Introduction to Radar System and Component Tests White Paper

Transcription

1 Introduction to Radar System and Component Tests White Paper This White Paper provides a general overview of different military and commercial radar systems. It also covers some typical measurements on such systems and their components. Application Note Roland Minihold / Dieter Bues 08_2012-1MA207_0e

2 Contents Table of Contents 1 Abstract Overview of Typical Radar Applications and Common Radar Types Typical radar applications Radar Frequencies, - Bands, Wavelength and Applications Radar Equation Common Radar Types CW (Doppler) and FMCW (Doppler (Speed)/Range) Radar) Simple Pulse (Range) and Pulse Doppler (Speed/Range) Radar Pulse Doppler radar Pulse Compression Radar (FM Chirp and Phase Coded) Frequency-Agile Radar (FAR) - Suppression of Jamming and Improved Clutter Rejection Stepped-Frequency Radar (Imaging Application) Moving-Target Identification (MTI) Radar Monopulse Radar (Phase or Amplitude Comparison) / (Range and Angle Measurement) Phased-Array Radar, Digital Beamforming Synthetic-Aperture Radar (SAR) Bistatic, Multistatic Radar Passive Radar Multimode Radar The Future of Radar Developments Common Radar Abbreviations Literature Additional Information MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 2

3 1 Abstract It was German engineer Christian Huelsmeyer who first used the radar principle to build a simple ship detection device intended to help avoid collisions in fog (Reichspatent Nr ). First widely used radar technology was developed for military purpose during World War II. Today, more than half a century later, there is a much wider radar application area beyond the military one. Radar is needed for weather forecast, airport traffic control and automotive applications such as car distance surveillance and pedestrian detection. Additionally radar technology today is affordable on a mass production basis due to highly integrated signal processing components which make it possible to detect even low power signals in applications where at former times much more RF energy was needed. Low power radar components automatically mean savings in costs and size. In addition there are a lot of CAD tools available for the development of such systems and to deal with higher frequencies up to 110 GHz and beyond. R&S created two complementary papers, application note 1MA127 and white paper 1MA207 regarding current radar technology in order to demonstrate its contribution to test and measurement of radar systems and components. The white paper gives an overview on radar Systems and important measurements on them. The corresponding application note 1MA127 goes into details in explaining radar test technology along with the specific products needed to perform the tests. Both documents, 1MA127 and 1MA207 are addressing students who want to become familiar with radar issues as well as radar professionals who want to solve certain test and measurement tasks. 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 3

4 2 Overview of Typical Radar Applications and Common Radar Types 2.1 Typical radar applications Typical radar applications are listed here to give an idea of the huge importance of radar in our world. Surveillance Military and civil air traffic control, ground-based, airborne, surface coastal, satellitebased Searching and tracking Military target searching and tracking Fire control Provides information (mainly target azimuth, elevation, range and velocity) to a firecontrol system Navigation Satellite, air, maritime, terrestrial navigation Automotive Collision warning, adaptive cruise control (ACC), collision avoidance Level measurements For monitoring liquids, distances, etc. Proximity fuses Military use: Guided weapon systems require a proximity fuse to trigger the explosive warhead Altimeter Aircraft or spacecraft altimeters for civil and military use Terrain avoidance Airborne military use Secondary radar Transponder in target responds with coded reply signal Weather Storm avoidance, wind shear warning, weather mapping Space Military earth surveillance, ground mapping, and exploration of space environment Security Hidden weapon detection, military earth surveillance 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 4

5 2.2 Radar Frequencies, - Bands, Wavelength and Applications Radar Bands, -Frequencies, -Wavelengths and their Applications Band Frequency Wavelength Application HF 3 to 30 MHz 10 m to 100 m Coastal radar systems, over-the-horizon (OTH) radars; 'high frequency' P 30 to 300 MHz 1m to 10 m 'P' for 'previous', applied retrospectively to early radar systems UHF 300 to 1000 MHz 0.3 m to 1 m Very long range (e.g. ballistic missile early warning), ground penetrating, foliage penetrating; 'ultra high frequency' L 1 to 2 GHz 15 cm to 30 cm Long-range air traffic control and surveillance; 'L' for 'long' S 2 to 4 GHz 7.5 cm to 15 cm Terminal air traffic control, long-range weather, marine radar; 'S' for 'short' C 4 8 GHz 3.75 cm to 7.5 cm Satellite transponders; a compromise (hence 'C') between X and S bands; weather radar X 8 12 GHz 2.5 cm to 3.75 cm Missile guidance, marine radar, weather, medium-resolution mapping and ground surveillance; in the USA the narrow range GHz ± 25 MHz is used for airport radar. Named X band because the frequency was kept secret during World War 2. Ku GHz 1.67 cm to 2.5 cm High-resolution mapping, satellite altimetry; frequency just under K band (hence 'u') K GHz cm K band is used by meteorologists for detecting clouds and by police for detecting speeding motorists. K band radar guns operate at ± GHz. Automotive radar uses GHz. Ka GHz 0.75 cm to 1.11 cm Mapping, short range, airport surveillance; frequency just above K band (hence 'a'); photo radar, used to trigger cameras that take pictures of license plates of cars running red lights, operates at ± GHz mm 40 to 300 GHz 1 mm to 7.5 mm Millimeter band, subdivided as below. The letter designators appear to be random, and the frequency ranges dependent on waveguide size. Multiple letters are assigned to these bands by different groups. Q 40 to 60 GHz 5 mm to 7.5 mm Used for military communications V 50 to 75 GHz 4 mm to 6 mm Very strongly absorbed by the atmosphere W 75 to 110 GHz 2.7 mm to 4 mm 76 GHz LRR and 79 GHz SRR automotive radar, high-resolution meteorological observation and imaging 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 5

