77 don't go to heaven 76 the box of Tricks 75 taken alive what the hell am I talking about let's find [Music] out hi I'm Grant and welcome to the 11th class in the radio navigation Series today we're going to be having a look at secondary surveillance radar this is a more advanced version of radar the air traffic controllers use to help guide us safely through the skies and today we're going to have a quick look at how it all works secondary surveillance radar is a type of radar that air traffic control can use normally in combination with a primary radar the main advantage of SSR is that more data can be sent secondary radar works by sending out an interrogation wave from the ground and waiting for the response from the aircraft depending on the type of Transporter transponder or the mode that it is selected to which we'll see next the aircraft can send back different information to this ground antenna another advantage of SSR is that each signal only has to travel One Direction in contrast to primary radar where the signal and the navigation rely on reflections of that primary radar beam that is sent out this means that both transmitters can be a bit smaller and cheaper and the system is less susceptible to erroneous Reflections so how does this all work then well if you remember the class on DME it uses very similar concept the ground station will send out an interrogation signal using pulses at 130 MHz 1030 MHz the spacing of these pulses determines the information sent back the transponder the spaces of these pulses is almost like a little question the response from the transponder in the aircraft is sent back on a different frequency to prevent interference and that is on 1090 1090 mahz the signal is also sent back in little pulses which will convey the information requested but it is sent out in all directions the direction and distance information is achieved in the same way as with a primary radar the beam of interrogation signals is rotated around and it's also waiting for responses and whenever it picks up those responses whatever Direction that's in that's the direction it thinks the aircraft's in and the time it takes to come out and back will determine the distance using speed distance time calculations the most basic form of secondary surveillance radar is called moday in this mode the aircraft transponder is set up to reply to the interrogator with a four-digit code which is s sent as a 12-bit binary code and what that means is there are 4,096 unique codes this is used for aircraft identification each digit can only be selected from 0 to 7 it is this way for some weird reason to do with octal numbering where it's base8 for counting instead of base 10 like normal counting but it's all very technical and it's not really that important to know right now just know that numbers eight and nine are not used in transponder codes this transponder code is called a squat code and it is usually given with a departure clearance or when leaving an airport and this way Air Traffic Control know who they're looking at on their displays as you fly off and that information can be handed along to the next Air Traffic Control area along with any other relevant details like aircraft type people on board Etc there are a few unique squat codes that are set by the pilots without instruction from Air Traffic Control these are not assigned codes but they are used when we're in some sort of bad situation they are 7500 760 and 770 75 standing for H Jack 76 standing for communication failure and 77 is for emergencies that's what I was getting at in the intro 75 taken alive 76 the Box tricks 77 don't go to heaven so when an air traffic controller sees these codes appear on their screen it will alert them and give them more information if you have 76 set and you aren't responding to any Radio Calls then they know what's up if you have 7 s set and you are rapidly descending you might have a problem meaning you can't stay up at an altitude and they'll clear the aircraft From Below you so that you can pass down through the flight levels without risk of flying into any other aircraft with mode C transponders which is the next level up of transponder we get that four-digit code functionality as well as altitude information for the controller the feed of this altitude is taken directly from the altimeter and is always referenced to the standard 1013 hect pass altimeter setting which gives pressure altitudes as a readout for the controllers rounded to the nearest 100 ft when the controller gets the altitude reading they will check it is within 300 ft of any assigned clear level they have given you and if it's greater than this then there might be an error either in your flying or in the system if the error is in the system you might need to revert and not use altitude reporting mode anymore and go back to mode a and it might have other KnockOn effects for certain airspace if you are not able to be confirmed as flying as the right level another thing that both mode A and C can do is something called ID denting this is a little button on the transponder that sends out an additional pulse of information which is sent out after the normal reply of those pulses which tells the controller the squat code Andor the altitude if requested by the controller the aircraft can ident which sends out a pulse on the screen of the controller making it easier to find this can be useful in busy areas of airspace to alert the controller of a new aircraft taking off or joining the airspace in a specific location the air traffic control might say squawk ENT and Report level passing and you'll squawk gu in and say we're passing level 3,000 and then they'll see a little pulse on their screen and the aircraft that's at 3,000 ft and they'll say okay that's the aircraft that's just joined my airspace now I'm aware of that one as well as all the other ones I have on my screen mode s is the third and final type of transponder and they're more complicated and have a lot more functionality these types of transponders have a unique 24-bit address this means that instead of the normal 4,096 available codes there's over 16 million codes this code is hardwired into the transponder unit and cannot be changed by the pilots this allows for Selective interrogations to be sent to spefic specific aircraft these transponders also store loads of data and parameters by the aircraft which they can then reply to specific requests from the air traffic controllers and from those radar systems by using a unique code and very specific pulses of information this allows for more complicated information to be transmitted and received the most common information is to do with speed heading aircraft registration the call sign the vertical speed the altitude and of course that four-digit code the mode s transponder uses the same frequencies as mode a and mode C so mode s is backwards compatible a ground station that only has mode a capability will still