Introduction to GPS

Fundamental Problem• How to know my location precisely ?

– In any condition– At any time– Everywhere on earth (at least outdoors!)

• How to locate a landmark or target precisely ?-- Guidance or Navigation

How far or which route?

Satellite based navigation definitionSatellite-based Navigation means Satellites areused to estimate the position of a point by usinga group of satellites.

Navigation Types• Landmark-based Navigation– Stones, Trees, Monuments

Limited Local use• Celestial-based Navigation– Stars, Moon

Complicated, Works only at Clear Night• Sensors-based Navigation– Dead Reckoning

Gyroscope, Accelerometer, Compass, OdometerComplicated, Errors accumulate quickly

• Radio-based Navigation– LORAN, OMEGA

Subject to Radio Interference, Jamming, Limited Coverage• Satellite-based Navigation– GPS, GLONASS, GALILEO, QZSS

Global, Difficult to Interfere or Jam, High Accuracy

Current Satellite Navigation Systems• GPS

– USA– Global Positioning System– First Experimental Launch Feb, 1978– First Operational Launch 1989 and Operational Capability in

Feb, 1993

• GLONASS– Russia– Global Navigation Satellite System

Current Satellite Navigation Systems• Galileo

– European Union– New services like SAR (Search and Rescue)– Higher Accuracy for Civil Community– Tentative Plan of Launch : 2005 – 2006– Availability of Service : 2008?

• QZSS( Quasi-Zenith Satellite System)– Japan– Basically, communication satellite but also transmit GPS like

signals for navigation– Communication– Broadcasting– Navigation

What is GPS?

The Global Positioning System (GPS) is a precise worldwide radio-navigation system, and consists of a constellation of satellites and their ground stations, operated and maintained by the US Department of Defense (DoD).

IntroductionUsing the Global Positioning System the following

two values can be determined anywhere on Earth

1. One’s exact location (longitude, latitude and height co-ordinates) accurate to within a range of 20 m to approx. 1 mm.

2. The precise time (Universal Time Coordinated, UTC) accurate to within a range of 60ns to approx. 5ns.

Basic Functions of GPS

Use of GPS• GPS receivers are used for positioning,

locating, navigating, surveying and determining the time and are employed both by:

Individuals (e.g. for leisure activities, such as trekking, balloon flights and cross-country skiing etc.)

Companies (surveying, determining the time, navigation, vehicle monitoring etc.)

GPS Signal Transit Time• There are currently 28 operational

satellites orbiting the Earth at a height of 20,180 km on 6 different orbital planes. Their orbits are inclined at 55° to the equator, ensuring that at least 4 satellites are in radio communication with any point on the planet.

Satellite Orbital Plane

Determining Transit Time• By comparing the arrival time of the

satellite signal with the on board clock time the moment the signal was emitted, it is possible to determine the transit time of that signal

Determining Signal Transit Time

Determining a Position in 3-D Space

• If the distance to the three satellites is known, all possible positions are located on the surface of three spheres whose radii correspond to the distance calculated. The position sought is at the point where all three surfaces of the spheres intersect

Determining a Position in 3-D Space

Determining a Position in 3-D Space

• Assumption: Signal transit time can be precisely measured

Impossible

If the transit time is out by just 1 s this produces aμpositional error of 300m.

As the clocks on board all three satellites are

synchronized, the transit time in the case of all

three measurements is inaccurate by the same

amount.

Determining a Position in 3-D SpaceIf the time measurement is accompanied by a constant unknown error, we will have four unknown variables in 3-D space:  

• longitude (X) • latitude (Y) • height (Z) • time error ( t) Δ 

It therefore follows that in three-dimensional space four satellites are needed to determine a position. Despite receiver time errors, a position on a plane can be calculated to within approx. 5 – 10 m.