6 3 Radar Equation The acronym RADAR stands for RAdio Detection And Ranging. Figure 1 shows the basic principle. Figure 1: Basic principle of Radar and its parameters An electromagnetic wave of power Pt is transmitted to a flying object, for example to a plane and is partly reflected back to the antenna with the receiving power Pr. From the time delay between the transmitted and received signal the distance to the plane can be calculated. Additional information can be gained from the frequency shift of the received signal, which is proportional to the speed of the plane. Receiving a signal of sufficient power by an adequate power to noise ratio is the biggest challenge of radar systems. The so called Radar Equation gives hints on the power relations within the system as indicated in Figure 1. The Radar Equation delivers the received power Pr as result. According to the Radar Equation following independent parameters determine the received power P r : P r 2 2 Pt G * 3 4 (4 ) * R (Formula 1) P t : The power transmitted by the antenna, dimension is dbm. Numeric examples : 63 dbm for real world Radar applications, 13 dbm for laboratory tests G: Gain of the transmitting antenna, dimension in dbi. The parameter determines how much the radiation beam of the antenna is focused toward the direction of the target. Numeric examples are 12 dbi for a BiQuad antenna and 70 dbi for a highly focusing parabolic antenna. 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 6

7 λ: The wavelength of the transmitted signal, dimension in meter. The wavelength can be directly calculated from the frequency. Numeric examples: 0.03 m for a 10 GHz signal and 0.12 m for a 2.54 GHz signal б: Radar cross section, RCS, is a virtual area representing the intensity of the reflection. Not all of the radiated power is reflected back to transmitting antenna, as indicated by the small waves close to the plane in Figure 1. The Sigma ( ) of the objects determines the virtual area of the reflecting object (plane) from which all of the incoming radiation energy is reflected back to the antenna. The dimension is square meter, m 2 in short. Practical examples are 12 m 2 for a commercial plane, 1 m 2 for a person or 0.01 m 2 for a bird. Refer to [18], page 6665 for further examples. R: Distance between the transmitting antenna and the reflecting object. Dimension in m. Numeric examples are 8000 m for real world applications or 5 m for laboratory conditions. It has to be stressed that this parameter reduces the result, i.e. the received signal by the power of 4, with the effect that far distant objects are providing only a small amount of received power. Example Parameter Abbreviation Value, Example 1 Value Example 2 Unit Transmitted power Pt dbm Gain of transmit antenna G dbi Wavelength (frequency) (f) 0.03 (10*10 9 ) 0.12 (2.5*10 9 ) m(hz) Radar cross section 12 0,3 m 2 Distance R m Received power, linear Pr *10 3 pw Received power, logarithmic Pr log dbm Table 1: Parameters of Radar Equitation and two examples Example 1 shows a a real world example, derived from [Pozar], example 2 shows a radar application which can be realized under laboratory conditions for example in an anechoic chamber. Example 1 read in clear text : A radar transmitting antenna with gain of 28 dbi is transmitting an electromagnetic wave at 10 GHz with a power of 63 dbm to a plane in a distance of about 8000 m. The plane has a radar cross section of 12 m 2. By means of the Radar Equation the received power back at the antenna is calculated to -90 dbm. Example 2 read in clear text: In a radar test laboratory implemented in an anechoic chamber a test transmitter provides 13 dbm to a matched antenna of 12 dbi with a frequency of 2.5 GHz. The reflecting object with a cross section of 0.3 m 2 is located in 5 m distance from the transmitting antenna. According to the Radar Equation the test receiver is going to receive a reflected signal of -48 dbm. 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 7

8 When comparing example 1 to example 2 we can conclude that despite much bigger transmitting power, better transmit antenna gain and bigger radar cross section in example 1 the received reflected power of example 1 is almost 50 db lower than the received signal of example 2. The reason is the smaller wavelength lambda which affects the result by a power of 2 and especially the bigger distance R of example 1 which affects the result by a power of 4. Small wavelengths, i.e. high frequencies are aimed for in most radar systems, especially in antenna arrays, because of the resulting small antenna size. It is obvious also, that in radar technology one has to deal with very small receiving power especially for far distant objects. 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 8

