1. Field of the Invention
This invention relates to bistatic rotor tip synthetic aperture radars and more particularly to a relay link for transferring wide band information to or from a moving rotor tip and a second location wherein the relative motion provides a doppler shift to the wide band information requiring compensation.
2. Description of the Prior Art
Rotor tip synthetic aperture radar (SAR) relates to radar mapping and surveillance systems in which transmit and/or receive apertures on a rotating radial arm utilize their motion to trace out long synthetic apertures in space from which radar signals can be sequentially transmitted and/or received so as to produce very fine angular resolution normal to the axis of rotation. Applications may be to helicopter rotors for navigation, obstacle avoidance, target detection, and other mapping functions and to fixed rotating radial arm towers for radar surveillance and ground testing and demonstration of synthetic aperture radar functions. Radar configurations may be monostatic with both transmission and reception from the rotating aperture or bistatic with only transmission or reception from the rotating aperture with the corresponding transmission or reception function performed by a fixed or non-rotating aperture.
One embodiment of a rotor tip SAR configuration may involve a radiating and/or receiving aperture at the tip of the radial arm, wave guide or coaxial transmission line down the arm, and microwave rotating joints through the arm rotation mechanism to the radar transmitter/receiver equipment at a fixed or non-rotating location. In applications to helicopters, especially, incorporation of rotating microwave joints in the transmission line down the blade or radial arm, may be constrained by considerations of flight performance and safety, and impose difficult and costly mechanical design problems. In addition, the long transmission line down the radial arm may impose a substantial loss in the signal level.
In communication systems where the transmitter is moving relative to the receiver, a doppler frequency shift according to equation 1 is observed. EQU f.sub.D =V f.sub.c /C (1)
where f.sub.D =the doppler frequency shift, V represents the relative frequency between the transmitter and receiving stations, f.sub.c represents the frequency of the signal coupling the stations and C represents the velocity of light. Methods to compensate for the doppler frequency shift in communication systems have been described such as, for example, in U.S. Pat. No. 3,182,131, which issued on May 4, 1965 to R. R. Barnes. In U.S. Pat. No. 3,182,131, a pilot signal source having a predetermined frequency such as 62 kilohertz is superimposed on a carrier signal of 6 gigahertz along with information in the range from 562 kilohertz to 1298 kilohertz and transmitted to a receiving station. The receiving station has a local oscillator which is used to remove the carrier signal of 6 gigahertz. The pilot signal and information signals pass through respective filters. The pilot signal as received with the doppler frequency shift is multiplied by a predetermined amount to provide the doppler frequency shift of the information signals in the band. The doppler frequency shift is combined with a predetermined frequency from an oscillator in a mixer, which is presented to a second mixer, where the doppler frequency shift for the center of the predetermined channel is subtracted from the signals in the predetermined band, and thereby provides the information with a reduced frequency shift which is centered about the midpoint of the predetermined frequency band. In U.S. Pat. No. 3,182,131, the pilot signal travels with the information signal superimposed on a carrier signal.
A doppler cancellation scheme is described in U.S. Pat. No. 3,325,736 wherein a signal is transmitted from a first station and received by a second station wherein it is processed to provide a signal with the frequency shift subtracted therefrom. The signal is then retransmitted from the second station to the first station wherein the doppler frequency shift added upon reception provides a signal where the first order of doppler frequency shift cancels. In U.S. Pat. No. 3,325,736, the transmitted and retransmitted frequencies may be sufficiently separated to permit continuous operation without feedback from the transmitting antenna to the receiving antenna of the same station. As shown in the drawing, each communication station must generate a predetermined frequency with respect to each other for proper operation.
In U.S. Pat. No. 3,450,842, a doppler frequency spread correction device is described which adds a doppler frequency to the base frequency.
It is therefore desirable to provide a relay link having one station in the rotor tip, and a second station mounted in a convenient structure, such as on the cab of a helicopter or on the supporting structure of the rotor for relaying radar related information from the rotor tip which may, for example, contain a receiver, to the cab of a helicopter which may, for example, contain a transmitter and signal processing equipment for processing a received signal.
It is further desirable to provide compensation for doppler frequency shifts arising from the relative motion of the first and second stations.
It is further desirable to transmit an auxiliary reference signal or pilot signal from the cab to the rotor tip and back to the cab where it may be used to cancel the one-way doppler frequency shift imposed on the radar data transferred from the rotor tip to the cab.
It is further desirable to phase lock the pilot signal with the radar signal being transmitted.
It is further desirable to offset in frequency the pilot signal being retransmitted at the rotor tip to the cab to facilitate simultaneous transmission and reception at both the cab and the rotor tip.