What is GPS
The Global Positioning System (GPS) is a space age navigational system that can pinpoint your position anywhere on the globe, usually within a few meters. This amazing technology is available to everyone, everywhere, day and night, and best of all, at no cost for use of the navigational data. GPS uses a constellation of 24 satellites in precise orbits approximately 11,000 miles above the earth. The satellites transmit data via high frequency radio waves back to Earth and, by locking onto these signals, a GPS receiver can process this data to triangulate its precise location on the globe. GPS operates 24 hours a day, in all weather conditions, and can be used worldwide for precise navigation on land, on water and even in the air. Some of its many current applications include: boating, fishing, hunting, scouting on land or from the air, hiking, camping, biking, rafting, pack trips by horseback, hot air ballooning, general aviation, snowmobiling and skiing, search and rescue, emergency vehicle tracking, 4 wheeling, highway driving and a host of other outdoor activities where accurate positioning is required

How GPS Determines Your Position
GPS uses satellite ranging to triangulate your position. In other words, the GPS unit simply measures the travel time of the signals transmitted from the satellites, then multiplies them by the speed of light to determine exactly how far the unit is from every satellite it's sampling.

By locking onto the signals from a minimum of 3 different satellites, a GPS receiver can calculate a 2D positional fix, consisting of your latitude and longitude. By locking onto a fourth satellite, the GPS can compute a 3D fix, calculating your altitude as well as your latitude/longitude position.

In order to do this GPS receiver makers use a 12 parallel channel receiver in all of its current products. Three of the channels lock on to satellites for triangulation. Another channel locks on to a fourth satellite for 3D navigation, which lets the unit calculate altitude in addition to latitude and longitude. These four channels continuously and simultaneously track the four satellites in the best geometrical positions relative to you. The additional eight channels track all other visible satellites, and then add this data to the data from the original four satellites. The unit then over-resolves a solution, creating an accuracy-enhanced reading. The additional channels also ensure reliable, continuous, and uninterrupted navigation, even in adverse conditions such as valleys or dense woods.

Accuracy of GPS
GPS was conceived in the 1970s, and is controlled by the United States Department of Defense. Although GPS was initially envisioned for military use, the Government realized early on that there would be numerous civilian applications as well. Subsequently, the Department of Defense created two transmission codes; the P code (Precision code) for military use, and the C/A code (Civilian Access code) for civilian use.

The highest accuracy levels were to be reserved for the military so as to prevent hostile enemy attacks against the U.S. using our own navigational system. However, once in operation, the civilian GPS receivers using the C/A code proved to be more accurate than the D.O.D. had intended. Consequently, the military developed a system for randomly degrading the accuracy of the signals being transmitted to civilian GPS receivers. This intentional degradation in accuracy is called Selective Availability or S/A. This reduced the civilian GPS accuracy levels to being within 100 meters or less, 95% of the time. However, typical accuracy for most users averaged between 20 and 50 meters the majority of the time. You could easily see the effects of S/A on a GPS receiver when you were not moving. Typically there would be random movements in speed, altitude and position readings, along with slow position "wandering" on the plotter trail, easily seen when you are on a .1 or .2 mile Plotter scale, and not moving. For example, while parked at the dock in your boat, you would see unexplainable changes in your digital speed-readings up to a few miles per hour, even though you were not moving.

Effective May 2, 2000 selective availability (S/A) has been eliminated. The United States Department of Defense (DOD) now has the technology to localize the control system to deny GPS signals to select areas. It is not often that your electronics products increase in value after you've purchased them. Now boaters, hikers, hunters, and outdoor enthusiasts of all types can locate their position up to ten times more precisely (within 10 to 20 meters) and navigate their way through unfamiliar terrain. Anglers can now return to their favorite spot on a lake or river instead of just their favorite area.

The decision to allow civilians so much accuracy in location information was finally completed because GPS is continually playing a more important role in the lives of people around the world - it's becoming a national utility. GPS is the global standard in navigation because it is completely free of charge to the public.

GPS Timing
GPS uses the triangulation of signals from the satellites to determine locations on earth. GPS satellites know their location in space and receivers can determine their distance from a satellite by using the travel time of a radio message from the satellite to the receiver. After calculating its relative position to at least 3 or 4 satellites, a GPS receiver can calculate its position using triangulation. GPS satellites have four highly accurate atomic clocks on board. They also have a database (or almanac) of the current and expected positions for all of the satellites that is frequently updated from earth. That way when a GPS receiver locates one satellite, it can download all satellite location information, and find the remaining needed satellites much more quickly.

My GPS device takes a long time (5 Min +) to fix, what is the problem?
GPS devices will take a significantly longer to get their first fix, after being purchased or after a long period of inactivity. This is perfectly normal and it is due to the time that it takes the unit to receive an updated almanac (which contains information about the GPS satellites that the unit needs to function).

What is the difference between Cold, Warm and Hot starts?
GPS units use information about satellite position and characteristics that changes over time. Most GPS units store this information in RAM or ROM, thus not requiring this information, called an almanac, to be re-downloaded each time the unit is used. When this information is present, and it is used to expedite an initial fix, that is a hot start. Most users will want to perform a hot start.

In some cases, the almanac information is missing, incorrect or outdated. When the information is missing a warm or cold start is performed. A warm start retains many settings and some almanac information and just tries to collect a limited amount before fixing. On a cold start, the unit is basically reset to factory-default conditions. Cold starts are generally used to resolve a problem or if the GPS unit has been moved significantly (3000 miles +) since its last use.

The GPS fix goes in and out every few seconds for a while after I start my GPS, is this normal?
Depending on your surroundings, this can be quite common after initially starting your GPS unit. Generally this is caused by having a very weak fix on one or more satellites that is lost and regained a number of times. In areas with dense clouds, tall buildings or other objects obstructing the sky, this may continue to happen while using the GPS.

I am having trouble getting a Bluetooth GPS to work with my Pocket PC, how can I fix this?
As there are now many different configurations and Bluetooth managers available, you will need to consult your PDA's manual to determine how to create a serial connection over Bluetooth and what COM port this will use. Once you know this COM port, start your connection, and choose the appropriate COM port.