A REPORT ON DME - DISTANCE MEASURING EQUIPMENT Operating Principle, Description and Error Analysis Warsaw University of Technology Faculty of Power and Aeronautical Engineering ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis DME (Distance Measuring Equipment) – Principle of operation, description of selected type and error analysis Course Title ANS647 - Attitude and Navigation System Report submission by – Sultan Mahmood Mukut Index: 302613, Semester II, Winter 2018, MEiL, WUT Submitted to – Janusz Gajda Senior Lecturer, Department of Automation and Aeronautical Systems, MEiL Warsaw University of Technology Warsaw, 18th January, 2019 1 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis List of Symbols and Derivations DME GPS ARNS ATC VHF UHF FIR INS NAVAID ICAO TACAN IFF VOR ADF ILS SSR FAA VFR IFR VOR/DME VORTAC ILS/DME LOC/DME LORAN MHz Distance Measuring Equipment Global Positioning System Aeronautical Radio Navigation System Air Traffic Control Very-High Frequency Ultra-High Frequency Flight Information Region Inertial Navigation System Navigational Aid International Civil Aviation Organization Tactical Air Navigation Identification friend or foe VHF Omnidirectional Range Automatic Direction Finding Instrument Landing System Secondary Surveillance Radar Federal Aviation Administration Visual Flight Rules Instrument Flight Rules VHF Omnidirectional Range and Distance Measuring Equipment VHF Omnidirectional Range and Tactical air navigation system Instrument Landing System and Distance Measuring Equipment Localizer and Distance Measuring Equipment Long Range Navigation Megahertz ≤ ± ε Σ 𝜃 less or equal plus-minus Epsilon Sigma Theta 2 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis TABLE OF CONTENTS TITLE PAGE Chapter 1 Introduction 4 Introducing DME 4 Background and Early Development 5 Installation and components of DME 6 Chapter 2 Principle of Operation of DME 8 Working Principle of DME 8 Chapter 3 Description and selected type of DME 11 Interrogator - DME Onboard Aircraft 11 Transponder - DME installed on Ground Station 12 Chapter 4 Discussion of Error Analysis Assumption of the problem and best approximation 14 14 Chapter 5 Concluding Remarks 16 List of References 17 3 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis Chapter 1: Introduction Introducing DME In the last hundred years for the first time in the history of mankind we observed the technological advancement at a rate so fast that is unprecedented and exemplary compared to the progress of previous centuries. The most vital tool of any civilization, communication has existed since the very beginning of human history but it was not until the last century that scientists and engineers began to study the science behind it. Development of navigation system, information theory, mass communication progressed enormously as people started commuting and travelling more from one place of earth to another. As with the advancement of respective fields of technology we saw the fascinating development in the field of radio navigation technology as well. Today a world so connected and alert is quite unthinkable without air transportation system. As the science and engineering of aeronautics and aerospace progressed so did the advancements of avionics and space communication system. GPS, and satellite communication revolutionized the mode of technology. Today we see the rapid growth of privatization, commercialization and militarization of Aircrafts and Spacecrafts. And with this increasing number it is inevitably crucial to know the position, location and orientation of each and every flying machine functioning in the world continuously. And to do that it is fundamental to measure and calculate the distance between the traffic controller station and the moving object, basically all the time. Luckily, we have developed the technology to do that instantly. This brings us to the topic of our report that is, DME. The full meaning of DME stands for Distance Measuring Equipment. From the elaboration of the abbreviation we get the general sense about the device. DME is a form of radar used by all kinds of aircrafts and is extensively used for navigation purposes. Basically, DME is an Aeronautical Radio Navigation System, which helps to determine the distance between an airplane and a ground station. In other word DME is like a secondary radar system that provides an aircraft’s flight crew with direct, continuous visual indication of distance from a selected ground station, which is situated at a known geographical location. 