Geography Quiz 3

GPS

global positioning system; a system to calculate a position or coordinate on the Earths surface

satellite/space segment

• constellation of at least 24 satellites (31 currently)

• each satellite follows one of six orbits

• altitude of around 12600 miles

• each satellite circles Earth in 12 hours

• constantly broadcasting codes to GPS receivers

Control Segment

• monitor stations continually monitor satellite position info

• master control station calculates orbit and clock info every 15 minutes

• ground antennas broadcast updated navigation info to satellites censor twixe a day

• satellites use updates in their broadcasts

User segment

• receivers continually listen for broadcast from satellites

• internal computer contains almanac of satellite positions

• calculate distance from each satellite

• use trilateration to determine position on Earth’s surface. 

satellite ranging

distance = speed of light + time difference

atmosphere delay

the GPS satellite signal bounces around when traveling through the ionosphere and the troposphere. This increases the time it takes to reach the Earth which alters the calculated position.

Multipath

The GPS signal is reflected off an object before it reaches the reciever

inaccuraces

• atmosphere

• orbit error

• electronic “noise”

• clock error

• receiver error

• geometric effects

• operator error

obstruction

some features of the landscape may block the GPS signals all together (GPS does not work very well in areas with too many obsticles)

differential GPS

involves the addition of a base station into the standard GPS. Base station calculates the difference between its true position and the position calculated from satellite signals

WAAS

Wide Area Augmentation System; a network of ground reference stations in North America, two master stations (one on each U.S. coast) and two geo-stationary stations. continually broadcasting correction information to mobile GPS receivers using standard GPS frequency

• typical accuracy less than 3 meters

Coordinate system

establishes a known origin point and a praticule (grid) upon which you con plot coordinate locations and make measurements

longitude and latitude

angles measured relative to the plane of the prime meridian and the plane of the equator respectively.

1 degree

1 minute

1 minute

60 seconds

Positional Dilution of Precision (PDOP) 

satellites close together