in everyday life we can use a ruler or maybe even a tape measure to help us measure distances but in aviation when the distances are a lot bigger we've got a far more useful bit of [Music] Kit hi I'm Grant and welcome to the sixth class in the radio navigation Series today we're going to be taking a look at DME or distance measuring equipment this is a handy bit of equipment that we can use to tell how far away we are from something and if we combine this with a VR or an NDB it can give us some pretty accurate information on where we are in space and how we can get to where we want to go distance measuring equipment is a system used in aviation to determine the exact distance between an aircraft and a ground station it relies on both equipment on the ground and in the airplane to function which makes it a form of secondary radar primary radar is where there's only one bit of equipment that transmits and receives like a bat using Sonar for example whereas secondary radar needs both the aircraft and the ground station to transmit and receive signals so the ground station is uh typically collocated with a navigational Aid like a VR NDB or an instrument Landing system ILS which we'll look at in the next class and in the aircraft there's a DME interrogator that transmits and also receives radio signals as well basically what happens is the DME interrogator on the aircraft sends out a stream of paired pulses as a signal to the ground station on a specific frequency the pulses are jittered uniquely to each plane what that means is that each pulse pair in between the pairs is separated by 12 microc seconds as standard but the gaps in between each pairs of pulses is unique this means that each aircraft has a unique signature in the form of the gaps in between the pulses on average though the amount of pulses per second is around 150 until the aircraft is locked on to a DME signal at which point it drops to 24 pulse pairs per second so the aircraft fires out this pulse pair signal which is unique in the forms of the gaps in between the pulse Pairs and the ground station transmits a reply back on a slightly different frequency but the jittering will be transmitted back so an aircraft receiver will be able to distinguish its own signal from the ones around it that other aircraft might be using as well the ground DME station sends back the signal 63 MHz different in frequency and it's a delay factored in of 50 microc seconds this means that the aircraft receiver can be sure that it isn't a reflection from a mountain uh of the signal just send out because the fact that the frequency is different means it can only come from the DME station the DME system therefore requires two frequencies to operate each separated by this 63 MHz difference and these frequencies exist in the ultra high frequency band which is between 300 mahz and 3 gahz uh and these frequencies are paired together those 263 need to be paired together into what we call channels and these channels are then linked to a VHF frequency so when you tune in the VHF frequency the DME channel is automatically tuned by the onboard aircraft equipment if there's a collocated DME that can be used for example so you tune up your VR it'll go there's DME there the aircraft will automatically choose the channel which is applicable to the uh collocated DME at that V for example hopefully that makes sense to then use the DME you would have to ident it in the same way that you do any other radio Beacon the DME ident will be sent over the same type of signal as an NDB VR or ILS but will play one in every four a slightly different pitch so you get three idents at a lower pitch then one at a more higher pitch which confirms that the DME is also tuned the DME interrogator in the aircraft measures the time it takes for the signal to go back and forth and then using this this time delay in between sending out and receiving a signal it calculates the slant range based on speed equals distance over time and obviously dividing the distance in half the distance measured is important to note is slant range not the horizontal distance over the ground this seems a bit useless because planes can fly all the way up from the ground up to 30,000 ft and more so surely this change in height will change the slant range and make the distance is inaccurate depending on what height you are and change depending on your height all trigonometry and Pythagoras and stuff well the point is it does although it doesn't really make all that much difference uh if we look at a basic example which I've set up here we're going to do some Pythagoras a s + b s = c^2 I've done the calculations off screen but we can know that for option one which is going to be aircraft uh a and aircraft B we're going to have two different figures we're going to convert them into the same units so we know there's 6,076 ft in every knock mile so for the first example we've got 67,68 s+ 20,000 squar and for the second example it's 67,000 600 squar plus 30,000 squar all the L calculations Al together you would get an answer in feet when you convert it back into nautical miles for the first example which is the 20,000 ft example you get a distance of to two decimal places 100.5 and for the example at 30,000 ft aircraft B there you get 10.12 nautical miles so the 10,000 ft difference makes what 0.7 worth of DME distance 0.07 sorry and compared to the horizontal distance we've got maximum .12 nautical miles off so it's calculating slant range but at long distances it doesn't really matter obviously when we getting closer and closer it's going to make more and more of a difference um because of the angle being greater however when we need to be completely accurate and precise when we're close into the aircraft such as for landing or takeoff we're going to be flying on specified procedures called standard instrument departures SIDS and standard terminal arrivals stars and also on approach procedures these procedures will of often have a specified DME distance to make turns on certain VR courses to be on and certain altitudes to be at if these are designed using the slant range from a certain DME then as long as we follow it and all the other aircraft are also following the DME slant range then it doesn't matter if the leg in between two points is 10 DME long then we fly until our DME says 10 then make the turn it doesn't matter if it's 10 ntic miles over the ground because the procedure wasn't around designed around a 10 n mile distance over the ground but a 10 DME distance on the aircraft which is slant range so it doesn't matter as long as you fly the DME distance that it says on the procedure it'll all work out because it's not bothered about ground range it's bothered about slant range another cool feature that can be built into a DME system is an offsetting of the zero DME distance this would normally occur overhead the DME where the time delay is really really short and therefore the distance measured is too small for our equipment to display properly it might be a measurement of 0.05 no miles for example 0.05 yes but DME distance is normally only ever given to one decimal place so the minimum distance we could read is 0.1 and therefore it would reset to maybe uh reading 0.0 on the DME with offsetting the zero DME point we can do some useful things the