7/29/2019 Es 201ppt Gps

1/33

Introduction to GPS

7/29/2019 Es 201ppt Gps

2/33

GPS - a product of Strategic Defense Initiative(SDI) (also known as Star Wars of Ronald

Regan) GPS is funded and controlled by the U. S.

Department of Defense (DOD) but can be used

by civilians for:- positioning

- georeferencing

- navigation

- time

- frequency control

7/29/2019 Es 201ppt Gps

3/33

The GPS is an earth-orbiting-satellite based

system fully operational since 1993 Signals available anywhere on the earth, day-

and-night

Used to determine:

position, altitude above the ellipsoid andprecise time

GPS satellites circle the earth twice a day,20,200 km above the earth a speed of about11300 km/hr (or about satellites moving at 4

km/s)

7/29/2019 Es 201ppt Gps

4/33

Segments of the GPS

1. Space Segment:

Six orbital planes(with nominally four

SVs in each) equally spaced (60

deg. apart)

inclined at about 55

deg. with respect tothe equatorial plane

12,600 miles altitude

7/29/2019 Es 201ppt Gps

5/33

Their configuration provides user between 5-8satellites visible from any point on the earth

The location of each satellite in space is known

The orbits are carefully planned and constantlyupdated so that actual location is never off by

much from the intended location

7/29/2019 Es 201ppt Gps

6/33

Each satellite constantly sends radiosignals from space, announcing its number,

and the time that signal was sent

The distance from each satellite to thereceiver is calculated by comparing the time

the signal says it was sent with the time thereceiver picks up the signal

The time difference is multiplied by the speed

of light to get the distance from satellite toreceiver

This is done for each satellite the receivercan see

7/29/2019 Es 201ppt Gps

7/33

The known distances and locations of eachvisible satellite are used to locate the position

of the receiver We can place ourselves anywhere on a

sphere around one satellite once we know thedistance to the satellite

Known distances from two satellites will placeus on a circle that is the intersection of twospheres

Known distances from three satellites willplace us in two points, which is theintersection of three spheres

7/29/2019 Es 201ppt Gps

8/33

We may be able to eliminate one point as being impractical, suchas out in space or deep undergound. With one gone, the othermust be correct

7/29/2019 Es 201ppt Gps

9/33

Multi-Satellite Ranging

1 range puts user

on the sphericalface of the cone.

Intersecting with

a 2nd rangerestricts user to

the circular arcs.

A 3rd range

constrains userto 1 of the 2

points.Which point is determined

by sanity 1 point

obviously wrong.Pictures courtesy http://giswww.pok.ibm.com/gps

7/29/2019 Es 201ppt Gps

10/33

Three satellites are sufficient, at leasttheoretically, to provide receiver location

More satellites simply add confirmation to thereceiver location

In practice, the more satellites the better Four satellites are the minimum to secure

only one, absolutely technically,trigonometrically unambiguous location (3Dposition)

7/29/2019 Es 201ppt Gps

11/33

2. Control Segment:

5 ground stations located around the world

monitor the GPS satellites

check operational health

master station transmits corrections tosatellites

7/29/2019 Es 201ppt Gps

12/33

7/29/2019 Es 201ppt Gps

13/33

7/29/2019 Es 201ppt Gps

14/33

3. User Segment:

The user segment consists of the GPS receivers

and the user community

Garmin Etrex Vista

$200-300

Trimble GPS systems

$5000-7000

7/29/2019 Es 201ppt Gps

15/33

The GPS receivers convert satellite signals into

positions, velocity and time estimates Four satellites are required to compute the four

dimensions of X, Y, Z (position) and T (time)

GPS receivers are used for navigation,surveying, time dissemination and otherresearch

Navigation receivers are made for aircraft, ships,

and ground vehicles and for hand carrying byindividuals

7/29/2019 Es 201ppt Gps

16/33

The distance from each satellite to thereceiver is calculated by comparing the time

the signal says it was sent with the time thereceiver picks up the signal

The time difference is multiplied by the speed

of light to get the distance from satellite toreceiver

This is done for each satellite the receiver can

receive signals The known distances and locations of each

visible satellite are used to locate the positionof the receiver

7/29/2019 Es 201ppt Gps

17/33

7/29/2019 Es 201ppt Gps

18/33

Clock data parameters describe the SV atomic clockand its relationship to GPS time

