Page 424 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Global positioning technology C HAPTER 14.1

a GPS-enabled application running on a single MCU that time measurement equipment to perform. The timing
is supported by this type of tool suite, refer to Fig. 14.1-8. accuracy of most GPS receivers is more than adequate
for commercial applications such as intelligent vehicle
systems.
14.1.4.3 Performance considerations
14.1.4.3.2 Time-to-first-fix
There are many parameters used by the industry to assess
TTFF is the measure of the time required for a receiver
the performance of a GPS receiver, and to evaluate the
to acquire satellite signals and calculate a position. The
relative performance of comparable receivers. The most
common parameters being used to evaluate GPS receiver three variants of a TTFF measurement, which depend
performance include positioning and timing accuracy, upon the condition of the GPS receiver when the TTFF
time-to-first-fix (TTFF), reacquisition time, and receiver is measured, are referred to as hot start, warm start, and
sensitivity. 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
14.1.4.3.1 Positioning and timing accuracy each satellite, and calculate a position fix.
The most obvious of these parameters is positioning Hot start occurs when a GPS receiver has recent
accuracy – how accurate are the positions calculated by versions of almanac and satellite ephemeris data, and
an autonomous GPS receiver, based on the number of current time, date and position information. This con-
satellites that can be seen by that receiver? This is dition might occur when a receiver has gone into
typically measured by performing a mapmatching test, a power-conserving stand-by mode due to application
where positions calculated by a receiver for landmarks requirements. In this situation, most receivers should be
on a map are compared to their known positions. This is able to acquire a position fix within 15 seconds.
a standard test that is often used to compare the accu- Warm start occurs when a GPS receiver is powered on
racy of multiple GPS receivers simultaneously. When after having been off or out of signal range for several
the S/A feature was still enabled, the accuracy of the hours to several weeks. In this condition, the receiver has
SPS signal served as the baseline for positioning accu- an estimate of time and date information, and a recent
racy for commercial GPS receivers. With the discon- copy of satellite almanac data, but no valid satellite
tinuation of S/A, the accuracy of autonomous GPS ephemeris data. In this state, a receiver can begin
receivers has increased significantly, but as of this tracking satellites immediately, but must still receive
writing there is no new accepted baseline for the updated ephemeris data from each satellite, which is
measurement of post-S/A receivers, except perhaps for only valid for approximately four hours. Under these
the PPS signal accuracy. conditions, most receivers should be able to acquire
A performance parameter closely related to position- a position fix within 45 seconds.
ing accuracy is timing accuracy – how close to UTC is the Cold start occurs when a GPS receiver has inaccurate
time calculated by the GPS receiver. This parameter date and time information, no satellite ephemeris data,
essentially measures the deviation of the calculated time and no almanac data, or data which is significantly out of
from UTC as maintained by the US Naval Observatory. date. In this state, the receiver must perform a search for
However, since the accuracy of the time component of the available satellite signals, and can take 90 seconds or
the SPS signal with the S/A feature enabled was within more to acquire a position fix. This condition is en-
340 ns, this is obviously a test requiring sophisticated countered when the GPS receiver is powered up for the

Fig. 14.1-8 GPS-enabled application software architecture.

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