Determining a Position in 3-D Space

GPS SegmentsThe Global Positioning System (GPS) comprises three segments (Figure 6): • The space segment (all functional satellites) • The control segment (all ground stations involved in the monitoring of the system: master control station, monitor stations, and ground control stations) • The user segment (all civil and military GPS users)

GPS Segments

Space Segments

Space Segments

Space SegmentsThe following information (navigation message) is transmitted by the satellite:

• Satellite time and synchronization signals • Precise orbital data (ephemeris) • Time correction information to determine the exact satellite time • Approximate orbital data for all satellites (almanac) • Correction signals to calculate signal transit time • Data on the ionosphere • Information on satellite health

Control SegmentsThe control segment (Operational Control System) consists of a Master Control Station located in the state of Colorado, five Monitor Stations equipped with atomic clocks that are spread around the globe in the vicinity of the equator, and three Ground Control Stations that transmit information to the satellites.

Control SegmentsThe most important tasks of the control segment are: • Observing the movement of the satellites and computing orbital data (ephemeris) • Monitoring the satellite clocks and predicting their behavior • Synchronizing on board satellite time • Relaying precise orbital data received from satellites in communication • Relaying the approximate orbital data of all satellites (almanac) • Relaying further information, including satellite health, clock errors etc.

Control Segments

GPS Control

User SegmentsThe signals transmitted by the satellites take approx. 67 milliseconds to reach a receiver. As the signals travel at the speed of light, their transit time depends on the distance between the satellites and the user. In order to determine the position of a user, radio communication with four different satellites is required. The relevant distance to the satellites is determined by the transit time of the signals. The receiver then calculates the user’s latitude ϕ, longitude λ, height h and time t from the range and known position of the four satellites. Expressed in mathematical terms, this means that the four unknown variables ϕ, λ, h and t are determined from the distance and known position of these four satellites.

GPS Navigation MessageThe navigation message is a continuous stream of data transmitted at 50 bits per second. Each satellite relays the following information to Earth:

• System time and clock correction values • Its own highly accurate orbital data (ephemeris) • Approximate orbital data for all other satellites (almanac) • System health, etc.

GPS Navigation MessageThe navigation message is needed to calculate the current position of the satellites and to determine signal transit times. Transmission time for the entire almanac is 12.5 minutes. A GPS receiver must have collected the complete almanac at least once to be capable of functioning (e.g. for its primary initialization).

Calculating PositionCalculations are effected in a Cartesian, three-dimensional co-ordinate system with a geocentric origin. The range of the user from the four satellites R1, R2, R3 and R4 can be determined with the help of signal transit times Δt1, Δt2, Δt3 and Δt4 between the four satellites and the user. As the locations XSat, YSat and ZSat of the four satellites are known, the user co-ordinates can be calculated.

Calculating Position

Sources of Errorsseveral causes may contribute to the overall error: • Satellite clocks: although each satellite has four atomic clocks on board, a time error of just 10 ns creates an error in the order of 3 m. • Satellite orbits: The position of a satellite is generally known only to within approx. 1 to 5 m. • Speed of light: the signals from the satellite to the user travel at the speed of light. This slows down when traversing the ionosphere and troposphere and can therefore no longer be taken as a constant.

Sources of Errors• Measuring signal transit time: The user can only determine the point in time at which an incoming satellite signal is received to within a period of approx. 10-20 ns, which corresponds to a positional error of 3-6 m. The error component is increased further still as a result of terrestrial reflection (multipath).

• Satellite geometry: The ability to determine a position deteriorates if the four satellites used to take measurements are close together. The effect of satellite geometry on accuracy of measurement is referred to as GDOP (Geometric Dilution of Precision).

