|The following article is a fast and simplified narration of the evolution of the SSR system towards Mode-S for all these people that are not familiar with the subject . It also helps young controllers or non controllers to have a quick understanding of the whereabouts of those systems , their problems and mainly their use ._|
From SSR to Mode-S
by Ioannis Mansolas
classical radar known as Primary Radar
was introduced it appeared like a miracle as it was the first time that
ships and aircraft could be detected far away or at night . After the WW2
it was introduced currently in Air Traffic Control operations following
the war experiences .
When later on the aerodromes and surrounding airspace became busy it was then realised that Primary radar was not at all an easy tool to use , as :
The Secondary Radar is introduced
1950-55 and due to similar problems of the military aviation to
discriminate the known/friendly aircraft from the unknown/enemy ones , an
additional type of message was added to the existing basic primary radar
information by using another type of equipment . This became known as the Secondary
Radar and although it did not start as a radar system it
finally developed as one because
it had to provide means of identification of a target at a specific
position . Thus this new device could provide :
The Secondary Radar is 'worshiped' !
|The merits of
the SSR were being proved ever since and when technology stepped into the
computer era the SSR become the 'must' of the ATC for a large
number of reasons :
Problems of the Secondary Radar
Controllers were extremely happy for this technical achievement as the work was drastically simplified . But it had to be simplified now because all the more the airspace was becoming congested and the old Primary Radar was showing its limits as an operational tool .
Alas ! technology , as life itself , is a compromise .....Under certain conditions and traffic congestion the SSR problems started becoming known ....
Unfortunately good co-ordination with all neighbouring ATC units is NOT the rule and the proper SSR code allocation is NOT respected even now by all adjacent units . The feasibility of a technical achievement in theory gets worse in implementation due to the ineffectiveness of our ATC systems due to the disharmony of the varying technologies implemented so far .
The Monopulse SSR
that tried to solve Garbling problems around the 70s came to
the idea that SSR should at least improve in directional accuracy . The
radar ability to discriminate directions derives from the angular width of
its radiation beam . Despite the existence of a parabolic reflector the
beam is not focusing exactly in one line but scans at the same time about
2 to 3 degrees of directions . It is thus natural that aircraft found at a
less difference of directions will not be seen as two different flights by the SSR and
their replies will be confused and again lost .
The achievement of technology here came from a combination of electronics and mathematics that allowed for an analysis and comparison of the phase of the arrival of the pulse elements of the signal of an SSR reply . As a result a computer programme that is linked to the SSR receiver is analysing the signals received and can measure and detect direction differences of about 0.5 to 0.8 degrees within the 2 to 3 degrees wide radar beam . This achievement alone has 'De-Garbled' by almost 90% the garbling problems of the old conventional SSR .
But that was not all ! Additionally , since the position was calculated it did not have to happen for all the pulses of the reply . In theory one only pulse was enough . This fact gave the name MONOPULSE SSR or MSSR ( MONO is the Greek for one-alone ) . However this reduced the need for many replies ! The old SSR was asking for replies at a rate of 450 in a second and the Monopulse could do its work at about 50 per second thus reducing the chances for FRUIT to happen . This new SSR finally :
The direction precision and the reduction of Garbling allowed as well for less separation minima . Present Multi-Radar systems that changed their SSRs to MSSRs reduced separation from 10 NM in area control to 5 NM and in APP from 5 NM to 3 NM . You may note here , though , that the separation criteria do not depend only on the quality of SSR but on the multi-coverage conditions , the update rate and the multi-tracking method which are not identical in all systems - even if the same terms are used , or rather ... abused .
is born !
implementation of MSSR allowed engineers to hope further . There were some
old ideas that now had a chance to materialise . Why finally get only 2
elements of information from an SSR or MSSR , that is Identity and Flight
Level since we may downlink almost any other ? Why
use the MSSR only to receive a reply from an aircraft and not to send a
message to an aircraft as well ? In other words , an Air-to-Ground
Data-Link could become reality to save us from the constant occupation of
the ever increasing radio contact with pilots
The answer was : Yes , it can happen provided the signal of the MSSR reply becomes longer to contain additional pulses for all that extra stuff . And so it did . The old SSR , MSSR reply of about 21 ms and 14 pulses had now to become 112 ms with 56 pulses but thanks to the ability of the MSSR to de-garble overlapping replies that did not look as terrible in the beginning . Of course there is no longer room for the old SSR in such a business and the end of 1999 has signed the death of the SSR in the 21st century .
