Performance considerations for electronic tracking devices

There are many parameters used by the industry to assess the performance of a GPS receiver, and to evaluate the relative performance of comparable receivers. The most common parameters being used to evaluate GPS receiver performance include positioning and timing accuracy, time-to-first-fix, reacquisition time, and receiver sensitivity.

Positioning and timing accuracy

The most obvious of these parameters is positioning accuracy – how accurate are the positions calculated by an autonomous GPS receiver, based on the number of satellites that can be seen by that receiver? This is typically measured by performing a mapmatching test, where positions calculated by a receiver for landmarks on a map are compared to their known positions. This is a standard test that is often used to compare the accuracy of multiple GPS receivers simultaneously. When the S/A feature was still enabled, the accuracy of the SPS signal served as the baseline for positioning accuracy for commercial GPS receivers. With the discontinuation of S/A, the accuracy of autonomous GPS receivers has increased significantly, but as of this writing there is no new accepted baseline for the measurement of post-S/A receivers, except perhaps for the PPS signal accuracy. A performance parameter closely related to positioning accuracy is timing accuracy – how close to UTC is the time calculated by the GPS receiver. This parameter essentially measures the deviation of the calculated time from UTC as maintained by the US Naval Observatory. However, since the accuracy of the time component of the SPS signal with the S/A feature enabled was within 340 ns, this is obviously a test requiring sophisticated time measurement equipment to perform. The timing accuracy of most tracking device is more than adequate for commercial applications such as intelligent vehicle systems.

Time-to-first-fix

Time-to-first-fix (TTFF) is the measure of the time required for a receiver to acquire satellite signals and calculate a position. The three variants of a TTFF measurement, which depend upon the condition of the GPS receiver when the TTFF is measured, are referred to as hot start, warm start, and cold start. These TTFF measurements include the amount of time it takes the GPS receiver to acquire and lock each satellite signal, calculate the pseudorange for each satellite, and calculate a position fix. Hot start occurs when a GPS receiver has recent versions of almanac and satellite ephemeris data, and current time, date and position information. This condition might occur when a receiver has gone into a power-conserving stand-by mode due to application requirements. In this situation, most receivers should be able to acquire a position fix within 15 seconds.

Warm start occurs when a GPS receiver is powered on after having been off or out of signal range for several hours to several weeks. In this condition, the receiver has an estimate of time and date information, and a recent copy of satellite almanac data, but no valid satellite ephemeris data. In this state, a receiver can begin tracking satellites immediately, but must still receive updated ephemeris data from each satellite, which is only valid for approximately four hours. Under these conditions, most receivers should be able to acquire a position fix within 45 seconds. Cold start occurs when a GPS receiver has inaccurate date and time information, no satellite ephemeris data, and no almanac data, or data which is significantly out of date. In this state, the receiver must perform a search for the available satellite signals, and can take 90 seconds or more to acquire a position fix. This condition is encountered when the GPS receiver is powered up for the first time after leaving the factory, or in other situations where the device has not been powered up or used for long periods of time. Many GPS tracking device allow the user to enter time, date and even current position information, which can reduce the TTFF in a cold start situation down close to that of a warm start.

More GPS tracking solutions at http://www.jimilab.com/ .