A proposed tracking receiver system containing three suitably positioned antenna elements and special signal-processing equipment would determine the direction of incidence of a microwave signal containing spread- spectrum digital data modulation. If the system were to contain two sets of antenna elements separated by a known baseline, it could determine the location of the transmitter as the intersection of the lines of incidence on the two antennas. Such systems could be used for diverse purposes in outer space and on Earth, including tracking astronauts and small robotic spacecraft working outside a spacecraft or space station, and locating cellular telephones from which distress calls have been made. The principle of operation does not require the transmission of a special identifying or distress signal by the cellular telephone or other transmitter to be tracked; instead, the system could utilize the data signal routinely sent by the transmitter, provided that the signal had the characteristics needed for processing as described below.
In its simplest form, the system would include three microstrip-patch antennas positioned to define an isosceles right triangle and a Cartesian coordinate system as depicted in Figure 1. The element at the origin of coordinates would be termed the reference element. The antenna elements on the x and y axes would both be separated from the reference element by a distance of half a wavelength at the carrier frequency of the signal to be tracked.
The basic equation for the phase of the incident signal can readily be manipulated to obtain the following equations for the colatitude (θ) and the azimuthal angle (Φ) that specify the direction of incidence:
where Φ1 is the difference between the signal phases at the x and reference antenna elements and Φ2 is the corresponding phase difference for the y and reference elements. The tracking problem thus becomes one of determining Φ1 and Φ2. In the presence of multipath interference and noise, the tracking problem is complicated by the need to compute a separate pair of phase differences and the corresponding direction for each path.
In a transmitter to be tracked by a preferred version of the proposed system, the baseband data to be conveyed would be orthogonally coded, then spread into two channels by use of two independent pseudonoise (PN) codes. The resulting baseband signals in the two channels would be used to modulate the microwave carrier signal by quaternary phase-shift keying (QPSK).