Use of CSB/SBO technique in the Instrument Landing System (ILS)

there are lots of websites which talk about how the ILS the instrument landing system works however they all seem to skate over one particular aspect that's exactly how the modulation the space modulation of the signal is produced using the CSB and SPO technique in this video I'm going to try to clarify that and on the way show how elegant and sophisticated this application of analog radio engineering really is first let's have a look at a conceptual model of a localizer system this shows what result we're trying to get but it won't be a practical system to VHF AM transmitters operating around 110 megahertz are provided one modulated with a hundred and fifty Hertz sinusoidal tone the other with ninety Hertz tone both transmitters use the same radio frequency source this could be a low frequency crystal oscillator followed by frequency multiplying stages to reach the very high frequency used by this means the transmitter app was a lock together and can form a coherent transmission the transmitters feed into two directional antenna systems mounted either side of the runway centerline these might consist of several yagi or log periodic antennas in a broad side array in this instance that antennas are physically offset so the peak of the radiation pattern is to one side or the other of the runway centerline I showed because both transmitters are fed from the same drive source the two radio beams form a coherent transmission but with the modulation depth of the two turns varying depending on where the aircraft is in relation to the runway centerline this is so-called space modulation of the transmission the mod depth depends on where the receiver is in the space in front of the antenna system went to the right of the centerline the 150 Hertz beam is stronger whereas to the left it's the 90 Hertz beam down the center line the two tones are equally strong and this is the indication that the pilot uses for guidance to the runway this simple system has a major flaw if one of the transmitters has a higher power or greater modulation depth than the other as shown here the line of equal modulation depth shifts away from the center line this could lead to an aircraft flying closer to obstructions and also to not being adequately lined up when finally becoming visual with the runway there is also a practical problem with adjusting and setting up the ILS changed to the course width for example require physical to the antenna system in angle or phasing of the elements both of which are not at all convenient the system provides a steering signal called the course signal which drives an indicator in the aircraft the indicator is called the CDI or course deviation indicator here's a runway with the ILS localizer at the left end the course signal extends in a triangular shape as shown here a few degrees wide an aircraft outside or on the edge of the course receives a full-scale fly left signal as shown when it is on course on the centreline the CDI needle is centered and it moves over to the full fly right indication at the other edge of the course and beyond the ILS course in effect funnels the aircraft towards the runway with the course signal becoming narrower in actual width as the aircraft nears the runway although the angular width remains the same this ensures that the pilot is close to the runway centreline when eventually becoming visual with the runway lights and he does not have to make large changes of course a slow level close to the runway in the real world the CDI also has a horizontal needle which provides guidance in the vertical plane using the glide slope transmission this picture is of a CDI that shares the indications with a vor that's why it's got a nought to 360 degree scale on it modern aircraft use similar format displays but using computer graphics the system in place in practical present-day ILS installations uses two signals CSB and SPO in a very elegant implementation of the required space modulation CSB means carrier and side bands SPO means side bands only to understand how the variable modulation depth signal is formed you need to understand three analog radio concepts first is the sum and difference technique the second is the characteristics of double sideband suppress carrier signals and then some antenna theories specifically what happens when you feed broadside antenna arrays in anti-phase I'll deal with each of these concept in turn there'll be a minimum of maths and a maximum use of graphics which I hope will make grasping the concepts easier and the overall aim of this technique is to make a stable reliable system where the course guidance is defined almost exclusively by the antenna system which has a low dependency on the absolute output power of the ILS transmitter equipment I should point out again that I'll be talking here only about the localizer aspect of the ILS the glideslope uses the same CSP and SPO technique but with somewhat different antenna arrangements but the object is similar to form a stable equal tone course signal with varying depth of tone either side of the correct light slope first the sum and difference technique this concept by the way is also used for example in analog stereo broadcasting on FM transmissions which have also developed mid twentieth century here we'll consider just the modulating tones on the two signals 90 and 150 Hertz sine wave tones the csb signal is modulated by the ninety and a hundred and fifty hertz tones added together the SPO signal is modulated by the 90 Hertz tone added to the anti-phase 150 Hertz that is 150 Hertz signal is inverted or shifted in phase by 180 degrees f1 and f2 represent the tones 90 and 150 Hertz actually the instantaneous value of the signals for example voltage but remember in practice now varying sinusoidally with time at some point in front of the ILS antenna system the csb