every adventure a successful one. This means teaching how to tell time more than how the make a clock. We are not techno-geeks and will not try to write like one. If interested in the scientific information, there are a number of other resources which provide the highly technical details. We have covered the brain numbing technical information for you and have tried to extract the information that is actually needed.
How GPS works
Studying this information with your product’s instruction manual will provide an understanding of how GPS works, and the equipment’s many capabilities. It is also very important to learn how to use a map and compass. Using GPS in conjunction with a map and compass will substantially increase navigation skills, allowing GPS receivers to be utilized to their fullest potential. Practice will further increase confidence by reducing the chance of becoming lost.
This is both good and bad because we will provide enough information to allow you to actually know what you’re doing, which of course will make you dangerous. The information is also somewhat general, providing examples of what features are available and their possible application in the field. Every brand of GPS receiver, (although sharing most of the same features), may work a little differently. We highly recommend an ongoing study and practice of your navigation skills, which includes getting out the product’s instruction manual and actually reading it.
Now a word from the legal department: Traveling and navigation are potentially dangerous activities. This information is not a substitute for reading a GPS receiver’s instruction manual. Using GPS can be danger in at least two regards. Watching the screen instead of the road, and relying on GPS as the only means of navigation. So keep your eyes open, pay attention and be responsible enough to understand your equipment and abilities.
Navigation 101, Defense spending pays off for civilians
GPS, an acronym for “Global Positioning System,” is a U.S. military technology that has been adopted for civilian use. Twenty-four orbiting Department of Defense satellites broadcast radio signals that through a GPS receiver, provides precise coordinate information worldwide.
The system was first conceptualized in the 1960’s for worldwide U.S. military navigation. By the mid 1970’s it became a joint effort by various branches of the U.S. armed services, and was referred to as Navstar. Despite the official name, it was the term GPS that stuck. The system’s first major military debut was in 1991 when it contributed to the overwhelming success of Desert Storm. With the system still in its infancy, only 16 satellites were utilized and were specially located over the Persian Gulf area. Hand held receivers helped allied forces navigate and maneuver around enemy positions in an unfamiliar desert without landmarks. The system was considered fully operational in 1995. Like its “Humvee” cohort, GPS has numerous practical civilian applications. For more military related information be sure to check out our section GPS at War.
U.S. tax dollars were hard at work as Uncle Sam spent 17 billion to develop this system and we can use it for free. That’s right, no set up or monthly fees. Commercial, recreational, friend and foe alike, anyone with a GPS receiver can access and use the system. That is why GPS units are referred to as receivers, they simply (or not so simply) receive the radio signals continually being broadcast from the 24 satellites. We say not so simply because the way GPS works is rather scientific requiring the explanation of considerable technical information. This includes the satellites broadcasting two sets of signals, one military and one civilian.
The Department of Defense’s ground control system operates the 24 orbiting satellites, placing 12 in each hemisphere. GPS receivers need to read at least three at a time to “triangulate” the equipment’s exact location. Four are needed to provide a more actuate three-dimensional fix.
The GPS should not ever be shut off to civilians. Despite the satellites broadcasting separate military and civilian signals, even in time of crisis or war, it is unlikely that civilians will ever be denied access to the system. This is because of the great numbers of users that rely on the system including the airlines, shipping, trucking, law enforcement and various emergency response agencies. Even a Presidential Decision Directive was signed that guarantees GPS will always be available to civilians.
Hand held receivers are about the size of a TV remote control. They include a built in antenna and view screen. A vehicle mount unit looks similar to a marine fish finder with a larger screen and a remote antenna that looks like a hockey puck. Portable units typically have a 2” screen, and vehicle fixed mount units have a 4” screen. For visual ease of operation, the largest unit practical should be used.
Accuracy remains about the same regardless of the size or price, as long as the antenna has a clear view of the sky. A GPS receiver should be able to lock onto its location anywhere in the world within 0 to 15 meters. With the government’s Selective Availability now shut off, the accuracy of most units averages approximately within two to three meters. Not perfect, but somewhere within 42 feet should be close enough to find your truck at the trailhead.
Thankfully in May of 2000, the government eliminated Selective Availability. This was a U.S. military safeguard that deliberately made the civilian signal inaccurate up to 100 meters. It was designed to prevent unfriendlies from using our own technology against us in targeting bombs and missiles. Up to a 100 meter inaccuracy was not that big of a deal unless you happen to be one of the unfortunates who died trying to find their shelter during a blizzard. The government got around the security problem by shutting down the signal around sensitive sites. That is why your screen will go blank while driving by a military base or while spying on Area 51.
For those users that require absolute accuracy such as land surveyors or ship captains negotiating reefs, there is a system know as Differential Correction or DGPS. Land based radio signals are used in conjunction with satellite signals to greatly improve the accuracy. To take advantage of this option, simply purchase a receiver capable of accepting DGPS signals.