9 distance measuring equipment

bài giảng môn Hệ thống dẫn đường mật đất hàng không, Tài liệu gồm những nội dung như: điều chế AM, điều chế FM, Điều chế không giang, hiệu ứng Doppler, Tổng quát về dẫn đường hàng không (Navigation), Hệ thống dẫn đường sử dụng sóng vô tuyến (Non visual navigation aids),Hệ thống dẫn đường bằng mắt, Yêu cầu tối thiểu về trang bị các thiết bị dẫn đường tại Việt Nam, Các tiêu chuẩn áp dụng cho hệ thống dẫn đường

1/9/2021 DISTANCE MEASURING EQUIPMENT | McGraw-Hill Education - Access Engineering (https://www.mheducation.com/) (/)  Search AccessEngineering here    Within this book  Browse AccessEngineering content by Show less  Course Subject Industry Books (/search?query=&f%5B0%5D=content_type%3ABooks&f%5B1%5D=book_component%3ATitles) Other () Airport Ground Navigation Systems Dr Arjun Singh  (/content/bookI/S9B7N8:090778000770047405494)59 Publication Date & Copyright: 2012 McGraw-Hill Education Private Lim- ited Show more  Table of Contents Figures (20) Tables (1) View: Table of Contents Tools 9 DISTANCE MEASURING EQUIPMENT Introduction DME measures the distance with respect to a ground transponder beacon by the length of time elapsing between the transmission of a pulse 'interrogation' signal from the aircraft and the reception of a similar pulse 'reply' from the beacon Traffic 'To and From' several beacons is channeled by a combination of frequency and pulse-pair coding wherein each signal consists of a pulse-pair of distinctive spacing The aircraft component is referred to as interrogator and the ground component as transponder Circuits are designed in such a way that they recognize only pulse-pairs whose leading edges are separated by the proper time interval (of the order of 10 to 25 μs) Data is presented of the ex- tent to such pulse-pairs can be used to permit several transponder beacons, within overlapping service areas, to operate on common interrogation and common reply frequencies when serving a multiplicity of interrogator respon- sors of the automatic searching and tracking type The DME should provide for continuous and accurate indication in the cockpit of the slant range distance of an equipped aircraft from an equipped ground reference point The system comprise of two basic components one fitted in the aircraft, the other installed on the ground In operation interrogators will in- terrogate transponders which in turn transmit the replies synchronous with in- terrogating pulses thereby providing means for accurate measurement of dis- tance Capability should exist for displaying information relating to distance https://www.accessengineeringlibrary.com/content/book/9780070704459/chapter/chapter9 1/51 1/9/2021 DISTANCE MEASURING EQUIPMENT | McGraw-Hill Education - Access Engineering and rate of change of distance While the DME operates with only 52 channels, the experience obtained with this equipment has been used to standardize in- ternationally the same channeling method to obtain 100 channels 9.1 HISTORICAL BACKGROUND The introduction of intercept equipment on-board aircraft early in the Second World War brought into use for the first time, the extensive application of equipment that would indicate directly the distance to a point It was not long until it was determined that a beacon (transponder) located on the ground could be used in conjunction with the early airborne radar to furnish the valu- able navigational information Transponder beacons were necessary for use in conjunction with early airborne radar because the very high frequency em- ployed for intercept purpose, did not permit distance reading at low altitudes purely through target reflections The usefulness of distance information fur- nished by the combination of airborne radar interrogators and ground transponders led to war time experience for development of distance-direction system, It was probably the Engineers of the 'WRIGHT FIELD' Laboratories of the United States Air Force, who foresaw the advantages of a distance direc- tion system (Polar Navigational System) which frame of the reference is con- sidered to be the earth Such a system, they reasoned, will permit to generate unlimited numbers of fixed flight paths This group had urged the development of Distance Measuring Equipment (DME) that could be used in conjunction with VOR that later on was developed by Civil Aeronautical Administration (CAA) In 1949, the ICAO recognized UHF DME in its standards and recommended practices with the following paragraph appearing under the recommendation for short distance aids '… It is intended that UHF DME (distance measuring equipment) will become a basic component of VOR at the earliest date practicable, to be added to all VOR which has been installed before that date….' The ICAO document referred to includes a development specification for a DME operating frequency in the 960 to 1215 MHz range 9.2 PRINCIPLE OF DME SYSTEM The basic principle of the distance measuring system is explained with help of Fig 9.1 The system consists of airborne equipment known as interrogator-re- sponder, commonly shortened to interrogator, and ground equipment referred to as