Classification and application of radar

电子科技大学 格拉斯哥学院 2017级 梅渊宗
Brief introduction of Radar
Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna (often the same antenna is used for transmitting and receiving) and a receiver and processor to determine properties of the object(s). Radio waves (pulsed or continuous) from the transmitter reflect off the object and return to the receiver, giving information about the object’s location and speed.
Radar was developed secretly for military use by several nations in the period before and during World War II. A key development was the cavity magnetron in the UK, which allowed the creation of relatively small systems with sub-meter resolution. The term RADAR was coined in 1940 by the United States Navy as an acronym for Radio Detection And Ranging or Radio Direction And Ranging. The term radar has since entered English and other languages as a common noun, losing all capitalization.
The modern uses of radar are highly diverse, including air and terrestrial traffic control, radar astronomy, air-defence systems, antimissile systems, marine radars to locate landmarks and other ships, aircraft anti-collision systems, ocean surveillancesystems, outer space surveillance and rendezvous systems, meteorological precipitation monitoring, altimetry and flight control systems, guided missiletarget locating systems, ground-penetrating radar for geological observations, and range-controlled radar for public health surveillance. High tech radar systems are associated with digital signal processing, machine learning and are capable of extracting useful information from very high noise levels.
Common radar types
1.Continuous wave radar
Fixed frequency CW radar systems can be used to measure speed. However, it does not provide any distance information. An antenna transmits a signal at a fixed frequency. The signal reflected back from a moving target, such as a car, creates a Doppler shift. That is, it receives a reflected signal at a slightly offset frequency. By comparing the frequency of the incoming and outgoing signals, we can determine the radial velocity (rather than the distance) of the target. Based on this principle, a typical application is traffic monitoring radar.Radar mobile sensors are based on the same principle, but they must also be able to detect slowly changing field intensitudes due to the potential for varying interference environments. The speed-traps, which traffic police use, USES the same technology. If a target at a given distance over-speeds the speed limit, the camera takes a picture.
Military applications:
Continuous wave radar is also used for target radar beam irradiation. This is a simple application: using a target tracking radar, the radar beam remains on the target. Guided antiaircraft missiles use the reflection of this target.
Continuous wave radars are difficult to detect, so they are classified as low probability of interception radars.
Continuous wave radar is ideal for low-flying aircraft that try to overcome enemy defenses by flying close to the ground. Pulse radar is difficult to distinguish ground echoes from reflections from low-altitude aircraft. Continuous wave radar overcomes this because it can ignore slowly changing ground echo surfaces and accurately locate only the reflected signals of moving targets. The captured information can then be transmitted to the cooperative pulse radar for further analysis and processing.
FM continuous wave radar
The disadvantage of continuous wave radar systems is that they cannot be used to measure distances due to the lack of a time reference. However, the distance of a stationary target can be measured using the time reference generated by an FM cw radar. The principle of this method is as follows: sending a signal with periodic frequency variation, when the street receives the echo signal, it will get a delay similar to that of pulse radar. The delay can be determined by comparing the frequency of sending and receiving signals so as to obtain the distance. More complex FM modes (such as noise radar) can be used to obtain the maximum fuzzy-free measurement distance within the same repetition cycle. However, the simplest case is to use the basic saw-tooth or triangular wave frequencies, which can only obtain relatively small fuzzy measurement distances.
Such ranging principles have the following applications: for example, to measure altitude on an aircraft (a wireless altimeter) or to maintain it with ground-tracking radar
Such ranging principles have applications such as measuring altitude on an aircraft (a wireless altimeter) or using ground-tracking radar to maintain a fixed altitude above the ground. Compared with pulse measurement radar, it has the advantage of providing continuous measurement results (relative to discrete time at various pulse repetition frequencies).
Frequency-modulated continuous wave radar is also commonly used in other civil ranging applications such as position indicators.
