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Pulse-repetition Frequency
The pulse-repetition frequency (PRF) is the number of pulses of a repeating signal in a specific time unit. The term is used within a number of technical disciplines, notably radar. In radar, a radio signal of a particular carrier frequency is turned on and off; the term "frequency" refers to the carrier, while the PRF refers to the number of switches. Both are measured in terms of cycle per second, or hertz. The PRF is normally much lower than the frequency. For instance, a typical World War II radar like the Type 7 AMES Type 7, GCI radar had a basic carrier frequency of 209 MHz (209 million cycles per second) and a PRF of 300 or 500 pulses per second. A related measure is the Radar signal characteristics#pulse width, pulse width, the amount of time the transmitter is turned on during each pulse. After producing a brief pulse of radio signal, the transmitter is turned off in order for the receiver units to detect the reflections of that signal off distant targets. Since the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radar
Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. 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 objects. Radio waves (pulsed or continuous) from the transmitter reflect off the objects and return to the receiver, giving information about the objects' locations and speeds. Radar was developed secretly for military use by several countries in the period before and during World War II. A key development was the cavity magnetron in the United Kingdom, which allowed the creation of relatively small systems with sub-meter resolution ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Doppler Effect
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842. A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession. The reason for the Doppler effect is that when the source of the waves is moving towards the observer, each successive wave crest is emitted from a position closer to the observer than the crest of the previous wave. Therefore, each wave takes slightly less time to reach the observer than the previous wave. Hence, the time between the arrivals of successive wave crests at the observer is reduced, causing an ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Look-down/shoot-down
A radar system has look-down/shoot-down capability if it can detect, track and guide a weapon to an air target that (as seen by the radar) is silhouetted against the ground. Problem and naming Airborne intercept radar relying exclusively on time domain radar techniques is effectively blind any time the radar's antenna is aimed towards the Earth's surface. That is because pointing the radar at the ground produces a large reflection. That reflection and the ensuing "cluttered" display overwhelms human operators and computing systems (see ground clutter). Radar systems of this type are essentially useless when pointed ‘down’ at the surface, with the zone of weakness near and below the horizon. This zone can be actively utilized by enemy combatants wishing to hide from radar tracking utilizing a technique known as terrain masking. Frequency domain signal processing combined with time domain signal processing, as in pulse-Doppler radar, is a way to eliminate that vulnerability. L ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Pulse-Doppler Radar
A pulse-Doppler radar is a radar system that determines the range to a target using pulse-timing techniques, and uses the Doppler effect of the returned signal to determine the target object's velocity. It combines the features of pulse radars and continuous-wave radars, which were formerly separate due to the complexity of the electronics. The first operational Pulse Doppler radar was in the CIM-10 Bomarc, an American long range supersonic missile powered by ramjet engines, and which was armed with a W40 nuclear weapon to destroy entire formations of attacking enemy aircraft. Pulse-Doppler systems were first widely used on fighter aircraft starting in the 1960s. Earlier radars had used pulse-timing in order to determine range and the angle of the antenna (or similar means) to determine the bearing. However, this only worked when the radar antenna was not pointed down; in that case the reflection off the ground overwhelmed any returns from other objects. As the ground moves at ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radar Scalloping
Scalloping is a radar phenomenon that reduces sensitivity for certain distance and velocity combinations. The name is derived from the appearance of areas that are scooped out of graphs that indicate radar sensitivity. Moving objects cause a phase-shift within the transmit pulse that produces signal cancellation. This phenomenon also has detrimental effect on moving target indicator systems, where the detection scheme subtracts signals received from two or more transmit pulses. Definition There are two different types of radar scalloping. * Intra-pulse * Inter-pulse Intra-pulse Scalloping This occurs when the radial velocity of the target induces a phase shift near 360 degree within the reflected pulse, which induces signal cancellation in the receiver. Intra-pulse radar scalloping begins to become a concern above the following velocity. :Velocity > \left (\frac \right) Scalloping occurs at 150 km/s for an L-band radar with a 1 microsecond pulse. Inter-pulse Scallopin ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Moving Target Indicator
