User's Manual

DRAFT TrimTrac 1.5 Technical Manual v0.3ab 49
2. that are SET_ALERT_STATE messages in which a priority alert status is
set “0=Clear” but the underlying condition that triggered the original alert
message has not been returned to its “normal” condition.
Different network operators may also handle queued up SMS messages
differently. For instance, if the TrimTrac 1.5 happens not to be in the QUERY
state when the server application sends a SMS message, this SMS message will be
held at the network operator’s Short Message Service Center (SMSC) for future
delivery. In some cases, the longer the SMS is held at the SMSC, the lower in the
delivery priority the message may become. As a consequence, the SMSC may
not send the queued SMS message before the QUERY Timeout, T5, has expired,
especially if the TrimTrac 1.5 has not been registered on the GSM network for a
long time. Therefore, if it can be reasonably expected that an application will
have extended periods of time between QUERY states, then extending the
QUERY Timeout, T5, should be considered.
Reporting Frequency
As shown in Figure 2, Expected Reporting Frequencies on page 50, the frequency
at which the TrimTrac 1.5 will report a position, while the unit is in motion, is
largely determined by:
DELAY Timeout, T4, setting
GPS Signal Strength
The time it takes for a TrimTrac 1.5 to compute a new position fix is inversely
proportional to available GPS signal level. When the TrimTrac 1.5 has an
unobstructed view of the sky, GPS signal strength is approximately –130 dBm.
This would equate to approximately 40 seconds to compute a new position fix as
shown in Figure 3, Warm Start TTFFs on page 51. For the most part, this is
equivalent to the unit being placed on the dashboard or rear package shelf of a
vehicle; provided, however, the window has no metalized coatings that would
attenuate the GPS signal.
A unit placed in the glove box of a typical vehicle, on the other hand, may only
see –136 dBm of GPS signal. In this circumstance, the unit may take upwards of
400 seconds to compute a new position fix and this assumes that objects, such as
buildings and trees, outside of the vehicle, do not further obscure view of the sky.