Air Traffic Systems

SECONDARY SURVEILLANCE RADAR Air Traffic Systems SECONDARY SURVEILLANCE RADAR INTRODUCTION AND BASIC PRINCIPLES SSR click What is the purpose of SSR?

Basic Primary Radar RX TX Radar Picture Transmitter Pulses & PROCESSING Received Signal Timing & Control Radar Picture

DISADVANTAGES OF PRIMARY RADAR High Power Clutter Permanent Echo's Weather i.e. Rain etc. Identification Q. List the advantages of SSR. click A. Positive ID. Clutter elimination (TX/receive on different frequencies). Lower power for equivalent range. Accurate height information. Emergency ID.

Basic Secondary Radar RX TX Radar Picture Timing & Control Tx 1030 MHz SSR Rx 1090 MHz Decoder Secondary returns TX RX & PROCESSING Timing & Control Radar Picture ABC 123

Modes and Pulse Spacing Military A/C Ident - Mode 1 3µs Military Mission Ident - Mode 2 5 µs Joint Military & Civil A/C Ident - Mode 3/A 8 µs The modes of operation and associated pulse spacing P1 to P3 (leading edge to lading edge) are: Mode 1 3S (military identification) Mode 2 5S (military identification) Mode 3/A 8S (military/civil identification both modes are the same) Mode B 17S (civil identification) Mode C 21S Height, (read from A/C altimeter) Mode D 25S Not in use at this time Mode 4 This introduces an additional pulse, it used by the Americans, but very limited within RAF, it is not covered on this course. Civil A/C Ident - Mode B 17 µs Altitude Data - Mode C 21 µs

SSR Questions (Modes) Who are you ? (Mode 1 and 3/A) What is your altitude ? (Mode C) What is your mission number ? (Mode 2) Are you FRIEND or FOE ? (Mode 4)

Mode Interlacing The ability of the system to automatically change modes. This enables more diverse information to be gathered. In order to gather as much information as possible from an aircraft the interrogator will need to ask more than one question. The interrogator asks more than one question by interlacing the transmission modes. This means that a different mode is transmitted by the interrogator on each PRF.

MODE INTERLACING h1 h2 Military transponder set to :- Mode 1, code = 1100 Mode 3/A, code = 4170 Mode C, code = h1 Civil transponder set to :- Mode 3/A, code = 2210 Mode C, code = h2 Mode interlace pattern 1, 3A, C 1 3/A C Military Civil No 2210 h2 1100 4170 h1 Coded Replies Because there are all sorts of a/c flying at any given period, if we were to limit our interrogations to one question only, there would be a danger of some a/c not responding because they have not been asked. This could result in a danger of collisions because the controller would not know that there was more than a few a/c out there. We get round this by asking different questions every PRF. This is called MODE INTERLACING. Consider the two different A/C. Every PRF we ask a different question. Explain click

EMERGENCY REPLIES CIVILIAN EMERGENCY GENERAL EMERGENCY - 7700 RADIO FAILURE - 7600 HI-JACK - 7500 But now we only send the codes in the first frame and then TX three empty frames spaced at 4.35us. click

ADVANTAGES OF SSR OVER PRIMARY RADAR POSITIVE IDENTIFICATION CLUTTER ELIMINATION LOWER POWER FOR EQUIVALENT RANGE ACCURATE HEIGHT INFORMATION EMERGENCY INFORMATION There are several disadvantages of SSR. click 1 interrogations via Sidelobes 2 FRUIT 3 garbling 4 track wander

DISADVANTAGES OF SSR INTERROGATIONS VIA SIDE LOBES GARBLING FALSE REPLIES UNCORRELATED IN TIME (FRUIT) GARBLING TRACK WANDER There are several disadvantages of SSR. click 1 interrogations via Sidelobes 2 FRUIT 3 garbling 4 track wander

This would be the AE pattern that would be radiated, Aircraft A 80 miles Aircraft 200 Miles C This would be the AE pattern that would be radiated, As you can see, a/c A is being interrogated by the Main Beam, but a/cs B and C are also being interrogated but this time by the Sidelobes. click Aircraft B

INTERROGATION SIDE LOBE SUPPRESSION Aircraft A 80 miles Aircraft C 200 Miles This process is called ISLS As you can see the Control beam is only radiated as far as the EOR of the Sidelobes. Now, any a/c in the Sidelobe will receive a P1 P2 and P3 pulse. click Aircraft B P1 P2 P3 2 s 

F.R.U.I.T. Here we have Site A interrogating an a/c Site B unwanted replies PRF 260Hz Interrogation Site A Here we have Site A interrogating an a/c The a/c sends back a reply from it’s omnidirectional AE but, since all replies are transmitted at 1090MHz, Site B will also receive it’s replies. This is what the target will look like on both displays click PRF 250Hz F.R.U.I.T.

