GPS 101
What is GPS?
GPS stands for Global Positioning System. It consists of satellites and receivers that can precisely measure a location on Earth.
Brief History
The United States Department of Defense began developing GPS in 1973 for military use. Although the United States Air Force began launching GPS satellites in 1978, the GPS receivers were too bulky and heavy for a person to carry. Portable military GPS receivers, called Manpacks, appeared in 1988. These backpack-mounted receivers weighed 17 pounds and cost about $45,000 each.
Civilian access to GPS began as early as 1983, primarily with GPS receivers that had to be transported in vehicles. Over time, the receivers became smaller and cheaper. The first commercially-available handheld GPS receiver was introduced in 1989.
Today, GPS receivers are highly miniaturized and inexpensive, often found as microchips in everyday devices, such as cell phones, automobiles, and laptops.
How GPS Works
GPS works by trilateration, a method to determine the two intersecting points of three or more overlapping spheres. Because you only need to know where you are on Earth (which acts as another sphere), the intersecting point in the space above Earth is not used.
In order for a GPS receiver to make these calculations, it needs to know two things: 1) the distance between it and each of four GPS satellites and 2) the locations of those satellites in the sky above it.
- Distance to each satellite—Every GPS satellite
transmits a radio signal that contains a unique digital code.
The code repeats after a brief amount of time. The receiver
knows when the code repeated because of other information in the
satellite's data message. The receiver uses all this information
to figure out how far away the satellite is by timing how long
it took for the signal to arrive.
- Location of each satellite—The receiver knows where each satellite is above the surface of Earth at any given moment because it has an almanac with data about every satellite's orbit. The almanac is constantly updated by additional data contained in each satellite's transmission. Each satellite broadcasts this almanac for all the satellites, and a very precise almanac for itself that is updated every few hours.
NOTE: Technically, signals from only three GPS satellites are needed for a rough location reading. Almost all GPS receivers use the signals from four or more satellites to calculate a reading that is accurate enough for practical purposes. The fourth satellite observation is needed because initially the GPS receiver does not know time accurately enough to determine ranges to the satellites based on the difference between time of reception and time of transmission of the individual satellite signals.
GPS Accuracy
GPS receivers are usually accurate to within 10 to 15 meters. However, there are some conditions that can lead to inaccuracy:
- Geometry. GPS accuracy is limited to the geometry of
satellites, which refers to where the satellites are located
relative to each other. If all the located satellites are in the
same area, the reading can be rather skewed. However, if all
four satellites are in a North, South, East, and West location,
then a reading will be very accurate.
- Blockage/Urban Canyon. Blockage occurs when a GPS signal is blocked in a vehicle or in a parking garage. Buildings and mountainous terrain can also block the satellites' reception. GPS can be inaccurate when you are in an urban setting with many tall buildings, also known as an urban canyon.
- Multipath. Multipath occurs when a radio signal reflects off an object and travels further than intended. This confusion of signals due to reflection can make GPS positions inaccurate.
TeleNav and GPS
TeleNav has developed applications that utilize GPS technology to provide a variety of useful features for both personal and business purposes. We encourage you to take a tour through our web page at www.telenav.com to see what TeleNav and GPS can do for you.