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.

Figure 1. Three Antenna Elements would be positioned to define a coordinate system for measuring the angles, q and f, that specify the direction of incidence of a radio signal.

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:

Image

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).

In the receiver (see Figure 2), the intermediate-frequency (IF) signals from the three antenna elements would be digitized, then processed through quadrature down-converters, then processed through fingers (defined in the next sentence) and combiners, the use of which would simplify the hardware needed to track the multipath components. As used here, "finger" signifies a time-multiplexed functional block that enables (at the expense of memory) the use of the same circuitry to track the signals arriving on multiple paths. Each finger would manage its own spreading-sequence timing and would perform its own multipath acquisition and tracking, despreading, and maximum-likelihood detection of orthogonal symbols. The combiners would provide the appropriate delays to align the symbols from the fingers for addition.

Figure 2. Signals From the Antenna Elements Would Be Processed to extract phase differences from modulation. The phase differences would be used to compute the angles q and f in Figure 1.

The multipath components would be resolved and the phase differences needed to compute the direction to the transmitter would be determined in a signal-processing scheme, the complexity of which admits of only a brief summary here: The scheme would involve noncoherent demodulation; that is, it would not rely on the generation, in the receiver, of a reference signal coherent with the phase of the carrier signal. Instead, it would rely on aligning the phases of local (receiver) PN code generators with the phases the transmitter PN code generators. The scheme would involve correlations of received modulation symbols with each and every one of the possible orthogonal symbols (of which there would be a total of 64 in the preferred version).

This work was done by G. Dickey Arndt, Phong Ngo, Henry Chen, and Chau T. Phan of Johnson Space Center; Brent Hill of Modem Links, LLC; Brian Bourgeois of Antech; and John Dusl of Lockheed Martin. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Computers/Electronics category.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Johnson Space Center, (281) 483-0837. Refer to MSC-23193