Dilution of PrecisionThe accuracy with which a position can be determined using GPS in navigation mode depends, on the one hand, on the accuracy of the individual pseudo-range measurements, and on the other, on the geometrical configuration of the satellites termed as DOP (Dilution of Precision). There are several DOP designations in current use: • PDOP: Positional DOP (position in 3-D space) • HDOP: Height DOP (position on a plane) • VDOP: Vertical DOP (height only)

Dilution of Precision

• DOP is the Geometric Orientation of Satellites with respect to the Antenna• Value of DOPs are used for GPS measurement quality. Smaller values suggest Good DOPs – Values greater than 5 are consideredto be poor PDOP• Ideal condition is One Satellite directly above the Antenna and other Three Satellites are spread at 120 degree apart

Ideal Satellite GeometryIdeal Satellite GeometryN

S

W E

Good Satellite GeometryGood Satellite Geometry

Poor Satellite GeometryPoor Satellite GeometryN

S

W E

Poor Satellite GeometryPoor Satellite Geometry

Height Accuracy and Selective Availability

Position Accuracy and Selective Availability

Noise, Bias, and Blunder

Single GPS Survey/ObservationStatic Observation

– Antenna is fixed at a point– Gives higher accuracy since observation is done for long time period– A few meters level accuracy

Kinematic Observation– Antenna is moving– Just a few or single observation at a particular point– Accuracy is lower– Sometimes error is too large, few hundreds of meters

Coordinate System

World Coordinate System

Latitude, Longitude, & Height

Data Formats

• NMEA 0183 format• NMEA 2000 format

NMEA sentences are all ASCII Each sentence begins with a dollarsign ($) and ends with a carriage

return linefeed (<CR><LF>) Data is comma delimited. All commas must be included as they act

as markers Following the $ is the address field aaccc

aa is the device idGP is used to identify GPS dataccc is the sentence formatter or sentence name

Example :

$GPGGA,hhmmss.ss,llll.ll,a,yyyyy.yy,a,x,xx,x.x,x.x,M,x.x,M,x.x,xxxx*hh

NMEA (National Marine Electronic Association)

Sample GPS data in NMEA Format

$GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47– GGA Global Positioning System Fix Data– 123519 Fix taken at 12:35:19 UTC– 4807.038,N Latitude 48 deg 07.038' N– 01131.000,E Longitude 11 deg 31.000' E– 1 Fix quality: 0 = invalid– 1 = GPS fix (SPS)– 2 = DGPS fix– 3 = PPS fix– 4 = Real Time Kinematic– 5 = Float RTK– 6 = estimated (dead reckoning) (2.3 feature)– 7 = Manual input mode– 8 = Simulation mode– 08 Number of satellites being tracked– 0.9 Horizontal dilution of position– 545.4,M Altitude, Meters, above mean sea level– 46.9,M Height of geoid (mean sea level) above WGS84 ellipsoid– (empty field) time in seconds since last DGPS update– (empty field) DGPS station ID number– *47 the checksum data, always begins with *

GGA Sentence Format

$GPGSA,A,3,04,05,,09,12,,,24,,,,,2.5,1.3,2.1*39

– GSA Satellite status– A Auto selection of 2D or 3D fix (M = manual)– 3 3D fix - values include: 1 = no fix– 2 = 2D fix– 3 = 3D fix– 04,05... PRNs of satellites used for fix (space for 12)– 2.5 PDOP (dilution of precision)– 1.3 Horizontal dilution of precision (HDOP)– 2.1 Vertical dilution of precision (VDOP)– *39 the checksum data, always begins with *

GSA Sentence Format

$GPGSV,2,1,08,01,40,083,46,02,17,308,41,12,07,344,39,14,22,228,45*75– GSA Satellite status

– GSV Satellites in view– 2 Number of sentences for full data– 1 sentence 1 of 2– 08 Number of satellites in view– 01 Satellite PRN number– 40 Elevation, degrees– 083 Azimuth, degrees– 46 SNR - higher is better– for up to 4 satellites per sentence– *75 the checksum data, always begins with *

GSV Sentence Format

$GPGLL,4916.45,N,12311.12,W,225444,A,*31– GLL Geographic position, Latitude and Longitude– 4916.46,N Latitude 49 deg. 16.45 min. North– 12311.12,W Longitude 123 deg. 11.12 min. West– 225444 Fix taken at 22:54:44 UTC– A Data Active or V (void)– *31 checksum data

GLL Sentence Format