GPS tracker-based positioning solutions

Enhanced client-assisted GPS positioning

The enhanced client-assisted GPS positioning technique is a hybrid between autonomous GPS and server-assisted GPS. This type of solution is similar to the serverassisted GPS, with the location server providing the mobile unit with a list of visible satellites on demand. However, in an enhanced client-assisted system, the mobile unit does the complete PVT calculation rather than sending pseudorange information back to the location server. This technique essentially requires the same processing power and capabilities as an autonomous GPS solution, in addition to a communication link between the mobile unit and the location server. However, the amount of time required to complete the PVT calculation is much less than with an autonomous GPS solution, because of the satellite view information provided by the location server, and fewer exchanges with the location server are required than with a server-assisted solution.

Dead reckoning

Dead reckoning (DR) is a technique used in conjunction with other GPS-based positioning solutions to maintain an estimate of position during periods when there is poor or no access to the GPS satellite broadcasts. DR is used primarily to enhance navigation applications, since maintaining an accurate position in real time is crucial to the performance of a navigation system, and there may be times during a trip when the GPS-derived position may be intermittent, or not available at all. These GPS outages can be caused by a variety of environmental and terrain features. Examples of areas where GPS coverage could be interrupted include:

1. tunnels through mountains or in urban areas, which prevent signal reception 
3. urban canyons, such as downtown areas populated by tall buildings, which can result in either blocked signals, or multipath errors caused by signal reflection 
3. heavy foliage, where overhanging trees or bushes block reception of the signal broadcasts 
4. interference/jamming, which can be caused by either harmonics of commercial radio transmissions, or by transmissions specifically designed to interfere with the reception of the satellite broadcasts for security reasons 
5. system malfunction, where the electronic tracking device itself is functioning intermittently.

When a positioning data outage of this sort is encountered, a system that is DRenabled will monitor inputs from one or more additional sensors in order to continue to track the direction, distance and speed the unit is moving. The system will process that data starting from the last known position fix, which will enable it to keep a running estimate of its position. The system will continue to monitor these sensor inputs and update its estimated position until the GPS receiver can again obtain an accurate position fix. At this point, the system updates its position with the satellite-based data For example, in an intelligent vehicle with an autonomous navigation system, the GPS receiver normally calculates the position data used by the navigation algorithm to determine the progress of the vehicle along the desired path. However, when driving in some environments, the portable tracker may have trouble maintaining a continuous satellite lock, resulting in intermittent periods where the vehicle’s position cannot be determined based on valid satellite data. In situations like this, DR is used to ‘fill in the gaps’, providing a method for estimating the current position based on the vehicle’s movements since the last known positioning fix.