As stated earlier , radar beam width play a vital role in their angular accuracy characteristics because as long as targets stay within the radar beam, there will be reflection, the problem is if several targets fly close enough that their angular separation is smaller than the radar beamwidth, all the return echoes will be blended into one return, and radar will only display a single target on screen.
To display two distinct radar returns of 2 target close to each other, radar beam needs to be able to pass between them without causing a return.
Elevation-azimuth resolution is the ability of a radar to display two targets flying at approximately the same range with a certain angular separation, such as two fighters flying line-abreast tactical formation.
The elevation-azimuth resolution capability is usually expressed in nautical miles and corresponds to the minimum angular separation required between two targets for separate display.
Angular resolution in nautical miles (1.852km) can be estimated by equation below
It important to remember that a radar vertical beamwidth is not necessary the same as it’s horizontal beamwidth. Hence, the azimuth and elevation resolution may be different.
Range Resolution
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. The range resolution capability is determined by pulse width.
A radar pulse in free space occupies a physical distance equal to the pulse width multiplied by the speed of light, which is about 984 feet per microsecond.
If two targets are closer together than one-half of this physical distance, the radar cannot resolve the returns in range, and only one target will be displayed.
The range resolution of the radar is usually expressed in feet and can be computed using the equation below. It is the minimum separation required between two targets in order for the radar to display them separately on the radar scope so smaller value for range resolution is desirable
As explained earlier, the longer the pulse, the worse the resolution would be. Range resolution is proportional to pulse width and inverse proportional to bandwidth
Shorter pulse will improve range resolution but will also reduce the power of the transmitted radar wave, thus reduce radar detection range.
To improve radar detection range and range resolution at the same time, a technique called pulse compression was invented.
Pulse compression
Pulse compression comes from the need to have large enough pulse without sacrifice range resolution. The basic principles of pulse compression are simple.
Instead of transmitting a square pulse with the same characteristic from start to finish, radar instead transmits very long pulses that can be divided into several sub pulses after matched filtering.
Thus, radar range resolution would then depend on the length of the sub pulses rather than total pulse length. Pulse compression can be either phase or frequency coded. One side benefit of code compression is that it makes radar much more resistance to jamming.
One of the first form of pulse compression is chirp compression (also known as linear frequency modulated pulse) where the frequency of pulse either increase or decrease with time.
Because chirps compression is predictable for ECM and often require very wide bandwidth, another form of pulse compression called binary phase coded compression were invented. In phase coded compression, the phase is used to distinguish sub pulses instead of the frequency.
It is important to remember that chirp and binary phase coded pulse compression are only the two most basics forms of pulse compression, modern radar use various different kind of pulse compression such as Pseudo random noise sequences, Polyphase, Quadriphase, Costas code, Welti codes, Huffman code.
Comparison between phase coded vs frequency coded pulse compression:
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