transponder or beacon, but more appropriately called transponder The cycle of the events that results in furnishing of an accurate indication of dis- tance, begins with the modulator in the interrogator equipment In the modu- lator, pulses are generated of few microseconds' lengths The rate at which these pulses are generated is rather low and seldom exceeds 150 per seconds These pulses are generated in pairs The output of the modulator is applied to a radio frequency generator that incorporates provisions for operating on num- ber of different radio frequency channels The output of the radio frequency generator, which occurs at a frequency f1, is connected to an antenna This an- tenna is common to the airborne receiver, but pulses do not disturb it since this receiver operates on a frequency fT that is appreciably different from f1 It is ne- cessary to connect the input circuit of the receiver to the antenna by means of an efficient filter usually known as a pre-selector Figure 9.1 Block diagram of an interrogator-responder and a ground transponder  https://www.accessengineeringlibrary.com/content/book/9780070704459/chapter/chapter9 2/51 1/9/2021 DISTANCE MEASURING EQUIPMENT | McGraw-Hill Education - Access Engineering  Open in new tab (/mhe-lookup/tab-link/ch09fig01/6076385)  Share (/#copy_link) On leaving the aircraft antenna, the pulses travel to the ground, where the transponder antenna picks them up The transponder antenna has non-direc- tional characteristics in the horizontal plane, but has a directional pattern in the vertical plane The antenna is connected to a receiver pre-selector and then to a receiver After detection, the signals are amplified by a video amplifier and passed through a delay network having a calibrated period and it is passed to a modulator The modulator amplifies the pulses from the receiver and im- presses them on a transmitter operating at a carrier frequency 'fT' From the transmitter, the signals are connected to the same antenna used for receiver as well From the ground antenna, the signals are transmitted to the aircraft and passed through the pre-selector to the receiver From the output of the receiver, the signals are amplified by a video amplifier and connected to a complex cir- cuit called the search and tracking unit and is also supplied with energy from the modulator The search and tracking unit examines all of the signals re- ceived and determines which ones have a fixed time-constant with respect to the transmitted signals A number of aircraft may interrogate the same transponder It is therefore ne- cessary to determine which of the pulses replies to the specific aircraft inter- rogator The modulator has been designed to produce pulses having an inten- tional jitter so that there is little probability that several aircrafts can continue to send pulses at exactly the same time The received pulses are due to its in- terrogator, the search and tracking unit locks on to these and continuously measures the time that has elapsed from the time of transmission to the time of reception At the same time, the search and tracking circuit causes the num- ber of pulses being transmitted to be greatly decreased The operation is per- formed to reduce the load on the transponder The time has elapsed from the time of transmission to the time of reception is converted to distance through knowledge of the fact that radio waves travel at a speed of 186,000 miles per second (3 × 108 meters per second), and is indicated directly to the pilot as dis- tance to the transponder, with accuracies sometimes within 0.10 mile The time elapsed between the transmitted pulses and received pulses by the interrogator is measured to obtain the distance between the aircraft and transponder The range time for one nautical mile (NM) is 12.36 μs Range time is two way travel time i.e., time from interrogation transmitter till the replies re- ceived to those interrogations 9.2.1 Basic System Theory  https://www.accessengineeringlibrary.com/content/book/9780070704459/chapter/chapter9 3/51 1/9/2021 DISTANCE MEASURING EQUIPMENT | McGraw-Hill Education - Access Engineering In the transmit mode the transponder emits replies to the interrogation pulse pairs either from an aircraft or from the signal generator, identification pulse pairs and squitter pluses The squitter pulse rate is set at 2700 pps when there are no incoming interrogations from either the aircraft or from the monitor sig- nal generator As the incoming interrogations increase or the signal generator interrogates the transponder, the squitter rate generation as ON correspond- ingly decreases and a constant duty cycle of 2700 pps is maintained The transponder is encoded at the rate of 2700 pps by the Ident signal The con- stant duty cycle reduces the surge on the transponder and also conditions the ground control system of the airborne receiver The aircraft interrogation pulse pairs are picked up by the antenna or the signal generator interrogation pulse pairs, routed through the directional coupler circulator/duplexer to the pre-se- lector mixer, and are converted to the first IF of 63 MHz and