UNIT - IV UNIT - IV GPS SURVEYINGGPS SURVEYING

BASIC CONCEPTS

Satellite positioningZ

X

Y

Greenwich Meridian

Point

Geocentre

Position vector

Mean rotation axis

Point Positioning

Satellite positioning

Z

X

Y

Greenwich Meridian

Geocentre

Mean rotation axis

interstation vector

Relative Positioning

Satellite observations

Directions: Photograph satellite against a star background. Interpolate direction to satellite from known co-ordinates

(right ascension, declination) of stars. No longer used.

Ranges: Pulsed laser (SLR), or time codes superimposed upon

microwave radio carrier signals (GPS)

Range Rate: Doppler shift in frequency of received radio signal can be

integrated to obtain change in range – related to relative position of transmitter and receiver (DORIS, Argos, SARSAT)

Basic concepts of GPS

Developers are US military and for USSR Joint Use Policy since 2004 (Defence,

Transportation) Position, Navigation & Timing (http://pnt.gov) Fully operational since 1995

How does a GPS work?Triangulation is used by surveyors to map objects and works on the following principles: suppose you measure a distance from one satellite and find it to be 21,000 kms. Given that the satellite has only a certain range or view of the earth (rather like we can see only part of the moon surface at any one time) this narrows down our possible location to a radius of 21,000 kms and centred around the satellite.

How does a GPS work?We now determine thedistance to a secondsatellite and find that tobe 22,600 kms. Thiswill also have only aselected footprint on theearth and the effectiveintersection of these towfootprints will narrowdown our position on theearth.

How does a GPS work?

Taking a measurementfrom a third satellite whichmight be 23,400 kmsaway it narrows ourposition down evenfarther, to the two pointswhere the 23,400 kmsphere cuts through thecircle formed by theintersection of the first twospheres. Consequently wecan now determine thatwe are somewhere on thecircle where these twospheres intersect.

Basic concepts of GPS

Four GPS satellites Four Ranges 3D Position & Time

How doe these satellites provide positional information?

Each satellite broadcasts its orbital position in “pseudo code”

The receiver on the ground calculates the time the signal (pseudo code) took to get from the satellite to ground and turns these time units into distance based on the speed the light travels at (“pseudorange”)

Using information from 3 to 4 satellites allows triangulation to the GPS receivers position.

Basic concepts of GPS

Observation Equation:

( ) ( ) ( )2 2 2

i i P i P i P Pc t x x y y z z - c t×∆ = − + − + − ×δ

Four unknowns – solve for xP, yP, zP, δtP

Kwajalein Atoll

US Space Command

Control SegmentControl Segment

Hawaii

Ascension Is.

Diego Garcia

Cape Canaveral

Ground AntennaMaster Control Station Monitor Station

Tasks of the ground segment:

Controlling and managing the telemetry and control stations.

Computation of ephemerids (orbit parameters) for each satellite.

Ordering satellite maneuvres. Computing the data for the almanach Determine the GPS time (Atomic hr) Communication link to the satellites

Space SegmentSpace Segment

24 satellite vehicles24 satellite vehicles Six orbital planesSix orbital planes

Inclined 55Inclined 55o o with respect to equatorwith respect to equator Orbits separated by 60Orbits separated by 60oo

20,200 km elevation above Earth20,200 km elevation above Earth Orbital period of 11 hr 55 minOrbital period of 11 hr 55 min Five to eight satellites visible from any point on Five to eight satellites visible from any point on

EarthEarth

The GPS ConstellationThe GPS Constellation

GPS Satellite VehicleGPS Satellite Vehicle

Four atomic clocksFour atomic clocks Three nickel-cadmium batteriesThree nickel-cadmium batteries Two solar panelsTwo solar panels

Battery chargingBattery charging Power generationPower generation 1136 watts1136 watts

S band antenna—satellite controlS band antenna—satellite control 12 element L band antenna—user 12 element L band antenna—user

communicationcommunication

GPS Satellite VehicleGPS Satellite Vehicle

WeightWeight 2370 pounds2370 pounds

HeightHeight 16.25 feet16.25 feet

WidthWidth 38.025 feet including wing span38.025 feet including wing span

Design life—10 yearsDesign life—10 yearsBlock IIR satellite vehicle assembly at Lockheed Block IIR satellite vehicle assembly at Lockheed Martin, Valley Forge, PAMartin, Valley Forge, PA

GPS Satellite VehicleGPS Satellite Vehicle

User segment

GPS receivers track L1 and/or L2 frequencies track C/A code for at least 4 satellites, and demodulation Time synchronization (Quartz clocks in the receivers) Decrypt satellite data from the code observations (orbit, etc.) receive P(Y) code (US Army) Compute the pseudo-range to each satellite Compute the time offset (receiver clock error) Compute the position.

