Satellite Communications-IIISatellite Radio Navigation and GPSSatellite Radio Navigation and GPS

Dr. Nasir D. Gohar

http://electronics.howstuffworks.com/gps.htm

http://www.colorado.edu/geography/gcraft/notes/gps/gps_ftoc.html

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WHAT IS SATELLITE NAVIGATION? Navigation- art or science of plotting, ascertaining, or directing of movements (knowing your whereabouts and being able to find your way around)

Celestial Navigation – Direction and distances determined with timed sighting of stars Wandering – Technique used by most of us while at new place

Piloting – Fixing position and direction wrt familiar and significant landmarks Radio / Electronic Navigation – Position is determined by measuring the travel time of radio wave as it moves from Tx to Rx

Terrestrial Systems such as Decca, Omega, Loran etc. Satellite Systems such as LEO based Navy Transit GPS, MEO based Navstar GPS and Russian Counter Part,….

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LORAN – LOng RAnge Navigation Most Effective, Reliable, and Accurate Terrestrial System LORAN-A, Developed during World War –II LORAN-C, developed in 1980’s and used for recreational aircrafts and ships

Principle: The elapsed time of coded signals from four land-based Txs, whose locations are known, at any Rx determines the position of the Rx based on Tri-laterationTri-lateration

Problems & Limitations:

Atmospheric Conditions and Multipath Transmission No Global Coverage

U r here.

Satellite Communications-IIINavstar GPS

Navigation System with Time and Ranging & Global Positioning System

Satellite based Navigation, 3D positioning, and Time-Distribution System Owned by USA DoD (maintained by US Air Force), 1994 (formally declared 1995) Provides continuous, highly precise position, velocity, and time information to any user with a GPS Rx, at any time, at any place (land, sea, air, space) in all weather conditions

Satellite Communications-IIINavstar GPS

Navigation System with Time and Ranging & Global Positioning System

Navstar GPS Services – Two level service or accuracy

Standard Positioning Service Civil users worldwide use the SPS without charge or restrictions. Most receivers are capable of receiving and using the SPS signal. The SPS accuracy is intentionally degraded by the DOD by the use of Selective Availability. SPS Predictable Accuracy

100 meter horizontal accuracy 156 meter vertical accuracy 340 nanoseconds time accuracy

Precise Positioning Service Authorized users with cryptographic equipment and keys and specially equipped receivers use the Precise Positioning System. U. S. and Allied military, certain U. S. Government agencies, and selected civil users specifically approved by the U. S. Government, can use the PPS. PPS Predictable Accuracy

22 meter Horizontal accuracy 27.7 meter vertical accuracy 200 nanosecond time (UTC) accuracy

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Navstar GPS Segments Space Segment-1

The Space Segment of the system consists of the 24 GPS satellites (21 in Operation, 3 as spare) These space vehicles (SVs) send radio signals from space GPS Satellites orbit the earth in 12 hours The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day The orbit altitude (20, 200 km) is such that the satellites repeat the same track and configuration over any point approximately each 12 hours (4 minutes earlier each day) Six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about fifty-five (55) degrees with respect to the equatorial planeFive and eight SVs are visible from any point on the earth

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Navstar GPS Segments Space Segment-2

Satellite Relative Positions

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Navstar GPS Segments Space Segment-3

The Mercator Projection of Navstar GPS Satellite Orbits: 3 GPS satellites provide horizontal (two-dimensional) location of a GPS Rx where as four GPS satellites provide its 3D position (including altitude)

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Navstar GPS Segments Control Segment

The Control Segment consists of a system of tracking stations located around the world The Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado These monitor stations measure signals from the SVs which are incorporated into orbital models for each satellites The models compute precise orbital data (ephemeris) and SV clock corrections for each satellite The Master Control station uploads ephemeris and clock data to the SVs The SVs then send subsets of the orbital ephemeris data to GPS receivers over radio signals

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Navstar GPS Segments User Segment

Navigation in three dimensions is the primary function of GPS GPS User Segment consists of the GPS receivers and the user community such as aircrafts, ships, ground vehicles, and for hand carrying by individuals GPS receivers convert SV signals into position, velocity, and time estimates Four satellites are required to compute the four dimensions of X, Y, Z (position) and Time GPS receivers are used for navigation, positioning, time dissemination, and other research projects Precise positioning is possible using GPS receivers at reference locations providing corrections and relative positioning data for remote receivers - Surveying, geodetic control, and plate tectonic studies are examples Time and frequency dissemination, based on the precise clocks on board the SVs and controlled by the monitor stations, is another use for GPS - Astronomical observatories, telecommunications facilities, and laboratory standards can be set to precise time signals or controlled to accurate frequencies by special purpose GPS receivers Research projects have used GPS signals to measure atmospheric parameters

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GPS Satellite SignalsThe SVs transmit two MW carrier signals:-

The L1 frequency (1575.42 MHz) carries the navigation message and the SPS code signalsThe L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by PPS equipped receivers

Three binary codes shift the L1 and/or L2 carrier phase :- The C/A Code (Coarse Acquisition) modulates the L1 carrier phase

