GPS processing techniques & some applications

PROCESSING TECHNIQUES

Vrince Vimal,CSED BAHIR DAR
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PSEUDO-RANGE PROCESSING

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•It is a measure of the range, or distance, between the
GPS receiver and the GPS satellite

OR
•It is the time shift required to line up a replica of the code

generated in the receiver with the received code from the
satellite multiplied by the speed of light

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•Either the P-code or the C/A-code can be used
for measuring the pseudo-range
•When the PRN code is transmitted from the satellite,
the receiver generates an exact replica of that code.
•Pseudo-ranges are measured by a correlation detector
that controls a delay lock loop which maintains the
alignment (correlation peak) between the receivergenerated code replica and the actual incoming code
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•Let the satellite sends a signal at a certain time tsi
•Signal will be received later at time tu
•The distance between the user and the satellite i

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Carrier-phase measurements

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•Carrier beat phase is the phase of the signal which
remains when the incoming Doppler-shifted satellite
carrier is differenced (beat) with the constant
frequency generated in the receiver.
•The ranges to the satellites can be obtained through
the carrier phases.
•The precision of carrier beat phase measurements is
much higher than the precision of code pseudo-ranges
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•There is, however, one problem. The carriers are just
pure sinusoidal waves, which means that all cycles look
the same.
•when it is switched on, cannot determine the total number
of the complete cycles between the satellite and receiver.
It can only measure a fraction of a cycle very accurately
(less than 2 mm),

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•The initial number of complete cycles remains unknown,
this is known as the initial cycle ambiguity, or the ambiguity

Bias
•If the initial cycle ambiguity parameters are resolved,

accurate range measurements can be obtained,
which lead to accurate position determination

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Ambiguity-Resolution Techniques
•Centimeter-level positioning accuracy could be achieved
with the carrier-phase observables.

• This requires successful determination of the initial
integer ambiguity parameters, This process is commonly
known as ambiguity resolution.

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•Earlier high-precision GPS relative positioning with
carrierphase observables was carried out using long
observational time spans (typically a few hours)
•This allows for the receiver-satellite geometry to
change considerably, which helps in separating the
ambiguity parameters from the baseline solution

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The correct integer values were simply obtained by
rounding off the real-valued numbers to the nearest

Integers.
•But this method is very time consuming.

•To over come this two methods are used:1. Receiver initialization

2. Antenna swap methods

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Receiver initialization
•If the base line is known, The ambiguity parameters are
determined by simply occupying the two end points of the
known baseline with the base and the rover receivers
for a short period of time.
•Than the rover receiver can move to the points to be
surveyed

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Antenna swap methods
•It used for a fast and reliable determination of

the initial ambiguity parameters.
•Initially receiver is set over the known point while setting
up the rover within a few meters from it
•About 1-minute simultaneous GPS data is then to be
collected at both receivers.

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•After the data has been collected, the two Antennas are
exchanged.

•With this new setup, another simultaneous 1-minuteGPS
data, at the previous rate, is collected by both receivers.

•Then the receivers are returned to their original setup
•Finally the data is downloaded into the PC processing

software

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On-the-fly ambiguity resolution
•Initially base and rover measurements are combined in

the double differenced mode and an initial adjustment
•Outcome of this initial adjustment is an initial rover

position along with estimates (real values) for the
ambiguity parameters and their uncertainty values,
or the covariance matrix.

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•This matrix can be represented geometrically to form a
region, known as the confidence region, around the

estimated real-value ambiguity parameter
•The confidence region takes the shape of an ellipse if the
number of the estimated parameters is two, and an
ellipsoid if it is three. For more than three, a
hyperellipsoid is obtained

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Dilution of Precision (DOP)
•DOP is measure of the geometry of the visible satellite

constellation.

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Good DOP value is to have one satellite directly above and
the other three evenly spaced around the receiver and

elevated to about 25 to 30 degrees

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•When the satellites are widely spaced, the overlap area of
the two zones of possible satellite range error is relatively
small, called the “area of positional ambiguity.
•It result in a good, or low Dilution of Precision value.
When the satellites are closely spaced, the overlap area of
the two zones of possible satellite range error is

considerably larger
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The overall GDOP, or Geometric Dilution of Precision
includes:
PDOP, or Precision Dilution of Precision, is the dilution of
precision in three dimensions.

HDOP, or Horizontal Dilution of Precision, is the dilution of
precision in two dimensions horizontally

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•VDOP, or Vertical Dilution of Precision, is the dilution of
Precision in one dimension, the vertical

•TDOP, or Time Dilution of Precision, is the dilution of
Precision with respect to time.

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Applications

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GPS Surveying

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•Surveying means precision positioning’
•This kind of data collection might include property lines
or construction surveying.
•GPS can survey points in minutes that conventional
surveys might require hours or even days to perform.

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Geographical positioning

GP means attaching coordinates to objects of interest: e.g.
control monument, gravity data point

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IMPACT OF GPS ON CONTROL NETWORKS
A control network is a set of monumented points with
Coordinates attached to them
The aim of a control network is to transfer coordinates
to other features, that is, to allow positioning of other
features with respect to the control points.

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Wt GPS now possible to establish a control point anywhere
without the necessity of having to connect it to already

existing control points through chains of intervisible points.
•the necessity of having any control points at
all is disappearing.

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Height & Geoid determination

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•GPS can naturally provide heights and/or height
differences above a selected ellipsoid.

•orthometric heights can be measured accurately by
geometric levelling techniques
However, this is a slow and expensive process a direct
and accurate measure of the geoid can be obtained by

N=h-H.

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A-4,, Least square methods, Relation positioning,
Ground transportation, orbit determination
climate research

Thank you
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GPS processing techniques & some applications

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