Pulse and pulse Doppler radar
A simple pulse radar system can only provide the distance information of the target by measuring the time difference between pulse sending and receiving, and it cannot determine the speed of the target. The pulse width determines the spatial resolution.
When receiving a pulse, each instantaneous rotating antenna points to a specific direction of radiation, and therefore direction information (phi with respect to azimuth) is available. The main tests of such (incoherent) radar equipment include range accuracy and resolution, receiver automatic gain control (AGC) processing time, peak power, frequency stability, local oscillator phase noise, and all pulse parameters.
Pulse Doppler radar
In addition to providing target distance information (and direction information), pulse Doppler radar also provides target radial velocity information. When the radar transmitter and receiver work together, the velocity information can be obtained from the phase change between pulses. I/Q demodulation is usually used. In order to avoid ambiguity of distance and speed, the latest pulse Doppler radar adopts the technology of variable pulse heavy frequency (PRF) as required, and the pulse repetition frequency generally varies from several hundred Hz to 500 KHz.
In addition, the more advanced pulse Doppler radar system USES a “staggered” pulse repetition frequency (PRF), which alternates the pulse repetition frequency according to the detection process. To achieve high performance of pulse Doppler system, very low LO phase noise, low receiver noise and low I/Q gain phase unbalance (to avoid false target information) are required.
Pulse compression radar
Traditional pulse radar and pulse doppler radar, in order to obtain high range resolution, need to send very short pulse, but short pulse means that the transmitted signal energy is low, the action distance is reduced. Increasing the pulse power can increase the operating distance, but the improvement of transmitting power is very limited, and the cost will be very high. There is a contradiction between the long acting distance and the high resolution.
The pulse compression system USES the modulation within the pulse to solve the contradiction between the range and the range resolution to a large extent. The advantages of large range provided by wide pulse and high resolution provided by short pulse are fully utilized. And can use low pulse power.
By modulating the pulses, a temporal reference is established between them, similar to the case of FMCW. Common modulation methods:
• linear frequency modulation
• nonlinear frequency modulation
• pulse phase coding
• multiphase modulation and time-frequency coding modulation
Although pulse compression radar has the advantages of long operating distance and high resolution at low pulse power, it also has an obvious shortcoming. The shortest operating distance is limited by pulse width, and the receiver is blocked during pulse transmission time. In the application of air traffic control, due to the shortage of pulse compression radar, two technologies are often used. Frequency modulation pulse is used in the long distance, while very short pulse is used in the short distance. However, large transmitting power is not required in the short distance.
-linear frequency modulation is the most widely used;

• non-linear frequency modulation, despite its many advantages, has been rarely used so far;
• pulse phase coding is widely used, especially Barker code (Barker) modulation with length of 11 and 13 bits;
• in advanced military radar systems, the application of specially coded multiphase modulated pulse compression technology is gradually increasing.
  Agile frequency conversion radar (FAR) (anti-jamming and clutter suppression)
  Frequency hopping is an effective method for radar systems to deal with jamming and electronic countermeasures (ECCM).
  FAR also has the function of clutter suppression. Typical parameters: switching time less than 1us, hopping bandwidth of hundreds of MHz in X band, and hopping bandwidth of 2GHz in W band (95GHz).
  Other far-related measurement parameters include frequency switching time, frequency hopping sequence, switching spurious and wideband amplitude phase stability.
  Step frequency hopping radar
  Stepping frequency hopping radar is commonly used in imaging applications. The frequency band width ranges from hundreds of MHz to 2GHz, and the resolution reaches 10cm.
  Pulse to pulse, the frequency changes in a fixed step. A typical application has a jump period consisting of 128 pulses. The advantage of stepping frequency hopping is that the frequency hopping within the broadband range can obtain very wide bandwidth, thus obtaining high resolution without a large instantaneous bandwidth.
