4 DISADVANTAGES OF PRIMARY RADAR High PowerClutterPermanent Echo'sWeather i.e. Rain etc.IdentificationQ. List the advantages of SSR.clickA. Positive ID.Clutter elimination (TX/receive on different frequencies).Lower power for equivalent range.Accurate height information.Emergency ID.
6 Modes and Pulse Spacing Military A/C Ident - Mode 13µsMilitary Mission Ident - Mode 25 µsJoint Military & Civil A/C Ident - Mode 3/A8 µsThe modes of operation and associated pulse spacing P1 to P3 (leading edge to lading edge) are:Mode 1 3S (military identification)Mode 2 5S (military identification)Mode 3/A 8S (military/civil identification both modes are the same)Mode B 17S (civil identification)Mode C 21S Height, (read from A/C altimeter)Mode D 25S Not in use at this timeMode 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 B17 µsAltitude Data - Mode C21 µs
7 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)
8 Mode InterlacingThe 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.
9 MODE INTERLACING h1 h2 Military transponder set to :- Mode 1, code = 1100Mode 3/A, code = 4170Mode C, code = h1Civil transponderset to :-Mode 3/A, code = 2210Mode C, code = h2Mode interlace pattern 1, 3A, C13/ACMilitaryCivilNo2210h211004170h1CodedRepliesBecause 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.Explainclick
10 EMERGENCY REPLIES CIVILIAN EMERGENCY GENERAL EMERGENCY - 7700 RADIO FAILUREHI-JACKBut now we only send the codes in the first frameand then TX three empty frames spaced at 4.35us.click
11 ADVANTAGES OF SSR OVER PRIMARY RADAR POSITIVE IDENTIFICATIONCLUTTER ELIMINATIONLOWER POWER FOR EQUIVALENT RANGEACCURATE HEIGHT INFORMATIONEMERGENCY INFORMATIONThere are several disadvantages of SSR.click1 interrogations via Sidelobes2 FRUIT3 garbling4 track wander
12 DISADVANTAGES OF SSR FALSE REPLIES UNCORRELATED IN TIME (FRUIT) INTERROGATIONS VIA SIDE LOBESFALSE REPLIES UNCORRELATED IN TIME (FRUIT)GARBLINGTRACK WANDERThere are several disadvantages of SSR.click1 interrogations via Sidelobes2 FRUIT3 garbling4 track wander
13 This would be the AE pattern that would be radiated, AircraftA80 milesAircraft200 MilesCThis 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.clickAircraftB
14 INTERROGATION SIDE LOBE SUPPRESSION AircraftA80 milesAircraftC200 MilesThis process is called ISLSAs 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.clickAircraftBP1P2P32 s
15 F.R.U.I.T. Here we have Site A interrogating an a/c Site Bunwanted repliesPRF 260HzInterrogationSite AHere we have Site A interrogating an a/cThe a/c sends back a reply from it’s omnidirectional AEbut, 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 displaysclickPRF 250HzF.R.U.I.T.
16 FRUIT Same as the basic MTI principle Correctreply(Frame pulses + 2 information..)FRUITSecond PRPRepliesFirst PRPRepliesSame as the basic MTI principleDelay 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.clickCoincidence1 PRF DELAYGateDe-fruitedvideo O/PDEFRUITER PRINCIPLE
17 GARBLE OVERLAPPING Overlapping replies End of Beginning of pulse C2C4F1End ofpulsetrain fromaircraft ABeginning ofpulse trainfromaircraft BOverlappingrepliesOVERLAPPINGOverlapping 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 GenerationWhereby a clock pulse is generated every time there is a Leading Edgea 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.
18 TRACK WANDER1stsweep2ndsweep3rdsweepIf, in the unlikely even that an a/c would respond to all the interrogations,the display would show a perfect eyebrow reply.click4thsweep
19 TRACK WANDER but, on the next revolution, 1stsweep2ndsweepbut, 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 TurningCapture by other SSRCLICKsweepsweeptrack
20 THE MONOPULSE SYSTEM ELIMINATES TRACK WANDER BECAUSE ONLY ONE PULSE FROM A REPLY TRAIN IS NEEDEDTO OBTAIN AN ACCURATE BEARING.This was why the MONOPULSE system was developed.clickIt eliminates Track Wander,because only one pulse from the reply is needed to produce an accurate bearing.
21 COMBINED SUM & DIFFERENCE PATTERNS BORE SIGHTThis 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 3CLICK (next slide)
26 This is my discrete address MODE SI am able to reply toyour “ALL CALL”because I have Mode SMode 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 preambleindicating that it is acknowledging receipt of a Mode S interrogation,and then sends a data block with all the information that is required.clickThis is my discrete addressData block 56 s or 112 sPreamble 8.0 sBit 1Bit 2Bit 3Bit 4Bit N111110.5 s0.5 s
27 Mode S – becoming compulsory under EASA rules The future of SSRMode S – becoming compulsory under EASA rulesCSSR and MSSR don’t support Mode S