Yet , this new channel of contact between the controller and the pilot can not accept probabilities of garbling even at a rate of some ... % and should be of course unique , that is it should refer to one specific aircraft at a time and vice versa . This is why a new method of 'calling' the aircraft was found by which into the signals of the 'calling' (interrogating) radar and to the answering aircraft their identity is included .
There is one identity for the radar that is calling ( a number obviously ) and one other similar from the aircraft that answers . In such a way a unique one-to-one contact can be established . All other radars or aircraft that do not identify their own numbers are not involved in this 'discussion' . There is a Selective Addressing here like as you call one mobile telephone number that belongs to one person . The word Selective provided its first letter S for this new way of contact or mode , becoming thus the Mode-S . In actual fact Mode-S is the continuation or extension of the old SSR idea and can be considered as a 'Super-SSR' . It can be implemented , however , only by the use of MSSRs .
This unique addressing , the one-to-one contact , is by definition the total lack of garbling and FRUIT since there is only one reply at a time to be processed by the MSSR and no other to confuse it !! So there was the perfect solution for the old SSR !!
But what information was to be down and/or up-linked between controller and aircraft . Well the list is impressive :
And last but not least , the identity via the Mode-S system is supported by 24 pulses which allow for a total number of 224 = (4096)2 = 16999716 codes in all !!! That means that all aircraft in the world may permanently have their own unique code for addressing that never changes . Correlation is done by the Flight Plan that contains both the call sign and the unique aircraft code via the Flight Plan Processing . So the SSR codes are no longer to be used and no squawks are now necessary !
And you may think this is a dream but I tell you this has already happened !! A full trip of one experimental aircraft , a BAC-111 , was completely controlled silently via a Mode-S link in 1993 ! The world's first !
Problems with Mode-S
The VHF Link
eh ? But ....there are always buts ....
The Mode-S system needs not so much on the air as on the ground . Nowadays the air part is actually the Mode-S transponder which equips all modern aircraft . On the ground installations , however , all the existing systems had to change accordingly and the cost was very high . The basic tool the MSSR is gradually replacing almost all known SSRs and the new buyers order only MSSRs no doubt . But their systems had to be expanded and redesigned drastically which meant much for all service providers !
Additionally the Mode-S had to use this selective calling (Roll-Call) for known aircraft but at regular intervals had to interrogate in the classic SSR (All-Call) way just to detect the 'new comers' into the system yet unknown to it . And even then all linking had to rely on the radar rotation . That is an instruction to the pilot input by a controller would have to wait until the radar faces towards the aircraft and then send it .
And while Mode-S was on the table the satellites gained the floor and impressively have shown that most of the Air-to-Ground linking can take place simply via the satellites and standard VHF transmissions . Just like using a mobile phone . The system becomes then more simple as the aircraft itself can transmit and receive the data like the user at the other end of a line without depending on the radar rotation . It also looks cheaper as an idea .
It was then that Mode-S received a blow on the face and it looked like this would be its end . Additionally the ADS (Automatic Dependent Surveillance) based again on the satellites gave a simple solution on the aircraft detection and communications : The position instead of deriving from the radar it could be known simply by a transmission made by the aircraft itself - by the automatic pilot or rather its FMS ( Flight management System) .
The 2 solutions competed bravely but the VHF Data-Link appeared first and was implemented easily while ADS is still on the fight to gain its position in ATC ! Such an experimental VHF-Data Link is existing in MASS UAC of Eurocontrol from 1996 and works in a simple way - at the moment for about 40% of the casual traffic that is equipped with the new transponders .
Mode-S reborn !
thought of Mode-S differently . Yes , to reduce the need for R/T, the
radio contact between pilot and controller , the VHF Data-Link is better .
The ADS can replace radar at least in areas impossible to cover with and may provide a radar display to pilots for self monitoring
of the traffic around for advanced traffic information .
Mode-S is still there with its other advantages :
And so ...the Mode-S was reborn !
You may note however that :
For more technical information try the two websites indicated below which are a very good reference on this matter - by clicking on the pictures :
The Mode-S page of Eurocontrol projects
Mode S... in a few
The Engineering Institute of Aviation in France - based in Toulouse in the ENAC premises
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