OTHER TYPES OF MTI

MTI AND PULSE DOPPLER RADAR

4.12 OTHER TYPES OF MTI

Two-frequency MTI. ⁷⁸· 79

The first blind speed of an MTI radar is inversely proportional to the carrier frequency, as described by Eq. (4.8). This can result in the appearance of many blind speeds in conventional MTI radar that operate at the higher microwave frequencies. One of the methods sometimes suggested for increasing the first blind speed is to transmit two carrier frequencies./~ and

J;i +

llf and extract the difference frequency

llf

for MTI processing. The resulting blind speeds will be the same as if the radar transmitted the difference frequency rather than the carrier. For example, if

llf

=

O.lf

0 , the first blind speed corresponding to the difference frequency is 10 times that of an MTI radar at the carrier frequency

Jo.

Thus, it would seem that the advantages of a VHF or UHF MTI might be obtained with radars operating at the higher microwave frequencies. A two-frequency MTI transmits a pair of pulses, either simultaneously or in close sequence, at two separate carrier frequencies. The two received signals are mixed in a nonlinear device and the difference frequency is extracted for normal

MTI signal processing.

The advantage of the greater first blind speed obtained with the two-frequency MTI is accompanied by several disadvantages.⁷³ H the ratio of the two frequencies is ,. < l, the standard deviation of the clutter doppler spectrum a, for a single-frequency MTI is increased to rr,( 1

+

r² )1

'2 in a two-frequency MTI. (This assumes that the clutter-velocity spectrum width a,. is the same for both carriers, or a₁

=

^ra2 where _{11 1} and _{11 2} are the clutter doppler- frequency spreads. This results in less improvement factor for a two-frequency MTI as compared with a single-frequency MTI. Note also that the clutter doppler spread of a radar which actually radiated a carrier frequency !if would be less than that of a radar at carrier frequency

.f~. by

the amount 1!,.f!f~. The two-frequency MTI might have the blind speeds of a

radar at the difference frequency but it has none of its other favorable clutter characteristics.

Although the first blind speed is greater in a two-frequency MTI, there may he deeper nulls than one might desire in the doppler response characteristic, just as there would be in a staggered MTI with only two pulse repetition frequencies. The two-frequency MTI has the advantage of being less sensitive than a single-frequency MTI to a mean clutter-doppkr- frequency other than de, assuming the single-frequency MTI employs no compensation such as TACCAR. This also results, however, in the loss of detection of targets with low doppler- frequency shift that otherwise would have been detected with the single-frequency MTI.

In general, the two-frequency MTI does not offer any obvious net advantage over properly designed single-frequency MTI systems for most MT[ radar applications.

Area MTI. This form of MTI does not use doppler information directly as do the other MTI techniques discussed in this chapter. The early area MTI systems stored a co(\lplete scan of radar video in a memory, such as a storage tube, and subtracted the stored video scan to scan.

Instead of subtracting successive scans, the subtraction can be on a pulse-to-pulse basis with much less memory required, if a short' pulse is used.⁷⁷ The pulse widths required for aircraft detection are of the order of nanoseconds. This technique relies on the fact that the echoes from moving targets change range from pulse to pulse and those from stationary and slowly moving targets do not. The short-pulse area MTI has no blind speeds and can be designed to have no range ambiguities. It is more attractive for application at the higher microwave frequencies where the available bandwidths are large and the normal MTI suffers from exces- sive blind speeds.

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