User's Manual

Chapter 4 Operation
GPS Self Navigation & Lost Coverage Alarms
While in the Reading mode using either L/L or GEO modes, the MMI depends on the
continual position updates provided by the GPS receiver. Since the GPS receiver is
listening to satellites at various positions in the sky, objects such as tall buildings, trees
and high-growing foliage may block the view of some of the necessary satellites. When
such a blockage occurs, the GPS receiver is temporarily unable to provide Lat/Lon
coordinates. This naturally presents a serious problem while in L/L or GEO modes.
An option known as self-navigation is available that can continue producing Lat/Lon
position updates during brief GPS blockages. While in self-navigation mode, each time
the MMI receives a reply from a transponder, it takes the stored Lat/Lon coordinate for
that transponder and updates the vehicle position. While in self-navigation mode, the
Dynamic Window (see the performance enhancement section for a description) feature is
disabled, since the vehicle position may have a greater amount of error than normal. As
long as the MMI is continually receiving transponder replies, and the time between
successive reads is brief, then the self-navigation mode will continue. If however, a
period of no read activity occurs, then the self-navigation mode will be terminated, and
the FIFO mode will become active. Once self-navigation mode is terminated, it will not
resume again until the GPS status transitions from active to inactive. At the time valid
GPS position data becomes available again, the previous reading mode will become
active, and features such as Dynamic Window will be resumed, if enabled.
The MMI will alert the operator whenever the GPS becomes unable to produce position
data for any reason while in either L/L or GEO modes. Any time the GPS data becomes
unavailable, both an audible and message alert will be presented on the handheld
terminal. When the GPS position becomes unavailable, and self-navigation mode has not
been enabled, then a message will be displayed indicating that the GPS is blocked, and
FIFO mode is being initiated. If self-navigation mode was enabled at the time of GPS
blockage, then another message will be displayed indicating that GPS data is unavailable,
and the self-navigation mode is now active. If self-navigation mode is unable to continue
to produce position data, and GPS data is still unavailable, then an alert will be given
indicating that no position data is available, and FIFO mode is being initiated. Finally,
when GPS position data is restored, then another alert will be issued indicating this status.
Upon the resumption of GPS data, the reading mode which was active prior to the loss of
position data, as well as any performance enhancement features will be restored.