Rapiscan TSA TPM-903B Operation And Service Manual - 4.0  Theory Of Operation

TSA TPM903 Operations and Service Manual 

Doc: # 5058 Rev. A 

_____________________________________________________________________________________________________________________

_____________________________________________________________________________________________________________________ 

This document is the exclusive property of Rapiscan Systems. 

Unauthorized use or distribution is strictly prohibited 

19 

4.0  THEORY OF OPERATION 

4.1. OVERVIEW 

The portal monitor makes its decisions for radiation alarms in the following manner. A level for 

N*sigma is selected using the keypad. Whenever the occupancy detector senses that the monitor 

is occupied, the monitor starts making alarm comparisons based on the parameters that have been 

stored in the controller’s NVRAM (FAST COUNT mode). 

When unoccupied, the portal monitor constantly updates the background count to reflect changes 

in the environment. The background is accumulated in 5-second increments, with the current 

background reading equal to the one-second average of the last four 5 second intervals. This 

updates the background completely every 20 seconds. When the unit is occupied, it ignores the 

current 5-second background interval, and goes into FAST COUNT mode. 

The monitor collects its counts in 200 millisecond (0.2 sec.) intervals. For example, if the number 

of intervals is set to 5, the alarm comparison will be based on 1.0 second counts. This sum of 

counts is then compared to an alarm level which is normalized to that number of intervals. 

The number of intervals should be selected based on an average walk speed of 1.5 meters/second 

while passing through the monitor. The summed count of the chosen number of intervals should 

reflect total occupancy time, and thus offer the maximum probability of detecting an alarm 

condition. 

While the monitor is occupied, it makes an alarm comparison every 200 milliseconds, based on 

adding together the most recent n 200 millisecond intervals. The intervals are stored 

continuously, so that as soon as the monitor is occupied, it waits for the current interval to end, 

then adds up the selected number of intervals and makes an alarm comparison. This means that if 

the monitor is set to five intervals, it is effectively starting to monitor the passage 1 second before 

the monitor has been occupied. This is called "look back." The monitor will continue to make 

comparisons until the "occupancy hold-in" time has expired after the end of the occupancy. This 

is called "look after." 

The "occupancy hold-in" forces the unit to continue to make alarm comparisons after the 

occupancy detector has cleared (look after). The amount of time selected for this parameter is 

based on the estimated speed of passage and pillar spacing.