signal has the value of M f1 plus f2 where m is a factor depending on the transmitter power and antenna gain towards that point similarly the SPO signal has a value of n f1 minus f2 where n likewise depends on transmitter power and antenna gain towards that particular point if we arrange to add and subtract the CSP in SPO signals and we'll come to see later how this happens we get the following effect taking the sum we substitute the formulae take away the brackets collect the terms for f1 and f2 this term is bigger than this term therefore F 1 is greater than f2 and so the 90 Hertz signal is stronger at that point in space similarly for the difference the same process now this term is smaller than that one so f1 is less than F 2 F 2 is greater than F once 150 heard signal is stronger at that point in space this shows the centre line of the localiser signal above the center line and the difference is taken between CS BN SPO to give a stronger 150 Hertz signal below the centre line that the sum is taken causing the 90 Hertz tone to predominate now we'll look at a block diagram of the ILS transmitter to see how these signals are produced two sine wave audio generators one on 90 years and the other on 150 Hertz are provided to give the CSB modulating signal the two tones are simply added together this could be as simple as a couple of resistors the SPL signal also consists of the two tones being added together however in this case one of them is inverted before being added to the other tone in the diagram I've shown the 150 Hertz signal to be inverted these are the two tones generated and added together for the CSB signal and here's the anti phase 150 Hertz signal added to the 90 Hertz to provide the SPO modulating signal the RF signals are generated as follows there's a VHF carrier signal generator this might be a low frequency crystal oscillator with frequency multiplying stages to reach the required frequency between 108 and 112 megahertz I've shown a hundred and nine point one megahertz in the diagram for the CSB signal we need to produce a double sideband full carrier signal that is conventional AM signal can be done by using a balanced modulator to give a double sideband suppress carrier signal then adding the carrier back in at a suitable level to give the required modulation depth the CSB signal passes through a linear RF amplifier which gives a few watts output to feed to the antenna system the SPO signal is simply fed to another balanced modulator and the resultant double sideband suppressed carrier DSPs C signal is again amplified to provide the required few watts of power to the antenna system in the SPO signal chain is a variable attenuator which controls the output power of the SPO component and incidentally thereby adjust the course with which we'll look out later and also to a vert or phase shifter this is nominally set to 90 degrees like this variable over a small range to allow for tolerances between the PSB an SPI RF path to be compensated for due to differences in feeder lengths and so on another signal is added to the CSP transmission as shown here a 1020 Hertz sine wave generator is keyed with Morse code signals to form the ident signal for the eyeless so the pilot can confirm that he's heading for the correct runway there is also a provision in the ILS spec for speech to be added to the transmission this is rare to see in practice although it could be used for example to give information to a pilot whose comm equipment had failed or to give weather information here's a brief recording of an ILS transmission as received on a simple AM receiver the low frequency tones the 1950 Hertz will probably only be audible if you're using headphones or a reasonably large loudspeaker now let's look at some characteristics of double-sideband suppressed carrier signals the SPO is this type of signal first look at the spectrum of an conventional AM signal there's the carrier in the center and two sidebands upper and lower each side if we take away the carrier we're left with a double sideband suppress carrier or D SB SC signal this shows the wave form of an AM signal with carrier a conventional AM signal if you put this through an envelope detector such as a simple diode which in this case chops off the negative going part of the waveform and you filter the output you recover the modulating signal now this is a DSP signal with the same modulating signal I've shown the modulating signal as a blue line if this way from was put through a dye detector you can see that the modulating waveform is not recovered in fact to receive this signal properly with a diode detector you have to add the carrier signal back in in the appropriate phase another interesting thing about the DSP SC signal is that the carrier reverses phase as the modulating waveform passes through 0 as shown here you can see the kinks in the carrier wave form at the zero crossing points to show how this happens the balance modulator in the transmitter can be considered to be an analog multiplier with a carrier input a modulating signal input and the output which is the product of the two V out equals VC times VM if we constrain VM to be plus or minus one we can see if the ami is plus 1 the output is VC VM the modulating signal is minus 1 the output is minus VC which means that the carrier is inverted if we look at this with some scope displays on the port's VM is plus 1 the output is the carryin phase VM at minus 1 and the carrier phase is inverted now we can show what happens when the VM signal varies plus or minus as it goes through 0 the carrier phase inverts let's look back at the DSBs see signal again we'll concentrate on just the area highlighted where the modulating signal falls through zero to a negative value so here is the expanded view for simplicity I've shown the carrier as a triangular wave but the principles apply equally to a sine wave that's the DSP SC