GPS SURVEYING FROM SATELLITE GEODESY BY GUNTER SEEBER

1. UNIT - IVUNIT - IV
GPS SURVEYINGGPS SURVEYING

2. BASIC CONCEPTS
Satellite positioning
Z
X
Y
Greenwich
Meridian
Point
Geocentre
Position
vector
Mean rotation
axis
Point Positioning

3. Satellite positioning
Z
X
Y
Greenwich
Meridian
Geocentre
Mean
rotation
axis
interstation
vector
Relative Positioning

4. 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)

5. 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

6. 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.

7. How does a GPS work?
We now determine the
distance to a second
satellite and find that to
be 22,600 kms. This
will also have only a
selected footprint on the
earth and the effective
intersection of these tow
footprints will narrow
down our position on the
earth.

8. How does a GPS work?
Taking a measurement
from a third satellite which
might be 23,400 kms
away it narrows our
position down even
farther, to the two points
where the 23,400 km
sphere cuts through the
circle formed by the
intersection of the first two
spheres. Consequently we
can now determine that
we are somewhere on the
circle where these two
spheres intersect.

9. Basic concepts of GPS
Four GPS satellites

Four Ranges

3D Position & Time

10. 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.

11. 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

13. Kwajalein Atoll
US Space Command
Control SegmentControl Segment
Hawaii
Ascension
Is.
Diego Garcia
Cape Canaveral
Ground AntennaMaster Control Station Monitor Station

14. 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

15. Space SegmentSpace Segment
24 satellite vehicles24 satellite vehicles
 Six orbital planesSix orbital planes
 Inclined 55Inclined 55oo
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 onFive to eight satellites visible from any point on
EarthEarth

16. The GPS ConstellationThe GPS Constellation

17. 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—user12 element L band antenna—user
communicationcommunication

18. 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 years
Block IIR satellite vehicle assembly at LockheedBlock IIR satellite vehicle assembly at Lockheed
Martin, Valley Forge, PAMartin, Valley Forge, PA

19. GPS Satellite VehicleGPS Satellite Vehicle

20. 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.

21. 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 informationThe location of the satellite or some information
on how to determine iton how to determine it
 Information regarding the amount of timeInformation 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

22. 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 satisfyNumber and type of codes necessary to satisfy
system requirementssystem requirements

23. Overview of Satellite TransmissionsOverview of Satellite Transmissions
 All transmissions derive from a fundamentalAll transmissions derive from a fundamental
frequency of 10.23 Mhzfrequency of 10.23 Mhz
 L1 = 154L1 = 154 •• 10.23 = 1575.42 Mhz10.23 = 1575.42 Mhz
 L2 = 120L2 = 120 •• 10.23 = 1227.60 Mhz10.23 = 1227.60 Mhz
 All codes initialized once per GPS week atAll 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

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

25. GPS Signal CharacteristicsGPS Signal Characteristics

26. Digital Modulation MethodsDigital Modulation Methods
 Amplitude ModulationAmplitude Modulation (AM) also known as(AM) also known as
amplitude-shift keying. This method requires changingamplitude-shift keying. This method requires changing
the amplitude of the carrier phase between 0 and 1 tothe 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 thefrequency-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 tokeying. 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.

27. 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)

28. Binary message formatBinary message format
Header formatHeader format
10001000 0001 1111 11110001 1111 1111
M L M LM L M L
Message IDMessage ID
Data word countData word count
DCL0 QRANDCL0 QRAN
Header checksumHeader checksum

29. 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(MsgStatus 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

30. 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 formatHeader 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 set48 00 query bit set
32 0d check sum32 0d check sum
In response to this the GPS outputs User settings outputIn 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 9bit in the 9thth
word of the above msg gives the statusword of the above msg gives the status
of pinningof pinning

31. Binary messageBinary message
 I/p message to change status of pinningI/p message to change status of pinning
 In the headerIn 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 7bit of the 7thth
word in the data portion is set to 1 to disableword in the data portion is set to 1 to disable
the pinningthe pinning
 The header checksum and data check sum must be correctThe 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 sendingWhether 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 ---- ----

32. 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 dataRMC: 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

33. 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> )

34. 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 bycode 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 codeThey 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 theBy 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 pseudorangeThis 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

35. GPS Satellite Signal:GPS Satellite Signal:
 L1 freq. (1575.42 Mhz) carries the SPS code and theL1 freq. (1575.42 Mhz) carries the SPS code and the
navigation message.navigation message.
 L2 freq. (1227.60 Mhz) used to measure ionosphereL2 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 ofThe Navigation message which is a 50 Hz signal consisting of
GPs satellite orbits . Clock correction and other systemGPs satellite orbits . Clock correction and other system
parametersparameters

36. 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 theerrors imposed within the ephemeris data sent in the
broadcast messagebroadcast message
 De-activated 2, May, 2000.De-activated 2, May, 2000.

37. Hand Held GPSHand Held GPS

38. Hand Held GPSHand Held GPS

39. GPS Surveying
 Traversing
 Triangulation
Base - Rover Methods

42. Thank YouThank You