What is GPS ?

The official USDOD name for GPS is NAVSTAR

he first GPS satellite was launched in 1978.

A full constellation of 24 satellites was achieved in 1994.

Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.

A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.

GPS satellites are powered by solar energy, but they have backup batteries onboard, in case of a solar eclipse.

Transmitter power is only 50 Watts or less.

GPS satellites transmit at least 2 low-power radio signals.

The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects, such as buildings and mountains.

However, modern receivers are more sensitive and can usually track through houses.

Pseudorandom code is an I.D. code that identifies which satellite is transmitting information.

You can see which satellites you are getting signals from on your device's satellite page.

Ephemeris data is needed to determine a satellite's position and gives important information about the health of a satellite, current date and time.

Almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day and shows the orbital information for that satellite and every other satellite in the system.

Ionosphere and troposphere delays: Satellite signals slow as they pass through the atmosphere. T

The GPS system uses a built-in model to partially correct for this type of error.

Signal multipath: The GPS signal may reflect off objects such as tall buildings or large rock surfaces before it reaches the receiver, which will increase the travel time of the signal and cause errors.

Receiver clock errors: A receiver's built-in clock may have slight timing errors because it is less accurate than the atomic clocks on GPS satellites.

Orbital errors: The satellite's reported location may not be accurate.

Number of satellites visible: The more satellites a GPS receiver can "see," the better the accuracy.

When a signal is blocked, you may get position errors or possibly no position reading at all.

GPS units typically will not work underwater or underground, but new high-sensitivity receivers are able to track some signals when inside buildings or under tree-cover.

Satellite geometry/shading: Satellite signals are more effective when satellites are located at wide angles relative to each other, rather than in a line or tight grouping.

Selective availability: The U.S. Department of Defense once applied Selective Availability (SA) to satellites, making signals less accurate in order to keep 'enemies' from using highly accurate GPS signals.