best example of this is to offset the DME distance so that it reads zero at the runway threshold even though the DME itself isn't sat at the end of the runway but somewhere else on the airport this way we get a nice distance countdown as you approach the runway on a procedure such as an ILS or maybe even a VI approach this offsetting is achieved by reducing the time delay from the standard 50 micros seconds to make this easy to understand I'm going to use some rough numbers in reality it would be fractions of seconds for these distances but say for example at the 50 nautical mile range the interrogation and response to 5 Seconds to go and come back this is with the 50 microc delay included so the calculation is made speed equals distance over time divide it in two and we get 50 nautical miles if we reduce the 50 microsc Gap uh or delay in the reply signal to maybe 25 microsc then the pulse and response now take slightly less time so it will maybe cause the DME to show 49 nautic miles even though the aircraft is physically still at 50 nautical miles out so if we do this close into the runway we can reduce the 50 microc delay just by just the right amount to trick the DME into reading zero when the aircraft passes over the runway so there's a few errors in DME systems that can happen but generally they're pretty accurate bits of Kit so the first one is basically a signal loss so if we're flying towards a DME station and we lose the signal temporarily for some reason then if we keep traveling at the same rate the DME will reduce or increase as it has been according to that rate so as long as we keep the same speed it will remember it has a memory function built in that if we lose the signal it'll keep counting down at the same rate we're flying towards it and then we get the signal back it'll carry on or if we don't have a signal for more than 10 seconds that will cause maybe a failure flag to pop up or a 0000 distance or a dashes of lines that point will be when the DME has failed another thing that can happen is saturation so basically a DME station could become overwhelmed with lots of aircraft interrogating this would mean that there's lots of aircraft that would not receive the correct pulse pairs back and pretty useless information would be provided this is why dmes come with basically what we call an automatic shedding system so it sheds off signals it gets rid of signals it disregards them that aren't as strong so maybe they're too far away or from a weak transmitter this means that in practice Only the strongest 100 signals uh and receive replies and distance information ground Reflections happen if the interrogation signal reflects off a mountain or maybe bounces up to a DME that's slightly higher on the ground like this example before getting to the DME and then the reply will be sent to the airc CRA but will have taken longer to do so because the signal has taken longer to reach the DME and that will show the aircraft being further away because speed equals distance over time that's a larger time if we have a clear path to DME as well then the reflected signal will always drive back at the aircraft later because it's traveling further so the plane is built in with a system that only picks up the First Signal the first reply from the DME station per pulse pair this aircraft will fire out or the aircraft sorry will fire out about 150 uh reducing to 24 pulse pairs per second and the first one to come out out come back out of each of those individual pulses will be the one that is listened to and used so as soon as the first pulse pair reply is received then the receiver stops listening and then it waits the appropriate amount of time delay that Gap in the jittered pattern and then it switches on holds its ear back up and says I'm listening again another one comes back it stops listening it waits for the appropriate Gap it listens again and the signal uh guarantees that only the first one is ever received aka the shortest distance the one without Reflections so there's various systems in place to keep the accuracy of the DME system quite High we've got the sort of the listening thing here we' got the saturation we've got the unique jittering we've got the memory function and other things which make it very accurate on Old systems it's accurate to be within 0.25 KN miles plus or minus 1.25% and on newer systems it's accurate to be2 NAU miles 95% of the time so pretty reliable bits of equipment to summarize then DME uses the ultra high frequency band from 300 MHz to 3 GHz it uses two frequencies one that gets sent out 63 MHz different from the one that uh is received by the aircraft they're paired into channels and those channels are linked to VHF frequencies and beacons and the aircraft will automatically tune the channel that is associated with the VHF frequency the plane will send out a unique signature of signals pulse pairs the Gap in between each pulse is 12 micros seconds but the Gap in between the pulses is unique to each aircraft so it sends this out to the DME station the DME Waits 50 micros seconds and then sends back a signal matching the time delays but 63 MHz difference so you the aircraft knows that it's getting its signal back instead of another aircraft's back and it knows it's not a reflection because there's this 63 MHz difference speed distance time calculations are then used you want to find the distance and then divide it by two obviously to get the the slant range between the aircraft and the receiver uh if you calculate the slant range compared to the ground range it's going to be very very similar because of the distances we're talking about at long range but as we get closer in it's going to make more of a difference and it doesn't matter when we're close range and being accurate because all procedures will be designed around slant range rather than horizontal distance over the ground one of the cool things you can do is you can incorporate a time delay so instead of the 50 microc delay you reduce it down to 0.25 or you could even extend it out to make the zero point of the DME in a different location so basically what you're doing is you're normally the time element here has 50 micr seconds if you have a shorter delay the time is going to be less and that's going to influence the distance it's going to make the distance less as well there's a few errors signal loss uh just losing the signal but there's a memory system built in that will hold on to a signal uh hold on to a DME distance counting down at a certain rate for 10 seconds saturation only the 100 strongest signals get a reply from a DME station ground Reflections the signal on the way to the DME station bounces off a mountain or the ground for example and that means it takes longer to travel to the DME so the overall time will be longer than the shortest possible direct route so the aircraft only listens to the first of each pulse pair reply meaning it's only getting the shortest possible distance between itself and the ground station these are accurate bits of Kit so you're looking at 0.25 KN miles plus or minus 1.25% for older bits of equipment and for newer bits of equipment you're looking at 0.2 KN miles 95% of the time so DME pretty good stuff give you some pretty accurate information on distance for how far away you are from the station