  Because transmitter and receiver require large rf bandwidth, these subsystems must have very good amplitude and phase stability to obtain high resolution. Therefore, it is very important to measure the amplitude phase stability between pulses. Just like the agile frequency radar (FAR), the setting time of local vibration in the jump process is also an important measurement parameter.
  Moving target indicating radar (MTI)
  The basic idea of active target indicating radar (MTI) is to suppress the reflection of fixed or slow moving targets, such as buildings, mountains, clouds, water waves and other clutter, so as to obtain the reflection and indication of moving targets, such as flying objects and vehicles. At this point, due to the doppler effect, there is a frequency difference between the target echo moving radially relative to the radar and the transmitter frequency, which is proportional to the relative radial velocity (for LFM radar). For the pulse radar system, the phase change between the moving target echo and the transmitted signal is generated.
  The optimal application of MTI requires some experience, such as staggered PRF (the time interval between pulses can change according to a certain law) to avoid the so-called blind speed. For optimized MTI or clutter suppression, important measurement parameters include: amplitude phase stability between transmitted signal pulses and pulses; The phase noise and high stability of local oscillator signal are especially important for the detection of slow moving targets.
  Phased array radar
  Unlike reflection antenna, which has only one radiation unit, phased array radar antenna has hundreds or even thousands of independent radiation units. The amplitude and phase of signals fed to each radiation unit can be independently controlled, so that any desired radiation direction shape (direction diagram) can be obtained. In practical application, the direction of radiation can be within plus or minus 60 °. Different from the traditional mechanical scanning antenna, the adjustment of the directional pattern of phased array antenna is realized by changing the feeding amplitude and phase of each element, which requires very short time and almost no delay. Array control cost is very high, mainly used in military field and synthetic aperture radar (SAR) satellite applications. Active phased array (ASEA) each radiating unit has its own emission/reception module (T/R), while passive phased array (PESA) USES common
  With the same RF signal, each unit is adjusted by a phase shifter.
  For ASEA, the amplitude-phase consistency of different T/R modules is very important, which requires accurate testing and measurement.
  Mono-pulse radar
  In mono-pulse radar system, at least two sets of antennas separated by space are needed. By comparing and differentiating channels, the target within the irradiation range of radar beam can be located. Of left and right channels coupled to form differential channels (Δ Az), namely the azimuth difference channel. In the direction pointing, and the channel get the maximum, and the difference between the channel minimum. Due to poor and channel (Σ) and channel (Δ) in a pulse echo can get the result. Therefore, one pulse is sufficient to calculate the target position. This combination of antennas is often referred to as a mono-pulse antenna. The ratio of and channel to difference channel provides the deviation degree of the actual target direction and antenna axis (" forward view "). The difference between the antenna forward view and the actual azimuth of the target is known as “forward view deviation Angle”.
  Three-dimensional radar system, the pitching Angle measurement using the same technology, the need for a pitch difference channels as the second differential (Δ EI). The consistency between channels is very important for multi-channel systems such as object mono-pulse radar. For this reason, the phase deviation of phase parameter synthesizer is usually required to be adjustable.
  Phased array radar
  Unlike reflection antenna, which has only one radiation unit, phased array radar antenna has hundreds or even thousands of independent radiation units. The amplitude and phase of signals fed to each radiation unit can be independently controlled, so that any desired radiation direction shape (direction diagram) can be obtained. In practical application, the direction of radiation can be within plus or minus 60 °. Different from the traditional mechanical scanning antenna, the adjustment of the directional pattern of phased array antenna is realized by changing the feeding amplitude and phase of each element, which requires very short time and almost no delay. Phased-array cost is very high, mainly used in the military field and synthetic aperture radar (SAR) satellite applications. Active phased array (ASEA) each radiating unit has its own emission/reception module (T/R), while passive phased array (PESA) USES common
  With the same RF signal, each unit is adjusted by a phase shifter.
  For ASEA, the amplitude-phase consistency of different T/R modules is very important, which requires accurate testing and measurement.
