Fundamentals of Ground Radar for Air Traffic Control Engineers and Technicians
This is a standard reference for FAA and military air traffic control engineers and maintenance technicians focused on ground-based radar systems. With the evolution and increasing sophistication of modern radar systems, now more than ever a single-source reference is needed that contains simply understandable information on MTI, MTD, and Air Traffic Control Radar Beacon systems. There is a downside to the drive away from knowledge-based problem solving...massive increases in costs and waste inherent in the idea of lowest replaceable units. Fewer technicians are expected to manage more systems while knowing less about their equipment. This unique training and reference text will not only provide answers for day-to-day tasks, it will also help allay cost increases and waste.
Inspec keywords: Doppler radar; radar displays; radar transmitters; radar receivers; radar detection; air traffic control; transmission lines
Other keywords: radar synchronizers; radar history; ground radar; air traffic control; secondary radar systems; radar displays; microwave transmission lines; radar transmitters; moving target detector; radar receivers; Doppler effect; moving target indicator
Subjects: Radar equipment, systems and applications; Signal detection; Display technology; General electrical engineering topics; Transmission line links and equipment; Air traffic control and navigation
The quality of a professional that distinguishes him from a tradesman is his formal knowledge, which includes an understanding of the background and origins of his business. A chemist should know about Robert Boyle, Antoine Lavoisier, John Dalton, Amadeo Avogadro, Henry Louis Le-Chatelier, and many, many, more. A medical doctor should know about Hippocrates, Anton van Leeuwenhoek, Louis Pasteur, Ignaz Philipp Semmelweis, Joseph Lister, and more. If for no other reason, those who made contributions to the science must never be forgotten, for to forget them is to dishonor them, and to dishonor the roots of the science is to discredit the profession. Science is a characteristic of a highly developed civilization and intellect; its growth and maintenance relies upon perpetual regard for the past. In most cases, the founders of science were geniuses who made incredible discoveries with only primitive knowledge; we owe them eternal gratitude, for there are very few of us, if any at all, who could achieve what they have achieved. It is the responsibility of every member of a profession to keep his or her memory and respect alive. Although most think of radar as a precipitate of World War II, its beginnings are traceable to the late nineteenth and early twentieth centuries, a brief period of unparalleled discovery, ingenuity, and invention in the history of man. And although the US government was the major entity responsible for the development of modern microwave radar as we know it today, this credit is due to the imposition of urgency of World War II, and to the gift of the multicavity magnetron by the British government in 1940.
The word professional is regularly misused today, and its use in the title of this chapter may appear to the outsider to be yet one more abuse. However, the work of a radar technician truly falls within proper definitions of the term. Some work is called a 'trade' and some is called a 'profession'. A 'trade' is defined as skilled work, but is differentiated from a profession by dictionary definitions. Definitions for 'profession' include such concepts as 'involving mental rather than manual work' Generally, 'trade' would describe such occupations as plumbers, electricians, welders, mechanics, and machinists where skill in performing tasks is more necessary than extensive knowledge. 'Profession' would describe such occupations as doctors, nurses, dentists, chemists, physicists, engineers, attorneys, etc. A Webster's Dictionary definition of the word 'technician' is 'one versed in the technicalities of some subject, specifically, an artist, writer, musician, etc. who has great technical skill or knowledge. The 'knowledge' part of the general definition of 'technician' places it in the 'professions' category. A 'profession' may require certain manual skills, but it is characterized principally by formal, academic, or scientific knowledge, and by a requirement to arrive at conclusions through application of that knowledge. The radar technician's job most certainly falls within that definition. He must be able to correctly theorize the source of a problem, even when he cannot see it. He must be able to document his actions in a manner adequate to allow readers to unquestionably understand him; this requires the correct and accurate use of accepted technical and scientific terms and definitions. He must also be able to speak of technical and scientific matters with the same precise, accurate, and correct use of terms and definitions, as problem solving and correction may often involve telephone communication with other technicians in locations perhaps hundreds of miles away, or even more.
This chapter discusses radar equipment theory of operation and maintenance methods. Everyday work in radar requires the frequent use of logarithms and deciBels. The use of deciBels must therefore be reviewed before proceeding with any further discussion.
This chapter is a very basic introduction to the principle of pulsed radar. Obviously, getting a beginning concept of the main principle is one objective. Another is to introduce many terms commonly used in the radar language. Those terms which are the most essential to remember are printed in bold italics to assist you. A transmitter emits a burst of RF energy, and objects in its path will reflect the pulse back to the transmitting antenna as an echo. A steering device, called a duplexer, allows the transmitter burst to go to the antenna without damaging the receiver, and then isolates the transmitter during a listening time. The listening time is usually called live time. During live time, the echoes from any objects, called targets, are routed into the receiver.
This chapter presents the radar frequency transmitting systems. The magnetron is the most economical among radar transmitting tubes. In earlier long-range magnetron systems, a microwave crossed-field amplifier (CFA), also called an amplitron, tube was used to boost the magnetron power. The magnetron also may produce poor or unbalanced frequency spectrums, and there is usually little that can be done to correct this, other than replacing the tube.