9 4 Common Radar Types This section lists the most common types of radar systems with brief explanations of how they work. 4.1 CW (Doppler) and FMCW (Doppler (Speed)/Range) Radar) A continuous wave (CW) radar system with a constant frequency can be used to measure speed. However, it does not provide any range (distance) information. A signal at a certain frequency is transmitted via an antenna. It is then reflected by the target (e.g. a car) with a certain Doppler frequency shift. This means that the signal's reflection is received on a slightly different frequency. By comparing the transmitted frequency with the received frequency, we can determine the speed (but not the range). Here, a typical application is radar for monitoring traffic. Radar motion sensors are based on the same principle, but they must also be capable of detecting slow changes in the received field strength due to variable interference conditions that may exist. Radar speed traps operated by the police use this same technology. Camera systems take a picture if a certain speed is exceeded at a specified distance from the target. Figure 2: Mobile traffic monitoring radar MultaRadar CD - Mobile speed radar for speed enforcement from Jenoptic 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 9

10 There are also military applications: CW radars are also used for target illumination. This is a straightforward application: The radar beam is kept on target by linking it to a target tracking radar. The reflection from the target is then used by an antiaircraft missile to home in on the target. CW radars are somewhat hard to detect. Accordingly, they are classified as low-probability-ofintercept radars. CW radars lend themselves well to detecting low-flying aircraft that attempt to overcome an enemy's air defense by "hugging the ground". Pulsed radar has difficulties in discriminating between ground clutter and low-flying aircraft. CW radar can close this gap because it is blind to slow-moving ground clutter and can pinpoint the direction where something is going on. This information is relayed to co-located pulse radar for further analysis and action. [7] FMCW radar The disadvantage of CW radar systems is that they cannot measure range due to the lack of a timing reference. However, it is possible to generate a timing reference for measuring the range of stationary objects using what is known as "frequency-modulated continuous wave" (FMCW) radar. This method involves transmitting a signal whose frequency changes periodically. When an echo signal is received, it will have a delay offset like in pulse radar. The range can be determined by comparing the frequency. It is possible to transmit complicated frequency patterns (like in noise radar) with the periodic repetition occurring at most at a time in which no ambiguous echoes are expected. However, in the simplest case basic ramp or triangular modulation is used, which of course will only have a relatively small unambiguous measurement range. Figure 3: Basic principle of FMCW radar. The target s velocity is calculated based on the measured delay t between the transmit signal and the received signal, whereas the frequency offset f gives the range This type of range measurement is used, for example, in aircraft to measure altitude (radio altimeter) or in ground tracking radar to ensure a constant altitude above ground. One benefit compared to pulse radar is that measurement results are provided continuously (as opposed to the timing grid of the pulse repetition frequency). FMCW radar is also commonly used commercially for measuring distances in other ways, e.g. level indicators. Automotive radar is in most cases FMCW radar too. 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 10

11 4.2 Simple Pulse (Range) and Pulse Doppler (Speed/Range) Radar Figure 4: Basic principle of a simple pulse radar system A simple pulse radar system only provides range (plus direction) information for a target based on the timing difference between the transmitted and received pulse. It is not possible to determine the speed. The pulse width determines the range resolution. Figure 5: Direction information with azimuth angle determination in a radar system with a rotary antenna The direction information (azimuth angle φ) is determined from the time instant of the receive pulse with reference to the instantaneous radiation direction of the rotating antenna. The important measurements on (non-coherent) radar equipment of this sort are the range accuracy and resolution, AGC settling time for the receiver, peak power, frequency stability, phase noise of the LO and all of the pulse parameters. The AGC circuit of the receiver protects the radar from overload conditions due to nearby collocated radars or jamming countermeasures. The attack and decay time of the AGC circuit can be varied based on the operational mode of the radar. Since the roundtrip of a radar signals travels approximately 150 meters per microsecond, it is important to measure the response of the AGC for both amplitude and phase response when subject to different overload signal conditions. The measured response time will dictate the minimum detection range of the radar. 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 11

12 4.3 Pulse Doppler radar A pulse Doppler radar also provides radial speed information about the target in addition to range information (and direction information). In case of coherent operation of the radar transmitter and receiver, speed information can be derived from the pulse-to-pulse phase variations. I/Q demodulators are normally used. The latest pulse Doppler radar systems normally use different pulse repetition frequencies (PRF) ranging from several hundred Hz up to 500 khz in order to clarify any possible range and Doppler ambiguities. More advanced pulse Doppler radar systems also " use "staggered PRF, i.e. the PRF changes on an ongoing basis to get rid of range ambiguity and reduce clutter as well. Important criteria for achieving good performance in pulse Doppler radar systems include very low phase noise in the LO, low receiver noise and low I/Q gain phase mismatch (to avoid "false target indication") in addition to the measurement parameters listed above. When measuring the pulse-to-pulse performance of a radar transmitter, it is important to understand the variables that can impact the uncertainty of the measurement system for accurate Doppler measurements: Signal-to-noise ratio of the signal - the better the signal to noise ratio of the signal, the lower the uncertainty due to noise contribution. Bandwidth of the signal - the bandwidth of the IF acquisition system must be sufficient to accurately represent the risetime of the pulsed signal, however too much bandwidth can result in added noise contribution uncertainty. Reference (or timebase) clock stability. Jitter or uncertainty due to the measurement point of the rising edge of the signal rising edge interpolation or signals that have changing edges impact this uncertainty. Overshoot and preshoot of the rising and falling edges any ringing on the rising and falling edges can impact the measurement points adversely on a pulse to pulse basis. It is important that the measurement point, or the average set of measurement points, are sufficiently far away in time from the leading and falling edges of a pulse. Applying a Gaussien filter to smooth the impact of the rising and falling edges can reduce this phenomena and is often implemented in the Doppler measurement system of a radar receiver. Time between measured signals due to the PRI of the measured signal, the close-in phase noise of the measurement system needs to be considered due to the integration time at lower offset frequencies. The same variables can also contribute to the uncertainty in the signal generator when testing the receiver circuit and Doppler measurement accuracy. 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 12

