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Electronic tracking devices - about Multipath error

Январь 5th, 2016 by homesecuirtycameras and tagged , , ,

Multipath is a major error source for both the carrier-phase and pseudorange measurements. Multipath error occurs when the GPS ( personal GPS tracking devices ) signal arrives at the receiver antenna through different paths. These paths can be the direct line of sight signal and reflected signals from objects surrounding the receiver antenna. Multipath distorts the original signal through interference with the reflected signals at the GPS antenna. It affects both the carrier-phase and pseudorange measurements; however, its size is much larger in the pseudorange measurements. The size of the carrier-phase multipath can reach a maximum value of a quarter of a cycle. The pseudorange multipath can theoretically reach several tens of meters for the C/A-code measurements. However, with new advances in 32 Introduction to GPS receiver technology, actual pseudorange multipath is reduced dramatically. With these multipath-mitigation techniques, the pseudorange multipath error is reduced to several meters, even in a highly reflective environment.

Under the same environment, the presence of multipath errors can be verified using a day-to-day correlation of the estimated residuals. This is because the satellite-reflector-antenna geometry repeats every sidereal day. However, multipath errors in the undifferenced pseudorange measurements can be identified if dual-frequency observations are available. A good general multipath model is still not available, mainly because of the variant satellite-reflector-antenna geometry. There are, however, several options to reduce the effect of multipath. The straightforward option is to select an observation site with no reflecting objects in the vicinity of the receiver antenna. Another option to reduce the effect of multipath is to use a chock ring antenna (a chock ring device is a ground plane that has several concentric metal hoops, which attenuate the reflected signals). As the GPS signal is right-handed circularly polarized while the reflected signal is lefthanded, reducing the effect of multipath may also be achieved by using an antenna with a matching polarization to the GPS signal. The disadvantage of this option, however, is that the polarization of the multipath signal becomes right-handed again if it is reflected twice.

When a cell phone operator calls 911 to report emergency situation, it is desirable to locate the caller automatically. Often the caller is inside a building. If the building is equipped with a geolocation system, the caller’s location inside the building can be identified. The location of the building must also be identified. This combination of information is sent to the emergency unit receiving the call. Another approach is to use the GPS signal to locate the caller. However, inside a building the GPS ( GPS Tracking Device ) signal strength may be very weak.

One of the approaches is to use a GPS base station in the neighborhood to receive signals at normal strength. The GPS signals of normal strength are acquired and tracked. In other words, the initial phase of the C/A code, the carrier frequency, and the navigation data can all be obtained from the signals of normal strength. This information can be transmitted through the cell system to the cell phone users in the vicinity. The cell phone unit will include a GPS receiver. This GPS tracker can perform acquisition based on the received GPS information. Since the carrier frequency of the GPS is known only a very narrow frequency range need to be searched.

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