1

02/11/2003 12.080 GPS 1

Applications of the GlobalPositioning System

Prof. Thomas HerringDepartment of Earth, Atmosphere

and Planetary Sciences12.080 Seminar Spring 2003.http://www-gpsg.mit.edu/~tah

02/11/2003 12.080 GPS 2

Overview

• Original design of the Global PositioningSystem (GPS)

• Use by “non-authorized” users• Applications to:

– Tectonic studies– Meteorology– Other applications

2

02/11/2003 12.080 GPS 3

GPS Original Design• Started development in the late 1960s

as NAVY/USAF project to replaceDoppler positioning system

• Aim: Real-time positioning to < 10meters, capable of being used on fastmoving vehicles.

• Limit civilian (“non-authorized”) users to100 meter positioning.

02/11/2003 12.080 GPS 4

GPS Design• Innovations:

– Use multiple satellites (originally 21, now ~28)– All satellites transmit at same frequency– Signals encoded with unique “bi-phase,

quadrature code” generated by pseudo-randomsequence (designated by PRN, PR number):Spread-spectrum transmission.

– Dual frequency band transmission:• L1 ~1.5 GHz, L2 ~1.25 GHz

3

02/11/2003 12.080 GPS 5

Latest Block IIR satellite(1,100 kg)

02/11/2003 12.080 GPS 6

Measurements• Measurements:

– Time difference between signal transmission fromsatellite and its arrival at ground station (called“pseudo-range”, precise to 0.1–10 m)

– Carrier phase difference between transmitter andreceiver (precise to a few millimeters)

– Doppler shift of received signal• All measurements relative to “clocks” in

ground receiver and satellites (potentiallyposes problems).

4

02/11/2003 12.080 GPS 7

Measurement usage• “Spread-spectrum” transmission:

Multiple satellites can be measured atsame time.

• Since measurements can be made atsame time, ground receiver clock errorcan be determined (along with position).

• Signal

V(t, r x ) = Vo sin[2p( ft -r k .r x ) + pC(t)]

C(t) is code of zeros and ones (binary).Varies discretely at 1 or 10 MHz

02/11/2003 12.080 GPS 8

Measurements• Since the C(t) code changes the sign of the

signal, satellite can be only be detected if thecode is known (PRN code)

• Multiple satellites can be separated by“correlating” with different codes (only thecorrect code will produce a signal)

• The time delay of the code is the pseudo-range measurement.

5

02/11/2003 12.080 GPS 9

PositionDetermination

(perfectclocks).

• Three satellitesare needed for 3-D position withperfect clocks. • Two satellitesare OK if height isknown)

02/11/2003 12.080 GPS 10

Positiondetermination:

with clockerrors: 2-D

case• Receiver clock isfast in this case, soall pseudo-rangesare short

6

02/11/2003 12.080 GPS 11

Positioning• For pseudo-range to be used for

positioning need:– Knowledge of errors in satellite clocks– Knowledge of positions of satellites

• This information is transmitted bysatellite in “broadcast ephemeris”

• “Differential” positioning (DGPS)eliminates need for accurate satelliteclock knowledge.

02/11/2003 12.080 GPS 15

Satellite constellation

• Since multiple satellites need to be seenat same time (four or more):– Many satellites (original 21 but now 28)– High altitude so that large portion of Earth

can be seen (20,000 km altitude —MEO)

7

02/11/2003 12.080 GPS 16

Current constellation• Relative sizescorrect (inertialspace view)• “Fuzzy” linesnot due to orbitperturbations,but due tosatellites beingin 6-planes at55o inclination.

02/11/2003 12.080 GPS 17

Ground TrackPaths followed by satellite along surface of Earth.

8

02/11/2003 12.080 GPS 18

Satellite AvailabilitySatellite Availability (smallest number above

15o minimum elevation)

02/11/2003 12.080 GPS 19

Pseudo-range accuracy• Original intent was to position using pseudo-

range: Accuracy better than planned• C/A code (open to all users) 10 cm-10 meters• P(Y) code (restricted access since 1992) 5

cm-5 meters• Value depends on quality of receiver

electronics and antenna environment.

9

02/11/2003 12.080 GPS 20

GPS Antennas (for precise positioning)

• Rings arecalled choke-rings (used tosuppressmulti-path)

Nearly all antennas are patch antennas (conductingpatch mounted in insulating ceramic).

02/11/2003 12.080 GPS 21

Positioning accuracy• Best position accuracy with pseudo-

range is about 20 cm (differential) andabout 5 meters point positioning.

• For Earth science applications we wantbetter accuracy

• For this we use “carrier phase” where“range” measurement noise is fewmillimeters

10

02/11/2003 12.080 GPS 22

Carrier phase positioning• To use carrier phase, need to make

differential measurements between groundreceivers.