13 4.4 Pulse Compression Radar (FM Chirp and Phase Coded) Classic pulse and pulse Doppler radar transmits extremely short pulses. Increasing the pulse power allows the radar system to achieve greater range results. Decreasing the duration of the transmit pulses also decreases the pulse volume and provides better range resolution for the radar system, i.e. closely spaced targets can be distinguished with smaller distances between them. Pulse compression combines the power-related benefits of very long transmit pulses (good range) with the benefits of very short transmit pulses (high distance resolution). Lower peak power can then be used. By modulating the transmit pulses, a timing reference is produced within the transmit pulse, similar to frequency-modulated continuous wave (FMCW) radar systems. Several different modulation techniques can be used. The most common are: Linear frequency modulation (FM chirp) Non-linear frequency modulation Encoded pulse phase modulation (e.g. Barker code) Polyphase modulation and time-frequency coded modulation Although pulse compression technique has various benefits such as low pulse power with good range and distance resolution, there is a significant disadvantage: The minimum measurement range is degraded depending on the pulse length, since the radar receiver is blocked during the transmit pulse. As this is a major disadvantage for radar systems used for air traffic control, they typically use both techniques: Between the frequency-modulated pulses for the larger range, small (very short) pulses are transmitted which only have to cover the nearby area and do not require very high pulse power. Linear FM is most common in older radar systems. An example is the air-defense radar RRP- 117 [4][4]. Non-linear FM (NLFM) is becoming more practical use because of its various benefits such as inherently low range sidelobes which yields an advantage in SNR compared to Linear FM. [16] Encoded pulse phase modulation is very common, particularly Barker codes with lengths of 11 and 13 [15]. In advanced military radar systems, polyphase pulse compression is also used increasingly with special codes [14]. Pulse compression radar signal require baseband IQ collection of the signal covering the BW of the pulse risetime, wideband analog FM demodulation or vector demodulation and new displays of the information for analysis (amplitude, frequency, and phase vs. time), and digital demodulation/evm measurement for BPSK/QPSK modulations. 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 13

14 4.5 Frequency-Agile Radar (FAR) - Suppression of Jamming and Improved Clutter Rejection Frequency hopping is an effective technique for a radar system to circumvent jamming and electronic counter-countermeasures (ECCM). It is typically used in military radar applications. Clutter rejection is also possible using FAR. Sub-microsecond switching times and bandwidths ranging from several hundred MHz in the X band to over 2 GHz at 95 GHz are typical. Other measurement parameters that are relevant with FAR include the frequency switching/settling time, hop sequence, switching spurious and broadband amplitude and phase stability. This type of radar should consider the test of the radar not under static conditions, but the hopping conditions across the BW of interest. Oscilloscopes with FFT analysis often need to be employed to assess hopping performance and anomalies due to hopping sequences. 4.6 Stepped-Frequency Radar (Imaging Application) Stepped-frequency radar systems are used in imaging applications. With typical bandwidths ranging from several hundred MHz to 2 GHz, resolutions of <10 cm can be achieved. Figure 6: Timing diagram for stepped-frequency radar The frequency is increased by a fixed value from pulse to pulse. Typical bursts contain 128 pulses. The benefit of a stepped-frequency radar system is that one can obtain wide bandwidth and thus good resolution without needing a large FFT capture bandwidth [17]. Due to the wide RF bandwidth of the transmitter and receiver, these subsystems must exhibit excellent stability in order to obtain the desired high resolution. Impacted stepped frequency radar is reduced cost of testing each pulse, but added cost of pulse to pulse coherent analysis (magnitude and phase stability is most important). As was the case with frequency agile radar, the settling time of the local oscillator is also an important measurement parameter. 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 14

15 4.7 Moving-Target Identification (MTI) Radar The idea behind MTI radar is to suppress reflected signals from stationary and slow-moving objects such as buildings, mountains, waves, clouds, etc. (clutter) and thus obtain an indication of moving targets such as aircraft and other flying objects. Here, the Doppler effect is exploited, since signals reflected by targets moving radially with respect to the radar system exhibit an offset vs. the transmitted frequency which is proportional to their speed (e.g. in linear FM radar). In pulse radar systems, the pulses reflected by moving objects have a variable phase from pulse to pulse referenced to the phase of the transmitted pulses. Figure 7: Moving target indication: Moving targets are indicated by continuously changing amplitude while fixed targets show constant amplitude Optimizing MTI requires the use of very sophisticated techniques such as staggered PRF (a variable pulse interval from pulse to pulse) in order to offset "blind velocities" or make them visible. Important measurement parameters when optimizing MTI or the clutter suppression include the following: Good pulse-to-pulse phase and amplitude stability for the transmit signal Highest possible phase stability or lowest possible phase noise for the LO in the radar system, particularly for MTI involving targets with low radial speeds 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 15

