GLOBAL POSITIONING SYSTEM

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GLOBAL POSITIONING SYSTEM

24hrs all weather world-wide navigation and positioning system developed & maintained by the US Department of Defence (DOD); there are 24 and more high altitudes (20,000 km) satellites.

The System

It is divided into 3 segments:

Control Segment

(brain of the GPS) owned by (DOD) (monitor and adjust) (orbital parameters, clocks, security, stations world-wide)

Space Segment

Constellation of Satellite Vehicles (SV) broadcasting low power signals (24-28 operational satellites with a 7.5 to 11 yrs life span)

Space Segment 2

*12 hrs orbit at 55° to the Equator, high orbit at 20 000km for accuracy coverage and survivability

User Segment

GPS signals are used to calculate position for multiple applications (SV's continuously send low power signals containing almanac, ephemeris and timing data)

Almanac

Contains files of information with pictures of the sky that approximate locations of all SVs for a particular time and place

Almanac 2

*The receiver automatically downloads and store current almanacs (updates monthly)

Almanac 3

(Bars for the sky plots) (Once the GPS is on board, positions are calculated quickly) (which is useful for planning data quickly)

Ephemeris

This data gives the exact orbital location of a particular satellite. Each SV broadcasts its own ephemeris and update it every 2 hrs.

Time

Imbedded in the coded signal, each satellite has 4 atomic clocks on board at nanosecond accuracy. Since each SV continuously broadcasts orbital and timing information, they act as precise reference points for GPS receivers; also used to trilaterate positions (ranging = how long signal takes to travel).

Time 2

The distance is calculated by how long the signal takes to travel back and forth.

Time 3

when there is a differences of 1/600th seconds it requires atomic clock

Mechanism of GPS

(SV's transmit location and time data) (receivers can calculate precise positions from the signals)

Mechanism of GPS 2

1 SV: Provides only a range (distance) of position on the sphere (x)

Mechanism of GPS 3

2 SV: Position at the intersection of 2 spheres; has an ellipse shape

Mechanism of GPS 4

3 SV: Provide a 2-D position (x, y) of distance and horizontal but NO ELEVATION

Mechanism of GPS 5

4 SV provides a 3-D position (x, y, z) of precise distance, elevation and horizontal

Precise Positioning

(The receiver uses the signal from 3 satellites to trilaterate an approximate 2-D position) (Determined by an elegant timing trick as the receiver locks an atomic clock, the signal from the 4th satellite is used to synchronize the receiver's clock to the atomic clock of the satellite) (The receiver then solves for a unique time correction that results in an exact point-solution for the 3 satellites)

GPS Accuracy

(The first GPS satellite was up in 1978 but fully operational in April 1995) Selective Availability (SA) kept civilian accuracy to about +/- 100 m

GPS Accuracy 2

Deliberate error for tactical advantage (Epsilon 'ephemeris fibbing' and Dither 'clock variation')

GPS Accuracy 3

Discontinued by Presidential Order

GPS Accuracy 5

(errors from the satellite clocks) = (ephemeris) (atmosphere particles) (receiver noise, quality of calculation)

GPS Accuracy 6

definition is(Circular Error Probability) radius of a circle defines percentage of location(usually 50 or 95%)

Multipath

Signals can reflected by an object before they reach the receiver (e.g. buildings)

DGPS (Differential Global Positioning System)

2 GPS receivers using the same satellites at the same time have same relative errors.

DGPS (Differential Global Positioning System) 2

Surveyors establish a base station and put the GPS receiver at a precisely known location and collect time-paired data nearby with a rover (mobile up to 300 km). The base station program is to calculate a position each second and compare it to the known location from the rover and record the 'differential' (correction).

Differential Correction

A GPS receiver is set up at a known location (Base station) and knows precisely where it is (so the error can be calculated each second)  Absolute reference position

Differential Correction 2

The rover corrects the data and at any second the errors occurring at one location are occurring everywhere in the same sky vicinity (The error calculated at the base station can be applied to the rover position) (Doesn't correct multipath)

Real Time DGPS

DGPS in real time with a radio link (RTDGPS) Field workers use post-processing (downloading a correction file requires special GPS software to time-pair the data) (DGPS can give sub-cm accuracy using survey-grade instruments) (Sub-meter with our Trimble Geo-XT's)

WAAS

(Wide Area Augumentation System)

WAAS 2

Implemented by US FAA (25 ground reference stations used to rebroadcast a correction signal to GPS receivers) (newer GPS are WAAS compliant) (The original system allowed us to see some satellites in Canada but the signal was low in equatorial orbits and ineffective where trees/terrain limited the view of the horizon (multipath)

WAAS 3

WAAS introduced to Canada & Mexico in 2007 and coverage to the northern areas were included (Stations in Winnipeg, Iqualut, Gander and Goose Bay)

Accuracy for waas

(WAAS allows 3 m accuracy) on GPS, CEP 95 and improved vertical positioning

Accuracy for waas 2

(Needs view of the sky) (reduced accuracy in urban canyons)

Accuracy for waas 3

SV position is important

Accuracy for waas 4

Poor conditions in tight valleys

Accuracy for waas 5

• Degraded or no reception under forest canopy

Accuracy for waas 6

Multipath (affects timing and accuracy)

Accuracy for waas 7

WEATHER CONDITIONS IS NOT A FACTOR!

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