USU 1360

It is an objective and universal form of communication of science

U.S. Department of Defense

12 billion

a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations.

better than a centimeter

The basis of GPS is triangulation from satellites

To triangulate a GPS receiver measures distance using the travel time of radio signals

To measure travel time, GPS needs very accurate timing which it achieves with some tricks.

Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret.

Finally you must correct for any delays the signal experiences as it travels thru the atmosphere.

four

By measuring how long a radio signal takes to reach us from that satellite

Both satellite and receiver are generating same pseudo-random codes at exactly same time

how long psuedo-random code takes to reach us

travel time x speed of light = distance

200 miles

atomic clocks

50k-100k

an atomic accuracy clock

Because an extra satellite range measurement can remove errors (the fourth satellite)

The air force has injected each GPS satellite into a precise orbit with a 24th on reserve

five

By Department of Defense...they use very precise radar to check each satellite's exact altitude, position and speed

ephemeris errors - caused by gravitational pulls from moon and sun and by pressure of solar radiation on satellites and sent to satellites to be transmitted along with timing signals

• 1.)As GPS signal passes thru charged particles of ionosphere and then thru water vapor in troposphere it gets slowed down a bit
• 2.)when signal bounces off various local obstructions before it gets to our receiver - multipath error
• 3.)intentional errors - in order to avoid terrorist groups using satellite as weapon - selective availablity

• 1.)Predict what a typical delay might be on a typical day - modeling
• 2.)compare relative speeds of two different signals - dual frequency

To make GPS more responsive to civil and commercial users worldwide

Its a way to correct various inaccuracies in GPS system

involves cooperation of two receivers, one stationary and one roving around making position measurements

• 1.) internet
• 2.) inverted DGPS - GPS receivers at location with transmitter GPS transmit position back to tracking office. Computer will do the calculations

• Location
• Navigation
• Tracking
• Mapping
• Timing