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Portable GPS Tracking at present and in the future

A change in the navigation bit polarity interferes with coherent acquisition and coherent tracking algorithms. If a receiver knows the navigation bit in advance, it uses this information to extend the coherency interval, which drastically increases sensitivity. Therefore, a pilot channel is adopted, which has a fixed secondary code instead of a navigation message with data.

Normally, however, receivers are not concerned with this process. Instead, Small GPS Tracker use tracking to obtain the navigation message and resolve code ambiguity. This, however, comes at a price. The receiver would need to have a few seconds of uninterrupted and uncorrupted signal reception to acquire enough of the navigation message to derive a time mark. This would be a disadvantage for many applications, in particular for positioning in urban conditions, in high-multipath environments, and when a quick positioning fix is required. Often, it is not possible for a receiver that either is indoors or has an obstructed sky view to ensure uninterrupted tracking. In this case, a receiver may operate in snapshot mode.

We have defined snapshot positioning as positioning based on code phase measurements from an acquisition process without reading a time mark from the navigation message. As discussed previously, to be able to make a positioning without tracking requires approximations of time and user position. This information is called assist information and is the heart, or rather the blood supply, of Personal Tracking Devices technology. If we don’t achieve a time estimate in the receiver better than 1 ms, we cannot resolve this ambiguity without tracking and reading the navigation data.

There are two main approaches taken to obtain the time mark in order to calculate pseudorange measurements:

(1) the time mark can comprise part of the assistance data from a synchronized network;

(2) the time mark may be calculated from redundant measurements. The implementation of assist information via cellular networks is covered by multiple patents. For example, describes a method of supplying time information through a network, and describes AGPS positioning using an approximate position from a cellular network.

The most common AGPS application is for cellular phones. Approximate user coordinates are roughly estimated using information about particular cellular network stations used by the host phone. The stations are located with rather high densities in many countries, especially in urban environments where the harsh conditions call for instant positioning and therefore for Motorcycle GPS Tracker applications. A handset position estimate with an accuracy of a couple of kilometers is available from a cell ID. Using the information about a few of these stations allows us to use methods similar to positioning with GNSS to improve the positioning estimate even further. A rather precise time estimate can also be delivered to a user through a cellular phone network.

If this assist information is available, we can use ambiguous code estimates from the acquisition process to find a receiver’s position, using measurements available just a few milliseconds after the receiver is switched on. For snapshot positioning, however, accuracy, would suffer.All positioning estimates are calculated in real time using broadcast ephemerides and with a mobile device front end at a cost of about $6. GPS Tracker Device permits sub-meter accuracy with code phase measurements. Further improvements in accuracy may be achieved using carrier phase measurements.

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