GPS

What is GPS

A system to calculate a position on the earths surface

What three factors are needed for GPS (segments)

User segments, Control segments, and Satellite segments

How many orbits

6

What are the 4 GPS control segments

1. Monitor Stations

2. Master Control

3. Ground Antennas

4. Satellites

Monitor Stations

continually monitor satellite position information

Master Control

calculates orbit and clock information every 15 minutes

Ground Antennas

Broadcast updated navigation information to satellites one or two times a day

Satellites

uses updates in their broadcast

GPS User Segments (4 needs)

Continually listening for broadcasts from satellites 

Almanac of satellite positions (calendar of days)

Calculate distance 
trilateration to determine position of Earths Surface 

Satellite Ranging (equation)

Distance = Speed of light x time distance 

Satellite ranging

Measures the distance it takes for a signal to go from satellite to ground

How many satellites are needed?

3, but 4+ are ideal

What accounts for inaccuracies (8)

1. Atmosphere

2. Orbit Error

3. Electronic “noise”

4. Clock Error

5. Multi-path error

6. Receiver Error

7. Geometric Effects

8. Operator Error

Atmospheric Delay 

The GPS satellite signal bounced around when traveling through the ionosphere and the troposphere

• increase time it takes to reach the Earth = altered calculation 

Multi-path

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

Obstruction

Some features of the landscape may block the GPS signals altogether

• ex) forests, cities, steep mountains

Positional Dilution of Precision (high vs low)

High PDOP: satellites are close together

• Large area of uncertainty

Low PDOP: Satellites widely spaced

• Small area of uncertainty

Two methods of improving Accuracy

1. Differential GPS 

2. Wide Area Augmentation System (WAAS)

Differential GPS

Addition of a base Station into the standard GPS

Base Station

GPS receiver (Base) set up on a precisely known location

• Calculates the difference between its true position and the position calculated from satellite signals

Mobile Receivers

• The position calculated by the mobile GPS are adjusted according to the differential 

WAAS

Continually broadcasting correction information to mobile GPS receivers using standard GPS frequency

• 3 meters

GPS Data

• coordinates position on earths surface

• lat/long

Coordinate systems + LONG/LAT (+/-, line names)

• Specific points in the system 

• Lat

  • East and West

  • parallels

  • negative = West

  • Positive = East

    • Equator

    • Prime Meridian

    • Tropics

    • Artic Circles

• Long 

  • north and south

  • meridians

  • Negative = South

  • Positive = North

Earth degree/tilt 

23..5