This is the first comprehensive satellite-based radio navigation system. The name stands for Navigation Satellite Timing and Ranging system. It has been developed and operated by the US Dept of Defence.

Navstar uses a constellation of between 24 and 32 satellites that transmit precise microwave signals. This system is managed by the United States Air Force 50th Space Wing.

History
In the early 1940s US developed ground based navigation systems like LORAN and Decca Navigator which were useful during World War II. In 1957, when USSR launched Sputnik, US scientists found that they could track it’s exact location due to it’s radio waves; by a phenomenon known as Doppler’s effect. This inspired for a satellite based navigation system.

The US Navy tested Transit in 1960 with 5 satellites. This constellation fixed 1 position every hour. In 1967, Timation satellites fitted with accurate clocks were launched. This clock parameter is still in use today.

The first extensive system was launched in the 1970s, known by the name Omega. Advanced systems began to be manufactured by Rockwell International in 1978. Now satellite expertise is provided by Lockheed Martin, Raytheon and Boeing.

Present
GPS can be segregated into 3 segments

The Space Segment
Includes minimum 24 operational satellites which revolve with a 12 hour period. At this given moment there are minimum 6 satellites that would be above us transmitting Navstar GPS signals

The Control Segment
A master control station in Colorado Springs, with five monitor stations and 3 ground antennae; tracking satellites around the globe. The monitor stations collect information from these satellites and sends it to the master control. The system is updated and information is sent back to the satellites via the ground antennae.

The User Segment
Here antennae, processors and receivers allow operators to receive the broadcasted signals; and hence position, velocity and time is computed.

Navstar GPS signals are broadcasted at 1575.42MHz, 1227.60MHz and 1176.45MHz; the latter being in use today.

Future
It’s HI-GPS.
This denotes High Integrity Global Positioning System. The developing team includes Boeing Advanced Systems, Phantom Works, Iridium LLC, Rockwell Collins, Coherent Navigation and academic experts.

In this system, Iridium’s Low Earth Orbit satellites would work with GPS satellites to provide higher accuracy of position and time. HI-GPS has goals laid till 2011; if they’re met, then there would be more satellites available providing better precision.

This is a project worth to be watched.