Originally Posted by
ZOOKER
Ian W,
What exactly is a "cooperative surveillance system"?
In over 35 years in ATC, I have never heard that expression, and over the last month, (although not familiar with their local environments), have tried to imagine the mindsets of the offering and receiving controllers involved, many times.
A primary search radar illuminates a target and receives the reflection of its output. It uses the radial the radar antenna is pointing at and the length of time for the reflection to return as range - and then displays these as a primary return in the appropriate position on the controller's display.
A secondary radar sends out repeated queries on 1030Mhz and receives a 'transponder'response on 1090Mhz that is encoded with an octal code and is also followed by a pulse train containing altimeter and sometimes aircraft ID. The radar uses the angle of the antenna and the time of the response to provide bearing and range and puts up a label on the controller's display at the appropriate position with the decoded aircraft information. If the aircraft does not 'transpond' a response - the controller sees nothing - the aircraft is thus 'cooperating' in the surveillance.
Automatic Dependent Surveillance takes this a step furter. The aircraft transmits on 1090Mhz or SATCOM, its position, the time that position was valid, together with a pulse train that carries aircraft ID, Altimeter, and sometimes its future trajectory, etc . The ground system listens to the appropriate frequency and displays a position symbol and label at the position in the message from the aircraft. This is totally cooperative surveillance as without the aircraft transmitting nobody knows where it is. This is why it is called 'Dependent Surveillance' as the other users of the airspace and controllers are 'dependent' on the aircraft transmitting its position.
It would appear that MH370 went from a cooperative target to a non-cooperative target. Worldwide, ATC is more and more dependent on cooperation from the traffic it is controlling.
Hope that helps You can read
Automatic Dependent Surveillance for more detail