Variable pulse repetition frequency for the GPM dual-frequency radar

In space missions, the delay of a receiving signal to a transmission signal generally spans a few thousands microseconds due to long range observation. An efficient sampling can be achieved by transmitting pulses successively, as schematically shown in Fig. 1. For the later consideration, a catching integer n is to be introduced. In Fig. 1, the catching integer n is set at 11, for which a pulse transmitted on time 0 is received between the 10th and 11th pulse transmissions, and a pulse on time 1 is received between the 11th and 12th pulses, and so forth. In the GPM as well as the TRMM, the range varies from one swing angle to another, accompanied with the beam scanning. Consequently, even though the received signal for a small swing angle is received properly in-between transmissions, the received signal for a large swing angle such as 15◦ will be received with a longer delay time, and may be received in conflict with transmission. Furthermore the orbital altitude of a satellite varies due to the relation of the orbital shape to the oblateness of the Earth. These issues can be resolved by adopting a variable pulse repetition frequency (VPRF) [2],[3],[4]. In this paper, several VPRF operations are proposed for the GPM.