signal with the modulating signal passing through zero if we look at the redline both the carrier and the DSP SC Sigma are in phase reaching a positive peak value at the same time in the center the mod signal passes through zero and now to the right of that point if you look at the red line again it shows that the carrier phase is opposite to that the SB SC signal phase now if we add the carrier and DSPs C Sigma together which is what happens with the space modulation of the ILS signals in the far field take the value of the carrier and of the DSP SC Sigma at all points along the time line to get this resultant signal now we can pass this signal through a diode or other M verb detector and filter the output to recover the modulating signal you can see that it matches the modulating signal it's falling from left to right now back to the same scenario the carrier and the DSP SC signal and put on the red marker again but now we'll shift the carrier phase by 180 degrees that is will invert it now you can see on the left the trough of the carrier coincides with the peak of the DSP SC signal it's 180 degrees out of phase if we go through the same process addition process as before we get this waveform again pass it through an envelope detector smooth the output to get this recovered modulation envelope you can see that this record envelope is inverted compared to that before it's now rising instead of falling as we go from left to right so the carryin phase carry it out of phase you can see that the signal gets inverted depending on the phase of the carrier to summarize with double sideband suppress carrier signals inverting the phase of the added carrier inverts the recovered modulation phase when the resultant is rectified we'll see later how this effect is used to form the space modulation required in the ILS mission now to consider the techniques used in the ILS localizer antenna first let's look at the transmitter feeding into a single directional antenna like a yogi or a lot of periodic it might have a pattern like this I should add at this point that all the antenna patterns here should be taken with a pinch of salt they're only hand-drawn approximations of what exists in real life just for illustration purposes if we add another similar antenna and feed them both in phase the pattern narrows and the maximum signal level increases the gain goes up by 3 DBS or so but an interesting thing happens if we now add an extra half wavelength of feeder so that the two antennas are fit in anti-phase a sharp null appears on the center line a receiver located on the center line picks up equal amplitude but opposite phase signals from each half so the resultant cancels out to produce a deep null in the ILS this principle is used to define the runway centerline as shown here the two antennas are fed in phase to produce a beam as shown in orange this carries the CSB carrier and sideband signal the SPO sidebands only signal is fed to the two antennas in anti-phase which gives a deep null on the centerline it is this null which defines the course that the aircraft follows I'll show how this happens in due course remember that both CSP and SPO are derived from the same carrier source so they form a coherent signal with space modulation of the signal in the far field and the requirement is to feed the CSP signaling phase and the SPS signal in anti-phase to the turnt inner elements this can be done using a hybrid splitter this is a useful component having four ports shown here p1 and p2 are the two output ports which connect to the antennas there's a summing port input and a different sport some important and the difference port the Delta input if RF power is fed into the summing port Sigma input the signal is divided equally at half the power is passed to each p1 and p2 in phase the difference port is effectively isolated and no power is fed out of this port when RF power is fed into the difference or delta port it also split equally in power to the two output ports p1 p2 but this time they are hundred eighty degrees apart in Phase a possible implementation of this is the hybrid ring splitter also called a rat race this consists of a circle of feeder for example coaxial cable with an overall circumference of one and a half wavelengths the four ports are connected on the top half of the ring with quarter wavelength feeder section between the distance between the different sport and the p1 a per port is three-quarters of a wavelength if we now consider power fed into the summing port Sigma it travels to p1 fire quarter wavelength of feeder and also to p2 via the same length of feeder the outputs are therefore in phase and split equally in power instantly if the power goes all around the ring it still ends up at the output ports in phase if we look at the path between the summing and different ports the Sigma and delta ports there's a half a wavelength for feeder by this path and one wavelength by the other path signals therefore arrive at the Delta difference port a hundred eighty degrees out of phase and thus council so no power goes out of the Delta port the delta port is in effect isolated power fed into the difference Port Delta goes via three-quarters of a wavelength feeder to output P 1 and a quarter wavelength feeder to p2 these outputs are therefore a hundred eighty degrees out of phase same consideration applies to the path between the difference and summing inputs the signals cancel out thus we have the requirements to feed the CSP and SPR signals to the antenna array the CSP signals fed in Phase two the entire the SPS signals are fed out of phase but now we need to look at the phase relationship between the signals in the main beam csb and the two lobes of the SPO signal we do this by considering some vectors this is a plan view the two antennas are represented as point sources equally spaced either side of the center line with both a.m. beam