The intent of this chapter is to introduce the reader to the general configuration of FAA radar installations. The FAA radar network contains a wide variety of systems: Airport Surveillance Radar (ASR), Air Route Surveillance Radar (ARSR), Airport Surface Detection Equipment (ASDE), Precision Approach Radar (PAR) (use has been discontinued in favor of instrument landing systems (ILS)), and Terminal Doppler Weather Radar (TDWR). Until deployment of the ASR-9, ASR-11, and ARSR-4, the ASR and ARSR, up to the video outputs, were very similar. This chapter will begin with a hypothetical, generic, synthesis systems as illustrated in Figure 6-1. This system will roughly resemble the ASR-8, ARSR-3, FPS-20, FPS-64, 65, 66, and 67. The system could also be converted to a magnetron system with the addition of coho phase-locking circuitry and automatic frequency control, and would then resemble the ASR-4, 5, 6, and 7, and the ARSR-1 and -2. Additionally, the USAF uses a modified version of the ASR-8 containing a magnetron and the necessary additions; in this system, the magnetron tuning is driven by the afc.
Radar is an acronym for radio detection and ranging. Even though most of us assume radar to be an echoing system, radio detection and ranging can be achieved by another means. A radar can also be an answer-back system, in which an interrogator transmits a signal to all aircraft equipped with transponders. On receipt of the interrogation, the transponder transmits a coded reply at a frequency differing from the interrogator's. To differentiate between the answer-back systems and the conventional echoing radars, the latter are called primary radars and the former are called secondary radars. Secondary radar systems have been built both with ground interrogators for aircraft transponders and with aircraft interrogators for ground or shipboard transponders.
The author of this book makes only a limited claim to the originality of this chapter. It has been rewritten from Chapter 10 of US Department of the Air Force Manual 52-8, entitled Radar Circuit Analysis, published June 30, 1951. The entire chapter has been retyped, edited, supplemented, and reformatted. Some of this was done to more closely conform to latter-day technical writing standards, some to add more current information, some for clarification, and some to make it more closely resemble the style of the other chapters of this book. Most importantly, no information has been deleted or diluted, and useful information has been added. All the illustrations have been retouched or redrawn, and all the labeling in the illustrations has been retyped.
Synchronizers may differ substantially in systems, each producing the unique set of triggers and gates for the radar's purpose and design, and by the hardware available at the time of manufacture. Some fundamental design concepts applicable to many synchronizers will be discussed in this chapter, and one synchronizer necessary for the ASR-9 MTD system will be introduced to illustrate a more complex application. Figure 9-1 is a simple diagram of some basic timing signals required of an early ASR radar. Triggers and gates for use throughout the radar system may be produced by the synchronizer, or by other units which rely upon the synchronizer for timing triggers. Among the various timing signals are the transmitter triggers, the coho gate in magnetron systems, stc triggers, display triggers, live-time gates to enable the receiver outputs for 741 μs (for 60-mile ASR radars) or 2,471 μs (for 200-mile ARSR radars), deadtime triggers to initiate built-in-test (BIT) functions, and many others. Satisfactory operation of the MTI system places stringent requirements on the synchronizer design and stability that would not otherwise be necessary. In sophisticated tracking and range/elevation/azimuth three-dimensional systems, timing becomes so complex, and subjected to so many variables, that only a computer with several interrupts and complex subroutines can suffice.
The fundamentals of common radar-transmitter hardware were addressed in preceding chapters. This chapter will deal in greater detail with several aspects of the pulsed radar transmitter. One topic will be the origin and composition of the emitted energy, a wide spectrum of frequencies. Another subject is the modulator, which will apply a high-voltage pulse to a third area of interest, the final power amplifier, which may be a magnetron, a klystron power amplifier tube, a traveling wave tube (twt), or an amplitron, also called a cross-field amplifier (CFA). There are still other means to transmit the radar burst. A major component in the modulator is the pulse forming network, an artificial transmission line; considerable discussion of this device will be contained in this chapter.
This chapter describes the basic operation and components of a generic radar receiver consisting of low-noise amplifiers, signal mixers, preselector filters, among others.
When this author entered the radar field in 1955, moving target indicator (MTI) systems had been in existence for less than 10 years and were still viewed as new. The non-MTI AN/MPN-1 was still the world's most extensively used air traffic control radar. Of all radar subject material, technicians and engineers in 1955 saw MTI as the greatest mental challenge. With that challenge to his senses of responsibility and competence, the author has devoted the years since 1955 to learning about, writing, and teaching MTI theory. The approach in this chapter is purely his own, and represents years of work and study.
Moving target indicator (mti) theory was explained in detail in the preceding chapter. For practical application, the technician needs an exposure to an actual system. All systems bear great resemblances to each other, and a good knowledge of one makes all others understandable. The largest deployment of latter-day FAA canceler-type mti systems was the ASR-8, and that system will be used as a basis for the sample system to be described in this chapter. This sample system will differ somewhat from the ASR-8 in that auxiliary, or accessory, circuits, not essential to the processing of the target data, have been omitted.
There have been other systems called moving target detectors and the term could lead to confusion to some experienced technicians unfamiliar with air traffic control radar. MTI, discussed in previous chapters, is an abbreviation for moving target indicator, a broad class of radar systems which indicate moving targets. An FAA MTD system, the system type hereafter simply called MTD, is within the broader class of MTI radar systems, and may be called an MTI system in some literature.
Radar display techniques have passed through even more stages of evolution than radar itself. Many of those stages will be addressed in this chapter, but some latter-day display techniques involving computer operating systems such as Lynx, Unix, and others, or which may encompass changing television methods are not included, and will require study of other material.
This chapter presents antenna gain azimuth change pulse to bearing conversion, beacon received power calculation, decibels, Doppler shift of an aircraft at a radial velocity, echo power, Fourier harmonic series, maximum expected radar range, mds prediction, overall noise figure, and VSWR (voltage standing wave ratio) equations.
