CH 5: Radar Principles Flashcards
4 fundamental elements of a basic pulse radar system
Transmitter, the receiver, antenna, and synchronizer or master timer.
The transmitter, through the antenna, sends out a pulse of RF energy at a designated frequency. The presence of a target is revealed when the RF energy bounces off the target, returns to the radar antenna, and goes into the receiver. The master timer measures the time between the transmission of a pulse and the arrival of a target echo.
RF energy travels at the speed of..?
Basic radar range determination equation
Light (c) which is 3x10(8) m/s.
Target range= (measures time x c)/2
Radar mile definition
The round trip time for an RF wave to travel to and from a target one nautical mile away.
12.4 microseconds for a 1 NM (1853m) round trip.
Measured time=(target range x 2)/c
Second time around echo definition
Second time around echo occurs when a target echo associated with a particular radar pulse arrives at the antenna after another radar pulse has been transmitted.
The radar master timer always assumes the target echo is associated with the slat pulse transmitted. This makes the target echo ambiguous on range.
Range ambiguities caused by second time around echoes limit the maximum unambiguous range of a radar system.
Maximum unambiguous range equation
Maximum unambiguous range =PRT/12.4microseconds/nm
A radar system designed for long-range detection should transmit a radar signal with a large PRT. In addition, as the PRF of a radar signal increases, the PRT decreases, and the maximum unambiguous range decreases.
Range resolution definition
Pulse width definition
Range resolution equation
Range resolution is the ability of a radar to separate two targets that are close together in range and are at approximately the same azimuth.
Range resolution capability is determined by pulse width.
Pulse width is the time that the radar is transmitting RD energy. Measured in microseconds. A radar pulse in free space occupies a physical distance equal to the pulse width multiplied by the speed of light, which is 984 feet/microsecond.
If two targets are closer together than one-half of a pulse width or less, the radar cannot resolve the returns in range, and only one target will be displayed.
Range resolution= (pulse width x 984ft)/2
Beamwidth definition
Beamwidth of a radar system is the horizontal and vertical thickness of a the radar beam.
Beamwidth governs the azimuth and elevation accuracy and resolution capability of a radar system in the same way that pulse width governs radar range accuracy and resolution.
Azimuth determination based on..?
Azimuth determination is based on the position of the antenna when the target is being illuminated. If the antenna is referenced to true North, the azimuth of the target can be measured relative to true North.
Azimuth accuracy of a radar system is determined by horizontal beamwidth (HBW). The narrower the HBW, the better the azimuth accuracy.
Azimuth resolution definition and equation
Azimuth resolution is the ability of a radar to display two targets flying at approximately the same range with little angular separation, such as two fighters flying line-abreast tactical formation.
The azimuth resolution capability is usually expressed in nautical miles and corresponds to the minimum azimuth desperation required between two targets for separate displays.
Azimuth resolution depends on the HBW of the radar. A small HBW improves azimuth resolution.
Azimuth resolution = (HBW x range)/60
Altitude resolution definition and equation
Altitude resolution is the ability of a radar to display two targets flying at approximately the same range and azimuth with little altitude separation, use. As two fighters flying a vertical stack formation.
The altitude resolution is usually expressed in feet and corresponds to the minimum altitude separation required between two targets for separate display.
Altitude resolution= (vertical beamwidth x range)/60
Radar resolution cell definition
A radar’s pulse width, horizontal beamwidth, and vertical beamwidth form a 3D resolution cell (RC). This RC is the smallest volume of airspace in which a radar cannot determine the presence of more then one target. The RC of a radar is a measure of how well the radar can resolve targets in range, azimuth, and altitude.
The shorter the pulse width, the better the range resolution capability. The barrier the horizontal beamwidth, the better the azimuth resolution capability. The narrower the vertical beamwidth, the better the altitude resolution capability.
Pulse Doppler Radar Filters definition
The radar differentiates between the returning carrier frequencies and al mother harmonic frequencies by using clutter cancellers, or filters, at the known harmonic frequencies. The radar cannot process frequencies canceled by these filters. These filters create ‘blind speeds’ for the radar. The closer together the spectral lines(individual frequencies), the more ‘blind speeds’ the radar will have.
To separate the returning target frequency shifts from all other frequencies in the returning waveform, the pulse Doppler Radar employs filters to cancel the known harmonic frequency shifts and those with no frequency shift.
Power density definition and equation
Power density is the power of a radio wave per unit of area normal to the direction of propagation. The power density generated by a practical antenna can be expressed by:
Power density from antenna=(Pt x G)/(4pir2)
Pt=transmitted power
G= antenna gain
r= radius of the antenna
Power density at target = (Pt x G)/(4piR2)
R= range to target
Power density at antenna = (see textbook)
Maximum radar range definition
Maximum radar range (Rmax) occurs when the signal power density received just equals the minimum detect all signal (Smin) for the receiver.
See textbook for equation.
