GPSGlobal Positioning System

Diana Cooksey, Montana State University, LRES Department

Overview

• What is GPS & how does it work?– Satellites

– Radio signals

– Almanacs

– Timing

What is GPS?

• Satellites orbiting the earth

• Positioning, navigation and timing

• Operates 24 hrs/day

• Used for any application requiring location information

GPS Constellations

• United States– NAVSTAR GPS (Navigation Satellite Timing &

Ranging system); 28 satellites

• European Union– Galileo; 30 satellites

• Russia– Global Navigation Satellite System (GLONASS);

24 satellites (10 healthy)

GPS Segments

UserControl

Space

Space Segment: GPS Satellites

• Power– Sun-seeking solar panels– Nicad batteries

• Timing– 4 atomic clocks

Satellite Orbits

• Orbit the earth at approx. 20,200 km (11,000 nautical miles)

• Satellites complete an orbit in approximately 12 hours

Satellite Signals

• Radio signals, 2 frequencies

• Two levels of service– Standard Positioning Service

(SPS)– Precise Positioning Service

(PPS)

Satellite Signals

• Radio signals contain– Unique pseudorandom code– Ephemeris– Clock behavior and clock

corrections– System time– Status messages– Almanac

Satellite Signals

• Require a direct line to GPS receivers

• Cannot penetrate water, soil, walls or other obstacles

Satellite Almanac

• Sent along with position and timing messages

• Prediction of all satellite orbits

• Needed to run satellite availability software

• Valid for about 30 days

Control Segment: US DoD Monitoring

Colorado Springs

Hawaii

AscensionDiego Garcia

Kwajalein

Orbits precisely measured

Discrepancies between predicted orbits (almanac) and actual orbits transmitted back to the satellites

User Segment

How Does GPS Work? Calculating a Position

• GPS receiver calculates its position by measuring the distance to satellites (satellite ranging)

Measuring Distance to Satellites

• 1. Measure time for signal to travel from satellite to receiver

• 2. Speed of light x travel time = distance

• Distance measurements to 4 satellites are required to compute a 3-D position (latitude, longitude and altitude)

Measuring Satellite Signal Travel Time

• How do we find the exact time the signal left the satellite?– Synchronized codes

Timedifference

One measurement narrows down our position to the surface of a sphere

12,000 mileradius

A second measurement narrows down our position to the intersection of two spheres

11,000 mileradius

12,000 mileradius

A third measurement narrows down our position to just two points

12,000 mileradius

11,500 mileradius

11,000 mileradius

Correcting for Timing Offset

• The first three measurements narrow down our position

• A fourth measurement is needed to correct for timing offset (difference in synchronization between satellite and receiver clocks)– Satellites use highly accurate atomic clocks– Receivers use accurate quartz clocks

6 seconds4 seconds

AB

6 seconds4 seconds

5 seconds(wrong time)

7 seconds(wrong time)

AB

5 Things to Take Away

1. 3 GPS segments

2. Satellites transmit radio signals containing– Unique pseudorandom code

– Ephemeris

– Clock behavior and clock corrections

– System time

– Status messages

– Almanac

3. Formula for satellite ranging (D = t ∙ v)

4. 4 satellites to compute an accurate 3-D position (the 4th measurement is needed to correct for timing offset)

5. We are not the only country with a GPS system

Overview

• How accurate is GPS?

– Error sources

– Differential correction

– Accuracy levels

GPS Error

• Atmospheric effects

• Multipath

• Satellite geometry

• Measurement noise (receiver error)

• Ephemeris data

• Satellite clock drift

• Selective availability (SA)

Ionospheric & Tropospheric Refraction

Multipath

Satellite GeometryGeometric Dilution of Precision (GDOP)

• GDOP can magnify or lessen other GPS errors

• Wider angles better measurements

• Components of GDOP– HDOP; H=horizontal lat/long– VDOP; V=vertical altitude– TDOP; T=time clock offset

PDOP values

<=4 excellent

5-8 acceptable

>=9 poor

Dilution of Precision (DOP)

Ephemeris Data

• A satellite’s positions as a function of time

– Each satellite broadcasts its individual ephemeris

– Can contain orbital position errors

Selective Availability (SA)

• The accuracy of GPS signals was intentionally degraded by the DoD

•

• SA was the largest component of GPS error

• SA was turned off on May 1, 2000

GPS Error Budget

• Ionosphere..................................5.0 meters (0.4)• Troposphere................................0.5 meters (0.2)• Ephemeris data..............................2.5 meters (0)• Satellite clock drift........................1.5 meters (0)• Multipath....................................0.6 meters (0.6)• Measurement noise.......... ..........0.3 meters (0.3)• Selective availability.....................30-100 meters

• Total.................................................~ 10 meters

Differential Correction

• GPS receiver on the ground in a known location (base station)

• Acts as a static reference point1. Transmits error correction messages to other

GPS receivers in the local area (real-time)

2. Differential correction can be done on computer after GPS data are collected (post-processed)

Roving receiver(unknownposition)

Base receiver(knownposition)

Radio link for real-time DGPS

How accurate is GPS?• Recreational and mapping grade.........................10-15 m

– C/A code– Autonomous

• Recreational and mapping grade.............................1-5 m– C/A code– With differential correction

• Submeter mapping grade.............................10 cm to 1 m– C/A code & carrier– With differential correction

• Survey grade.............................................................1 cm– Dual frequency– Advanced survey methods

Six Main Sources of GPS Error

• Atmospheric effects

• Multipath effects

• Satellite geometry

• Measurement noise

• Ephemeris data

• Satellite clock drift

Things to Take Away

• 6 major sources of error affect the accuracy of GPS positions

– Atmospheric error largest source

– Previously SA

• Almanac and ephemeris data are different

• Differential correction increases accuracy