Aviation transponder interrogation modes

IMPORTANT NOTE:

This blog helps in comprehending the basic knowledge concerning the interrogation mode used by the air traffic control radar beacon system, moreover this blog give us a way to cover up an important topic  of our course NAVIGATION GUIDANCE AND CONTROL.

INTRODUCTION

                                             An aviation transponder interrogation mode is the format of a sequence of pulses from an interrogating Secondary Surveillance Radar (SSR) or similar Automatic dependent surveillance-broadcast (ADS-B) system. The reply format is usually referred to as a "code" from a transponder, which is used to determine detailed information from a suitably equipped aircraft.
In its simplest form, a "Mode" or interrogation type, is generally determined by pulse spacing between two or more interrogation pulses. Various modes exist from Mode 1 to 5 for military use, to Mode A, B, C and D and Mode S for civilian use. 

• Mode S Individual Interrogation

Figure 1: Mode S - short interrogation (56 bits - 16.25 μs)

The Mode S up link interrogation format starts with two pulses,   P 1 and P2, which are solely for the purpose of suppressing existing Mode A/C only transponders so that they are not aware of the main Mode S information. The Mode S interrogation data contained in the P6 data block is phase modulated. The first phase reversal is the timing point for the subsequent bits (chips) of information. The Mode S interrogation may be of short (56 bits) or long (112 bits) format.

Figure 2: Mode S - long interrogation (112 bits - 30.25 μs)

The Mode S side lobe suppression pulse  P5 is transmitted from the control beam like the P2 ISLS in the Mode A/C system. If P5 is of more power than P6 it has the effect of overwhelming the sync phase reversal of P6 so that the Mode S transponder cannot read the subsequent information.

• Mode S Up-link Formats

A conventional SSR interrogator may have a typical sequence of Mode A interrogation, followed by Mode C interrogation or other modes. This would be repeated continually at a high rate to ensure that a position/identity plot can be produced for all targets in line-of-sight range of the interrogator during each antenna revolution.
The Mode S ground station produces a larger variety of interrogation types. These types can be roughly classified into two types:

• All-call interrogations

• Roll-call interrogations          

Roll-call interrogations are selectively addressed to acquired Mode S equipped aircraft using the unique 24-bit address assigned to each aircraft. Only the addressed aircraft produce replies.

All-call interrogations obtain replays from all aircraft in the beam dwell, although, under certain circumstances, Mode S aircraft can be “locked out” to all interrogations so that they do not reply.

The first problem for the Mode S system is to find the addresses of aircraft that are in radar cover so that selective addressed interactions can be made with them. This is achieved by Mode S all-call interrogator witch are made periodically from the radar.

pulse width of P4	Mode S inter mode
0.8 µs	no replay	When the short P4 is used by Mode S radar, an aircraft fitted with a Mode S transponder will be detected solely by the Mode S formats - i.e. aircraft will be first detected by the Mode S formats all-call and subsequently followed by Mode S selective address interrogations.
1.6 µs	all-call reply	Although the long p 4 may be used to obtain Mode S all-call replies, its use is expected to be rather limited, in particular because the interrogator identity (ii) code and associated all-call lock features of Mode S are not relevant for this interrogation.
no P4	Mode A reply	This is the downwards compatible modes providing the Mode S transponder to reply on early interrogators.
no P4 but full P2	Mode S reply	The interrogator sends a P 2 with full amplitude as for the P 1 pulse. This activates a Mode S transponder to then look for a following P 6 pulse containing the Mode S specific information. An older Mode A/C transponder seems this as an ISLS- condition and don't reply.