16 4.8 Monopulse Radar (Phase or Amplitude Comparison) / (Range and Angle Measurement) Figure 8: Monopulse Radar Antenna In monopulse radar systems, at least two antenna groups arranged at spatially distinct locations are used [13]. By comparing the summation and difference channels, it is possible to localize the reflecting object within the radar beam. Using counterphase coupling of the left and right antenna groups, a difference channel (ΔAz) is formed ("delta azimuth"). The azimuth is determined by exploiting the fact that at this angle for a maximum of the summation channel, the difference channel must be at a minimum. Since the summation channel (Σ) and the difference channel can be formed from just a single echo, one pulse is enough to accurately compute the coordinates. (This is why this way of grouping antennas is also referred to as "monopulse antenna".) The ratio of the summation channel to the difference channel provides a measure of the offset of the real direction from the center axis of the antenna ("boresight"). The angular difference between the antenna boresight and the actual offset angle of the target is known as the "offboresight angle". In 3D radar systems, the elevation angle is also measured as the third coordinate. The same technique can be applied in this case too. The antenna is divided into upper and lower halves. The second difference channel (ΔEl) is now known as the "delta elevation". Channel matching of the different channels is critical in monopulse radar systems and must be measured. Multi-channel phase-coherent synthesizers with adjustable phase offsets are typically used for this purpose. Phase coherent multichannel analysis e.g. by means of a high performance digital oscilloscope with IQ interface becomes important for testing transmitter coherency 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 16

17 4.9 Phased-Array Radar, Digital Beamforming Radar applications normally need some kind of dynamic beam steering, because the area of a static electromagnetic beam is normally too small for object detection, especially within A&D applications. In classical radar systems mechanical devices have been used in order to move the radar beam to cover a certain area. The 360-degree rotating antenna in air surveillance systems is the best example. Older fighter planes also are using mechanically moving radar antennas. However, mechanical systems are heavy and failure-prone which both are drawbacks for equipment being used in safety-critical applications. Figure 9: 360-degree rotating antenna (left) of an air surveillance system and electronically steered antenna (right) High performance Digital Signal Processing along with affordable and small, highly integrated hardware systems have made possible another technique called "digital beamforming" (DBF) or AESA (Active Electronically Scanned Array). Figure 10 shows the basic principle; the literature [2] provides detailed information L mech Antenna 1 Antenna 2 Antenna 1 Antenna 2 Figure 10: Basic Principle of AESA (Active Electronically Scanned Array) or DBF (Digital Beamforming) 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 17

18 DBF always relies on antenna arrays. Modern systems sometimes include up to 1000 independent small antennas. To keep it simple, Figure 10 includes only 2 antennas. However the operation principle is the same for all bigger systems. All subsequent discussions are related to a single plane 2-dimensional antenna system only. The 3-dimensional real-world antenna system behaves in a similar way. In the example provided two sine signals of electrical energy are supplied to two isotropic antennas. An isotropic antenna is radiating equally in all directions. The two signals are of same phase and magnitude, so called coherent signals. The resulting power of the radiated sum beam is the sum of the two sine signals. Figure 10 shows the relation for the perpendicular direction with respect to our 2-Element antenna array. The behavior in this direction of transmission is shown in the left half of Figure 10. The situation for a certain side transmission, about 45 degrees, is shown in the right half. The upper part shows signal superposition, the lower half shows the same by using vectors. Blue and green shows the electromagnetic energy transmitted from the two antennas. The red parts of the drawings show the two superposed antenna signals. As indicated in the lower right, the side transmission causes a phase shift (delta) of both antenna signals which causes the red sum signal to be reduced compared to the sum signal without phase shift. The declined sum signal can be clearly seen in the signal representation in the upper part of Figure 10. It can be summarized now, that the antenna array has a specific maximum propagation direction, which is 90 degrees, because of isotropic antennas supplied by two signals of same phase and magnitude (= coherent signals). The resulting beam radiated to the side, for instance 45 degrees, is smaller due to a phase shift we are getting because of different propagation path lengths. We can summarize this in following table (3 rd line): Behavior of sum signals Two antenna supply signals 90 degrees sum signal x degrees sum signal In phase (coherent) Maximum Less than maximum Phase locked but not in phase(certain angle) Less than maximum Maximum Table 2: Behavior of sum signals dependent on phase of antenna supply signals The way in which the antenna array sum signal is declining along with the side angle depends on the distance between the antenna elements. It is therefore a property specific to the geometry of the antenna array. Additionally it depends on the wavelength, i.e. the frequency of the radar signal, because the phase shift is related to the wavelength. So far considered two antenna supply signals of same magnitude and phase have been considered, so called coherent signals. If a small phase shift is applied now in between both signals, we can reverse the effects so far described, i.e. decline the 90-degree beam and increase the 45 degree side beam to a maximum. If the phase between both signals is increased until a certain value the beam moves slowly ato 45 degree. Turning the beam by a certain angle is one way of so called "beamforming". The results can now be summarized again in Table 2, lowest line. The phase shift of antenna array supply signals can be generated in different ways. The simplest way is to use supply cables of different lengths. Another method is to implement phase-shifting elements in the appropriate antenna supply circuits. However, both methods are static and the phase shift can't be externally controlled. The Application Note 1MA127 related to this White Paper, describes an integrated circuit performing digitally controlled phase shifting. It is also shown how such phase shifters can be directly digitally controlled by test instruments, ex. network analyzers. A high performance digital scope can be suited to test the transmitted delay between up to 4 channels of T/R modules if the skew between the scope channels is sufficiently low (<< 1 ns). 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 18