• Simultaneous measurements allow phaseerrors in clocks to be removed i.e. the clockphase error is the same for two groundreceivers observing a satellite at the sametime (interferometric measurement).

02/11/2003 12.080 GPS 23

Phase positioning

• Use of carrier phase measurementsallows positioning with millimeter levelaccuracy and sub-millimeter ifmeasurements are averaged for 24-hours.

11

02/11/2003 12.080 GPS 24

Examples ofpositioning

results

-0.10

-0.05

0

0.05

0.10

8 10 12 14 16 18 20 22 24

North

(m)

Time (hrs)

Phase Differential Positioning

Scatter 0.008 m and 0.002 m

-200

-100

0

100

200

8 10 12 14 16 18 20 22 24

North

(m)

Time (hrs)

Pseudorange Point Positioning

Scatter 21 m and 23 m

-2.0

-1.0

0.0

1.0

2.0

8 10 12 14 16 18 20 22 24

North

(m)

Time (hrs)

Pseudorange Differential Positioning

Scatter 0.20 m and 0.15 m

SAon

02/11/2003 12.080 GPS 25

Summary

• Use of differential measurements withcarrier phase allows very preciseposition determination (independentlargely of security features).

• We use these measurements in Earthscience for deformation studies andatmospheric studies

12

02/11/2003 12.080 GPS 26

Schematic of signal propagation• GPS signals retardedby propagation throughatmosphere• Ionosphere (plasma)is accounted by linearcombination ofmeasurements at L1and L2 frequencies.

02/11/2003 12.080 GPS 27

Atmospheric delays• Delay of signal due to atmosphere refraction

is 2.3-30 meters, depending on elevationangle

• Elevation angle dependence can be used toestimate amount of delay due to atmosphere

• For GPS frequencies, this delay dependsgreatly on the amount of water vapor inatmosphere

13

02/11/2003 12.080 GPS 28

GPS Measurements of precipitable water vapor (real-time)

02/11/2003 12.080 GPS 29

Tectonic Deformation Results• “Fixed GPS” stations operate continuously

and by determining their positions each daywe can monitor their motions relative to aglobal coordinate system

• Temporary GPS sites can be deployed onwell defined marks in the Earth and themotions of these sites can be monitored(campaign GPS)

14

02/11/2003 12.080 GPS 30

Locations measured with GPS

• Red are campaign (~5000), other colors continuousGPS operated by different groups

02/11/2003 12.080 GPS 31

Example of motions measured in Pacific/Asia region

• Fastest motionsare >100 mm/yr• Noteconvergencenear Japan

15

02/11/2003 12.080 GPS 32

Motion in North America

• Note rapidchange inCalifornia• Remainderof fieldconsistentwith rotatingNA plate

02/11/2003 12.080 GPS 33

Detail in Western United States

16

02/11/2003 12.080 GPS 34

CaliforniaDetail

• Compilation ofcontinuous andcampaign GPS• Only s<1.5 mm/yrshown• In 100 years, fastestpoints move 5 m

02/11/2003 12.080 GPS 35

Detailed view of LA Basin

17

02/11/2003 12.080 GPS 36

Motion in Turkey and Aegean

02/11/2003 12.080 GPS 37

1999 Izmit EarthquakeBlack vectors are observed, Yellow model calculation

18

02/11/2003 12.080 GPS 38

Motion after Earthquakes. Example fromHector Mine, CA

Continued motion tells us about material characteristicsand how stress is re-distributed after earthquake

-40

-30

-20

-10

0

10

20

30

1999.0 1999.5 2000.0 2000.5 2001.0 2001.5 2002.0 2002.5 2003.0

Pre-Hector MinePost_Hector mine

Diffe

renc

e fro

m M

ean

(mm

)

Year

Time of Hector mine earthquake220 mm offset removed

RMS Scatter about linear trend 0.8 mm

02/11/2003 12.080 GPS 39

Atmospheric pressure loading

-20

-10

0

10

20

1999.95 2000.00 2000.05 2000.10 2000.15

JPLMITAtm Load

DRAO

Hei

ght (

mm

)

Year

19

02/11/2003 12.080 GPS 41

Relativistic Effects

-50

-25

0

25

50

0 4 8 12 16 20 24

PRN 03 Detrended; e=0.02Clock - trend (ns)GR Effect (ns)

Cloc

k er

ror (

ns)

Time (hrs)

02/11/2003 12.080 GPS 42

Other applications• Aircraft positioning, vehicle navigation, engineering(real-time differential GPS links shown)

20

02/11/2003 12.080 GPS 43

Conclusions

• GPS dual-use technology: Applicationsin civilian world widespread– Geophysical studies (mm accuracy)– Engineering positioning (<cm in real-time)– Commercial positioning: cars, aircraft,

boats (cm to m level in real-time)• System modifications to support civilian

users better underway