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USER’S MANUAL__________________________________________________________________

248 _________________________________________________________________ M211322EN-D

phase of the first trip return is different from the phase of the second trip

return. For a magnetron radar, the RVP900 measures the phase of the

transmitted pulse and the phase locking is done digitally as opposed to the

traditionally locking COHO. For a Klystron radar, the phase is controlled

by the RVP900 via a digital phase shifter that is precisely calibrated.

Typically the Klystron COHO is phase shifted so that each transmit pulse

has a different phase. The sequencing is controlled by the RVP900.

6.8.2 Algorithm

Figure 53 on page 250 shows a schematic of the data processing for

random phase. The figure shows the Doppler spectra for the first and

second trip in the various processing stages. The vertical scale is in dB and

the horizontal scale is velocity. In this example, the second trip echo is

shown as being stronger than the first trip echo (usually the reverse is true).

Ideal 1st and 2nd Trip Echoes

The ideal first and second trip echoes represent the echoes as they would

appear individually. The ideal 1st trip echo is the echo that would be

measured if there were no second trip echo interference. The ideal second

trip echo represents what would be measured if there were no 1st trip echo

interference. If there is no interference from the other trip, a standard

Klystron system can measure the ideal spectra, but there is no way to know

whether the echoes are in the first or second trip.

Raw 1st and 2nd Trip Echoes

This figure shows how the echoes from the first trip and second trip

interfere with each other. For the case of a standard magnetron system, the

first trip echo is coherent, while the second trip echo is incoherent (white

noise) since the phase of the second trip echo is random. This is because

the receiver is phase locked only to the first trip.

Another way to implement a magnetron system is to let the COHO free-

run (rather than phase locking to the transmit pulse), measure the phase of

each transmit pulse and digitally correcting for the transmit phase. Using

this digital phase locking technique, the RVP900 can phase lock or

"cohere" to either the first or the second trip.

Using this technique alone, it is possible to distinguish between 1st and 2nd

trip echoes for the case when the echoes are not overlapped. In other words,

the echoes will appear as the idealized first and second trip echoes. This

range de-aliasing effectively doubles the range of the radar. The problem

is that when echoes are overlapped, the noise contamination from the

stronger echo will make it impossible to measure the weaker echo. This is

illustrated in the figure. Thus if the first trip echo has a good signal-to-noise

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