be able to receive at least the four-digit code selected on the mode s transponder and yes the mode s transponder still has the availability to select a four digit code even though it's got those 16 million plus other codes and it's primarily for that backwards compatibility and also because of those very useful emergency codes that we all learn about so this is roughly what a transponder panel on a modern jet aircraft looks like and while there isn't a clear mode AC C and S selection the options will be available by selecting the relevant button on this sort of dial down here so you probably have two physical transponder systems running through one panel you just select the left one or the right one you've got the system for selecting the code a little failure light if the system fails the ENT button to give that ping and then over here you've got sort of standby which is off you've got alt off which should be your mode a there's no altitude information coming out you've got xndr transponder which is just a standard mode C transponder then you've got these two modes here ta and T R A and that is a great feature of mode s transponders it's this ta and t r a functionality what that means is it's different modes of something called tcas which stands for traffic collision and avoidance system this is a really clever system whereby all the transponders within a certain area talk to each other and they're sharing all their information about heading height speed Etc if they predict a collision might occur then they will arrange for one of the aircraft to climb and another one to descend to avoid each other and then they provide that information to the pilots of both aircraft to fly that avoidance maneuver I won't go into the details or the specifics of how this information is displayed or worked out because it is different from aircraft to aircraft and it's covered in a lot more detail in the instrument subject but tcast basically serves as a last resort to be used if a controller makes a big mistake and sets two aircraft on a collision course so there are a couple of errors in the transponder system that you might see generally with mode A and C transponders the first one is Fring and this this is when an aircraft is within range of two groundbased interrogation Radars and replies to one and the reply is heard by both because the signal is sent out omnidirectionally this means that the reply received by the ground station may be the one that is meant for the other station and this accidental receiving of the wrong signal is called fruiting because it is an acronym stands for false replies unsynchronized to the interrogator transmission the result of fruiting is incorrect information in bearing and distance or replies from aircraft not under control of the relevant Air Traffic Control station and ground radar station causing a lot of confusion the next error is something called garbing if two aircraft are lined up on the same bearing the reply signals might overlap and cause confusion as to which aircraft is at which distance quite a simple one that they're basically lined up shadowing each other or the first one Shadows the back one the replies get in the way of each other they can cause interference but these errors are generally solved by the mode s and that unique aircraft identification code that you have and this makes secondary surveillance radar a pretty reliable tool for aircraft and ATC to use so in summary then secondary surveillance radar uses a interrogator and response principle the interrogation is sent out on 10:30 MHz and the reply is on 10 90 MHz it's a pulse radar so these little pulses the spacing in between the two pulses determines the information that is replied to that's the question and the reply comes through the interrogator to that question it also has the same functionality as primary radar because the system spins wherever it feels as it spins wherever it feels the interrogator come back in from that's the direction the aircraft's in and the time it takes signal to go out and come back determines the distance you have three different modes of transponder you've got mode a C and S A is the most basic there's 4,096 different codes we can use from using digits 0 to 7 in mode a you just get a display on the screen with this unique code no other information other than bearing and range is received but you do have the functionality to set some unique codes 77 don't go to heaven 76 the box of tricks and 75 taken alive for emergencies radio failures or communication failures and 75 for a hijack situation in mode C you get the same functionality as mode a but there's also an altitude readout for the air traffic controllers the altitude is a pressure altitude because it's based off of 1013 hectopascals that's standard pressure setting and it's rounded to the nearest 100 ft and all these transponders you get the function to ident which is a little button that you push in the flight deck and it sends out a ping a pulse to the radar controllers so they can see easily who you are maybe if there's confusion over uh an aircraft that's just joined airspace or just taken off they might say right I like to have know all the aircraft that have just taken off from this airport so um I'm going to ask every single one to ID in just after takeoff mode s is the best and it has a unique each transponder has a unique code it's a 24bit code which means there's 16 million plus unique codes and because it's Unique it allows for specific interrogation from the ground radar which means that a lot more complicated information could be sent back still has the functionality of mode C and mode a including those codes but it also has the ability to provide speed heading registration information call signning information and a lot more um a lot more of that detail will be shown in the instrumentation class or course rather one of the best things about modes is because it has all this information it can share that with nearby aircraft and provide traffic collision and avoidance uh through the tcast system basically the aircraft talk to each other if they see they coming towards each other they'll say you climb and you descend and they'll arrange that tell the pilots to do it they'll do it and there won't be any Collision B there there's a few errors in the mode a and mode C that are kind of solved by the mode s and this very unique signal that is sent out the first one's fruiting uh there's two ground stations they both hear the reply to one interrogator so this gets a reply when it didn't ask for any it didn't interrogate for example so that can cause confusion in bearing distance and you might get a reply from an aircraft that's not within the air traffic controls region of control garbing is when they line up and the first aircraft basically Shadows the or cast a shadow on the second one and it might reply to information and they overlap and again that just leads to confusing information