amplified by 63 MHz preamplifier coupled to the main receiver The receiver converts the 63 MHz signal to the second IF of 10 MHz and the pulse pairs are demodulated (a) Decoding The video pulse pairs are applied to the decoder The decoder checks the pulse pairs spacing and if it is a valid interrogation, permits the received signal to be further processed Otherwise the signal is rejected The decoder is controlled by a 'dead time' gate to prevent interference caused by the reception near-field and far-field interrogation signal It varies the pulse spacing of interrogation spaced too closely e.g interrogations take a multiple path to the receiver The dead time is set to 55 microseconds (b) Encoding Valid interrogations are passed to the encoder, which establishes the priority between identification, reply transmission, squitter pulses and also sets pulse pairs spacing The reply delays period (station delay) and generates pulse pairs for identification signal The automatic overload control voltage is also pro- duced in this unit to desensitize the receiver and the 63 MHz preamplifier, when the transponder is over interrogated This unit also ensures the constant duty cycle of the transponder The output of the encoder is a combination of identi- fication, reply, and squitter pulses, and is also applied to the Gaussian pulse generator (c) Gaussian Pulse Generator The pair of the pulses from the encoder triggers the Gaussian generator to pro- duce a pair of Gaussian shaped pulses These are further amplified and ap- plied for power amplification (d) Transmitter RF Exciter The crystal frequency is multiplied 12 times by three triplers and the output is applied to the RF input A small portion of the RF signal from the second tripler is routed to the local oscillator tripler and output of the local oscillator is coupled to the pre-selector, mixer as the local oscillator injection signal The power output is routed through an RF transformer, the circulator, and antenna transfer relay to antenna for transmission (e) Operational Specification for DME The civil aviation community uses DME for indication of distance in Nautical Miles (NM) for a reference location in which a transponder is located to an air- craft operating within its service range The transponder will be either used in- dependently or collocated with different type of navigational aids for: Medium range operation up to 300 NM along with a VOR of ERP + 23dBw Terminal area operation up to 100 NM along with a VOR of the ERP +17dBw  https://www.accessengineeringlibrary.com/content/book/9780070704459/chapter/chapter9 4/51 1/9/2021 DISTANCE MEASURING EQUIPMENT | McGraw-Hill Education - Access Engineering For approach and landing operation of aircraft, the DME will be collocated along with either localizer or GP component of ILS 9.3 AIRBORNE DME TRANSPONDER After turning 'ON' the warm up or after tuning to a new channel, the equipment must vary its range looking for a pulse reply that matches its interrogation This is called search operation During this period, the dial will be rotating rap- idly and will be covered by a mask or 'flag' The flag notifies the pilot that the equipment is still in search, or that a tracked signal has been interrupted In either case, it precludes display of erroneous distance information DME sig- nals are sometimes momentarily interrupted To prelude the necessity of searching the full range and returning to the signal, memory capability is incor- porated in all airborne equipment The user has the choice of 'static' or 'dy- namic' memory Static memory holds the dials at the last reading prior to sig- nal interruption; dynamic memory causes dials to continuously move at the distance change rate in force at the moment of interruption This memory period is approximately 10 seconds on TVOR channel and 5 seconds on ILS channels Setting DME control to override will allow the airborne equipment to range beyond 50 NM limits on short range channels Short range facilities should be in pilot command for the equipment to override in an area where this condition exists; it would be possible for the system to track either station The airborne equipment warns the pilot of possible ambiguity by garbling one or presenting both station aural identifiers In standby, the airborne DME is turn 'ON' but not transmitting and ranging mechanisms are not constantly searching This saves unnecessary wear and tear on the system component, and at the same time allows the pilot to put the DME in operation without a warm up period The airborne DME has an auto- matic capability that places the system in standby when VOR facility without DME is tuned, or when the reply signal falls below a reliable level This feature is called 'signal controlled search' 'Self-Test' capability to check display accur- acy is also incorporated which electronically zeros the system ranging circuits Echo protection feature is also incorporated which causes the equipment to re- turn to zero under certain conditions before starting search, and in all cases to search outbound (reciprocating search) The timing and logic circuits provide the interrogation