Appendices
Copyright © 2015 Coda Octopus Products Ltd
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F180R MOTION Sensor User and Reference Guide
System Design
The USGPS satellite navigation system is divided into three segments. The space segment
represents the satellites from which the signal is propagated. The current satellite
constellation per February 2007 is made up of 30 satellites in six orbital planes with an
inclination of 55 degrees. The satellites circle the Earth every 12 hours and are operating at a
height of approximately 20200km. Each satellite is identified either by the Space Vehicle
Number (SVN) or the Pseudorandom Code Number (PRN) which is embedded in the
transmitted signal. Initially the satellites were set up to transmit the navigation information
on the L1 and L2 frequencies (1575.42 MHz and 1227.60 MHz). All satellites launched after
September 2006 include a functionality to transmit on a third frequency, the L5 signal 1176.45
MHz, which is expected to be fully operational by 2012.
The control segment consists of a number of monitoring and control stations which
continuously verify the integrity of the system. The final segment is the user segment which
presents the navigation information to the end user. This can be achieved with everything
from a low-end GPS receiver to high-grade multi-receiver systems.
GPS Measurement Models
The underlying mathematical model used in GPS navigation is trilateration where ranges to a
minimum of three satellites are needed to solve for the X, Y, Z components of the receiver
position. However, as these ranges are derived by measuring signal propagation times, a
fourth measurable is needed in order to solve for the time offset between the high accuracy
GPS satellite clock and the low accuracy receiver clock. Different approaches on how to
measure the ranges from receiver to satellite exist, depending on the desired level of
accuracy.
Pseudorange Measurements
The pseudorange measurement model estimates the ranges from the receiver to each
satellite by calculating the apparent transit time of the signal. This transit time is calculated by
measuring the amount of time shift that is required to align a copy of the navigation code
replicated on the receiver with the signal which is continuously transmitted by the satellite.
Since each satellite generates a signal containing a unique ID the receiver is able to isolate and
range to several satellites at the same time.
The pseudorange measurement model is the simplest GPS measurement model and is the
basic model found in consumer grade GPS receivers.
Carrier Phase Measurements
The carrier phase measurement model provides a more precise way of estimating the ranges
between the satellites and the receiver. This model uses the phase of the carrier signal rather
than the information modulated onto the signal as a tool to estimate the range to a much
higher degree than the pseudorange measurement.
The problem with this approach is that the phase measurement does not contain any
information regarding the number of whole cycles the signal has travelled in addition to the
measurable phase difference. This number of whole cycles is known as the integer ambiguity
and must be resolved by the receiver during the initialization of the system. During the
survey, the receiver maintains control over the integer ambiguity, however if the signal is
blocked or severely deteriorated by multipath for example, the integer ambiguity is lost and
the receiver is forced to repeat the initialization process.
Different approaches are used to solve for and maintain control over the integer ambiguity
and relevant literature should be consulted for further information on these techniques.
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