ENGRG 59910: Introduction to GIS

Lecture 10: GPS

Acknowledgment: Michael Piasecki

Introduction To GPS

• Satellites• Atomic clocks• Microchips and calculators• Radio Beacons and other transitions from astro methods to modern

methods. Ground based systems

The Technological Revolution

• 1973 – 1977: Proof of concept• 1976: US Congress approved development• 1978: First satellites launched• Assets belongs to US Government• Stewardship via US Dept of Defense• First real operational test for GPS technology was the Gulf War in 1990• June 26, 1993, the 24th satellite into orbit• Full constellation achieved in 1995• Selective Availability turned off by President Clinton in May 1st, 2000• But also authorization of GPS III modernization, starting 2017• Currently 31 satellites in orbit of various ages and type• Ultimately 62 are planned for• 12,000 miles above Earth• Orbit twice daily

History: NAVSTAR Global Positioning System (GPS)

NAVSTAR stands for NAVigation Satellite Timing And Ranging system

• GLONASS• Global Navigation Satellite System• Russian Federation Ministry of Defense• Has deteriorated over time; not complete• Being restored to full availability in partership with India

• GALILEO• EU + 6 non-EU countries: China, India, Israel, Morocco, Saudi Arabia and Ukraine• First 4 satellites in orbit by 09/12/2012 from Kazakhstan• 30 satellite (27 + 3)• Full operation expected in 2019

Non-US Positioning Systems

• Baidou: China’s regional system that China proposed to expand into a global system named COMPASS.

• QZSS: Japanese proposed regional system, adding better coverage to the Japanese islands;

• Indian regional Satellite System (IRNSS): India’s proposed regional system.

Non-US Positioning systems

• Position and coordinates.• The distance and direction between any two waypoints, or a position and a

waypoint.• Travel progress reports.• Accurate time measurement.

Four Primary Functions of GPS

Okay, you’re lost, you have amnesia, and you have absolutely no idea whereyou are. You go up to someone, and like a good amnesiac you ask, “WhereAm I?” This person tells you that you are 625 miles from Boise, Idaho. You say,sarcastically, “Wow, that’s some great information. Thanks Buddy.”

How GPS Works

• You still have absolutely no idea where you are, so once again you go up tosomeone and ask, “Where Am I?” This person tells you that you’re 690miles from Minneapolis. You just scratch your head, but then you start toremember what you learned in GPS class, and, with the US map in yourpocket to guide you, you proceed.

We’re Getting Closer

• You see a another smiling face and ask “where am I” one more time. Thistime you get the reply “you are 615 miles from Tucson”. The light pops onin your brain. It’s all clear now. You draw three circles extending in the radiiof 625 miles from Boise, 690 from Minneapolis, and 615 miles fromTucson. The circles intersect in Denver! Aren’t you glad that you took thisclass?

Now We Know!

• Animation of 24 GPS satellites with 4 satellites in each of 6 orbits.• It shows how many satellites are visible at any given time. This ensures

redundancy to ensure accuracy.

Satellites in orbit

• Stations on earth, and a GPS receiver, thedistances between each of these pointscan be calculated.

• The distance is calculated based on theamount of time it takes for a radio signalto travel between these points.

• Using satellites in the sky, ground allowsthe GPS receiver to know where you are,in terms of latitude and longitude, on theearth.

• The more satellites the GPSr can “see”,the more accurate your reading.

• The GPSr must “see” the satellites, so itdoes not work well in dense forests,inside caves, underwater, or insidebuildings.

How does GPS work?

Sources of Signal Interference

Earth’s Atmosphere

Solid Structures

Metal Electro-magnetic Fields

Obstruction

GPS for Navigation

• Each satellite of the network sends a signal indicating its position and thetime of the transmission of the signal.

• Since signals travel at the speed of light, the receiver can determine theradial distance of the satellite from the receiver.

• Many think that the receivers transmit information to the satellites,whereas in reality, it is the other way around.

• The receiver then uses basic math to determine its position.

Satellites and signals

• Although light travels at 186,000 miles per second, its path is impeded byvarious factors such as the ionosphere and the troposphere.

