FINAL REPORT - Stakeholders - Ofcom

amplitudes of each antenna (calculations of the sum and difference signals from these
antennas), the azimuth of the aircraft within the beam can be determined from a single
reply (and theoretically even from one single pulse in the reply).
Rx A
Rx B
Figure 3-8: Monopulse SSR (MSSR) technique
Monopulse SSR (known as MSSR) therefore has significant advantages over the early
sliding window methods – it is roughly 4 times more accurate and requires fewer replies.
This in turn means the interrogation frequency (pulse repetition frequency, or PRF) can
be reduced, leading to reductions in the overall level of RF interference. Note that the UK
has already in a previous update cycle replaced its sliding window radar with monopulse
radar – hence there is little further scope for increased spectrum efficiency.
In the current environment, three ‘modes’ of SSR can be implemented: A, C and S. Mode
S is an evolution of conventional MSSR Mode A/C in which more data transfer
capabilities are added to the system. Mode S enables the selective interrogation of
aircraft and the extraction of air derived data through which new ATM functionality can be
developed. It overcomes certain limitations of SSR Mode A/C:
• The limitations in the number of Mode A identity codes – Mode S uses the hardcoded
ICAO address of the aircraft for identification – each 24-bit address is
unique to its aircraft. For identification of the aircraft to the controller, this address
can then be uniquely correlated with the flight ID which is also down-linked by
Mode S.
• Altitude precision – Mode S provides higher altitude precision (from 100ft to 25ft
increments).
The standard Mode S level is known as Elementary Surveillance (with ICAO address, 25ft
altitude increments and position).
Mode S Enhanced Surveillance consists of Elementary Surveillance supplemented by the
extraction of airborne parameters known as Downlink Airborne Parameters (DAPs) to be
used in the ground Air Traffic Management systems. Some parameters are for display to
controllers, known as Controller Access Parameters (CAPs), and some are for (ATM)
system function enhancements, known as System Access Parameters (SAPs).
Mode S has been designed to be compatible with existing technology – i.e. Mode A/C and
ACAS. Mode S standards are defined in ICAO Annex 10 Volumes III and IV amendment
73.
3.3.3.4 SSR Modes
These definitions from ICAO Annex 10 Volume IV attempt to clarify the various roles of
each mode.
Page 65