Module 3 - Differential Data Processing Techniques Flashcards
The pseudo-range equation for GPS measurements is as follows:
(i) Identify and describe the main observable.
Ξπ‘τ― = measured time offset between the received signal from the satellite and the signal
generated in the receiver.
What does PDOP stand for?
Position Dilution of Precision
True or False: GPS is a range-rate system
False
True or False: In GPS pseudo-ranging, the term βpseudoβ is because time is measured rather than range?
False
Calculate the expected effect of an orbit bias of 3m over a baseline length of 30 km
30000*3/20000000 = 0.0045m = 0.005m
Name one other satellite dependent bias
Satellite clock bias
Systematic errors β reduced by modelling
dion, dtrop
Measured values
delta t (Ξπ‘τ―)
Unknown quantities
dxp, dyp, dzp, Ξ΄t
Initial estimates β provisional values
xp0, yp0, zp0
Correction supplied in the navigation message (not modelled)
cdt
Known values β constants, or determined from information contained in the navigation
message
c, xi, yi, zi
True or False: Refraction of GPS signals causes systematic errors which can be modelled in order to reduce
their effects on the measurements
True
True or False: Horizontal positions are more accurately determined than vertical positions using GPS due to
the refraction effects on the signal
True
What does GDOP stand for?
Geometric dilution of precision
Describe four (4) attributes of the GPS orbits.
55 degree inclination 20200 km above the earthβs surface 6 orbital planes 4 per orbital plane (24 constellation) Period of 11h58 min
Why is this process called pseudo-ranging and not simply ranging?
Ξt; pseudo ranging is uncorrected for clock offset
Where does PDOP come from and what does it mean?β
The DOP stands for Dilution of Precision and PDOP stands for Position Dilution of Precision. PDOP
is a value derived from the square root trace of the Qxx matrix, or N-1 or (AtPA)-1. With PDOP only
the first three elements are used as these are the position elements. This value indicates the
relative geometry of the satellites and the receiver. It ranges from about 2 to 6 β the lower the
PDOP the better the geometry.
What is the most likely reason for
inaccurate position fix?
Multipath
Tropospheric and ionospheric refraction are modelled
What is/are the common feature(s) of these models?
Explain this/these features.
Explain how a GNSS user (with an old code-correlating receiver using L1 only) can adopt a setting on
the controller/receiver to avoid these residual errors after modelling from becoming huge
The common term/feature is the relationship between the error and the secant of the zenith angle.
The errors increase to βinfinityβ near the horizon and the modelling thereof becomes unstable
near the horizon. So after modelling, there would still be huge residual errors. A simple method is
for the user to set the βsatellite cut-off angleβ to 10 to 15 degrees above the horizon. All satellite
signals received below this angle will be ignored.
Give at least three errors/biases that are not explicitly contained within the GPS pseudorange model above, and state, for each one, whether a DGPS system such as OmniStar would reduce their effect on the solution?
Multipath β not reduced through differencing;
Orbit Errors β DGPS will reduce
Receiver Resolution β no, DGPS will not help
Antenna Phase centre β not an issue for code phase measurements β accuracy level is not sensitive to this
Satellite constellation geometry β DGPS wont help much as PDOP at the base station and the rover is likely to be similar, unless there are obstructions at the rover, in which case DGPS will help.