UNIT - IV GPS SURVEYING

BASIC CONCEPTS Satellite positioning Point Positioning

Satellite positioning 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 ( Fully operational since 1995

How does a GPS work?

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: Four unknowns – solve for x P, y P, z P,  t P

Kwajalein Atoll US Space Command Control Segment Hawaii Ascension Is. Diego Garcia Cape Canaveral Ground Antenna Master 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 Segment 24 satellite vehicles Six orbital planes Six orbital planes Inclined 55 o with respect to equator Inclined 55 o with respect to equator Orbits separated by 60 o Orbits separated by 60 o 20,200 km elevation above Earth 20,200 km elevation above Earth Orbital period of 11 hr 55 min Orbital period of 11 hr 55 min Five to eight satellites visible from any point on Earth Five to eight satellites visible from any point on Earth

The GPS Constellation

GPS Satellite Vehicle Four atomic clocks Four atomic clocks Three nickel-cadmium batteries Three nickel-cadmium batteries Two solar panels Two solar panels Battery charging Battery charging Power generation Power generation 1136 watts 1136 watts S band antenna—satellite control S band antenna—satellite control 12 element L band antenna—user communication 12 element L band antenna—user communication

GPS Satellite Vehicle Weight Weight 2370 pounds 2370 pounds Height Height feet feet Width Width feet including wing span feet including wing span Design life—10 years Design life—10 years Block IIR satellite vehicle assembly at Lockheed Martin, Valley Forge, PA

GPS 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 Structure GPS Signal GPS Signal Method (code) to identify each satellite Method (code) to identify each satellite The location of the satellite or some information on how to determine it The location of the satellite or some information on how to determine it Information regarding the amount of time elapsed since the signal left the satellite Information regarding the amount of time elapsed since the signal left the satellite Details on the satellite clock status Details on the satellite clock status

Important Issues to Consider Methods to encode information Methods to encode information Signal power Signal power Frequency allocation Frequency allocation Security Security Number and type of codes necessary to satisfy system requirements Number and type of codes necessary to satisfy system requirements

Overview of Satellite Transmissions All transmissions derive from a fundamental frequency of Mhz All transmissions derive from a fundamental frequency of Mhz L1 = = Mhz L1 = = Mhz L2 = = Mhz L2 = = Mhz All codes initialized once per GPS week at midnight from Saturday to Sunday All codes initialized once per GPS week at midnight from Saturday to Sunday Chipping rate for C/A is Mhz Chipping rate for C/A is Mhz Chipping rate for P(Y) is Mhz Chipping rate for P(Y) is Mhz

Schematic of GPS codes and carrier phase

GPS Signal Characteristics

Digital Modulation Methods Amplitude Modulation (AM) also known as amplitude-shift keying. This method requires changing the amplitude of the carrier phase between 0 and 1 to encode the digital signal. Amplitude Modulation (AM) also known as amplitude-shift keying. This method requires changing the amplitude of the carrier phase between 0 and 1 to encode the digital signal. Frequency Modulation (FM) also known as frequency-shift keying. Must alter the frequency of the carrier to correspond to 0 or 1. Frequency Modulation (FM) also known as frequency-shift keying. Must alter the frequency of the carrier to correspond to 0 or 1. Phase Modulation (PM) also known as phase-shift keying. At each phase shift, the bit is flipped from 0 to 1 or vice versa. This is the method used in GPS. Phase Modulation (PM) also known as phase-shift keying. At each phase shift, the bit is flipped from 0 to 1 or vice versa. This is the method used in GPS.

