2. Topics to be Covered
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
3. At the end of 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
4. Position and Time from 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
5. Preamplifier and
Down Convertor
Clock
C/A Code
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
6. Pseudo-Range Navigation
1. Four satellites (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
7. 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
8. Geometric Dilution of Precision (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
9. 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
10. DGPS Introduction
1. The idea 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
11. •The rover receiver determines 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
12. 1. The accuracy degrades 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
13. 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
14. Sources of DGPS Corrections
1. Land-Based Beacons
2. Satellite-Based Augmentation Systems (SBAS)
3. Local Area Augmentation Systems (LAAS)
4. Internet-Based Services
DGPS
D2-S3 14
15. Post-Processed DGPS
1. Better latency
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