19 Phased-array radar antennas have hundreds or even thousands of individual radiating elements (as opposed to a reflector antenna with a single radiator). The magnitude and phase of the power fed to the elements can be individually controlled, making it possible for the overall antenna to produce wave fronts with nearly any desired shape. In real-world operation, the pattern can be turned by about ±60. The efficiency of the antenna drops at larger angles. Unlike a conventional antenna that is moved mechanically, a phased array can rotate its pattern in space with practically no delay. Figure 11: Active Electronically Scanned Antenna Schematic Diagram (from) Since phased-array antennas are very costly, they are used primarily in military and SAR satellite applications. The standard is now an active phased-array radar (or active electronically scanned array, AESA) based on many individual, small transmit/receive modules, whereas the passive variant (PESA) uses a common RF source whose signal is modified using digitally controlled phase shifter modules. What is important with AESA is the uniformity of the different modules in terms of the amplitude and phase, which involves considerable test and calibration effort. Very fast automated test systems are required to align an array of hundreds, sometimes thousands if elements to achieve target performance. 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 19

20 4.10 Synthetic-Aperture Radar (SAR) Synthetic-aperture radar (SAR), like the related real-aperture radar (RAR; see also side-looking airborne radar), belongs to the class of mapping radar systems. Such radar systems are deployed in aircraft or satellites to provide a two-dimensional view of a section of terrain by scanning the earth's surface using electromagnetic waves. In defense industry, inverse synthetic aperture radar imaging (ISAR) of moving objects is an important tool for automatic target recognition. Both SAR and ISAR have the same underlying theory, the main difference is the geometry configuration. In SAR imaging, the radar is flying in the space, and the object is stationary, while in ISAR imaging, the object is moving and the radar is stationary. But only the relative movement between the object and the radar is important. So the ISAR imaging problem can be found to be equivalent to the more easily understood SAR imaging problem. The basic principle behind SAR involves an antenna that can be moved perpendicularly to the radiation direction. The position must be precisely known at all times. The direction of motion is normally referred to as the "along track" or azimuth and the related cross coordinate as the "cross track" or range. The "footprint" is the area which the real antenna is currently covering. The "swath" is the strip of terrain which the footprint crosses due to the ongoing motion of the real antenna. SAR involves replacing the instantaneous snapshot produced by a large antenna with many snapshots produced using a small, mobile antenna. During the course of the related movement, each object in the target area is illuminated at a different angle of view and recorded accordingly. As long as the path of the real antenna is known with sufficient accuracy, the aperture of a large antenna can be synthesized on the basis of the magnitude and phase of the received radar echoes in order to attain a high spatial resolution in the direction in which the antenna is moving. Figure 12: Synthetic-aperture radar (SAR). The SAR antenna beam is moved back and forth while traveling along the azimuth 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 20

21 The best possible resolution that can be attained in the azimuth or flight direction using SAR is equal to half the length of the real antenna, i.e. for a decrease in the azimuth antenna length LAz (designated as L in Fig. 9) of the real antenna, its resolution capacity δ Az improves as follows: Az L 2 Az The resolution in the radial direction (slant range) is determined in principle by the signal bandwidth of the transmit signal that is used: S l c 0 2B R where C o is the speed of light For a resolution of 1 m, we thus need a signal bandwidth of 150 MHz. Today's SAR systems use a signal bandwidth of >1 GHz (2 GHz is desirable) in order to attain a resolution of <10 cm. The signal bandwidth is normally attained using pulse compression techniques such as linear frequency modulation. More advanced SAR systems also use stepped frequency, polarization switching and other complex techniques (e.g. intrapulse beamsteering, multiaperture recording in azimuth, spatiotemporal waveform encoding, etc., in the TerraSAR X). [5][6] [20] Test challenges are the bandwidth of interest for the generation and analysis tools. There is a trade-off between tools and stepped frequency vs. single frequency e.g. higher effort for pulse to pulse coherent analysis vs. higher analysis bandwidth needed Bistatic, Multistatic Radar In most cases, the transmitter and receiver use the same antenna through time-domain multiplexing. This type of radar system is known as "monostatic radar". A bistatic radar system has one transmitter location and one or more receiver locations with a large distance or offset angle between them. It is easy to turn a monostatic radar system into a bistatic radar system by setting up additional receiving sites. A bistatic radar system can also be created using two monostatic radar systems that operate on the same frequency. In a bistatic radar system, the transmitter and receiver are separated by a large distance and usually also have a large azimuth offset. This means that a signal will be received even if no power (or only very little power) is reflected directly toward the monostatic radar system due to the geometry of the reflecting object (stealth technology). This type of system has practical applications in weather radar [9] and the military, e.g. to intercept stealth flying objects. When using multiple spatially separated receivers, we use the term multistatic radar. These radar have an increased importance on timebase and network synchronization. Measurement systems must have better synchronization and lower clock jitter to for meaningful pulse-to-pulse measurements. This is especially important in low-phase noise applications i.e. slow moving target detection Passive Radar Passive radar is a radiolocation technology which, unlike conventional radar, does not actively transmit electromagnetic energy in order to detect and track the reflections that are produced. Instead, reflections and the Doppler Effect are evaluated for transmissions from known broadcast transmitters, mobile radio transmitters and other systems that produce constant signals. Passive radar systems are difficult to locate, since they do not transmit any signals. This is a decisive advantage in military applications. Another advantage is the ability to intercept stealth aircraft, which is very limited with existing active radar technology. [10] 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 21