4 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis DME system is based on the communication between two components, one installed on board of the airplane (interrogator) and another one placed on the ground (transponder). Later the components and uses of DME will be discussed. Where there is a connection between a ground station and an airplane, whether they are manned or unmanned, installation of DME in both the hosts, is a must. This report is divided into five chapters. In the first chapter along with the introduction we shed some light on DME introduction, background, early development information and the use of DME system. In this report we tried to focus only on DME system, and on later chapters we discuss the operating principle, description and error analysis of aircraft DME systems. Background and Early Development Aviation engineers have worked tirelessly year after year, to improve flight technology solely bearing in mind the aviation safety. The first DME, reportedly was developed in Australia. In mid 1940s, one such pioneering aviation engineer, James Hamilton Gerrand or James "Gerry" Gerrand, under the supervision of physicist Edward George "Taffy" Bowen, designed prototype equipment, which happens to some of the first distance measuring equipment, that measured an airplane’s distance from a target airdrome (a location from which aircraft flight operations take place). Later in the 1950s, other versions of DME system were developed, which operated in the 200 MHz VHF band. The Australian domestic version was referred to as DME(D) Figure 1 Typical DME installation, External view or DME(A), whereas later in the 1960s, the international version was adopted as DME(I) system by ICAO, which operated approximately at 1000 MHz band. The identification system IFF was designed for command and control, and it is used to identify aircrafts or vehicles. IFF is used by both civil and military air traffic control interrogation systems. And the DME system was a post second world war development of the IFF systems. 5 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis Installation and components of DME For air navigation the most basic requirement is the knowledge of an aircraft’s position, and this requirement is fulfilled by providing the pilot with bearing and distance information. Bearing information is generally derived via VOR or ADF systems, whereas distance information is derived by DME. DME complies with the standards prescribed by the ICAO and is installed at all international airports, at all capital city airports and many regional airports along international flight routes. DME was developed from a composite distance and bearing facility known as TACAN which was designed in the USA as an aid to military aircraft. As VOR fulfills the bearing requirements for civil aviation navigation, thus this component of the TACAN system is not used to assist civil air operations. A combined VOR/TACAN installation is commonly referred to as ‘VORTAC’. Where TACAN is not installed for military purposes then a DME, manufactured to the same specifications as the DME portion of TACAN, is installed. This is referred to as VOR/DME. DME Components: An aircraft uses DME to determine its distance from a ground-based DME (transponder) by sending and receiving pulse pairs – two pulses of fixed duration and separation. The ground stations are typically equipped with VORs or ILS systems. Usually sometimes VOR/DME refers to combined radio navigation station for an aircraft, which consists of two radio beacons, placed together that is a VOR and a DME. Other navigation systems established by the FAA are more of a combined system like, VOR/DME, ILS/DME, LOC/DME, VORTAC, which under a frequency pairing plan, also provides course and distance information from collocated components. DME usually operates on frequencies in the UHF spectrum between 960 MHz and 1215 MHz band. Figure 2 Showing a CONHAM ATC-DME Antenna 6 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis Figure 3 DME in the aircraft measures the time difference between transmission (1) and reception (2) and uses this to calculate the distance The receiving equipment onboard aircraft provides for automatic DME selection and assures azimuth (an angular measurement in a spherical coordinate system) and distance information reception from a common source when designated VOR/DME, ILS/DME, LOC/DME, VORTAC, are selected. Some aircraft have separate VOR and DME receivers, each of which must be tuned according to the appropriate navigation facility. The airborne equipment basically includes an antenna and a receiver. DME airborne components • Antenna and receiver The aviator-controllable features of the DME receiver onboard the aircraft includes: • • • • • Channel (frequency) selector – enables the pilot to select which VHF radio is channeling the DME, and having its own frequency selector DME, use the frequency of the associated VOR/DME or VORTAC station On/Off/Volume switch Mode switch (distance, groundspeed or time to station may be selected) Hold function - holds current DME channel (useful for ILS approach when DME is nearby but not collocated) Altitude - some DME correct for slant-range