phase at the receiving point are somewhere on the center line equal signals are received from both antennas and the vectors add to produce two times V a resultant with both a and B in anti-phase the two vectors oppose each other and no resultant signal appears there's an O on the centerline now consider a location are displaced above the centerline now there's a path length difference between each antenna and the receiving point it's shown here as alpha degrees phase lag from the B antenna showing this as vectors VA is here and the vectors are assumed to rotate anti-clockwise by the way Phoebe likes by alpha degrees we get the resultant signal by vector addition as shown the some signal likes by alpha over 2 at this particular displacement from the centerline if we now drive a and B in anti-phase vector VB is shifted 180 degrees as shown the resultant signal is shown here we now need to consider the phase angle between the in phase resultant and the out of phase resultant signals angle beta is the base angle of this isosceles triangle it's given by a 180 minus alpha all over 2 or 90 minus alpha over 2 degrees this angle is also beta so we can work out the phase angle between V R and V R - which is beta plus alpha over 2 or 90 minus alpha over 2 plus alpha over 2 which results in 90 degrees now we consider a point R displaced equally on the opposite side of the centerline now it's VA which has the extra path length alpha with a and B in phase resultant the same alpha over 2 lagging vector has appeared before however the resultant for the outer phase condition is now in this quadrant and it's 180 degrees different from that when the point R was above the centerline to summarize at a particular displacement either side of the centerline the summed in phase signal V R is always alpha over 2 degrees lagging whereas the outer phase signals are always a hundred eighty degrees apart and plus or minus 90 degrees from the in phase signal you could also say that one of the lobes is 90 degrees and there is 270 degrees from the in phase signal so there you can see perhaps why yes now that extra 90 degrees phase delay added in the transmitter so the CSB in SPO singles are either in phase or a hundred eighty degrees out of phase depending on which side of the center line you're on let's now look at practical localizer antennas I've shown here six directional antennas they could be Yogi's or more commonly these days locked periodic antennas the blue blocks represents the power distribution feeders which give the required power distribution and phasing to form a narrow radiation pattern a mirror image of antenna system is placed symmetrically about the center line as shown this picture is an eight element log periodic array at Coventry Airport just about the smallest practical array that's used these days at the other end of Coventry runway is this 14 element array of log periodic s-- the two halves are fed in phase with the CSV signal to form this beam a few degrees wide across the center line the two halves are fed out of phase with the SPI signal to produce this sort of pan with a deep knoll along the center line as we've already seen a hybrid splitter can be used to feed the two halves with appropriately phase signals from the CSP and espyo transmitter outputs the relative phase between the CSP and SPS signals is shown here the two lobes of the SPO signal have a hundred eighty degree difference in phase the carrier in the CSB signal has a zero or a 180 degree phase difference to the SPO signal depending on which lobe above or below the centerline is considered this phase difference by the way is ninety degrees due to the antenna characteristics we talked about before and ninety degrees due to the phase shift added to the SPO signaling in the transmitter on the centerline the deep null in the SPO pattern means that only the CSB signals Rah's is received with its equal ninety and a hundred fifty hertz turns this defines the center of the course towards the runway that the aircraft uses for guidance the sum and difference technique works so the CSB and SPO signal are summed in space to the left or below in this case the center line and the difference is taken to the right of the center line hence the ninety Hertz signal is strong below the center line here and above it where the difference to signal is taken 150 sigmund predominates now note that if the CSB signal reduces in amplitude as shown here it doesn't affect the centerline of the course signal that is solely defined by the deep null in the SB antenna this antenna can't rest it can be made very stable making for a reliable and safe long-term approach guidance system now in practical antenna implementations like this there are inevitably low-level spurious side lobes to the radiation pattern shown schematically here they're typically more than 15 DBS down on the main signal however there is the possibility of a false course being picked up by an aircraft as shown here especially if the side lobe is mainly of the CSB signal with its equal 90 and 150 Hertz tone modulation to prevent this false course happening an extra two signals are radiated using two or three of the localizer antennas try the side of the centreline these give a broad CSB and SPO patent with a radiated power about 10 DBS below that of the main course signal by this means the spurious signals from the side lobes are overpowered with a full-scale CDI signal fly left or fly right as appropriate this extra transmission is called the clearance signal as opposed to the main course signal as shown now some localizers use a separate radio frequency for the clearance signal this is often beneficial as the beamforming necessary can be done more easily for the two signals if they're on different frequencies this is especially applies where the ILS is used for category two or three systems where the greatest precision is required to