GPS Signal Structure Binary message format and NMEA format Binary message format and NMEA format Binary message format Binary message format Header portion (compulsory) Header portion (compulsory) Data portion (optional) Data portion (optional)

Binary message format Header format M L M L Message ID Message ID Data word count Data word count DCL0 QRAN DCL0 QRAN Header checksum Header checksum

Binary Messages Example of binary messages: Example of binary messages: Aim: To disable the pinning feature Status of pinning is seen in User setting Output(Msg ID 1012) O/P message Pinning is controlled using Nav configuration Pinning is controlled using Nav configuration (Msg ID 1221) I/P message (Msg ID 1221) I/P message

Binary messages I/p to the GPS to see the status of pinning I/p to the GPS to see the status of pinning Header format 81 ff sync word Header format 81 ff sync word 03 f4 Msg ID 03 f4 Msg ID data count data count query bit set query bit set 32 0d check sum 32 0d check sum In response to this the GPS outputs User settings output message. (least significant byte first) ff81 f ff81 f The 5 th bit in the 9 th word of the above msg gives the status of pinning

Binary message I/p message to change status of pinning I/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. 2 nd bit of the 7 th word in the data portion is set to 1 to disable the pinning 2 nd bit of the 7 th word in the data portion is set to 1 to disable the pinning The header checksum and data check sum must be correct for the message to be valid. The header checksum and data check sum must be correct for the message to be valid. Whether pining is disabled can be checked by sending the previous msg again. Now Whether pining is disabled can be checked by sending the previous msg again. Now ff81 f

NMEA messages These are standardized sentences used in context with the GPS These are standardized sentences used in context with the GPS Examples: O/P statements Examples: O/P statements GGA: GPS fix Data GGA: GPS fix Data GSA: GPS DOP and active satellite GSA: GPS DOP and active satellite GSV: GPS Satellite in view GSV: GPS Satellite in view RMC: recommended min GPS data RMC: recommended min GPS data I/P messages I/P messages IBIT Built In test command IBIT Built In test command ILOG log control ILOG log control INIT Initialization INIT Initialization IPRO Proprietary protocol IPRO Proprietary protocol

NMEA messages Sample Message $GPRMC,185203,A, ,N, ,E,0.00,121.7,221101,13.8,E*55 $ Start of sentence Type of sentence Type of sentence UTC UTC Validity Validity Latitude & orientation Latitude & orientation Longitude & orientation Longitude & orientation Speed Speed Heading Heading Date Date Magnetic variation and orientation Magnetic variation and orientation Checksum (followed by and ) Checksum (followed by and )

Anti-spoofing Anti- spoofing denies the P code by mixing with a W- code to produce Y code which can be decoded only by user having a key. Anti- spoofing denies the P code by mixing with a W- code to produce Y code which can be decoded only by user having a key. What about SPS users? What about SPS users? They use cross correlation which uses the fact that the y code are the same on both frequencies They use cross correlation which uses the fact that the y code are the same on both frequencies By correlating the 2 incoming y codes on L1 and L2 the difference in time can be ascertained By correlating the 2 incoming y codes on L1 and L2 the difference in time can be ascertained This delay is added to L1 and results in the pseudorange which contain the same info as the actual P code on L2 This delay is added to L1 and results in the pseudorange which contain the same info as the actual P code on L2

GPS Satellite Signal: L1 freq. ( Mhz) carries the SPS code and the navigation message. L1 freq. ( Mhz) carries the SPS code and the navigation message. L2 freq. ( Mhz) used to measure ionosphere delays by PPS receivers L2 freq. ( Mhz) used to measure ionosphere delays by PPS receivers 3 binary code shift L1 and/or L2 carrier phase 3 binary code shift L1 and/or L2 carrier phase The C/A code The C/A code The P code The P code The Navigation message which is a 50 Hz signal consisting of GPs satellite orbits. Clock correction and other system parameters The Navigation message which is a 50 Hz signal consisting of GPs satellite orbits. Clock correction and other system parameters

Selective Availabity Two components Two components Dither : Dither : manipulation of the satellite clock frequency manipulation of the satellite clock frequency Epsilon: Epsilon: errors imposed within the ephemeris data sent in the broadcast message errors imposed within the ephemeris data sent in the broadcast message De-activated 2, May, De-activated 2, May, 2000.

Hand Held GPS

GPS Surveying Traversing Triangulation Base - Rover Methods

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