Radar scalloping
Encyclopedia
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.
Intra-pulse radar scalloping begins to become a concern above the following velocity.
Scalloping occurs at 150 kM/s for an L-band radar with a 1 microsecond pulse.
Radar scalloping for MTI radar begins to become a concern when the radial velocity is greater than the following value.
This occurs near multiples of 15 m/s for an L-Band radar with pulse repetition frequency
of 1kHz (10m/s to 20m/s, 25m/s to 35m/s, 40m/s to 50m/s, and so on).
involves blind velocities created by the clutter rejection filter. A two PRF detection schemes will have detection gaps with a pattern of discrete ranges, each of which has a blind velocity.
Scalloping happens in a two PRF detection scenario when target velocity produces a blind velocity for one PRF while the target is at the blind range of the other PRF.
The blind velocity for a specific pulse repetition frequency (PRF) is an integer multiple of the following, which causes the signal to have zero doppler.
The blind range for a specific PRF is an integer multiple of the distance between transmit pulses, which is when the reflected pulse arrives at the same time as when the transmitter fires.
This leaves a series of detection gaps at discrete velocity and range combinations.
These detection gaps are filled in by using three or more alternating PRF in the detection scheme.
An L-Band transmitter using a 3 kHz and 4 kHz PRF pair has the following characteristics.
This system would fail to detect reflections at 50kM and 100kM that are moving 600kM/s or 1,200kM/s.
It would also fail to detect reflections at 37.5kM and 75kM that are moving 450kM/s or 900kM/s.
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Moving target indicator (MTI) radar reduces the impact of blind velocity by using redundant detectors that introduce a compensating reverse phase shift undoes the phase shift caused by Doppler from reflector motion. This is also improved by using 3 or more pulses with staggering pulse intervals.
Pulse-Doppler radar
systems compensate using 3 or more different pulse repetition frequency
in the detection scheme.
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
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.
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.
Scalloping occurs at 150 kM/s for an L-band radar with a 1 microsecond pulse.
Inter-pulse Scalloping
Inter-pulse radar scalloping involves two types of systems.- Moving Target Indicator
- Pulse-Doppler Radar
Moving Target Indicator
Scalloping occurs when the radial velocity of the reflector induces an integer multiple of a 360 degree phase shift between two or more transmit pulses.Radar scalloping for MTI radar begins to become a concern when the radial velocity is greater than the following value.
This occurs near multiples of 15 m/s for an L-Band radar with pulse repetition frequency
Pulse repetition frequency
Pulse repetition frequency or Pulse repetition rate is the number of pulses per time unit . It is a measure or specification mostly used within various technical disciplines Pulse repetition frequency (PRF) or Pulse repetition rate (PRR) is the number of pulses per time unit (e.g. Seconds). It...
of 1kHz (10m/s to 20m/s, 25m/s to 35m/s, 40m/s to 50m/s, and so on).
Pulse-Doppler Radar
Scalloping for pulse-Doppler radarPulse-doppler radar
Pulse-Doppler is a 4D radar system capable of detecting both target 3D location as well as measuring radial velocity . It uses the Doppler effect to avoid overloading computers and operators as well as to reduce power consumption...
involves blind velocities created by the clutter rejection filter. A two PRF detection schemes will have detection gaps with a pattern of discrete ranges, each of which has a blind velocity.
Scalloping happens in a two PRF detection scenario when target velocity produces a blind velocity for one PRF while the target is at the blind range of the other PRF.
The blind velocity for a specific pulse repetition frequency (PRF) is an integer multiple of the following, which causes the signal to have zero doppler.
The blind range for a specific PRF is an integer multiple of the distance between transmit pulses, which is when the reflected pulse arrives at the same time as when the transmitter fires.
This leaves a series of detection gaps at discrete velocity and range combinations.
These detection gaps are filled in by using three or more alternating PRF in the detection scheme.
An L-Band transmitter using a 3 kHz and 4 kHz PRF pair has the following characteristics.
- 3 kHz blind ranges are 50kM, 100kM, ...
- 3 kHz blind velocities are 450kM/s, 900kM/s, ...
- 4 kHz blind ranges are 37.5kM, 75kM, ...
- 4 kHz blind velocities are 600kM/s, 1,200kM/s, ...
This system would fail to detect reflections at 50kM and 100kM that are moving 600kM/s or 1,200kM/s.
It would also fail to detect reflections at 37.5kM and 75kM that are moving 450kM/s or 900kM/s.
Compensation Strategies
Intra-pulse scalloping is improved by shortening the transmit pulse or by modulating the transmit pulse. This can include frequency or phase shifting associated with pulse compressionPulse compression
Pulse compression is a signal processing technique mainly used in radar, sonar and echography to increase the range resolution as well as the signal to noise ratio...
.
Moving target indicator (MTI) radar reduces the impact of blind velocity by using redundant detectors that introduce a compensating reverse phase shift undoes the phase shift caused by Doppler from reflector motion. This is also improved by using 3 or more pulses with staggering pulse intervals.
Pulse-Doppler radar
Pulse-doppler radar
Pulse-Doppler is a 4D radar system capable of detecting both target 3D location as well as measuring radial velocity . It uses the Doppler effect to avoid overloading computers and operators as well as to reduce power consumption...
systems compensate using 3 or more different pulse repetition frequency
Pulse repetition frequency
Pulse repetition frequency or Pulse repetition rate is the number of pulses per time unit . It is a measure or specification mostly used within various technical disciplines Pulse repetition frequency (PRF) or Pulse repetition rate (PRR) is the number of pulses per time unit (e.g. Seconds). It...
in the detection scheme.