#5 user position computation & dgps

#5 user position computation & dgps

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  User Position Computation& DGPS D2-S3 1
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  Topics to beCovered User Position Computation • Position, and Time from GPS • GPS Receiver Block Diagram • Pseudo-Range Navigation • Geometric Dilution of Precision (GDOP) and Visibility DGPS • DGPS introduction • DGPS elements • Real time DGPS • Sources of DGPS Corrections • Post-Processed DGPS D2-S3 2
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  At the endof this session, trainee will be able to: Describe how to get position and time from GPS Explain pseudo range navigation and geometric dilution Explain DGPS working principle and different types 3D2-S3
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  Position and Timefrom GPS 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 bits. 2. The C/A code generator repeats the same 1023-chip PRN-code sequence every millisecond. PRN codes are defined for 32 satellite identification numbers. 3. The receiver slides a replica of the code in time until there is correlation with the SV code. Correlation Example shown below. 4. 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. User Position Computation D2-S3 4
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  Preamplifier and Down Convertor Clock C/ACode Generator Data Bit Alignment Data Parity Data Decoding Satellite Positions Pseudo -Range Corrections Pseudo – Ranges Receiver Position, Velocity and Time Computations Navigation Message Mixer Position, Velocity, Time GPS Receiver Working Principle Time Measurement Data Bit Demodulation & Code Control C/A Code Measurement Antenna GPS Receiver Block Diagram  The receiver PRN code start position at the time of full correlation is the time of arrival (TOA) of the SV PRN at receiver.  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. D2-S3 5
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  Pseudo-Range Navigation 1. Foursatellites (normal navigation) can be used to determine three position dimensions and time. Position dimensions are computed by the receiver in Earth-Centred, Earth-Fixed X, Y, Z (ECEF XYZ) coordinates. 2. Five or more satellites can provide position, time and redundancy. 3. More SVs can provide extra position fix certainty and can allow detection of out-of-tolerance signals under certain circumstances. User Position Computation D2-S3 6
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  Geometric Dilution (GD) 1.The geometric dilution can be calculated for any instantaneous satellite configuration as seen from particular user’s location. 2. The world median value of the geometric dilution factor (for the nominal constellation) is about 2.7. This quantity is usually called PDOP or Position Dilution Of Precision. 3. Typically dilution factor range from 1.5 to 8. 4. The variations in this dilution factor are typically much greater than the variation in ranging errors. User Position Computation D2-S3 7
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  Geometric Dilution ofPrecision (GDOP) and Visibility 1. The volume of the shape described by the unit-vectors from the receiver to the SVs used in a position fix is inversely proportional to GDOP. 2. Poor GDOP, a large value representing a small unit vector-volume, results when angles from receiver to the set of SVs used are similar. 3. Good GDOP, a small value representing a large unit-vector-volume, results when angles from receiver to SVs are different. 4. For planning purposes GDOP is often computed from Almanacs and an estimated position. 5. Estimated GDOP does not take into account obstacles that block the line-of-sight from the position to the satellites, which may not be realizable in the field User Position Computation D2-S3 8
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  GDOP vs visibility In general, ranging errors from the SV signals are multiplied by the appropriate GDOP term to estimate the resulting position or time error. Various GDOP terms can be computed from the navigation covariance matrix.  GDOP Components Position Dilution of Precision (3-D), sometimes the Spherical DOP PDOP Horizontal Dilution of Precision (Latitude, Longitude) HDOP Vertical Dilution of Precision (Height) VDOP Time Dilution of Precision (Time) TDOP User Position Computation D2-S3 9
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  DGPS Introduction 1. Theidea behind all differential positioning is to correct bias errors at one location with measured bias errors at a known position. 2. A reference receiver, or base station, computes corrections for each satellite signal. 3. DGPS implementations require software in the reference receiver that can track all SVs in view and form individual pseudo-range corrections for each SV. 4. 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. 5. Applying a simple position correction from the reference receiver to the remote receiver has limited effect at useful ranges. DGPS D2-S3 10
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  •The rover receiverdetermines its location as it move around, just as the basic GPS receiver determines its position. •A radio receiver in the rover to acquire the differential corrections from the base station. Extra software to combine these corrections with the rover’s computed pseudo-ranges. The Roving Receiver •The reference receiver or base station is a GPS receiver located at a fixed position that has been surveyed or exactly determined by post-processing or some other means. The Reference Receiver •Tracks all the satellites in view and measures their pseudo-ranges. •Solves the GPS problem in reverse to determine what the pseudo-ranges should be. •Generates a list of corrections needed to make the measured pseudo-range values accurate for all visible satellites. •Communicates the correction information to the rover(s). The base station DGPS elements DGPS D2-S3 11
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  1. The accuracydegrades gradually as the rover gets farther from the base station, primarily due to increasing differences in the atmospheric conditions. 2. Factors to consider in selecting the location for the base station include: 3. Differential corrections should be computed at the reference station and applied at the remote receiver at an update rate that is less than the correlation time of SA. Suggested DGPS update rates are usually less than twenty seconds. 4. DGPS corrections are often transmitted in a standard format specified by the Radio Technical Commission Marine (RTCM). 5. Differential corrections may be used in real-time or later, with post-processing techniques. 6. Private DGPS services use leased FM sub-carrier broadcasts, satellite links, or private radio-beacons for real- time applications Clear view to the sky Proximity to your working areas Absence of RF interference Minimal sources of multipath DGPS D2-S3 12
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  Real time DGPS 1.For real-time DGPS, the correction data usually is transmitted via a radio link from the base station to the rover. 2. Radio provides the fastest path (and usually the most convenient) and minimizes solution “latency. 3. The data typically is transmitted in a format defined by RTCM-104(Radio Technical Commission for Maritime Services). 4. The structure is similar to that used for the data transmitted by the GPS satellites. DGPS D2-S3 13
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  Sources of DGPSCorrections 1. Land-Based Beacons 2. Satellite-Based Augmentation Systems (SBAS) 3. Local Area Augmentation Systems (LAAS) 4. Internet-Based Services DGPS D2-S3 14
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  Post-Processed DGPS 1. Betterlatency 2. Post-processed DGPS can achieve better accuracy by using multiple base observations from before and after the measurement, as well as using more sophisticated algorithms. 3. Post-processing techniques require raw GPS base data to be stored in digital files that are later processed against raw GPS rover files by specialized software. DGPS D2-S3 15
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