Coherent Data Collectors: a Hardware Perspective
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COHERENT DATA COLLECTORS: A HARDWARE PERSPECTIVE Coherent Data Collectors: A Hardware Perspective Russell Rzemien For more than 15 years, the Applied Physics Laboratory has been designing and operating coherent radar data-collection instrumentation. In this article, the engineer- ing challenges and design approaches used to meet analysis requirements are described, examples of collectors are provided, and the nature of future development efforts is outlined. (Keywords: Coherent signal processing, Radar analysis, Radar instrumentation, Radar systems, Test and evaluation.) INTRODUCTION Radar1–3 is an acronym derived from the words radio processors that only use amplitude information may fail detection and ranging. The term reveals much about to detect these targets in such environments. the operation and use of early radars. Those first devices Far better performance is achieved by coherent ra- used radio waves to detect the presence of objects and dars, i.e., radars that use the phase or frequency infor- to measure the ranges of those objects. Modern radars mation of the return echoes and not just the amplitude frequently provide additional information, such as of the return signal. An echo’s phase (or frequency) angular position and inbound velocity of an approach- remains constant for stationary objects like clutter, ing target. However, the primary function of radar is whereas the phase varies for moving objects. It is this target detection; all else follows from this. The detec- changing phase, along with the amplitude of the echo, tion problem remains a challenge, particularly with the that the signal processors in coherent radars use to development of stealth design techniques that signif- discriminate targets from background clutter. In mod- icantly reduce the strength of the target’s echo. Today’s ern radars, the phase and amplitude information is Fleet radars must detect those echoes even when they frequently available as two digitized data streams called are buried in a background of “clutter” caused by re- the I (in-phase) and Q (quadrature-phase) channels. flections from objects in the environment such as the Phase is calculated as arctan(I/Q) and amplitude as sea, rainstorms, or land masses. IQ.22+ Surveillance radars use either coherent or noncoher- An effective coherent radar must generate a signal ent techniques (and occasionally both) when detecting stable in both amplitude and phase. Instabilities in the targets. Noncoherent processing uses the amplitude or transmitted pulse or spurious signals present in the strength of the echo to locate the target. Because clutter receiver will affect detection sensitivity by, for exam- returns may be orders of magnitude greater in ampli- ple, raising the noise floor or detection threshold in the tude than those from approaching aircraft or missiles, processor, thereby masking weak target returns. In some JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 18, NUMBER 3 (1997) 377 R. RZEMIEN cases, the processor may generate false detections (false data collection can be restricted to a particular region, alarms). Coherent data, collected and analyzed, pro- or sector, of the surveillance volume. Use of this pro- vide a way to measure system stability, a means to cedure significantly reduces the average data rate that determine target visibility in the presence of strong needs to be stored by the collector. clutter, and a database to use for checking the accuracy Characteristics of the radar to be instrumented great- of environment and target models used by the radar ly affect collector system requirements. For mono-pulse system designers. track radars, the number of channels can be 3 or more Instrumenting a radar to collect coherent data can times that required in a search radar because monopulse be complex. Unlike radar video, which is relatively easy radars use several beams, transmitted or received simul- to obtain and record, coherent data frequently require taneously, to precisely locate the position of a target. To special interface circuitry both in the radar and in the evaluate radar performance, several parallel data chan- collector. (IEEE Standard Radar Definitions defines ra- nels containing the results of this multibeam (but single dar video as the signal that remains after envelope or pulse) interrogation must be collected. Thus, track phase detection, which in early radar was the displayed radars are more difficult to instrument than search signal. One can think of this as a process that removes radars because the burst or instantaneous data rates are the phase or frequency information, leaving only the greater. Fortunately, for pulse radars, only a fraction of amplitude or “strength” of the echo.) Data rates are these data need be collected. Typically, a range gate, usually several times greater, in some cases more than equivalent to a window in time, is established about the an order of magnitude greater, than video rates. Addi- position of the