22 It is also important to understand that the collection systems need to cover the BW of the signals that need to be analyzed. For instances, 80 MHz near airports can cover each cellular infrastructure, however new digital video signals must be collected over wider spans, or individually over multiple analysis. The collection of passive radar signals is continually evolving as new wireless infrastructure, digital video, and digital audio terrestrial broadcast stations are deployed world wide Multimode Radar Many of the radar systems used nowadays in military applications (e.g. in aircraft) are multimode radar systems which must handle a wide range of tasks: Target searching and tracking Weapon guidance Altimeter High-resolution ground mapping Bad weather and terrain avoidance Electronic counter-countermeasures (ECCM) Different PRF modes are used in these applications, including FM chirp, Barker phase modulation or complex modulation, AESA antennas, SAR, frequency hopping, intrapulse polarization, etc. Example: Technical specifications for a military airborne data acquisition system[8]: Operates from airborne platforms Multi-mode Multi-band Fully Coherent Fully Polarimetric FREQUENCY BANDS A, B & C 100 to 600 MHz F 2.9 to 3.4 GHz I 9.0 to GHz J 15.5 to 16.0 GHz POLARISATIONS Linear vertical and horizontal Circular left and right-hand PEDESTALS Azimuth and elevation control using boresighted optical tracking system Sector scanning at up to 40 deg/s ANTENNAS Various (to suit application) 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 22

23 MODES Frequency-agile Polarisation-agile Pulse-by-pulse data recording to Ampex DCRSi or compatible media User-definable transmit waveforms up to 32 k samples 1 to 40 khz PRF User-selectable range ambits (768 to cells) Simultaneous two-channel, co and/or cross polar receiver Up to 500 MHz instantaneous bandwidth (2.25 GHz using eight 500 MHz 50% overlapped pulses) Data sampling at 100, 250 or 500 MHz DATA GATHERING MODES High Range Resolution 0.36 m using DPDPS linear chirp modulation over 500 MHz bandwidth 768 to contiguous 0.3 m range cells Ultra High Range Resolution 0.1 m using DPDPS linear chirp modulation over 2.25 GHz bandwidth (eight 500 MHz 50% overlapped pulses) 3072 to contiguous m range cells Testing a multimode radar system of this sort is complex and costly. Fast, fully automated test systems are needed. 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 23

24 4.14 The Future of Radar Developments In the future, we can expect to encounter multisensory systems that combine radar and infrared (or other) systems[11]. This will make it possible to combine the benefits of the different types of systems while suppressing certain weaknesses [11]. Military onboard radar systems will be increasingly confronted with the stealth characteristics of advanced aircraft. The contradiction between the different requirements imposed on aircraft must be solved (i.e. planes should exhibit stealth properties while not revealing their position through the use of onboard radar). One possibility involves the use of a bistatic radar system with a separate illuminator and only a receiver on-board the aircraft. In the future, radar antennas will in many cases no longer exist as discrete elements with suitable radomes. Instead, they will be integrated into the geometrical structure of the aircraft, ship or other platform that contains them. The next generation of AESA radars used on-board aircraft will have more than one fixed array in order to be able to handle greater spatial angles. Finally, the speed of the digital back-end equipment handling the radar raw data will need to increase i.e. through parallel processing in order to handle data rates as needed for highresolution radar operating modes.[12] 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 24

