PRF ( Pulse Recurrence Frequency ) is the number of pulses trasmitted in a second.
PRF is one of the main factors that determine the maximum theoretical unambiguous range of a pulse radar.
The trasmitter must remain “silent” while the receiver is “listening” for echoes.
If the design maximum range is 200NM the receiver must be allowed to “listen” for the period of time fro when a pulse has been trasmitted untill it can go to 200NM and than return.
That is a round trip of 400 NM, which would require a longer “silent period” than a radar whit a range of 100NM where the pulses have to travel only 200NM.
Therefore we can say that a lower PRF enables a longer maximum theoretical range: max therorical range is inversly proportional to the PRF.

The following formula may be used to calculate the max theoretical unambiguous range :

Range (in Km) = 300 000 ÷ ( PRF x 2 ), or if we need to find the required PRF based on a specific unambiguous range needed, we can rearrange the formula:
PRF = 300 000 ÷ ( Range (in Km) x 2)

In our case we can apply the second one.
1 NM = 1.852 km so
200 x 1.852 = 370.4 Km

PRF = 300 000 ÷ ( 370.4 x 2 )
PRF = 404.97 pps = approx. 405 pps