The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) Code This noise-like code modulates the L1 carrier signal, "spreading" the spectrum over a 1 MHz bandwidthThe C/A code repeats every 1023 bits (one millisecond) There is a different C/A code PRN for each SV. GPS satellites are often identified by their PRN number, the unique identifier for each pseudo-random-noise codeThe C/A code that modulates the L1 carrier is the basis for the civil SPS

The P-Code (Precise) modulates both the L1 and L2 carrier phases The P-Code is a very long (seven days) 10 MHz PRN code In the Anti-Spoofing (AS) mode of operation, the P-Code is encrypted into the Y-Code The encrypted Y-Code requires a classified AS Module for each receiver channel and is for use only by authorized users with cryptographic keys The P (Y)-Code is the basis for the PPS

The Navigation Message also modulates the L1-C/A code signal -The Navigation Message is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock

corrections, and other system parameters.

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GPS Satellite Signals

Satellite Communications-IIIGPS Satellite Data and its Format

Satellite Communications-IIIGPS Satellite Data and its Format The GPS Navigation Message consists of time-tagged data bits marking the time of transmission of each subframe at the time they are transmitted by the SV A data bit frame consists of 1500 bits divided into five sub-frames each carrying 300 bits

Data bit sub-frames (300 bits transmitted over six seconds) contain parity bits that allow for data checking and limited error correctionThree six-second sub-frames contain orbital and clock data

SV Clock corrections are sent in sub-frame one Precise SV orbital data sets (ephemeris data parameters) for the transmitting SV are sent in sub-frames two and three

Sub-frames four and five are used to transmit different pages of system data

A data frame is transmitted every thirty seconds An entire set of twenty-five frames (125 sub-frames) makes up the complete Navigation Message that is sent over a 12.5 minute period

Clock data parameters describe the SV clock and its relationship to GPS time (Clock Algorithm)Ephemeris data parameters describe SV orbits for short sections of the satellite orbits

Normally, a receiver gathers new ephemeris data each hour, but can use old data for up to four hours without much error The ephemeris parameters are used with an algorithm that computes the SV position for any time within the period of the orbit described by the ephemeris parameter set

Satellite Communications-IIIGPS Satellite Astronomical Almanac Almanacs are approximate orbital data parameters for all SVs The ten-parameter almanacs describe SV orbits over extended periods of time (useful for months in some cases) and a set for all SVs is sent by each SV over a period of 12.5 minutes (at least) Signal acquisition time on receiver start-up can be significantly aided by the availability of current almanacs The approximate orbital data is used to preset the receiver with the approximate position and carrier Doppler frequency (the frequency shift caused by the rate of change in range to the moving SV) of each SV in the constellation

Satellite Communications-IIIGPS Satellite Astronomical Almanac Almanacs are approximate orbital data parameters for all SVs The ten-parameter almanacs describe SV orbits over extended periods of time (useful for months in some cases) and a set for all SVs is sent by each SV over a period of 12.5 minutes (at least) Signal acquisition time on receiver start-up can be significantly aided by the availability of current almanacs The approximate orbital data is used to preset the receiver with the approximate position and carrier Doppler frequency (the frequency shift caused by the rate of change in range to the moving SV) of each SV in the constellation

Phase Delay due to Ionosphere - Each complete SV data set includes an ionospheric model that is used in the receiver to approximates the phase delay through the ionosphere at any location and time

GPS Time Offset from Universal Coordinated Time (UTC) - Each SV sends the amount to which GPS Time is offset from Universal Coordinated Time. This correction can be used by the receiver to set UTC to within 100 ns

Satellite Communications-IIIGPS Satellite Grouping Three Distinct Groups and one Sub-group of Navstar GPS satellites

11 Block-I Group satellites were prototypes and just for testing purpose Block-II Group satellites were first set of fully functional satellites with cesium atomic clocks

Can detect certain errors and provide alarms using coded messages Can operate for about 3.5 days between receiving updates and corrections from Control Segment

Block IIa satellites are more intelligent and can go for 180 days between uploads Block IIR satellites are similar to Block-IIa satellites except having autonomous navigation capabilities

GPS Satellite Identification Three Identifying Numbers

Navstar Number identifying the specific satellite onboard HW SV Number is space vehicle number assigned in the order of vehicle launch PRN Code Number is a unique integer number used for encrypting the signal from satellite

Satellite Communications-IIIGPS Satellite Receiver-1 The GPS receiver produces replicas of the C/A and/or P (Y)-Code Each PRN code is a noise-like, but pre-determined, unique series of bitsThe receiver produces the C/A code sequence for a specific SV with some form of a C/A code generator Modern receivers usually store a complete set of pre-computed C/A code chips in memory, but a hardware, shift register, implementation can also be used

