PROCESS OF AND APPARATUS FOR PROVIDING FILTER DISCRIMINATION BETWEEN RECEIVED DIRECT IMPULSES AND DELAYED PULSES AS IN PULSE NAVIGATION AND RELATED APPLICATIONS

摘要

1508372 Radio navigation INTERNATIONAL NAVIGATION CORP 24 March 1975 12152/75 Heading H4D An arrangement for suppression of multipath signals received in a LORAN-C receiver comprises a composite filter Fig. 2 comprising 3-pole 30 kHz wide band elliptical filter A which has a frequency response curve A in Fig. 1 which is linearly combined through amplifier 1, which it precedes so as to minimize saturation of the amplifier by unwanted proximal frequency sources, to 2-pole synchronous 40 kHz filter B which is tuned to have bell-shaped, substantially Bessel function response of greater centre frequency amplitude response than filter A, of wider bandwidth, ultimately intersecting and having greater attenuation than filter A at about 155 and 45 kHz. A tap-off is taken from the first pole B1 of the filter element B, to an envelope differentiator 3, the receiver noise, which is introduced thereat being alleviated by inclusion of a further pole filter B2SP1/SP, which is chosen to match the characteristics of second pole filter B2 which follows amplifier 2 in the main chain. It is stated that the resultant frequency response curve R, Fig. 1, the steep portion of which drops steeply much as response A provides adequate separation between sky-waves and ground waves of 30Ás delay while eliminating the outer band Decca frequencies (73 and 129 kHz) and the signals from more remote frequency high power signals in the vicinity.