generator with the re- quired signals to produce coded pulses to modulate the transmit signal The timing circuits provide the interrogation generator with the information required to code the pulse The logic circuits provide the interrogation generator with the information required to control the pulse interrogation rate The interroga- tion pulses from the interrogation generator are applied to the modulator The frequency synthesizer generates a signal at of the transmit frequency This signal is frequency multiplied and applied to the power amplifier The power amplifier increases the signal level of the output from the frequency multiplier to the required transmits level The signal is pulse modulated by the action of the modulator before transmission The ground station reply signal is applied to the receiver section where it is het- erodyned with an injection signal of the same frequency as the transmit signal The ground station reply signal frequency is either 63 MHz above or below the transmit frequency on all channels Further heterodyning is done in the receiver to produce a 10.7 MHz intermediate frequency The reply signal is demodu- lated, and the detected pulses are applied to the decoder The decoder separ- ates the distance replies from the noise and other information transmitted by the ground station, and applies the distance reply pulses to the computing cir- cuits The computing circuits use the information from the limiting circuits and measure the time elapse between interrogation and reply pulses and convert  this into distance information to derive the distance indicator https://www.accessengineeringlibrary.com/content/book/9780070704459/chapter/chapter9 5/51 1/9/2021 DISTANCE MEASURING EQUIPMENT | McGraw-Hill Education - Access Engineering The transponder in the aircraft is tunable by the pilot to any one of the several assigned DME channels as the aircraft flies from one DME to another in its cross country navigation The process of distance measurement originates in the airborne unit with the generation and transmission of pulse signal is called 'interrogations' The airborne transmitter repeatedly initiates and transmits pulse signals consisting of pulse pairs having 12 μs spacing, a pulse-width of 3.5 μs and a Gaussian or Sine squared shape These pulse-pairs are recovered by the transponder beacon receiver whose output triggers the associated transmitter into transmitting reply pulse pairs The reply pulse pairs are re- ceived by the airborne receiver and timing circuit automatically measure the round trip travel time (the time interval between interrogation and reply pulses) and convert this time into electrical signals which operates the distance indicator In normal system operation, a given ground beacon may be interrogated simul- taneously by a number of aircraft The ground beacon will reply to all aircraft, and each aircraft will receive all the replies that are transmitted To permit in- terference free operation under such conditions, each aircraft's interrogation pulses occurs at a rate i.e., intentionally permitted to vary in a random manner over a limited range In order to determine, which of the many reply pulses are replies to the given aircraft's interrogation pulse, an entirely automatic time se- lection process of a stroboscopic nature is employed Such a process locates the proper reply pulses by determining which reply indicates a consistent round trip travel time, as measure from each of the previous interrogation pulses Be- cause the interrogation pulses from other aircraft are varying in a random man- ner with respect to the given aircrafts interrogation pulses So two airborne units will become synchronized more than momentarily, hence there is little or no interference between units As discussed above, the process of distance determination originates the range circuits of the airborne interrogator from one station and transmission of an interrogation pulse pair The range circuits go into search condition auto- matically each time the airborne unit is initially tuned to a ground transponder beacon or if there is some major interruption in the received signals In the search condition, the airborne unit interrogates station at a rate varying between120 to 150 pulse pair per second The range circuits can progressively vary time delay intervals by means of a slide range gate which tests each time position for the number of successive reply pulses received within certain uni- form checking period If no replies are received the range gate is advanced to test a slightly longer time delay interval, and so on When at some particular time delay interval, a sufficient number of recurrent replies are detected, the search is completed and stopped The correct reply pulses are the only pulses which are received in synchronism with the given aircraft's own random inter- rogation pulse The search process is required up to a maximum of 20 seconds from comple- tion, depending the aircraft distance from the ground station Thereafter, the range circuit locks on to the proper pulses and the unit transfer to 'track' opera- tion During track