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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


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