Satellite Communications-IIIGPS Satellite Receiver-2 A GPS receiver uses the detected signal power in the correlated signal to align the C/A code in the receiver with the code in the SV signal Usually a late version of the code is compared with an early version to insure that the correlation peak is tracked. A phase locked loop that can lock to either a positive or negative half-cycle (a bi-phase lock loop) is used to demodulate the 50 HZ navigation message from the GPS carrier signal The same loop can be used to measure and track the carrier frequency (Doppler shift) and by keeping track of the changes to the numerically controlled oscillator, carrier frequency phase can be tracked and measured The receiver PRN code start position at the time of full correlation is the time of arrival (TOA) of the SV PRN at receiverThis TOA is a measure of the range to SV offset by the amount to which the receiver clock is offset from GPS time This TOA is called the pseudo-range

Data Bit Demodulation and C/A Code Control

Satellite Communications-IIIGPS Satellite Receiver-2 The C/A Code Generator

The C/A code generator produces a different 1023 chip sequence for each phase tap setting In a shift register implementation the code chips are shifted in time by slewing the clock that controls the shift registers In a memory lookup scheme the required code chips are retrieved from memory

C/A Code Phase Assignments The C/A code generator repeats the same 1023-chip PRN-code sequence every millisecond PRN codes are defined for 32 satellite identification numbers

C/A Code PRN Chips The receiver slides a replica of the code in time until there is correlation with the SV code.

Correlation Animation (250k)

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Satellite Communications-IIIGPS Satellite Ranging-1 The GPS Pseudo Ranging and Rx Clock Bias

Position is determined from multiple pseudo-range measurements at a single measurement epoch The pseudo range measurements are used together with SV position estimates based on the precise orbital elements (the ephemeris data) sent by each SVThis orbital data allows the receiver to compute the SV positions in three dimensions at the instant that they sent their respective signals Four satellites (normal navigation) can be used to determine three position dimensions and timePosition dimensions are computed by the receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ) coordinatesTime is used to correct the offset in the receiver clock, allowing the use of an inexpensive receiver clock SV Position in XYZ is computed from four SV pseudo-ranges and the clock correction and ephemeris data

Satellite Communications-IIIGPS Satellite Ranging-2 The GPS Pseudo Ranging and Rx Clock Bias Receiver position is

computed from the SV positions, the measured pseudo-ranges (corrected for SV clock offsets, iono-spheric delays, and relativistic effects), and a receiver position estimate (usually the last computed

receiver position)

Satellite Communications-IIIGPS Satellite Ranging-3 The GPS Rx 3D Position Calculation

Satellite Communications-IIIGPS Sources of Errors GPS errors are a combination of noise, bias, blunders

Noise errors are the combined effect of PRN code noise (around 1 meter) and noise within the receiver noise (around 1 meter) Noise and bias errors combine, resulting in typical ranging errors of around fifteen meters for each satellite used in the position solution

Satellite Communications-IIIDifferential GPS - The idea behind all differential positioning is to correct bias errors at one location with measured bias errors at a known position. A reference receiver, or base station, computes corrections for each satellite signal.

Because individual pseudo-ranges must be corrected prior to the formation of a navigation solution, DGPS implementations require software in the reference receiver that can track all SVs in view and form individual pseudo-range corrections for each SV. These corrections are passed to the remote, or rover, receiver which must be capable of applying these individual pseudo-range corrections to each SV used in the navigation solution. Applying a simple position correction from the reference receiver to the remote receiver has limited effect at useful ranges because both receivers would have to be using the same set of SVs in their navigation solutions and have identical GDOP terms (not possible at different locations) to be identically

affected by bias errors

Bias ErrorsSelective Availability (SA)

SA is the intentional degradation of the SPS signals by a time varying bias. SA is controlled by the DOD to limit accuracy for non-U. S. military and government users. The potential accuracy of the C/A code of around 30 meters is reduced to 100 meters (two standard deviations). The SA bias on each satellite signal is different, and so the resulting position solution is a function of the combined SA bias from each SV used in the navigation solution. Because SA is a changing bias with low frequency terms in excess of a few hours, position solutions or individual SV pseudo-ranges cannot be effectively averaged over periods shorter than a few hours. Differential corrections must be updated at a rate less than the correlation time of SA (and other bias errors).

Other Bias Error sources; SV clock errors uncorrected by Control Segment can result in one meter errors. Ephemeris data errors: 1 meter Tropospheric delays: 1 meter. The troposphere is the lower part (ground level to from 8 to 13 km) of the atmosphere that experiences the changes in temperature, pressure, and humidity associated with weather changes. Complex models of tropospheric delay require estimates or measurements of these parameters. Unmodeled ionosphere delays: 10 meters. The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized air. The transmitted model can only remove about half of the possible 70 ns of delay leaving a ten meter un-modeled residual. Multipath: 0.5 meters. Multipath is caused by reflected signals from surfaces near the receiver that can either interfere with or be mistaken for the signal that follows the straight line path from the satellite. Multipath is difficult to detect and sometime hard to avoid.

Blunders

Blunders can result in errors of hundred of kilometers Control segment mistakes due to computer or human error can cause errors from one meter to hundreds of kilometers User mistakes, including incorrect geodetic datum selection, can cause errors from 1 to hundreds of meters Receiver errors from software or hardware failures can cause blunder errors of any size