Abstract: A multiple channel demand assignment multiple access (DAMA) system is configured as a spoke-and-hub star network in which a satellite relays information between each of many small-aperture terminals (SATs), which may be portable terminals (PTs), and a hub network control terminal (NCT) which receives requests from the PTs, allocates channel resources, and broadcasts housekeeping messages. The system has a reservation protocol using at least three separate data channels to take advantage of the inherent lack of adequate radiated power at the PTs: a NCT outbound channel is tightly packed with data, without guard spaces, and it uses typically 98% of the allocated power; separate return channels for data and reservations use little satellite power, with no need for extremely tight data packing. Guard spaces are included in the reservation channel to overcome the total range of propagation delay uncertainty without requiring knowledge of the location of a particular PT.

Abstract: A method and apparatus for masking the presence and content of data transmissions includes a mobile transmitter which encodes the data by means of a pseudorandom spreading (PRS) code having a chip rate, and which transmits it at a carrier frequency. A masking transmitter remote from the mobile transmitter encodes a carrier at the same frequency with a second PRS sequence having the same chip rate, and transmits it at a power level higher than the power level of the mobile transmitter. A signal interceptor cannot separate the mobile and masking station carriers on a frequency basis. The relatively low-power mobile station PRS signal is difficult to detect in the presence of the high-power masking station code. The intended receiver (master station) receives an ensemble signal which is principally masking station signal. The master station regenerates the known masking station PRS code and subtracts it from the ensemble signal to leave a residue signal which contains the data-bearing mobile station PRS signal.

Abstract: An automatic direction finder antenna array having a plurality of pairs of oppositely positioned antenna elements arranged radially from a common center with each pair forming a cardioid radiation pattern when energized. Switching apparatus energizes successive antenna pairs to produce a rotating cardioid pattern but with the cardioid being switched back and forth 180.degree. at a higher switching frequency when each pair of antennas is energized during the rotating function. The antenna AX whose output bears the lowest ratio R to the output of its oppositely positioned antenna is determined as well as the antenna AZ having the largest output signal VZ. The precise angle of reception of a received signal is then determined from AX, AZ, and R.

Abstract: A transmit/receive space diversity system communicates between transmitter-receivers at two separate sites. Each site has at least two antennas. Each antenna at a first one of the sites transmits a pilot signal, at differing frequencies. At the second or other site, all the pilot signals are received by each antenna, and the pilot signals are evaluated to determine which pilot-signal-receiving antenna is part of the lowest loss path between the sites. A data source at the second site is routed to the identified antenna for transmission of data signals back to the first site. At the first site, the data signals are received by all the antennas, and the best of the received data signals is routed to utilization apparatus. A data signal from the first site is transmitted to the second site, at which it is received by the selected antenna.

Abstract: An arrangement for coarse and vernier measuring of the delay or phase shift introduced by a delay path by generating a recurrent pseudorandom signal at an original chip clock rate derived from a reference clock. At the receiving end of the delay path, a delay locked loop regenerates the chip clock and the pseudorandom signal at a phase established by the delay. The coarse count is measured by the number of chip clock cycles. The vernier delay is measured in terms of a fractional portion of a chip clock cycle. The vernier measurement is made by starting a counter which counts reference clock signals in response to a coincidence of the original chip clock and the reference clock, and by ending counting and latching the count in response to coincidence of the regenerated chip clock and the reference clock.

Abstract: A communication system transmits information which may be in the form of frames or bursts of suppressed-carrier data over a dispersive transmission path which introduces multipath distortion. Each frame is stored as it is received and processed by iteratively simulating the multipath distortion, subtracting the distortion from the stored frame to form a corrected signal, and evaluating the quality of the resulting signal. The quality of the resulting signal is determined by frequency multiplying the corrected signal and evaluating the total power of components other than the frequency multiplied carriers. The iterative procedure adjusts the phase and, if deisred, the amplitude of signals tapped from a delay line.

Abstract: A communications system for conveying information angle-modulated onto a carrier includes a modulator which angle-modulates the data onto the carrier and which also randomly changes the phase of the carrier. Since the carrier phase is randomly changed, an unauthorized receiver or interceptor of the signal who frequency-multiplies the signal and integrates over a long period in order to reconstruct the carrier produces a carrier which has a broader bandwidth and relatively less amplitude than in the case in which the carrier phase is not randomly changed. The probability of intercept and demodulation by the unauthorized receiver is thereby reduced. The authorized receiver frequency-multiplies and integrates for the known frame interval to recover the carrier.