• There are also errors due to the time mechanism.• The satellites are equipped with atomic clocks and so their timing is very

accurate.• The receivers do not have accurate clocks and so this also introduces errors

into the calculation.

GPS errors

• Signal transmission errors and noisy channels.• Relativistic errors due to gravitational fields.• Signal delays due to atmospheric disturbance.

Other errors

Commercial GPS Applications Span A Wide Range of Economic Activities

Satellite Ops:Ephemeris, Timing

Communications:Network Timing,Synchronization

Surveying &Mapping

Fishing &Boating

Off shoreDrilling

Recreation

Trucking & Shipping

Personal Navigation

Aviation

Railroads

Power GridInterfaces

Who uses GPS?

GPS Three Segments

Space Segment

Solar powered, backup batteries, rocket boostersHydrogen Maser Atomic Clocks

• These clocks lose one second every 2,739,000 million years• The GPS satellites are nothing more than a set of clocks in the sky

Hydrogen Master Clock

Passive hydrogen maser

https://www.youtube.com/watch?v=p2BxAu6WZI8

GPS Orbits

Consolidated Space Operations Center: CSOC• Master Control Station is located at the Consolidated Space Operations

Center (CSOC) at Schriever Air Force Station near Colorado Springs• Track the satellites for orbit and clock determination• Time synchronization• Upload the Navigation Message• Manage DOA (dead on arrival)

Control Segment

Control Segment

Kwajalein Atoll

US Space Command

Hawaii

Ascension Is.

Diego Garcia

Cape Canaveral

Ground AntennaMaster Control Station Monitor Station

• Military.• Search and rescue.• Disaster relief.• Surveying.• Marine, aeronautical and terrestrial navigation.• Remote controlled vehicle and robot guidance.• Satellite positioning and tracking.• Shipping.• Geographic Information Systems (GIS).• Recreation.

User Segment

User Segment

Consists of Receivers

User Segment: Military and Civilian

GPS on the Battlefield

In Rainforest, BrazilConflicting Interests• Military needs a secure system

• Civilians need unrestricted access

• Orbits of Satellites are exact and known by receivers (preprogrammed)• DOD monitors and updates ephemeris errors (orbit errors)• Pseudo Random Code includes navigation message with ephemeris error

information

User Segment: Position

GPS Signals: Navigation Message

… and also its health status

… orbital information

GPS: How does it work?

The GPS receiver and a satellite produce the same signal at a given time. Good clocks important…

Receiver measures the delay between generating a time signal and receiving the same time from the satellite. Simple!

Range distance = c * (t1 - t0)

Time Differencet1

t0

• Satellites carry highly accurate atomic (Cesium and Rubidium) clocksonboard.

• GPS receivers have much less accurate quartz clocks.• With positional fixes from 3 satellites, a timing error of only 1/1,000,000 of

a second between the satellite and the receiver equals a positional error of300 meters or more on the ground!

Why Four Satellites?

GPS: How does it work?

More Satellites

equalsBetter

Resolution

• GPS measurements are not totally accurate• Depending on your GPS receiver, you can achieve different levels of

accuracy.• More satellites used, more sophisticated processing = greater accuracy• More accurate receivers are more expensive

GPS Accuracy

Basic Positioning: Today

Before May 2000:25-100 m

6-10 m

• C/A Code on L1

1-5 m

• C/A Code on L1• L2C Code on L2• New Code on L5

Better resistance to interference

Eliminates need for costly DGPS in many non-safety applicationsDifferential Global PositioningSystem (up to 10cm accuracy)

US operates one via Coast Guard; ground based system correct satellite estimates.

Basic Positioning: Tomorrow

• Satellite clock error, ephemeris error• Atmospheric Refraction

• Ionospheric Refraction• Tropospheric Refraction

• Multipath• Signal Obstruction• Selective Availability• Receiver clock error• Human Error

Systematic Sources of GPS Error

• Sources of User Equivalent Range Errors (UERE)• Ionospheric effects ± 5 meter • Ephemeris errors ± 2.5 meter • Satellite clock errors ± 2 meter• Multipath distortion ± 1 meter• Tropospheric effects ± 0.5 meter • Numerical errors ± 1 meter

GPS Errors

• Signal speed varies: Refraction• Signal bounced by ionosphere: Reflection• Time of signal propagation affected: Error

Atmospheric Effects

• Occurs when GPS signals are reflected and the receiver detects two signalinstead of one at different times. This causes confusion in some low-endGPS units, but is generally easy to correct.

