User's Manual

Chapter 4 Operation
that does not meet the signal strength requirement will result in a NO_RD status. This is
the same status which occurs any time the MMI does not receive a reply from a
transponder. The default minimum signal strength setting will most not likely never need
to be altered.
Dynamic Interrogation Window
Another performance enhancement feature implemented in the MMI is the Dynamic
Interrogation Window. Ordinarily, the interrogation window is set to a particular fixed
radius, and is not altered. This is normally acceptable for routes that have a consistent
installation density. However, on routes where there are widely varying installation
densities, a fixed interrogation window size can actually cause performance degradation.
If an excessive non-adjustable window size is used on a dense route, then the MMI will
not have as much time to re-interrogate transponders that are close, but have not yet been
read. This would result in either missed transponder readings, or would require the
operator to slow the vehicle to an unacceptable speed. Likewise, if the non-adjustable
window size becomes too small, then some transponders may never be interrogated at all
due to the vehicle speed and errors in the Lat/Lon coordinate information. To overcome
these limitations, the dynamic interrogation feature may be enabled by the System Setup
menu within the Tools command. When enabled, this feature will automatically adjust
the interrogation window size once per second to try to maintain a fixed number of
transponders in the window. This ensures that in situations where there are a large
number of transponders in a small area, the interrogation window size will be kept
smaller, so time is not wasted on interrogating transponders that are farther away. Also, it
ensures that in situations where there is a small number of transponders in a large area,
the interrogation window size will be allowed to grow, which prevents transponders from
never being interrogated at all.
Frequency Scan
To accommodate older transponders whose reply frequency has drifted down beyond
normal limits, the frequency scan feature may be enabled. This feature is enabled in the
Frequency Control menu contained in the Tools command. This feature will tune
receiver channel 1 between two frequency limits defined in the Frequency Control menu.
This allows transponder replies at abnormally low frequencies to be received, without any
special or additional effort. However, one caution should be noted when using this
feature. Since a receiver channel 1 is changing frequencies quite often, a transponder
reply may not be received since the receiver was at one frequency, while the transponder
was at another. This results in a potentially increased read time for transponders that
have abnormally low reply frequencies. The amount of speed degradation is related to
how wide the frequency scan is. The wider the frequency scan range, the potentially
slower the response time will be for low frequency transponders. By default the upper
frequency limit for the scan is one channel below the lowest channel currently
programmed in channels 2-8. Both limits may be arbitrarily changed within the
Frequency Control menu.