  Synthetic aperture radar (SAR)
  Synthetic aperture radar (SAR), like real aperture radar (RAR), is an imaging radar. Such radar systems are installed on airborne or satellite-borne platforms and scan the earth’s surface with electromagnetic waves to obtain two-dimensional images of the ground. The basic principle of SAR is that it contains an antenna that moves Along a path perpendicular to the radiation direction, whose position is known throughout the whole process. The movement direction is usually called “Along Track” or azimuth direction, while the corresponding direction perpendicular to the movement is called “Cross Track”. A “footprint” is an area of real aperture exposure, and a “swath” is a swath of swath swept along the moving direction. SAR enables the radar to move in orbit and transmit radar signals at a certain repetition frequency, integrating signals from different positions continuously and increasing the time bandwidth product. It can be equivalent to that the antenna length increases in the direction of motion, the equivalent beam Narrows and the resolution improves. In the range direction, SAR signals can still use broadband signals to obtain high resolution. The resolution along the moving direction can reach half the size of the real antenna. The length of the real antenna is reduced by half and the resolution is improved by twice. If a resolution of 1m is required, the signal bandwidth is 150MHz. Modern SAR has a bandwidth greater than 1GHz (sometimes 2GHz bandwidth is required) and a resolution less than 10cm. Signal bandwidth is usually obtained by pulse compression, such as linear frequency modulation. The more advanced SAR USES stepping frequency hopping, polarization switches, and other complex techniques.
  Bi-static/Multi-modal radar
  In most cases, radar transmitter and receiver use the same antenna to realize multiplexing through time switching. This type of radar is called “single-base radar”. Bistatic radar has one transmitter and one or more receivers at another location. A very long distance or large space Angle between transceiver antennas. Single-base radar can easily form a multi-base radar by adding additional receivers. Alternatively, two single-base radars operating at the same frequency may form a multi-base radar. The multi-base radar transceiver part is far away or has a large space Angle. This means that in some cases, when the signal cannot be received by the single-base radar due to target reflection and other reasons, the multi-base radar can still receive the signal. Therefore, this kind of radar is often used in weather radar and military anti-stealth radar. When a system USES multiple distributed receivers, we call it a multi-base radar.
  Low interception rate radar
  Low interception rate (LPI) radar is a military radar system developed for electronic warfare environment. More or less, this LPI radar eludes detection by the electronic intelligence system (ELINT). LPI radar adopts the following technologies:
  • multi-base radar
  • ultra-low sidelobe antenna
  • uwb signal
  • long pulse
  Low power,
  • passive radar
  Multimode radar
  Nowadays, many military radar systems need to complete a large number of tasks, so they must adopt a variety of modes.
  • target search and tracking
  • weapons guidance
  • high-resolution ground mapping
  • severe weather forecast
  • electronic countermeasures
  Different pulse repetition frequency (PRF) and modulation modes are used in these applications. Frequency modulation pulse (Chirp), Barker code (Barker) modulation and complex modulation, AESA antenna, SAR, frequency hopping, polarization change, etc. Measure multiple mode thunder like this
  The system is complex and expensive.
  Future radar technology prospect
  In the future, we look forward to seeing multi-sensor systems including radar and infrared systems that closely link their strengths to overcome their specific weaknesses.
  Military airborne radar systems will continue to face the threat of stealth attacks by advanced fighters. In the future, a fighter plane must have both stealth attack function and can not expose itself when using airborne radar, which is a contradiction that must be solved. The solution to this problem may be to use bistatic radar, which means only transmitters or receivers on the aircraft. The radar antenna will no longer be a separate unit in the hood, but conformal to the geometry of an aircraft (ship or other platform). The next generation AESA airborne radar system will have multiple arrays of antennas to gain greater spatial scanning angles. The speed of the radar data processing part will be greatly improved (through parallel processing) to adapt to the processing of higher data rate, so as to obtain higher resolution.