• High-end receivers compensate for multipath• Mapping and Survey units use a hardware solution: a special semi-

directional antenna

Multi-Path Error

Multipath

• When something blocks the GPS signal• Areas of Great Elevation Differences

• Canyons• Mountain Obstruction• Urban Environments

• Indoors

Signal Obstruction

Obstruction

• Used to compensate for obstruction• Uses compass bearing and an offset distance to calculate the position of

the obstructed target.• Trimble Pathfinder Pro XR unit has this capability

Using Off-Set Positioning

Government introduces artificial errors to reduce GPS position accuracy• Discourages hostile forces from using GPS• Largest source of error• How does it work?

• Off-setting satellite clocks.• Introduction of ephemeris error by the Space Command control center• Only the military has the correction information.

Selective Availability (S/A)

Selective Availability (S/A) Error

100 meters

40 meters

20 meters100 meters

Will it be turned on in case of War?

• In 1996 it was announced that S/A would be phased out and turned offwithin 4 years

• On May 1st 2000, S/A was turned off by order from US President Clinton,but the military can still turn it on when necessary

• In 2006 S/A was turned off permanently• In 2007 US gov announced the production of future generation GPS

satellites, GPS III, without the S/A to be ever switched on again

S/A Status

• DOP = Dilution of Precision• DOP is a measurement of error• DOP Values usually range from 1 to 6• Low values are best

Measuring GPS Error: DOP

• It is best to have satellites spaced evenly around the receiver with onesatellite overhead and one low on the horizon.

• High DOP values are found when satellites are clustered together.

Dilution of Precision (DOP)

• PDOP (Position Dilution of Precision): the most commonly used, measuresDOP in 3D.

• HDOP (Horizontal Dilution of Precision): measures DOP in 2D (horizontalonly).

• VDOP (Vertical Dilution of Precision): measures DOP in the vertical only.• TDOP (Time Dilution of Precision): measures DOP with respect to time.

Measurements of DOP

• Differential correction• Uses a base station• Post-processing• Real-time

Error Correction

• Wide Area Augmentation System, WAAS• Uses reference stations to calculate

corrections (<3m accurate)• Even better than DGPS

• Obtain GPS position at a site with known position (base station)• Derive difference between actual and GPS positions

• Post-processing vs. Real-Time processing

Differential GPS (DGPS)

http://www.e-trimblegps.com/

DGPS: USCG Radio Beacon

http://www.navcen.uscg.gov/dgps/default.htm

The Coast Guard’s maritime DGPS service provides 10 meter (2 dRMS) navigation accuracy, integrity alarms for GPS and DGPS out-of-tolerance conditions within 10 seconds of detection, availability of 99.7% per month. Typically, the positional error of a DGPS position is 1 to 3 meters.

• Developed by Federal Aviation Administration (FAA) for aircraft navigation• Available to everyone in North America• 25 ground stations across US monitor GPS errors (2 master stations in

coast)• Corrections sent to geosynchronous satellites, or satellite with a fixed

position over the equator• WAAS-equipped GPS receiver uses corrections from satellites to improve

accuracy• 95% accuracy typically on order of 3 meters• Not expensive (do not require additional receiver)

Wide Area Augmentation System, WAAS

Wide Area Augmentation System

Confirmed accuracy performance of 1 – 2 meters horizontal and 2 –3 meters vertical throughout the majority of the continental U.S. and portions of Alaska.

Some amazing things to do with GPS

• Earth Tides• Measuring subsidence• Track the moving earth• Post Glacial Rebound• Geocaching• Emergency Rescue

• GPS completely changed positioning forever• GPS will continue to improve

Conclusion and discussion