Global Positioning System (GPS)

GPS Basics GPS stands for Global Positioning System which measures 3D locations on Earth surface using satellites GPS operates using radio signals sent from satellites orbiting the earth Created and Maintained by the US Dept. of Defense System as a whole consists of three segments Satellites (space segment) Receivers (user segment) Ground stations (control segment)

GPS History Development began in 1973 First satellite became operational in 1978 Declared completely functional in 1995 A total of 52 satellites have been launched in 4 phases 30 satellites are currently functional Managed by the U.S. DOD Originally developed for submarines Now part of many modern applications GPS History

Satellites At least 4 satellites are above the horizon anytime anywhere GPS satellites are also known as “NAVSTAR satellites” The satellites transmit time according to very accurate atomic clocks onboard each one The precise positions of satellites are known to the GPS receivers from a GPS almanac data Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin. 4

Satellites cont. The satellites are in motion around the earth Like the sun and moon satellites rise and set as they cross the sky Locations on earth are determined from available satellites (i.e., those above the horizon) at the time the GPS data are collected Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin. 5

Receivers Ground-based devices read and interpret the radio signals from several of the NAVSTAR satellites at once Geographic position is determined using the time it takes signals from the satellites to reach the GPS receiver Calculations result in varying degrees of accuracy that depend on: Quality of the receiver User operation of the receiver (e.g., skill of user and receiver settings) Atmospheric conditions Local conditions (i.e., objects that block or reflect the signals) Current status of system

Ground Stations Control stations Responsibilities Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin. Ground Stations Control stations Master station at Falcon (Schriever) AFB, Colorado 4 additional monitoring stations distributed around the world Responsibilities Monitor satellite orbits & clocks Broadcast orbital data and clock corrections to satellites

How GPS Works: Overview Satellites have accurate atomic clocks onboard and all GPS satellites transmit the same time signal at the same time Think “synchronize your watches” The satellite signals contain information that includes Satellite number Time of transmission Explain Triangulation 8

How GPS Works: Overview Receivers use an almanac that includes The position of all satellites every second This is updated timely from control stations The satellite signal is received, compared with the receiver’s internal clock, and used to calculate the distance from that satellite Trilateration (similar to triangulation) is used to determine location from multiple satellite signals Explain Triangulation 9

How GPS Works: Signal Processing Distances between satellites and receivers is determined by the time is takes the signal to travel from satellite to receiver Radio signals travel at speed of light (186,000 miles/second) All satellites send the identical time, which is also generated by the receivers Signal travel time = offset between the satellite signal and the receiver signal Distance from each satellite to receiver = signal travel time * 186,000 miles/second 1sec Satellite signal Receiver signal

How GPS Works: Trilateration Start by determining distance between a GPS satellite and your position

How GPS Works: Trilateration Adding more distance measurements to satellites narrows down your possible positions

How GPS Works: Trilateration

How GPS Works: Trilateration The 4th satellite in trilateration is to resolve any signal timing error Unlike GPS satellites, GPS receivers do not contain an atomic clock To make sure the internal clock in the receiver is set correctly we use the signal from the 4th satellite

GPS Error Sources Satellite errors Satellite position error (i.e., satellite not exactly where it’s supposed to be) Atomic clocks, though very accurate, are not perfect Atmospheric Electro-magnetic waves travel at light speed only in a vacuum Atmospheric molecules, particularly those in the ionosphere, change the signal speed

Multi-path distortion The signal may "bounce" off structures before reaching the GPS receiver – the reflected signal arrives a little later Receiver error: Due to the receiver clock or internal noise Selective Availability No longer an issue

Satellite Clock & Satellite Position Sources of Error Satellite Clock & Satellite Position Atomic clock errors +/- 2 meters of error Satellite is not in precise orbit +/- 2.5 meters of error 17

Atmospheric Delays/Bending Sources of Error Atmospheric Delays/Bending +/- 5 meters or error 18

Sources of Error Multi Path Interference (signal bouncing off of buildings, trees, etc.) +/- 1 meter of error 19

Receiver Timing/Rounding Errors Sources of Error Receiver Timing/Rounding Errors +/- 1 meter of error (depends on the quality of the GPS receiver) Quadruple Redundant Atomic Clocks Accurate to Nanoseconds 2:02:01.23456789012 Powered by 4 AA Batteries 2:02:01.2345 20

GPS - Selective Availability A former significant source of error Error intentionally introduced into the satellite signal by the U.S. Dept. of Defense for national security reasons Selective Availability turned off early May 2, 2000 21

Satellite Coverage: Position Dilution of Precision (PDOP) GPS Error: Position Dilution of Percision Satellite Coverage: Position Dilution of Precision (PDOP) Remember that satellites are moving, causing the satellite constellation to change Some configurations of satellites are better than others Poor PDOP Good PDOP

GPS - Error Budget Example of typically observed error from a consumer GPS receiver: Typical Observed errors (meters) satellite clocks 0.6 orbit (position error) 0.6 receiver errors 1.2 atmosphere 3.7 Total 6.1 Multiplied by PDOP (1-6) Total error ~ 6.1 - 36.6 meters Meters Atmosphere Receivers Orbit Error Satellite Clocks 6 12 18 24 30

Differential Correction GPS - Error Correction 2 Methods: Point Averaging Differential Correction 24

Point Averaging Point Averaging is one of the simplest ways to correct GPS point locations Collect many GPS measurements at the same location and then average them to get one point The averaged point should have greater accuracy than a single point measurement Accuracy varies with this method but you should have a position that is within 5 meters of its true location 95% of the time 25

GPS - Point Averaging Averaged Location This figure shows a successive series of 3-D positions taken using a receiver kept at the same location, and then averaged 26

GPS - Differential Correction Differential correction collects points using a receiver at a known location (known as a base station) while you collect points in the field at the same time (known as a rover receiver) Any errors in a GPS signal are likely to be almost the same among all receivers within ~ 300 miles of each other ~ 300 miles (~ 480 km) or less Base station (known location) Rover receiver 27

GPS - Differential Correction The base station knows its own location It compares this location with its location at that moment obtained using GPS satellites, and computes error This known error (difference in x and y coordinates) is applied to the rover receiver (hand-held unit) at the same moment Example: Base Station File Time GPS Lat GPS Long Lat. error Long. error 3:12.5 3:13.0 3:13.5 3:14.0 3:14.5 3:15.0 35.50 35.05 34.95 36.00 35.35 35.20 79.05 78.65 79.55 80.45 79.30 79.35 .5 .05 -.05 1.0 .35 .20 .5 -.35 .55 1.45 .30 .35 28

GPS - Differential Correction GPS error when using differential correction: 1 – 3 meters There are two ways that differential correction can be applied: Post-processing differential correction Does the error calculations after the rover has collected the points Requires downloading a base-station file Real-time differential correction Done in real time by receiving a broadcasted correction signal May require additional hardware

Strengths of GPS Easy To Incorporate into Project Once trained, just about anyone can use it Cheap Widely Available 30

Weaknesses Does require a training component Accuracy Issues Differential Correction may not be an option in many parts of the world 31

Other satellite navigation systems in use or various states of development include: GLONASS – russia's global navigation system. Fully operational worldwide. Galileo – a global system being developed by the european union and other partner countries, planned to be operational by 2016 (and fully deployed by 2020) Beidou – people's republic of china's regional system, currently limited to asia and the west pacific

COMPASS – people's republic of china's global system, planned to be operational by 2020 IRNSS – india's regional navigation system, covering india and northern indian ocean QZSS – japanese regional system covering asia and oceania

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