safely guide aircraft to land here I've shown the course signal on F 1 and the clearance signal F 2 F 1 and F 2 are typically separated by about 8 kilohertz so they still fall within the fairly wide nav receive a pass band fairly obviously these systems are called 2 frequency localizers the clearance frequency is fed to the inner two or three antenna elements each size are shown here in light blue there's also a possibility of using a completely different antenna array for the clearance ziggler shown this is possible because radio coherence isn't required as either the strong core signal or the weak clearance signal captured the receiver detector at any one time Caryn's is required however in the modulation frequencies so the 1950 hurt stones come from the same sources for both course and clearance transmitters we now look at a nav receiver I have passband it's quite wide at about 30 kilohertz left over from earlier times when VHF receivers and transmitters were a lot less stable than they are today on a single frequency localizer the ILS signal sits in the center of the pass band as shown for a to frequency of localizer each frequency is offset about four kilohertz from the nominal Center frequency so they end up about eight kilohertz apart when the aircraft is on course the course signal is stronger and captures the receiver detector and so drives the CDI indication because the clearance signal is about 10 DB x' down however the situation is reversed when the aircraft is in the clearance area course signal is weaker and the receiver AGC brings up the clearance signal to full level in the IAF so it captures the detector and drives the CDI now we look at course with what it means and how it's just it this shows an aircraft flying towards the ILS localizer an issue of the aircraft is around 40 degrees off the centreline localizer signal may be unusable so the CDI flag is showing as the aircraft reaches 35 degrees off central so the identical becomes audible and the CDI flag clears and the needle shows a full-scale flyer right this is maintained until about 5 degrees off center when the CDI needle comes off the end stop and starts moving across to the left passing to the center when the aircraft is on the centerline if it continues through the centreline the CDI shows increasingly fly left reaching full scale deflection at about plus 5 degrees in this case the full-scale signal is again given until the aircraft reaches plus 35 degrees when the flag shows and the ident signal becomes inaudible if we show the CDI position graphically it has a sector a full-scale fly right a nominally linear region in the center as the CDI pointer moves across the scale then full-scale fly left these are the clearance and course signals as shown the course width is shown it's the width of the linear region and this is a graph for a narrower course width now look at a runway with the localizer located on the left the course signal is shown here as a narrow fan triangular shape no this does not represent in any way the beam shape of the radio waves involved it merely shows where the CDI gives course steering information to the pilot CDI gives full-scaled indications when the aircraft is on the edge of the fan shape as shown here and the course width is defined as the width of the course thing left the runway threshold and eye care requirements stated should be 210 meters or so this means that the signal needs to be a narrower angle for longer runways of course width is set as follows let's look back at the transmitter block diagram I showed a variable attenuator in the SPO signal chain which sets the SPI output level fed to the antenna system this sets the course width as follows looking back at the sum and difference technique the SPO signal modulation component in the far field is given by n F 1 minus F 2 where F 1 and F 2 are they 90 and 150 Hertz tone levels respectively the factor n depends on transmitter power and antenna gain towards that point if we make n say bigger as shown here by increasing the SPO transmitter power you can see that this term F 1 gets bigger and the F 2 term gets smaller meaning the difference in mod depth is greater and so the CDI indication is more sensitive and reaches full scale deflection sooner that means that the course width is less let's show this graphically or vary the SPR signal power as shown by the blue patterns here as the SPO signal increases the course width reduces and it's relatively easy to set the required course width in this way there's one major drawback in the CSB sbo scheme of things here's an aircraft flying towards the eyeless with the CDI providing steering information unfortunately the SPS signal fails always turned off for some reason immediately the CDI centers as the receiver is getting the CSB signal only with this equal tone modulation this is a highly dangerous situation there's no matter where the aircraft is in relation to the centreline it receives an on course signal for this reason great care must be taken to prevent pilots using the ILS if the SPO signal is absent for any reason equipment failure is taken care of by comprehensive monitoring and an automatic changeover to standby equipment but sometimes during maintenance it's necessary to radiate just CSB signals alone appropriate warning to the pilots must be given in this case finally the glideslope system as I've mentioned before uses the same CSB SPO technique but with different antenna arrangements to take care of the vertical guidance necessary I think you'll agree that the eyeless presents a tour de force of analog radio engineering techniques and its sophistication and elegance has provided reliable and safe landing guidance for pilots for at least 65 years

Channel: leepd60

Views: 33,313

Keywords: ILS, Instrument Landing System, CSB, SBO, Avionics

Id: 8g6ZMdjnIDw

Length: 31min 53sec (1913 seconds)

Published: Wed Dec 02 2015