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

For more advanced information check the Eurocontrol and CENA links on the bottom of this page

 Primary Radar 


When the 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 :

  • It provided only a 2-dimensional position of aircraft , as no altitude can be detected by such a system 
  • Due to reflections on various aircraft around positions were detected but those of the aircraft to receive services had to be IDENTIFIED . In other words this radar can not understand the who-is-who of what it sees . The controller has to tell the difference by an identification method usually consisting of track deviations
  • The identification methods used were cumbersome and with some possibility of severe errors to happen 
  • Almost half of the observed 'targets' on the screen were not the controlled aircraft and separation between 'known - unknown' traffic was very important leading to continuous restrictions in maneuvers 
  • Apart from aircraft a number of other reflections caused by interference or hills , obstacles , towers , bridges etc. was cluttering the screens 
  • Intense radio contact with pilots was necessary to verify all related ATC elements like Altitude /  Flight level , track , speed etc.

The Secondary Radar is introduced

Around 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 :
  • A 2-Dimensional position PLUS the Flight level of the aircraft which corresponds to practically  a 3-Dimensional position 
  • Means of identity by setting a specific signal by the pilot , called SSR code and then observed by the controller on the screen




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 :
  • It provided a readable label attached to the aircraft position displaying the full flight number , the Flight level and many other data that one would like to see as Ground Speed , Direction / Track , Flight Path information and co-ordination data 
  • It helped the quicker and safer Tracking of computer systems of all the air movements around
  • It proved to be much cheaper which helped the installation of many such stations 
  • It could cover larger distances by about 100-150% covering by far more wide areas 
  • It rejected false reflections due to signal analysis and would provide a much clearer screen practically without any undesired clutter

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 .... 

  • When there are aircraft in close vicinity , that is close distance and/or close direction  , their SSR replies can overlap , the ground decoder is confused and finally their information is lost . This term is known as Garbling 
  • When there are many SSR stations around the aircraft replies received by other SSR stations that did not 'ask' for these replies , were received and calculated as valid ones resulting in confusion and finally rejection due to errors  . This phenomenon  is known as FRUIT and results from the fact that an aircraft SSR reply is received not only by the SSR that triggered it but by all the others around . The unexpected reply thus arriving to these other SSRs in the area results in wrong decoding and/or  inconsistent position measurements which finally force the computer to reject the SSR information 
  • The total number of different SSR codes you may allocate to various aircraft is consisted of 12 pulses and seems huge as it is 4096 ( result of 8X8X8X8=4096 ) or the same 212 (=4096 , for 12 pulses on a 2-state digit - pulse ON or OFF )  . But in practical terms and for the present needs it is already insufficient as a large number of them is reserved by different units and for traffic arriving at different times . If the same SSR code is used in neighbouring areas then the computer systems have a problem in allocating the same to 2 different flights and usually they do not , or they have to be redesigned to accept them both but always within ordered and well co-ordinated number limits ( 'code families' )  

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 


Engineers 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 :

  • Tripled the direction accuracy of the old conventional SSR
  • Reduced Garbling and FRUIT by about 90% 

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 . 



The Mode-S 

is born !  


The 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 :

  • Any clearance to the aircraft 
  • Any request by the pilot
  • All cockpit data appear in front the controller without having to ask 
  • All non-control data as ATIS , METAR ,Route-info,SLOTS etc. can be directly received by the pilot also without having to ask 
  • Many aircraft parameters can be fed to the tracking system and help the accuracy and speed of position calculations

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 


the ADS 



Fantastic , 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 !


Then people 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 . 

But ....

Mode-S is still there with its other advantages :

  • It has place with its 112 pulses for more information than the 2 others 
  • It avoids the need for SSR codes 
  • It helps the radar tracking to execute a faster and more accurate job 
  • And may work without embarrassing the controller and/or the pilot as these two will practically do nothing more . Mode-S works on the background without needing the human intervention .

And so  ...the Mode-S was reborn !

You may note however that :

  1. Mode-S is still using radar both as a detection tool as well as a link
  2. A VHF-Link is used only for communications and replaces the 'natural' R/T only
  3. The ADS tries to replace both - but it is still under development and it is not officially proven yet 




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 words
by Yves Sagnier


The Engineering Institute of Aviation in France - based in Toulouse in the ENAC premises



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