Specifications

BATS AND PORTABLE BATS OPERATION
3-29
plies against the entered repetition value. It accepts the first reply that arrives in a window 50 ms
prior to the expected arrival of the signal (beginning of time window) and up to 50 ms after the ex-
pected arrival. It then ignores replies later than 50 ms after the calculated time gate window. If no
replies are detected within a certain period, an error code #2 is flagged and the window opens up
to find the reply.
FILTER LEVEL/VELOCITY FILTER
Velocity filtering is a technique of rejecting any target position changes that are unlikely due to an
assumed maximum target velocity and elapsed time since last update. It is based on the differ-
ence between the previous X and Y position (averaged, if smoothing is enabled) and the present
targets X and Y position. If the present calculated position passes the velocity filter test it is then
sent to the smoothing function. If after a certain period of time there is no reply within the ex-
pected area then an error code #5 is flagged and the system opens up to find the reply.
NOTE: A fast-turning ship can cause the target to appear to be traveling at excessive
speeds and be rejected by the velocity filter, when the compass option is not in use.
SNR Threshold: The threshold setting is normally set to 20dB (default). Raise the threshold if the
system is triggering on noise, usually indicated by erratic slant ranges. Use caution when setting
threshold lower than 20dB. I.e., if you know the approximate position of the target and you want
to get the maximum range then it may be possible to lower threshold in the presence of low noise.
As the threshold level is increased, only stronger signals are accepted, and weak signals, which
could be caused by multi-path or extraneous noise in the water, are ignored. The lowest threshold
setting increases the sensitivity of detection by recognizing smaller signals. In a high-noise envi-
ronment this can lead to erratic readings. Higher threshold settings decrease the possibility of de-
tecting on noise, but can lead to occasional missed valid signals and also late detections causing
erratic bearings. A lower threshold setting is usually the best choice, but trial will lead to the opti-
mum threshold setting for a given application.
Averaging: The Averaging parameter is variable from 0 (off) to 10. It is independent of the Filter
Level setting in that you can select the buffer size in which to average a number of replies over (1-
10). This setting corresponds to the number of replies that are averaged along with the present
reply. The averaging routine is used in conjunction with the QF (Quality Factor) and the Filter Lev-
el. With a high quality factor the target position is weighted heavily both on the present reply and
on the previous replies in the averaging buffer. If the present reply has a high QF and the previous
reply had a low QF then the position with the high QF remains heavily weighted within the averag-
ing buffer. As new replies are received the high QF positions maintain their heavier weight while
the positions with lower QF’s have very little weight. The weights of the target positions are re-
duced as the newer replies are received. This approach biases the target position towards the bet-
ter QF and also towards the most recent reply providing a better representation of target position
than by using a mean or average of the target positions. At the higher filter levels (8-10) the aver-