operation the display of the range gets automatically and continuously from window and normal variation in the time delay of the proper reply pulses Such variations will occur if the aircrafts distance from the ground station is changing as a result of its flight path However, such variations are necessarily quite slow because of the relationship between the interrogation rate and the aircraft speed The rate of interrogation in track operation varies randomly between 24 and 30 interrogations per seconds During that time, the range gate is locked 'ON' to the proper reply pulses, the time delay setting of the range gate is a proportionate measure of the aircrafts distance from the ground station The mechanical position of the control device that varies the time delay of the range gate position the numerical indic- ators on the distance indicator by means of electrical control The time meas-  https://www.accessengineeringlibrary.com/content/book/9780070704459/chapter/chapter9 6/51 1/9/2021 DISTANCE MEASURING EQUIPMENT | McGraw-Hill Education - Access Engineering uring circuits have a memory provision to maintain the existing distance in- formation in the event that reply pulses are not received for approximately 10 seconds 9.3.1 Distance Measurement and System Timing The block diagram of the DME transponder is shown in Fig 9.2 The distance measurement function can be examined from the system point of view The range circuit of the airborne interrogation initiates the distance measuring pro- cess by formulating and transmitting an interrogation pulse-pair, which is re- ceived at a ground station antenna The pulse pairs received by the ground an- tenna are routed to the receiver of the ground transponder beacon where the pulses are amplified and detected into video pulse-pairs They are fed to the decoder where the pulses are examined for proper coding (spacing and width) and decoded if such proper coding exists The output of the decoder then trig- gers the encoder, which encodes a reply signal with proper pulse spacing The output of this unit (pulse pairs) is routed to the pulse shaper where the pulses are shaped, amplified, and routed to the transmitter section for modulation of the gated RF The output RF pulses are radiated into space as replies via the antenna The reply pulse are received by the aircraft, decoded by the airborne receiver, and examined the range circuit for synchronism with the airborne unit's own random generated interrogation pulse Figure 9.2 Block diagram of DME transponder  Open in new tab (/mhe-lookup/tab-link/ch09fig02/6076385)  Share (/#copy_link) The airborne unit actually measures the elapsed time between the transmis- sion of the interrogation pulse-pair and the receipt of the reply pulse-pair and converts this time by electrical means into distance indication In other word, the distance indication is a measurement of the range time of the pulse pairs This timing sequence is easily seen by means of the system timing diagram of Fig 9.3 Timing starts in the range circuits of the airborne unit with the first pulse of the interrogation pulse-pair After a time delay, depending upon the distance between the aircraft and the ground station, the interrogation pulses are received at the antenna of the ground transponder beacon The interroga- tion pulses are decoded and the reply is encoded and transmitted after a pre- set time delay (the reply delay of the ground station) This reply delay has dura-  tion of 50 microseconds which the airborne range circuits automatically ac- https://www.accessengineeringlibrary.com/content/book/9780070704459/chapter/chapter9 7/51 1/9/2021 DISTANCE MEASURING EQUIPMENT | McGraw-Hill Education - Access Engineering count for Thus, the total time elapsed for any interrogation response cycle is the sum of reply pulse spacing, the two way transit time (range time), and the reply delay as shown in Fig 9.3 Figure 9.3 System timing diagram  Open in new tab (/mhe-lookup/tab-link/ch09fig03/6076385)  Share (/#copy_link) EXAMPLE 1 The total time elapsed between an aircraft interrogation and receipt of a reply to that interrogation is 185.6 μs What is the distance between the air- craft and ground transponder in NM? The reply delay is adjustable and must be maintained 50 μs for an error in distance measurement results An error adjustment is 12.36 μs, which is an extra ordinary error and would results in an inherent system error of one NM The accuracy of the system distance readings depends upon the ac- curacy of the reply delay, the airborne time measuring circuits, the airborne indicators, and the rise time of the pulse used in the system The transpon- der beacon transmits at a rate of 1350 pairs per second, which are distance replies and squitter, with the total distance replies limited to 5400 pulse pairs per seconds It should now be clear about DME function of both air- borne and ground equipment Each has a specific function in the process of distance determination 9.3.2 Basic System Function of DME a Airborne Unit Producing and transmitting a properly coded interrogation signal on any of the 126 channels assigned to the