error Figure 3 Showing an onboard aircraft DME instrument 7 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis Chapter 2: Principle of Operation of DME Working Principle of DME In the DME system the interrogating equipment, known as the ‘Interrogator’, is installed in the aircraft and the target, located on the ground, is referred to as the ‘Transponder’ or ‘Ground Beacon’. A complete DME system loop requires both aircraft-installed and ground-based equipment for the exchange of information. The ground-based equipment is normally colocated with a VOR or ILS which provides the pilot with the slant-range distance to the DME transmitter. In radar terminology the definition of slant range (Fig 5) is the line-of-sight distance along a slant direction between two points which are not at the same level relative to a specific datum. Datum or a geodetic datum is basically a coordinate system and a set of reference points used for locating places on Earth. As the slant range distance also includes the aircraft’s height above the ground-based station, it will always be slightly greater than the flight planned distance to a DME station. For example, in Fig 4, the DME display in an aircraft 6,000 feet directly above a DME transmitter will read one nautical mile, so in this case 12,000 feet equals to two nautical miles. Figure 4 DME operating principle, elaborated 8 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis Figure 5 Slant range distance demonstration Although DME operates in a separate frequency band, its frequencies are paired with a VOR, ILS, or localizer (LOC) frequency. When the pilot of a DME-equipped aircraft tunes the frequency of a VOR or ILS with DME, the frequency of the co-located DME is automatically tuned. The aircraft interrogates the DME ground station with a pulsed signal, and the station replies. Aircraft equipment measures the time between transmission and reception to determine the distance; from that, groundspeed and time to station can also be derived. The basic components inside an interrogator and a transponder are as follows: The airborne DME (interrogator) includes: • UHF Transmitter / Receiver • Timing circuits • Distance indicator The ground-based DME (transponder) includes: • UHF Receiver / Transmitter • Decoder-encoder computer-time delay 9 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis Figure 6 Principle of Operation of DME The interrogator sends request to the DME ground stations broadcasting in all the direction towards the ground a pulse pair sequence; the ground beacons reply to the received pulse pair sequence with the same pulse pair sequence delayed of 50 µs in all the directions towards the sky, thus allowing the on board DME receiver to compute the range measurement based on a round trip time measurement. The most absolute thing would be to find the most accurate location of an aircraft using information from beacons, that is from ground-based station. This is basically the same problem a GPS system or a cellular phone has to solve on a daily basis. It should be noted that, distance given between the aircraft and the transmitter of DME, is the oblique angle and not the actual distance between the aircraft and DME. As mentioned before, DME works in the frequency range UHF 960 - 1215 MHz, where the beam does not depend on weather conditions and the transmission pattern by line of sight. We have already been familiarized with the information that DME on the aircraft consists of a UHF transmitter and receiver called interrogator and the DME at a ground-based station called transponder. We will discuss more about the two components in the next chapter. 10 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis Chapter 3: Description and selected type of DME Generally, to determine the distance from a ground transponder to an aircraft interrogator a DME system loop is used. Compared to other VHF/UHF NAVAIDs, a DME is very accurate and precise. The distance information can be used to determine the aircraft position or flying a track that is a constant distance from the station, and this is actually referred to as a DME Arc which we will discuss briefly later on. Conventional DME system loop consists of two physically separated sub-systems, an airborne interrogator and a ground transponder. Paired pulses, at a specific spacing, are sent out from the interrogator and are received at the transponder. The ground station (transponder) then transmits paired pulses back to the aircraft at the same pulse spacing, but on a different frequency. In this chapter, descriptions of interrogator and transponder are discussed in brief. Interrogator - DME Onboard Aircraft An interrogator is a radar transmitter that sends pairs of pulses (interrogation pulses) and triggers a transponder, to response. This system is usually combined in one unit with an answering device, which receives a