The filter dialogue box is used to restrict which targets are displayed on the radar, based on the altitude/level they are flying at. Targets may be filtered based on four different criteria:
TOP - Any targets with a current level higher than specified will not be displayed on the radar.
HIGH - Targets with a current level higher than specified, but below the TOP level, will be displayed but using a different style of “Filtered” label. This enables a limited set of information to be displayed for targets that are “just above” the vertical limits of interest to the controller.
LOW - Targets with a current level lower than specified, but above the BOTTOM level, will be displayed but using a different style of “Filtered” label. This enables a limited set of information to be displayed for targets that are “just below” the vertical limits of interest to the controller.
BOTTOM - Any targets with a current level lower than specified will not be displayed on the radar.
ENABLE UP - If this is highlighted then the TOP/HIGH settings will be enabled; if it is not highlighted then the TOP/HIGH settings will be ignored and there will be no upwards filter.
ENABLE DOWN - If this is highlighted then the LOW/BOTTOM settings will be enabled; if it is not highlighted then the LOW/BOTTOM settings will be ignored and there will be no downwards filter.
Please note, all references to vertical levels in the section above have been referred to simply as “level”, but refer to altitudes below the transition.