GPS Signal StructureGPS Signal Structure

GPS SignalGPS Signal

Method (code) to identify each satelliteMethod (code) to identify each satellite The location of the satellite or some information The location of the satellite or some information

on how to determine iton how to determine it Information regarding the amount of time Information regarding the amount of time

elapsed since the signal left the satelliteelapsed since the signal left the satellite Details on the satellite clock statusDetails on the satellite clock status

Important Issues to ConsiderImportant Issues to Consider

Methods to encode informationMethods to encode information Signal powerSignal power Frequency allocationFrequency allocation SecuritySecurity Number and type of codes necessary to satisfy Number and type of codes necessary to satisfy

system requirementssystem requirements

Overview of Satellite TransmissionsOverview of Satellite Transmissions

All transmissions derive from a fundamental All transmissions derive from a fundamental frequency of 10.23 Mhzfrequency of 10.23 Mhz L1 = 154 L1 = 154 •• 10.23 = 1575.42 Mhz 10.23 = 1575.42 Mhz L2 = 120 L2 = 120 •• 10.23 = 1227.60 Mhz 10.23 = 1227.60 Mhz

All codes initialized once per GPS week at All codes initialized once per GPS week at midnight from Saturday to Sundaymidnight from Saturday to Sunday Chipping rate for C/A is 1.023 MhzChipping rate for C/A is 1.023 Mhz Chipping rate for P(Y) is 10.23 MhzChipping rate for P(Y) is 10.23 Mhz

Schematic of GPS codes and carrier phaseSchematic of GPS codes and carrier phase

GPS Signal CharacteristicsGPS Signal Characteristics

Digital Modulation MethodsDigital Modulation Methods

Amplitude ModulationAmplitude Modulation (AM) also known as (AM) also known as amplitude-shift keying. This method requires changing amplitude-shift keying. This method requires changing the amplitude of the carrier phase between 0 and 1 to the amplitude of the carrier phase between 0 and 1 to encode the digital signal.encode the digital signal.

Frequency ModulationFrequency Modulation (FM) also known as (FM) also known as frequency-shift keying. Must alter the frequency of the frequency-shift keying. Must alter the frequency of the carrier to correspond to 0 or 1.carrier to correspond to 0 or 1.

Phase ModulationPhase Modulation (PM) also known as phase-shift (PM) also known as phase-shift keying. At each phase shift, the bit is flipped from 0 to keying. At each phase shift, the bit is flipped from 0 to 1 or vice versa. This is the method used in GPS.1 or vice versa. This is the method used in GPS.

GPS Signal StructureGPS Signal Structure

Binary message format and NMEA formatBinary message format and NMEA format Binary message formatBinary message format

Header portion (compulsory)Header portion (compulsory) Data portion (optional)Data portion (optional)

Binary message formatBinary message format

Header formatHeader format10001000 0001 1111 11110001 1111 1111

M L M LM L M L

Message IDMessage ID

Data word countData word count

DCL0 QRANDCL0 QRAN

Header checksum Header checksum

Binary MessagesBinary Messages

Example of binary messages:Example of binary messages:Aim: To disable the pinning featureAim: To disable the pinning feature

Status of pinning is seen in User setting Output(Msg Status of pinning is seen in User setting Output(Msg ID 1012) O/P messageID 1012) O/P message

Pinning is controlled using Nav configurationPinning is controlled using Nav configuration

(Msg ID 1221) I/P message(Msg ID 1221) I/P message

Binary messagesBinary messages I/p to the GPS to see the status of pinningI/p to the GPS to see the status of pinning Header format Header format 81 ff sync word81 ff sync word 03 f4 Msg ID03 f4 Msg ID 00 00 data count00 00 data count 48 00 query bit set 48 00 query bit set 32 0d check sum32 0d check sum

In response to this the GPS outputs User settings output In response to this the GPS outputs User settings output message. (least significant byte first) message. (least significant byte first)

ff81 f403 1000 0048 ---- ---- ---- ---- 0000 ---- ----ff81 f403 1000 0048 ---- ---- ---- ---- 0000 ---- ----The 5The 5thth bit in the 9 bit in the 9thth word of the above msg gives the status word of the above msg gives the status

of pinningof pinning

Binary messageBinary message I/p message to change status of pinningI/p message to change status of pinning In the header In the header

Msg Id becomes 04 C5 (nav configuration )Msg Id becomes 04 C5 (nav configuration ) Here the message also includes a data portion.Here the message also includes a data portion.

