Page 415 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 4. 1 Global positioning technology

14.1.2.2.3 Almanac and support data most users from taking full advantage of both GPS fre-
subframes quencies. Without a complete knowledge of the
encrypted L 2 frequency, only mathematical exercises
Subframes four and five of the navigation message data enable high accuracy applications of GPS such as sur-
frame contain comprehensive almanac data for the entire veying to take advantage of any information provided by
GPS constellation, along with delay parameters that the
L 2 .
receivers use for approximating phase delay of the
transmitted signal through the ionosphere, and correc-
tion factors to correlate GPS and Universal Time 14.1.3.1.1 Position calculation
Coordinated (UTC). The fundamental technique for determining position
The almanac data contains orbit and health in- with the GPS is based on a basic range measurement
formation on all of the satellites in the GPS constellation. made between the user and each GPS satellite observed.
GPS receivers use this information to speed up the These ranges are actually measured as the GPS signal
acquisition of SV signal transmissions. The almanac data time of travel from the satellite to the observer’s posi-
in subframe four contains health and status information tion. These time measurements may be converted to
on the operational satellites numbered 25 through 32, ranges simply by multiplying each measurement by the
along with ionospheric and UTC data. The almanac data speed of light; however, since most GPS receiver internal
in subframe five contains health and status information clocks are incapable of keeping time with sufficient
on the operational satellites in the GPS numbered accuracy to allow accurate ranging, the mathematical
1 through 24. PVTsolution must solve for errors in the receiver clock at
For a more detailed description of the information
contained in the Navigation Message, refer to the ICD- the time each observation of a satellite is made. Satellite
ranges are commonly called pseudoranges to include this
GPS-200c specification, which is available from the US receiver clock error and a variety of other errors inherent
Coast Guard Navigation Center.
in using GPS. These receiver clock errors are included as
one component in a least squares calculation, which is
14.1.3 Fundamentals of satellite- used to solve for position using a technique called
based positioning trilateration.
To calculate the values for PVT, the concept of tri-
angulation in two-dimensions as is commonly practised in
To understand the true value and cost of the positioning determining the location of an earthquake epicentre is
capabilities of the GPS, it is important for the user to extended into three-dimensions, with the ranges from
have a basic understanding of the science behind posi- the satellites prescribing the radius of a sphere (see
tioning, and the types of components and techniques that Fig. 14.1-2). This technique is known as trilateration,
may be used to calculate accurate positions. This section since it uses ranges to calculate position, whereas tri-
divides this discussion into three main areas: the basic angulation uses angular measurements. If a sphere
science behind GPS; the different unassisted and centred on the satellites’ position in space is hypotheti-
assisted position calculation techniques that may be cally created with the range from the user to each sat-
used, depending upon the needs of the specific appli- ellite as its radius, the intersection of three of these
cation; and the hardware and software components spheres may be used to determine a user’s two-
necessary for calculating a position.
dimensional position. While it may seem counterintui-
tive that ranges to three satellites will allow for only
14.1.3.1 The basic science of global a two-dimensional position, in fact one observation is
positioning needed to solve for each of latitude, longitude and
receiver clock error. Thus, to determine a user’s position
The design of the GPS makes it an all-weather system in three-dimensions a minimum of four satellites is
whereby users are not limited by cloud cover or in- required, in order to solve for altitude, as well as latitude,
clement weather. Broadcasting on two frequencies, the longitude and clock error.
GPS provides sufficient information for users to de- Once pseudoranges have been determined to three or
termine their PVT with a high degree of accuracy and more SVs, the user’s PVT can be calculated by solving
reliability. As mentioned previously, frequency L 1 is N simultaneous equations as a classic least squares
generally regarded as the civilian frequency while fre- problem, where N is the number of satellite pseudor-
quency L 2 is primarily used for military applications. anges measured. The relationship between the receiver
Applications and positioning techniques in this chapter and each satellite’s position can best be written by
will focus on GPS receiver technology capable of tracking extending the Pythagorean Theorem as illustrated in
L 1 only, as cost and security issues typically preclude equation 14.1.1, where i is the number of each satellite

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