reply signal from the transponder and produces the appropriate output. This is also called interrogator-transmitter. Figure 8 Showing a cockpit displayinstrument Figure 7 Showing an interrogator instrument (Collins 339F-12) 11 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis Transponder - DME installed on Ground Station A ground station based DME system or in short, a transponder (short for transmitterresponder), serves as a detector for signals which are received from an aircraft interrogator. The signal from the aircraft propagates towards the transponder as interrogation pulses. Received signal is then encoded and processed as the transponder has a series of encoding and decoding procedures. The transponders can only be triggered by a pair of received pulses that have the same duration and pause (match) with the frequency of the signal from the interrogator. After that the response is emitted again as an answer signal to the interrogator. The ground equipment consists basically of a high-powered transponder beacon, with accessory equipment in the form of an aerial array, and test equipment. A transponder (also abbreviated sometimes to TP, XPDR, XPNDR or TPDR) produces response upon receiving a radio-frequency interrogation. The transponder receives interrogation from the Secondary Surveillance Radar (SSR) on 1030 MHz and replies on 1090 MHz. SSR is referred to as "secondary", to distinguish it from the "primary radar" that works by passively reflecting a radio signal off the skin of the aircraft. Primary radar determines range and bearing to a target with reasonably high fidelity, but it cannot determine target elevation (altitude) reliably except at close range. SSR uses an active transponder (beacon) to transmit a response to an interrogation by a secondary radar. This response most often includes the aircraft's pressure altitude and a 4-digit octal identifier. Figure 9 Showing a ground DME system (SKYNAV N9000) 12 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis Transponder codes are 4-digit numbers transmitted by an aircraft transponder in response to a secondary surveillance radar interrogation signal to assist air traffic controllers or ATC with traffic separation. A discrete transponder code (often called a squawk code) is assigned by the ATC to identify an aircraft uniquely in a flight information region (FIR). This allows easy identification of aircraft on radar. Air traffic control units use the term "squawk" when they are assigning an aircraft a transponder code. Squawk basically means "select transponder code". A pilot may be requested to squawk a given code by an air traffic controller, over the radio. The pilot then selects the desired code on their transponder and the track on the air traffic controller's radar screen then becomes correctly associated with their identity. We can point out the basic processes carried out on a ground-based transponder: • Signal identification -this signal usually transmitted via Morse code serves to provide airport location information to the aircraft. • Replying signal – this signal is transmitted by the transponder to the aircraft when it receives interrogation signal in the same mode but with different frequency, which is based on the DME transponder working frequency; the length of time between the interrogator signal and the signal reply is (delay time) usually of 50 µs. • Signal squitter - Pulses carry reply pulses and will still be transmitted randomly even though when the transponder does not receive interrogation pulses and is not emitting ident pulses. Figure 10 Showing a ground DME system (TAJ Systems) Figure 11 Transponder in a private aircraft 13 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis Chapter 4 Discussion of Error Analysis DME system error is made up of components in the airborne equipment, the signal propagation path and the ground station. Since DME is based upon timing the reply to a pulse pair interrogation of a ground station by the airborne equipment, so any unaccounted delay will cause an error. There are four principle sources of error – • Bias errors in the ground station • Bias errors in the airborne equipment • Noise in the airborne equipment, and • Noise in the ground station However, in this report, we will not focus on the coding and software analysis of the whole error analysis process rather we will only discuss an assumed problem regarding DMEs in brief. In this particular chapter, at first the problem is described below. Assumption of the problem and best approximation Figure 12 shows a simplified typical situation of navigation with a modern aircraft. The assumed airplane receives signals from various beacons and is in an unknown position where every signal from the beacons is assumed to contain some error. The main goal of this problem is to develop a method for computing the most likely position of