Page 420 - Automotive Engineering Powertrain Chassis System and Vehicle Body
P. 420
Global positioning technology C HAPTER 14.1
For example, in an intelligent vehicle with an auton- 14.1.3.2.7 Additional GPS augmentation
omous navigation system, the GPS receiver normally techniques
calculates the position data used by the navigation algo- Additional techniques are being developed for increasing
rithm to determine the progress of the vehicle along the the accuracy of the positioning information derived from
desired path. However, when driving in some environ-
the GPS for certain applications. One technique, which
ments, the GPS receiver may have trouble maintaining
has been developed by the US Federal Aviation Admin-
a continuous satellite lock, resulting in intermittent
periods where the vehicle’s position cannot be deter- istration (FAA), uses transmissions from communication
mined based on valid satellite data. In situations like this, satellites to improve the positioning accuracy of GPS
DR is used to ‘fill in the gaps’, providing a method for receivers in aircraft. This technique, known as the
estimating the current position based on the vehicle’s WAAS, uses a network of wide area ground reference
movements since the last known positioning fix. stations (WRSs) and two wide area master stations
A variety of input sensors can be used to provide DR (WMSs) to calculate pseudorange correction factors for
capability. In the intelligent vehicle example, several dif- each SV, as well as to monitor the operational health of
each SV. This information is uplinked to communication
ferent sensor inputs can be made available to the navigation satellites in geostationary earth orbit (GEO), which
system to assist inDR calculation.The types ofsensors that transmit the information on the L 1 frequency, along with
could be used to enable DR in a vehicle system include:
additional ranging signals. This system has improved the
magnetic compass, which can provide a continuous, positioning accuracy of GPS receivers on board aircraft
coarse-grained indication of the direction in which to within 7 metres horizontally and vertically, allowing
the vehicle is moving the system to be used by aircraft for Category I precision
approaches. A Category I system is intended to provide
gyroscope, which can be used to detect the angular
movement of the vehicle an aircraft operating in poor weather conditions with safe
speedometer, which can provide the current speed vertical guidance to a height of not less than 200 feet with
of the vehicle runway visibility of at least 1800 feet.
Another method for improving positioning accuracy
odometer, which can provide continuous data on is known as carrier-phase GPS. This is a technique
the elapsed distance where the number of cycles of the carrier frequency
wheel speed sensors, such as Hall-effect or variable between the SV and the receiver is measured, in order
reluctance sensors (VRSs), which can provide to calculate a highly accurate pseudorange. Because of
fine-grained vehicle speed information
the much shorter wavelength of the carrier signal rela-
accelerometers, which can detect changes in the tive to the code signal, positioning accuracies of a few
velocity of the vehicle. millimetres are possible using carrier-phase GPS tech-
niques. In order to make a carrier-phase measurement,
Many of these sensors are already widely used in vehicles
for other applications. Accelerometers are being used standard code-phase GPS techniques must first be used
to calculate a pseudorange to within a few metres, since
today in impact detection (airbag) systems; wheel speed it would not be possible to derive a pseudorange using
sensors are being used in traction-control and anti-lock only the fixed carrier frequency. Once an initial pseu-
braking systems; and of course the trip meters available dorange is calculated, a carrier-phase measurement can
today in many cars use inputs from the speedometer, then be used to improve its accuracy by determining
odometer and compass to calculate distance travelled,
distance remaining and fuel economy. which carrier frequency cycle marks the beginning of
Systems that leverage inputs from remote vehicle each timing pulse. Of course, receivers that can perform
sensors to enable DR can certainly provide more consis- carrier-phase measurements will bear additional hard-
tent positioning information under some circumstances ware and software costs to achieve these improved
than may be possible with a single-point GPS receiver. accuracies.
However, depending upon the mix of sensor inputs used,
the accuracy of the resulting position data may vary. Some
of these sensors are more accurate than others, and most 14.1.4 GPS receiver technology
are subject to a variety of environmental, alignment and
computational errors that can result in faulty readings. In order to design and build a GPS receiver, the de-
Some vendors of DR-enabled positioning systems have veloper must understand the basic functional blocks that
been exploring methods of reducing the effects of these comprise the device, and the underlying hardware and
errors. The development of self-correcting algorithms and software necessary to implement the desired capabilities.
self-diagnosing sensors may help reduce the impact that The sections below describe the main functional blocks
sensor errors can have on these systems in the future. of a GPS receiver, and the types of solutions that are
427