FRUIT Same as the basic MTI principle Correct reply (Frame pulses + 2 information..) FRUIT Second PRP Replies First PRP Replies Same as the basic MTI principle Delay the first reply by 1 PRF and compare it to a undelayed reply. As long as it’s still there, we have a genuine reply from our interrogation. click Coincidence 1 PRF DELAY Gate De-fruited video O/P DEFRUITER PRINCIPLE

GARBLE OVERLAPPING Overlapping replies End of Beginning of pulse C2 C4 F1 End of pulse train from aircraft A Beginning of pulse train from aircraft B Overlapping replies OVERLAPPING Overlapping Replies. Consider replies from two passing a/cs. If you were to look at the replies on a scope, this is what you would see. Two 20.3us frames with some 0.45us Code Pulses spaced 1.45us apart but also wider pulses. When the processor looks at them, it will only recognise the 0.45us pulses, the pulses are wider than 0.45us, it will discard and so inadvertently produce errors. This is overcome by a process called Leading Edge Generation Whereby a clock pulse is generated every time there is a Leading Edge a clock pulse being 120ns. (Show on the Board clock pulse being 1/4 of Code Pulse.) The processor looks at the longer than usual pulse and decides whether it is more than one pulse, It if is, it will guess where about the second pulse starts and then insert a Leading Edge pulse to create two pulses from one.

TRACK WANDER 1st sweep 2nd sweep 3rd sweep If, in the unlikely even that an a/c would respond to all the interrogations, the display would show a perfect eyebrow reply. click 4th sweep

TRACK WANDER but, on the next revolution, 1st sweep 2nd sweep but, on the next revolution, it may only respond to some of the interrogations, so the radar would position the track by taking the average bearing from where the most hits occurred. This would position the track Left of the Boresight. Can be lost due to : Aircraft Turning Capture by other SSR CLICK sweep sweep track

THE MONOPULSE SYSTEM ELIMINATES TRACK WANDER BECAUSE ONLY ONE PULSE FROM A REPLY TRAIN IS NEEDED TO OBTAIN AN ACCURATE BEARING. This was why the MONOPULSE system was developed. click It eliminates Track Wander, because only one pulse from the reply is needed to produce an accurate bearing.

COMBINED SUM & DIFFERENCE PATTERNS BORE SIGHT This gives a combined SUM and DIFFERENCE receive pattern which is the principle of the MONOPULSE system. As we have a Sum and Difference pattern we also have a separate receive for both channels as well as a receive for the Control pattern. So to summerise we TX on 2 Channels. And Rx on 3 CLICK (next slide)

Mono-pulse Secondary Surveillance Radar MSSR

Cardion Secondary Surveillance Radar Antenna mounted on Tactical Watchman Mounted on T101 Air Defence Radar

SSR SYSTEM APPLICATIONS MILITARY AIRCRAFT CIVIL AIRCRAFT AIR TRAFFIC CONTROL MARITIME AIRCRAFT Just to show “every man and his dog” uses it. MISSILE NAVAL SYSTEMS

This is my discrete address MODE S I am able to reply to your “ALL CALL” because I have Mode S This is my discrete address Mode S will be an issue for our MSSR and CSSR systems because they don’t support Mode S. The a/c, if it has Mode S capability, sends back a 4 pulse preamble indicating that it is acknowledging receipt of a Mode S interrogation, and then sends a data block with all the information that is required. click Data block 56 s or 112 s Preamble 8.0 s Bit 1 Bit 2 Bit 3 Bit 4 Bit N 1 1 1 1 1 0.5 s 0.5 s

The future of SSR Mode S – becoming compulsory under EASA rules CSSR and MSSR don’t support Mode S

Airfield layout

Any Questions ?