Secondary Surveillance RADAR Advantages
Secondary surveillance RADAR effectively counteracts the following shortcomings of
The limiting effect of aircraft reflection areas that vary with aircraft size and
Displays degraded by weather conditions, especially precipitation
Impairment of the RADAR display due to ground clutter even though the RADAR
is equipped with MTI
Blind spots in the antenna coverage pattern
Identification Friend or Foe (IFF) electronically distinguishes between friendly and
hostile aircraft. Selective Identification Feature (SIF) is a form of IFF that enables the
IFF system to generate many variably coded replies.
The Interrogator Set consists of a beacon-type IFF/SIF system and processing units
that provide synthetic video for display on the RADAR PPI. It enhances the RADAR
operator's PPI display by replacing the conventional IFF response from the aircraft with
a target symbol that represents a variety of aircraft status conditions and with two sets
of numbers that provide direct identification and altitude.
The targets are continuously refreshed to prevent target fade and to provide the
operator with easy-to-read information. Target trail dots are available to portray course
history and/or to provide an indication of ground speed. The operator selects the
needed information display on the PPI. An aircraft emergency, hijack, or
communications failure is automatically displayed in addition to the information displays
that the operator selects.
The IFF/SIF system is known as ATCRBS. The ATCRBS is capable of making
interrogations in any four of the six different modes shown in Table 5-7. Mode A is the
civil and military air traffic control mode. Since civil mode A is the same as mode 3 in
military equipment, this common air traffic control mode is called mode 3/A. Modes 1, 2,
4, and 5 are military tactical modes. Mode 4 provides for positive secure friend
identification. Mode 5 provides for enhanced secure friend identification. Each platform
is assigned its own unique PIN. Mode B is a civil air traffic control mode but is not used
in the United States. Mode C is used for automatic altitude transmissions. Mode D has
been established, but its use has not been specified.