25 5 Common Radar Abbreviations Common Radar Abbreviations Abbreviation AESA AEW AFC AGC AM APAR ASR ASR-S ATC BARDS BSD BW CFAR CMOS COHO DBF DC DCRSi DOA DoD DPDPS DSP DTM DUT ECC ECCM EIRP ELINT EMPAR EMV ESA ESM ESP EW FCC FCC FCW FFT FMCW Meaning Active Electronically Scanned Array Airborne Early Warning Automatic-Frequency-Control Automatic Gain Control Amplitude Modulation Active Phase Array Radar Airport Surveillance Radar Airport Surveillance Radar Mode-S (Mode S is an extension to secondary radar. Mode S makes it possible to query additional information, e.g. the speed of the aircraft.) Air Traffic Control Baseband Radar Detection Sensor Blind Spot Detection Bandwidth (or Beamwidth) Constant False Alarm Rate Complementary Metal-Oxide Semiconductor Coherent Local Oscillator Digital Beam Forming Direct Current ( or Discrete Circuit) Digital Cassette Recording System Improved Direction of Arrival Department of Defense Dual Purpose Digital Processing System Digital Signal Processor Digital Terrain Model Device Under Test Error Correcting Code Electronic Counter-Countermeasures Effective Isotropic Radiated power Electronic Intelligence (electronic acquisition of radar parameters) European Multifunction Phased Array Radar Electromagnetic Vulnerability Electronically Steerable Array Electronic Warfare Support Measures Electronic Stability Program Electronic Warfare Fault Collection Unit Federal Communications Commission Forward Collision Warning Fast Fourier Transform Frequency Modulated Continuous Wave 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 25

26 Common Radar Abbreviations Abbreviation FSK FTLO GaAs GaN GCA HF IF LO LO LPI LRR LRU MMIC MTD MW NAFTA NRIET OTH PA PAR PAR PDF PESA PN PRF PRI PRT PSS RADAR RAM RBW RCS RDF RS RWR RX SAM SAR SNR SPI SRR SSPA SSR STALO Meaning Frequency Shift Keying Fast-Tracking Local Oscillator Gallium Arsenide Gallium-Nitride Ground-Controlled Approach High Frequency (3-30 MHz) Intermediate Frequency Local Oscillator Low Observability Low Probability of Intercept Long Range Radar Line-Replaceable Unit Microwave Monolithic Integrated Circuit Moving Target Detection Megawatt North American market Nanjing Research Institute of Electronic Technology Over-The-Horizon Power Amplifier Phased-Array-Radar Precision Approach Radar Pulse Desensitization Factor Passive Electronically Scanned Array Pseudo-Noise Pulse Repetition Rate or Frequency Pulse Repetition Interval Pulse Repetition Time Predictive Safety System Radio Detection and Ranging Random Access Memory, Radar Absorbing Material, Rolling Airframe Missile, or Reliability, Availability and Maintainability Resolution Bandwidth Radar Cross-Section Range and Direction Finding Ramp Slope Radar Warning Receiver Receive Surface-to-Air Missile Synthetic Aperture Radar Signal-to-Noise Ratio Serial Peripheral Interface Short Range Radar Solid-State Power-Amplifier Secondary Surveillance Radar Stable Local Oscillator 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 26

27 Common Radar Abbreviations Abbreviation STAP SWT T/R TBD TRM TWT TX UHF ULA UWB VCO VHF VSA Meaning Space-Time Adaptive Processing Software-Timer Transmit/Receive Track-Before-Detect Transmitter-Receiver Module Traveling Wave Tube Transmit Ultra High Frequency Uniform Linear Array Ultra Wideband Voltage Controlled Oscillator Very High Frequency Vector Signal Analyzer 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 27

28 6 Literature [1] Merrill I. Skolnik,1990, Radar Handbook, Second Edition McGraw-Hill [2] Merrill I. Skolnik,1990, Radar Handbook, Second Edition McGraw-Hill, Chapter 7 [3] [4] [5] [6] [7] [8] [9] [10] Silent Sentry Passive Surveillance [11] [12] radar-technology-looks-to-the-future.html [13] [14] [15] [16] [17] [18] David M. Pozar, Microwave Engineering, Third Edition, Wiley [20] Synthetic Aperture Radar Imaging.pdf 7 Additional Information This application note is subject to improvements and extensions. Please visit our website to download new versions. Please send any comments or suggestions about this application note to TM-Applications@rohde-schwarz.com. 1MA207_0e Rohde & Schwarz Introduction to Radar System and Component Tests 28

29 About Rohde & Schwarz Rohde & Schwarz is an independent group of companies specializing in electronics. It is a leading supplier of solutions in the fields of test and measurement, broadcasting, radiomonitoring and radiolocation, as well as secure communications. Established more than 75 years ago, Rohde & Schwarz has a global presence and a dedicated service network in over 70 countries. Company headquarters are in Munich, Germany. Environmental commitment Energy-efficient products Continuous improvement in environmental sustainability ISO certified environmental management system Regional contact Europe, Africa, Middle East customersupport@rohde-schwarz.com North America TEST-RSA ( ) customer.support@rsa.rohde-schwarz.com Latin America customersupport.la@rohde-schwarz.com Asia/Pacific customersupport.asia@rohde-schwarz.com China / customersupport.china@rohde-schwarz.com This white paper may only be used subject to the conditions of use set forth in the download area of the Rohde & Schwarz website. R&S is a registered trademark of Rohde & Schwarz GmbH & Co. KG; Trade names are trademarks of the owners. Rohde & Schwarz GmbH & Co. KG Mühldorfstraße 15 D München Phone Fax