the aircraft based on all the information available. We distinguish two kinds of beacons: very high frequency omnirange, VOR and distance measuring equipment, DME. The VOR beacons (Fig 12: VOR1, VOR2, VOR3) let the airplane to read the angles, 𝜃1 , 𝜃2 and 𝜃3 (known to the aircraft; standard measure of angles in aviation is clockwise from North in degrees) from which the signal is coming. The DME beacon, using a signal that is sent and bounced back, allows the distance from the aircraft to the beacon to be measured. In this example of Fig 12, the assumed distance is 864.3 km± 2.0 km. Each of the measurements is given with an estimate of its error. The standard notation for measurements and errors is m ± n. This means that the true value being measured lies between m − n and m + n. 14 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis As the problem is merely a geometrical problem, thus the problem is simplified by considering it in two dimensions (2D) only. That is, the altitude is not considered, which could be read from other instruments and would unnecessarily complicates this example. Unknown coordinates of the aircraft are denoted by x and y (Fig 12). It is to be observed that unless we are in a pathological situation, any pair of two VOR/DME readings give enough information to compute x and y. Figure 12 Example of an aircraft and four beacons Ultimately after calculation we get the aircraft position (x, y) and some mathematical operations are done to minimize the norm of total error (usual Euclidean norm ‖𝜀‖2 ; ‖𝜀‖2 : = 4 𝜀 2, 𝑖=1 𝑖 √∑ or ‖𝜀‖𝑚𝑎𝑥 ≔ 𝑚𝑎𝑥(|𝜀𝑖 |)) and the method of least squares (LS) is used as well. After that modelling of the problem mathematically is done, then it is solved analytically. Then finally, analyzing the solution i.e. the error analysis is done. 15 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis Chapter 5: Concluding Remarks For any flying machines whether it’s an aircraft or a spacecraft, ensuring its safe guidance and exact position, and positioning of air traffic is very important and crucial. So, the use and installation of respective ILS, DME systems are must. Along with ILS alarm system, the DME system contribute to an aircraft’s overall avionics system, ensuring that accurate guidance information is provided and maintained. The two physically separated sub-systems, the airborne interrogator and the ground transponder completes the DME system loop and ensures uninterrupted communication between the aviator and the ATC. Without distance measuring equipment any flight is simply impossible as DME data are used to update position, velocity, and wind measurements form inertial navigation system (INS) measurements. In modern DME systems, the principle error involves only the measurement of time, and can easily be calibrated. However, the main source of error is the effect of terrain on radiowave propagation, which can be neglected under most conditions where the aircraft is ≤ 100 km from the station. Figure 13 DME principle A DME system is almost universally available, reliable and very accurate even though it is limited to land-based navigation. LORAN, Omega and GPS are all inherently more accurate, but not always reliable and available, thus the advanced method of using Kalman filtering is developed to determine INS error from simple DME equipment. 16 ANS637 Attitude and Navigation System DME, its principle of operation, description of selected type and error analysis List of References [1] Gaston H. Gonnet, Ralf Scholl - “Determination of the accurate location of an aircraft”, Scientific Computation, 2009. [2] Alfred R. Rodi, James C. Fankhauser, Robin L, Vaughan - “Use of distance measuring equipment (DME) for correcting error in position, velocity and wind measurements from aircraft INS” - December 1991. [3] "Engineer exploded myths in many fields" – via The Sydney Morning Herald; 9th Jan, 2013. [4] “Operational Notes on Distance Measuring Equipment” – booklet published by Civil Aviation Authority Australia. [5] Luciano Musumeci, Jaron Samson, Fabio Dovis - “Experimental assessment of Distance Measuring Equipment and Tactical Air Navigation interference on GPS L5 and Galileo E5a frequency bands”, Conference paper, Jan, 2012. [6] Sherman Lo, Yu Hsuan Chen, Robert Erikson, Robert Lilley - “Distance Measuring Equipment Accuracy Performance Today and for Future Alternative Position Navigation and Timing (APNT)”, Aviation Management Associates, 2013. [7] Mike Collins – “How It Works: Distance Measuring Equipment GPS Is Pushing This Technology to Pasture”, Dec, 2017. [8] Instrument Flying Handbook, FAA-H-8083-15, 1999 [9] R. W. Latham, R. S. Towness – “DME Errors”, Navigation – Journal of the Institute of Navigation, Vol.24, 1975-76. [10] Websites – www.en.wikipedia.org www.airwaysmuseum.com www.onlinelibrary.wiley.com www.greggordon.org/flying www.flightlearnings.com ~ 17