target, and data need be collected only tional waveform and radar status data, such as trans- within this gate. The reduction of the average data rate mitter frequency, pulse code, and pulse repetition in- is equivalent to that achieved by angle sectoring of the terval, need to be recorded as well. Developing a device search radar’s surveillance volume. to perform this task and to meet program analysis and Environmental studies for search radars require glo- evaluation requirements is a significant technical chal- bal collection capability. The strength of a radar signal lenge. The following sections describe approaches that echo from clutter will fluctuate over time, and echo have been successful in the past. returns from different directions and at different ranges will also vary greatly. Statistical measures of the clutter, ESTABLISHING COLLECTOR such as the mean echo strength and the standard de- viation of the echoes’ returns, are used to help char- REQUIREMENTS acterize the clutter environment. A small sample of Before design can begin, it is important that the data, e.g., from one or two pulses, over a limited range system requirements be stated clearly. Is the collector will not be sufficient to determine these statistics, nor needed to evaluate target detectability, or is there an is it possible to process such data and accurately deter- interest in studying aspects of the radar environment, mine the false alarm rate that would result from oper- or both? Are specific performance parameters to be ation in this environment. It is also difficult to deter- measured, such as how small a target, relative to the mine where best to position the collection “slice” or to clutter, can be detected? This parameter is frequently convince oneself that the slice of data collected is truly called the subclutter target visibility. Are the data representative of the environment (Fig. 1). needed to determine how pulse compression affects More fundamentally, even if analysis could obtain target visibility in clutter? (Pulse compression is a tech- the correct parameters to specify, for example, the nique whereby a long-duration radar pulse can be pro- probability distribution describing the clutter’s normal- cessed to provide a high-range resolution view of either ized cross section, such a representation is somewhat a target or the environment. The compression tech- misleading. The radar processor will not see the long- nique usually leaves some residual “noise,” referred to term average clutter but must deal with the spatially as range sidelobes, around echoes, and this residue may clumpy nature of the clutter. This is particularly true be greater than the echo of a small target of interest.) for land clutter, where shadowing effects cause echoes Answers to these questions will determine how much, to be zero for extended ranges followed by areas of large how fast, and what type of data should be collected. clutter returns. Sea clutter tends to drop off in ampli- Another requirement might be to gather sufficient tude, more or less smoothly, with range from the radar. data for playback in studies of system performance on Figure 2 shows these characteristics, though the range a global level (e.g., effectiveness of different processing scale does not allow one to appreciate the relatively schemes used in automatic track-while-scan proces- smooth fall off of sea clutter. sors). This requirement makes it necessary to collect all The collector designers must take all of these con- or most of the data at a continuous rate for tens of siderations into account. The data rates from the radar minutes. If, on the other hand, interest is limited to largely determine the speed of the input circuitry. The search radar performance against a particular target, average data rate and length of data collection events, 378 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 18, NUMBER 3 (1997) COHERENT DATA COLLECTORS: A HARDWARE PERSPECTIVE EFFECT OF THE RADAR ON DESIGN 4 1220283644526068 The rate at which the collector receives data is Amplitude (dB) determined primarily by the radar set’s analog-to-digital 550 (A/D) converters. Conversion rates for these devices are chosen to ensure capture of the return echo, regard- less of the time at which it is received. Therefore, at a minimum, samples must be taken with spacing equal 490 to the transmitted pulse width. (Radar front-end pro- cessing typically stretches the received pulse so that the processed echo will not “hide” between samples.) Some loss in signal amplitude results from this minimum 430 sampling, but the loss is normally not enough to justify oversampling. (Oversampling occurs when the time between A/D conversions is less than the time duration of the transmitted pulse.) 370 The conversion rate multiplied by the number of Range cell number bits per converter determines the data rate from each channel. The number of bits chosen for use in the radar depends on many factors, such as system dynamic range 310 requirements, subclutter visibility, and type of signal processing performed in the radar.