22ndnd bit of the 7 bit of the 7 thth word in the data portion is set to 1 to disable word in the data portion is set to 1 to disable the pinningthe pinning

The header checksum and data check sum must be correct The header checksum and data check sum must be correct for the message to be valid.for the message to be valid.

Whether pining is disabled can be checked by sending Whether pining is disabled can be checked by sending the previous msg again. Nowthe previous msg again. Now

ff81 f403 1000 0048 ---- ---- ---- ---- 7800 ---- ----ff81 f403 1000 0048 ---- ---- ---- ---- 7800 ---- ----

NMEA messagesNMEA messages

These are standardized sentences used in context with the GPSThese are standardized sentences used in context with the GPS Examples: O/P statementsExamples: O/P statements

GGA: GPS fix DataGGA: GPS fix Data GSA: GPS DOP and active satelliteGSA: GPS DOP and active satellite GSV: GPS Satellite in viewGSV: GPS Satellite in view RMC: recommended min GPS data RMC: recommended min GPS data

I/P messagesI/P messages IBIT Built In test commandIBIT Built In test command ILOG log controlILOG log control INIT InitializationINIT Initialization IPRO Proprietary protocolIPRO Proprietary protocol

NMEA messagesNMEA messages

Sample MessageSample Message$GPRMC,185203,A,1907.8900,N,07533.5546,E,0.00,121.7,221101,13.8,E*55$GPRMC,185203,A,1907.8900,N,07533.5546,E,0.00,121.7,221101,13.8,E*55

$ $ Start of sentenceStart of sentence Type of sentenceType of sentence UTCUTC ValidityValidity Latitude & orientationLatitude & orientation Longitude & orientationLongitude & orientation SpeedSpeed HeadingHeading DateDate Magnetic variation and orientationMagnetic variation and orientation Checksum (followed by <CR> and <LF> )Checksum (followed by <CR> and <LF> )

Anti-spoofingAnti-spoofing

Anti- spoofing denies the P code by mixing with a W-Anti- spoofing denies the P code by mixing with a W-code to produce Y code which can be decoded only by code to produce Y code which can be decoded only by user having a key.user having a key.

What about SPS users?What about SPS users? They use cross correlation which uses the fact that the y code They use cross correlation which uses the fact that the y code

are the same on both frequenciesare the same on both frequencies By correlating the 2 incoming y codes on L1 and L2 the By correlating the 2 incoming y codes on L1 and L2 the

difference in time can be ascertaineddifference in time can be ascertained This delay is added to L1 and results in the pseudorange This delay is added to L1 and results in the pseudorange

which contain the same info as the actual P code on L2which contain the same info as the actual P code on L2

GPS Satellite Signal:GPS Satellite Signal:

L1 freq. (1575.42 Mhz) carries the SPS code and the L1 freq. (1575.42 Mhz) carries the SPS code and the navigation message.navigation message.

L2 freq. (1227.60 Mhz) used to measure ionosphere L2 freq. (1227.60 Mhz) used to measure ionosphere delays by PPS receiversdelays by PPS receivers

3 binary code shift L1 and/or L2 carrier phase3 binary code shift L1 and/or L2 carrier phase The C/A codeThe C/A code The P codeThe P code The Navigation message which is a 50 Hz signal consisting of The Navigation message which is a 50 Hz signal consisting of

GPs satellite orbits . Clock correction and other system GPs satellite orbits . Clock correction and other system parametersparameters

Selective AvailabitySelective Availabity

Two componentsTwo components Dither :Dither :

manipulation of the satellite clock frequencymanipulation of the satellite clock frequency

Epsilon: Epsilon:

errors imposed within the ephemeris data sent in the errors imposed within the ephemeris data sent in the broadcast messagebroadcast message

De-activated 2, May, 2000.De-activated 2, May, 2000.

Hand Held GPSHand Held GPS

Hand Held GPSHand Held GPS

GPS Surveying

Traversing Triangulation

Base - Rover Methods

Thank YouThank You