Abstract: A novel maximum likelihood sequence detector is provided for decoding linear cyclic error correction codes. The detector comprises one cyclic correlator for each two shaft sets of the code which have weights greater than one, and a serial correlator for detecting the shift sets of all zero's and all one's. The number of cyclic correlators required to decode linear codes is reduced to less than half the number of shift sets which define all the codewords instead of half the number of codewords where the number of shift sets is always less than the number of codewords.

Abstract: An accurate radio frequency ranging system is provided for measuring the time required for a signal to be transmitted from a ground station to a remote station and to be returned to the ground station. The ground station is provided with a master reference clock that is employed to drive a first pseudonoise generator. The ranging system in the ground station is started by a unique and predetermined start epoch signal produced by the first pseudonoise generator. The first pseudonoise generated signals are transmitted to the remote station where they are tracked and sensed. When the original start epoch signal is sensed at the remote station, it is employed as a trigger or read signal to initiate the generation of a stop epoch signal. The stop epoch signal is retransmitted to the ground station and stored in a register where it is compared with pseudonoise generated signals being retransmitted from the remote station to the ground station.

Abstract: A non-coherent receiving system is provided with a novel triple-bit matched filter detector system which provides two outputs indicative of the data stream being detected. A preliminary or soft decision is made concerning the state of the bits in the data stream. One of the outputs provides a best estimate of state of the data bit being detected and the other output provides the second best estimate of the state of the data bits being detected. The incoming data stream is processed and modified as it is shifted through a plurality of shift registers in real time employing the data bits subsequently detected. A controller is provided to make a plurality of predetermined evaluations of the state of the bits in the data stream and to provide a final or hard decision of the correct state of the data bits previously detected.

Abstract: In a digital data acquisition system, there is provided a coherent direct sequence spread spectrum receiving system. The coherent receiving system comprises an independent carrier tracking loop and three independent branches for detecting and locking on to a received psuedonoise data signal. The psuedonoise data signal being transmitted is provided with a time slot or discrete time portion during which no data is modulated onto the carrier. The carrier signal which is not phase modulated with digital data is tracked by the tracking loop so that the absolute phase of the carrier is never lost. The psuedonoise data signals are now recoverable in a coherent direct sequence spread spectrum receiver without the requirement of absolute phase determination or non-coherent detection.

Abstract: A ranging apparatus is provided having an encoded waveform signal which is generated by completely random sequence generator without second order statistics so that the waveform signal has a remote statistical probability of being analyzed. The modulated radio frequency waveform is transmitted to an object and the reflected signal is received by the ranging apparatus which comprises a digital solid state read-write memory arranged to have information written into the memory at a fixed frequency and arranged to have the information read out of the solid state read-write memory at a variable frequency to provide the equivalent of a rotating memory. The digital logic employed in the ranging apparatus is extremely accurate and not subject to drift and changes which could occur in analog type systems. The implementation of the ranging apparatus is accomplished with economical commercially available components thus providing an improved digital ranging apparatus.

Abstract: Apparatus is provided for decreasing the acquisition time for recovering spread spectrum codes. The transmitted spread spectrum code is preceeded by a sync signal to establish system time at the receiver. The received coded data signals are applied to a matched filter or a delay line such as a surface acoustic wave device. The number of chips in the delay line is made an integral fraction of the number of chips in the word to be detected. The signals stored in the delay line are detected chip by chip as the coded data passes through the delay line. The resultant output analog signals from the delay line is digitized and stored chip by chip in a digital memory, such as a shift register, having the same chip storage capacity as the sequence group being transmitted. The values of chips in the digital storage memory are recirculated back to the input of an adder.

Abstract: Undesirable signals which are similar to desirable signals occur in transmitter-receiver systems, phased array antenna systems and null or steering antenna systems and are eliminated by a novel adaptive processor. The undesired signal is electronically cancelled by causing the weak undesired replica signal to track the stronger undesired reference signal. The circuitry structure employs a phase tracking loop which generates a phase tracking error signal. The phase tracking error signal is applied to a voltage controlled oscillator and a fixed delay element in the phase tracking loop path. The phase tracking loop is coupled to the weak signal path to shift the phase of the weak undesired replica signal so that it is in phase with the stronger undesired reference signal and may be electronically cancelled. The system may be employed to eliminate undesired signals and leave weak desired signals uneffected and easily detectable.

Abstract: A system for transmitting desired information is provided which has a very low probability of being intercepted. The system combines a well known narrow bandwidth directional antenna for limiting the interception of the main beam information signal with a novel random noise generated signal which guarantees that the sidelobe signals which also contain the desired information cannot be distinguished as information signals.

Abstract: A novel interleaver-de-interleaver is provided which is adapted to store bits of a data stream after being error encoded. The data bits are stored in a random access memory in addresses identifiable by an array of columns and rows. The interleaver comprises address pointer means and logic for reading the data bits out of the memory addresses in a predetermined reordered sequence to provide a quasi-random pattern sequence of data bits which when transmitted are substantially immune to periodic bursts of radio frequency interference signals.

Abstract: A spread spectrum carrier wave device, such as an altimeter, is provided having a transmitting antenna in close proximity to the receiving antenna. The reference signals being transmitted are electromagnetically coupled into the receiving system as undesirable signals. When the strength of the undesirable signals becomes stronger than the desired signals, they mask the desired signals and make them difficult to recover. The undesirable signals are removed from the receiving path by generating a replica of the undesired signals and applying them to an input of an adaptive processor with the signals in the receiving path. After the undesired signals are removed by the process of adaptive nulling, the desirable signals are recovered and processed.

Abstract: A coding system produces a code with enhanced acquisition security by generating a plurality of linear component codes, C.sub.1, C.sub.2, . . . C.sub.n, combining the component codes in accordance with a modulo-2 addition rule to form a linear first composite code, nonlinearizing the first composite code to form a nonlinear second composite code, time delaying the component codes, C.sub.1, C.sub.2, . . . C.sub.n-1, and combining the time delayed codes with the nonlinear second composite code in accordance with a Boolean majority voting rule to form a nonlinear acquisition composite code. Acquisition of the nonlinear acquisition composite code is achieved by generating a plurality of linear reference component codes, R.sub.1, R.sub.2, . . . R.sub.n, that correlate respectively with the linear acquisition component codes, C.sub.1, C.sub.2, . . . C.sub.

Abstract: Electronic digital apparatus for computing an approximation of the arctangent of a given tangent number, N, being in the range of 0 to 1 and in binary form, operates in two stages to provide a solution of the expression: ##EQU1## During the first stage, steps are taken by the apparatus to find out where within the range of 0 to 1, divided preferably into four equal increments, the known tangent number N is located. When the correct increment represented by one of a plurality of increment numbers I.sub.j stored in a ROM unit of the apparatus is found, then one of a like plurality of center numbers C.sub.k of that increment also stored in the ROM unit may be readily selected, as can also be the corresponding one of a plurality of stored numbers A.sub.i representing the arctangents of the stored center numbers C.sub.k. During the first stage, the product number C.sub.k (N) is also formed. In the second stage of operation of the apparatus, the dividend and divisor numbers N-C.sub.k and 1+C.sub.