The GPS Navigation Message consists of time-tagged data bits marking the time of itstransmission by the satellite and includes:

Ephemeris data parameters describe SV orbits for

short sections of the satellite orbits An ionospheric model that is used in the receiver

to approximates the phase delay through theionosphere at any location and time

The amount to which GPS Time is offset fromUniversal Coordinated Time. This correction canbe used by the receiver to set UTC to within 100

nanoseconds

7/29/2019 Es 201ppt Gps

19/33

GPS Satellite Signals and Data

The satellites transmit two microwave carriersignals

The L1 frequency (1575.42 MHz) carries the

navigation message, the SPS code signals knownas the C/A (coarse acquisition) Code, the P(precise) Code used for the PPS

The L2 frequency (1227.60 MHz) carries the P

Code used for the PPS. The phase differencebetween the P-Code on L1 and L2 is used tomeasure the ionospheric delayby PPS equippedreceivers tracking both frequencies

7/29/2019 Es 201ppt Gps

20/33

7/29/2019 Es 201ppt Gps

21/33

How position is determined?

Position dimensions are computed by the

receiver in Earth-Centered, Earth-Fixed X, Y, Z(ECEF XYZ) coordinates

Position in XYZ is converted within the

receiver to geodetic latitude, longitude andheight above the ellipsoid

Latitude and longitude are usually provided in

the geodetic datum on which GPS is based(WGS-84)

7/29/2019 Es 201ppt Gps

22/33

7/29/2019 Es 201ppt Gps

23/33

Receivers can often be set to convert to otheruser-required datums

Position offsets of hundreds of meters can resultfrom using the wrong datum

Receiver position is computed from the SV

positions, the measured pseudo-ranges, and areceiver position estimate

Four satellites allow computation of three positiondimensions and time

Three satellites could be used determine threeposition dimensions with a perfect receiver clock

7/29/2019 Es 201ppt Gps

24/33

In practice this is rarely possible and three SVsare used to compute a two-dimensional,horizontal fix (in latitude and longitude) given an

assumed height

This is often possible at sea or in altimeterequipped aircraft

Five or more satellites can provide position, timeand redundancy

Twelve channel receivers allow continuoustracking of all available satellites, includingtracking of satellites with weak or occasionallyobstructed signals

7/29/2019 Es 201ppt Gps

25/33

GPS accuracy

Selective Availability (or SA) - introduced

intentional errors of up to a hundred meters intothe publicly available navigation signals

disabled since 1 May 2000

WAAS (Wide area Augmentation System), since

2000 - accuracy to 2m horiozontal (not in India)

DGPS (Differential GPS): within cm accuracy

7/29/2019 Es 201ppt Gps

26/33

GPS Errors

GPS errors are a combination of noise, bias,

and blunders

Noise Errors: are the combined effect of PRN(pseudo-random noise ) ( 1 m) and noise

within the receiver noise ( 1 m)

Bias Errors: SV clock errors uncorrected byControl Segment can result in one meter

errors in position.

Tropospheric delays: 1 m position error

7/29/2019 Es 201ppt Gps

27/33

7/29/2019 Es 201ppt Gps

28/33

Multipath: 0.5 m of position error

Multipath is caused by reflected signals from

surfaces near the receiver that can eitherinterfere with or be mistaken for the signal thatfollows the straight line path from the satellite

Blunders: can result in errors of hundred of kms

User mistakes, including incorrect geodeticdatum selection, can cause errors from 1-100 m

Receiver errors from software or hardwarefailures can cause blunder errors of any size.

7/29/2019 Es 201ppt Gps

29/33

Different Geometric Dilution of Precision (GDOP)

7/29/2019 Es 201ppt Gps

30/33

3

20

30

5

1

21

14

26

25

N

Horizon

45 above horizon

SATELLITE SKYPLOT

20

25

Satellite is unavailable

Satellite is available but not used

Satellite is available and used

7/29/2019 Es 201ppt Gps

31/33

7/29/2019 Es 201ppt Gps

32/33

7/29/2019 Es 201ppt Gps

33/33