interrogation function Receiving and processing replies transmitted by a transponder beacon assigned to any one of the 126 channels designated for reply purposes Examining the reply coding and rejection of those signals which do not possess the proper pulse spacing Recognizing the correct replies by a sorting process  https://www.accessengineeringlibrary.com/content/book/9780070704459/chapter/chapter9 8/51 1/9/2021 DISTANCE MEASURING EQUIPMENT | McGraw-Hill Education - Access Engineering Measuring the elapsed time between an interrogation and its reply and converting this time measurement to a distance indication Recognizing and reproducing, as an audible indicator to the pilot, the identification signal transmitted by the transponder beacons b Transponder Beacons Receiving interrogations on one assigned frequency of the 126 fre- quencies available Examining the coding of the interrogation pulses and initiating replies only to those which possess the proper spacing Generating coded reply pulse-pairs at appropriate power level on the assigned reply frequency Maintaining a 50 μs reply delay for system accuracy Arranging a signal priority system 9.4 CHOICE OF OPERATING FREQUENCY There are three major considerations that determine which band of frequen- cies is optimum for use by pulse type DME The band chosen must have sufficient bandwidth to permit the efficient transmission of pulses on the number of channel dedicated by the require- ments of the aviation application The propagation conditions attending the frequencies in the chosen band must be such that they will permit reliable transmission of information with use of minimum power The band chosen must be such that the available techniques will permit design of equipment with minimum weight In free space the power input to a radio receiver will be (9.1) where, Pt is the transmitted power, Gt is the gain of the transmitting antenna, Gr is the gain of the receiving antenna and; r is the distance between receiver and transmitter λ is the transmitting wavelength There is a minimum power at a radio receiver is able to respond, and to furnish this power to the receiver, the minimum required transmitter power will be (9.2) The DME antennas employed on the ground have an omni-directional pattern in the horizontal plane and a directive pattern in the vertical plane For such an ar-  ray having a length L and employing elements spaced less than one https://www.accessengineeringlibrary.com/content/book/9780070704459/chapter/chapter9 9/51 1/9/2021 DISTANCE MEASURING EQUIPMENT | McGraw-Hill Education - Access Engineering wavelength apart, the gain, to a first approximation, will be (9.3) Substituting Equation (9.3) in Equation (9.2), the minimum power required will be (9.4) From Equation (9.4), it appears that it is possible to maintain the minimum power required at all frequencies at exactly the same value merely by changing the length of the receiving array The beam width becomes narrow as the array is lengthened The angle in the vertical plane eventually reaches such a small value that a large overhead cone of no signal is introduced, thereby no service will be supplied to the aircraft other than those at long distances A linear array of about 7 feet has been found to be the most practical length for any fre- quency that would be suitable for distance measurement use The aircraft antenna will always be limited by aerodynamic considerations, which are determined by physical size Therefore, it is not unreasonable to as- sume that Gt at all frequencies will be approximately same The simplified form is (9.5) f is transmitting frequency in MHz K is constant derived from Equation (9.4) Considering the frequencies suitable for DME and setting the minimum power at 10,000 MHz at zero dB, the Table 9.1 shows the power required at various frequencies Table 9.1 Minimum required power versus frequency Frequency in MHz 10,000 5,000 3,000 1000 500 200 Pt (min) in dB 0 –3 –5.2 –10 –13 –17  Open in new tab (/mhe-lookup/tab-link/ch09table01/6076385)  Download data (/mhe-lookup/download-link/ch09table01/6076385)  Share (/#copy_link) It indicates that equipment operating at a 200 MHz would require 17-dB of 10/51 power less than that operating at 10,000 MHz This power difference is very significant, particularly since the power available from airborne equipment is limited In Equation (9.5) the calculations are based on free space conditions, but this condition does not prevail in actual operating conditions For long dis- tance, quasi-optical path, however the attenuation will increase with frequency Therefore, the most advantageous frequency for DME operation would be be- low about 1500 MHz In consideration of equipment design, it is possible to produce equipment within the range of about 200 to 1500 MHz with approximately the same tech- nique Connection to the antenna may be made through solid dielectric lines At frequencies above 1500 MHz it becomes necessary to use magnetrons as frequency generators, and connection to the antenna is made through wave- https://www.accessengineeringlibrary.com/content/book/9780070704459/chapter/chapter9