29-06-2012, 11:34 AM
RADAR SIGNAL PROCESSING
[attachment=26045]
Carrier
The carrier is an RF signal, typically of microwave frequencies, which is usually (but not always) modulated to allow the system to capture the required data. In simple ranging radars, the carrier will be pulse modulated and in continuous wave systems, such as Doppler radar, modulation may not be required. Most systems use pulse modulation, with or without other supplementary modulating signals. Note that with pulse modulation, the carrier is simply switched on and off in sync with the pulses; the modulating waveform does not actually exist in the transmitted signal and the envelope of the pulse waveform is extracted from the demodulated carrier in the receiver. Although obvious when described, this point is often missed when pulse transmissions are first studied, leading to misunderstandings about the nature of the signal.
Pulse repetition frequency (PRF)
In order to build up a discernible echo, most radar systems emit pulses continuously and the repetition rate of these pulses is determined by the role of the system. An echo from a target will therefore be 'painted' on the display or integrated within the signal processor every time a new pulse is transmitted, reinforcing the return and making detection easier. The higher the PRF that is used, then the more the target is painted. However with the higher PRF the range that the radar can "see" is reduced. Radar designers try to use the highest PRF possible commensurate with the other factors that constrain it, as described below.
Staggered PRF
Staggered PRF is where the time between interrogations from radar changes slightly. The change of repetition frequency allows the radar, on a pulse-to-pulse basis, to differentiate between returns from itself and returns from other radar systems with the same frequency. Without stagger any returns from another radar on the same frequency would appear stable in time and could be mistaken for the radar's own returns. With stagger the radar’s own targets appear stable in time in relation to the transmit pulse, whilst the 'jamming' echoes are moving around in time (uncorrelated), causing them to be rejected by the receiver.
Clutter
Clutter (also termed ground clutter) is a form of radar signal contamination. It occurs when fixed objects close to the transmitter—such as buildings, trees, or terrain (hills, ocean swells and waves)—obstruct a radar beam and produce echoes. The echoes resulting from ground clutter may be large in both areal size and intensity. The effects of ground clutter fall off as range increases usually due to the curvature of the earth and the tilt of the antenna above the horizon. Without special processing techniques, targets can be lost in returns from terrain on land or waves at sea.
Clutter is used by the military to jam radars by the use chaff. Chaff is small reflective material used to hide troop, ship, or aircraft movements by creating many returns and overwhelming the radar's receiver with spurious targets.
Sensitivity time control (STC)
STC is used to avoid saturation of the receiver from close in ground clutter by adjusting the attenuation of the receiver as a function of distance. More attenuation is applied to returns close in and is reduced as the range increases.
[attachment=26045]
Carrier
The carrier is an RF signal, typically of microwave frequencies, which is usually (but not always) modulated to allow the system to capture the required data. In simple ranging radars, the carrier will be pulse modulated and in continuous wave systems, such as Doppler radar, modulation may not be required. Most systems use pulse modulation, with or without other supplementary modulating signals. Note that with pulse modulation, the carrier is simply switched on and off in sync with the pulses; the modulating waveform does not actually exist in the transmitted signal and the envelope of the pulse waveform is extracted from the demodulated carrier in the receiver. Although obvious when described, this point is often missed when pulse transmissions are first studied, leading to misunderstandings about the nature of the signal.
Pulse repetition frequency (PRF)
In order to build up a discernible echo, most radar systems emit pulses continuously and the repetition rate of these pulses is determined by the role of the system. An echo from a target will therefore be 'painted' on the display or integrated within the signal processor every time a new pulse is transmitted, reinforcing the return and making detection easier. The higher the PRF that is used, then the more the target is painted. However with the higher PRF the range that the radar can "see" is reduced. Radar designers try to use the highest PRF possible commensurate with the other factors that constrain it, as described below.
Staggered PRF
Staggered PRF is where the time between interrogations from radar changes slightly. The change of repetition frequency allows the radar, on a pulse-to-pulse basis, to differentiate between returns from itself and returns from other radar systems with the same frequency. Without stagger any returns from another radar on the same frequency would appear stable in time and could be mistaken for the radar's own returns. With stagger the radar’s own targets appear stable in time in relation to the transmit pulse, whilst the 'jamming' echoes are moving around in time (uncorrelated), causing them to be rejected by the receiver.
Clutter
Clutter (also termed ground clutter) is a form of radar signal contamination. It occurs when fixed objects close to the transmitter—such as buildings, trees, or terrain (hills, ocean swells and waves)—obstruct a radar beam and produce echoes. The echoes resulting from ground clutter may be large in both areal size and intensity. The effects of ground clutter fall off as range increases usually due to the curvature of the earth and the tilt of the antenna above the horizon. Without special processing techniques, targets can be lost in returns from terrain on land or waves at sea.
Clutter is used by the military to jam radars by the use chaff. Chaff is small reflective material used to hide troop, ship, or aircraft movements by creating many returns and overwhelming the radar's receiver with spurious targets.
Sensitivity time control (STC)
STC is used to avoid saturation of the receiver from close in ground clutter by adjusting the attenuation of the receiver as a function of distance. More attenuation is applied to returns close in and is reduced as the range increases.