Moving target indication (MTI) is a mode of operation of a radar to discriminate a target against the clutter. It describes a variety of techniques used for finding moving objects, like an aircraft, and filter out unmoving ones, like hills or trees. It contrasts with the modern stationary target indication (STI) technique, which uses details of the signal to directly determine the mechanical properties of the reflecting objects and thereby find targets whether they are moving or not. Early MTI systems generally used an acoustic delay line to store a single pulse of the received signal for exactly the time between broadcasts (the pulse repetition frequency). This stored pulse will be sent to the display along with the next received pulse. The result was that the signal from any objects that did not move mixed with the stored signal and became muted out. Only signals that changed, because they moved, remained on the display. These were subject to a wide variety of noise effects that ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Weather Radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the intensity of the precipitation. Both types of data can be analyzed to determine the structure of storms and their potential to cause severe weather. During World War II, radar operators discovered that weather was causing echoes on their screen, masking potential enemy targets. Techniques were developed to filter them, but scientists began to study the phenomenon. Soon after the war, surplus radars were used to detect precipitation. Since then, weather radar has evolved on its own and is now used by national weather services, research departments in universities, and in television stations' weather departments. Raw images are routinely used and special ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Frequency Ambiguity Resolution
Frequency ambiguity resolution is used to find the true target velocity for medium pulse repetition frequency (PRF) radar systems. This is used with pulse-Doppler radar. Definition Radial velocity aliasing occurs when reflections arrive from reflectors moving fast enough for the Doppler frequency to exceed the pulse repetition frequency (PRF). Frequency ambiguity resolution is required to obtain the true radial velocity when the measurements is made using a system where the following inequality is true. :Radial \ Velocity > 0.5 \left (\frac \right) The radial velocity measurements made in this way produce a modulo function of the true radial velocity. :Apparent \ Velocity = (True Velocity) MOD \left (\frac \right) Theory Radar pulsing causes a phenomenon called aliasing, which occurs when the Doppler frequency created by reflector motion exceeds the pulse repetition frequency (PRF). This concept is related to range ambiguity resolution. Doppler frequency shift is intr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
L-Band
The L band is the Institute of Electrical and Electronics Engineers (IEEE) designation for the range of frequencies in the radio spectrum from 1 to 2 gigahertz (GHz). This is at the top end of the ultra high frequency (UHF) band, at the lower end of the microwave range. Applications Mobile service In Europe, the Electronic Communications Committee (ECC) of the European Conference of Postal and Telecommunications Administrations (CEPT) has harmonized part of the L band (1452–1492 MHz), allowing individual countries to adopt this spectrum for terrestrial mobile/fixed communications networks supplemental downlink (MFCN SDL). By means of carrier aggregation, an LTE-Advanced or UMTS/HSDPA base station could use this spectrum to provide additional bandwidth for communications from the base station to the mobile device; i.e., in the downlink direction. In the Americas, mobile services are operated between the 1.7 GHz to 2.1 GHz range in the PCS and AWS bands. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Range Ambiguity Resolution
Range ambiguity resolution is a technique used with medium Pulse repetition frequency (PRF) radar to obtain range information for distances that exceed the distance between transmit pulses. This signal processing technique is required with pulse-Doppler radar. The raw return signal from a reflection will appear to be arriving from a distance less than the true range of the reflection when the wavelength of the pulse repetition frequency (PRF) is less than the range of the reflection. This causes reflected signals to be folded, so that the apparent range is a modulo function of true range. Definition Range aliasing occurs when reflections arrive from distances that exceed the distance between transmit pulses at a specific pulse repetition frequency (PRF). Range ambiguity resolution is required to obtain the true range when the measurements are made using a system where the following inequality is true. :\text > \left (\frac \right) Here ''c'' is the signal speed, which fo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Unambiguous
Ambiguity is the type of meaning in which a phrase, statement or resolution is not explicitly defined, making several interpretations plausible. A common aspect of ambiguity is uncertainty. It is thus an attribute of any idea or statement whose intended meaning cannot be definitively resolved according to a rule or process with a finite number of steps. (The '' ambi-'' part of the term reflects an idea of " two", as in "two meanings".) The concept of ambiguity is generally contrasted with vagueness. In ambiguity, specific and distinct interpretations are permitted (although some may not be immediately obvious), whereas with information that is vague, it is difficult to form any interpretation at the desired level of specificity. Linguistic forms Lexical ambiguity is contrasted with semantic ambiguity. The former represents a choice between a finite number of known and meaningful context-dependent interpretations. The latter represents a choice between any number of poss ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Laser Range Finder
A laser rangefinder, also known as a laser telemeter, is a rangefinder that uses a laser beam to determine the distance to an object. The most common form of laser rangefinder operates on the time of flight principle by sending a laser pulse in a narrow beam towards the object and measuring the time taken by the pulse to be reflected off the target and returned to the sender. Due to the high speed of light, this technique is not appropriate for high precision sub-millimeter measurements, where triangulation and other techniques are often used. Pulse The pulse may be coded to reduce the chance that the rangefinder can be jammed. It is possible to use Doppler effect techniques to judge whether the object is moving towards or away from the rangefinder, and if so, how fast. Precision The precision of the instrument is determined by the rise or fall time of the laser pulse and the speed of the receiver. One that uses very sharp laser pulses and has a very fast detector can range ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