Abstract: Data signals are angle-modulated onto a carrier in such a fashion as to suppress the carrier. Such modulation may be BPSK, MSK, QPSK, or OPQSK. The carrier may be subject to anomalous phase shifts, as may occur for example when the transmitter and receiver are in relative motion. In order to regenerate the carrier, the received signal is frequency-doubled (or quadrupled in the case of QPSK). The frequency doubled signal is demodulated by means of locally generated double-frequency demodulating signals to form baseband I and Q signals. The I and Q baseband information is evaluated over a predetermined time interval to establish the relative phase between the carrier of the double frequency signal and the double-frequency demodulating signal. At the end of each predetermined interval, the phase of the double-frequency demodulating signal is adjusted to be in-phase with the double-frequency carrier.

Abstract: An MSK modulator includes a source of digital sawtooth signals having a recurrence rate equal to the desired recurrence rate of the unmodulated carrier. The digital sawtooth signal is applied to an adder together with a phase control digital signal. The phase control digital signal phase-shifts the digital sawtooth signal. The phase-shifted digital sawtooth signal is applied to a sine ROM to produce a digital sinusoid with an unmodulated recurrence rate equal to the recurrent rate of the digital sawtooth signal. When the phase control signal is an accumulated signal, the digital sinusoid is frequency-modulated. The phase control signal for MSK modulation is a ramp signal generated by a controlled accumulator. The accumulator includes a controllable clocked adder/subtractor, the output of which is coupled by way of a register back to an input. The other input of the adder/subtractor is coupled to receive a fixed ramp rate controlling word.

Abstract: A system for adjusting the antenna matching network of a radio system in the presence of an interfering signal in the matching network being adjusted comprises a loop from which the interfering signal is cancelled by an adaptive interference cancellation (AIC) system. The AIC system receives an interference sensor signal having the frequency of the interfering signal. Means responsive to the interference sensor signal and phase and amplitude control signals provides a modified interference sensor signal having a phase and amplitude determined by the control signals. This modified interference sensor signal is combined with a mismatch signal which is derived from the signal in the matching network to produce a modified mismatch signal.

Abstract: A phase modulator includes a digital frequency word generator, an adder and a register arranged to generate recurrent digital sawtooth signals at a carrier rate. A second digital adder is coupled to receive the sawtooth signals and also receives digital information signals. The adder produces recurrent digital sawtooth signals phase-modulated by the information signal. A pair of adders receive the digital sawtooth signals and mutually sign-reversed digital information signals to produce a pair of oppositely phase-modulated constant-amplitude signals in a pair of channels. A sine memory is addressed by the phase-modulated digital sawtooth signals to produce phase-modulated sinusoidal-representative digital signals. The digital signals are then converted to analog signals. Since the two channels contain signals which are phase-modulated but not amplitude-modulated, the signals may be amplified by nonlinear amplifiers.

Abstract: A receiver in a communication system employing a transmitted signal consisting of a succession of N chip PRS data codes randomly selected from M PRS data codes and each multiplied by an N chip master PRS code to form a succession of encoded PRS codes which together form a continuous encoded PRS signal, and in which the PRS data codes are formed from M successive and overlapping segments of a reproducible source PRS code with each successive PRS data code beginning at a chip in the source PRS code X chips removed from the preceeding PRS data code. The receiver decodes the continuous encoded PRS signal by logic including a first correlator for removing the master PRS code from each received encoded PRS code to recover the generated PRS data code.

Abstract: A decoder for decoding received pseudo random sequence (PRS) codes modulated upon a carrier signal and having an original chip rate f.sub.c, and for correcting for unknown frequency shifts in the received PRS codes. The decoder includes a local oscillator for demodulating the carrier signal and detecting the received PRS codes and a signal source for generating X correction PRS codes each having a pattern of chips similar to the chip pattern of a predetermined one of the received PRS codes and each shifted in chip rate from f.sub.c by amounts which will provide a spectrum of frequencies spanning any expected frequency shift of a received PRS code. Also provided is a correlator for correlating each of the X correction PRS codes with each received PRS code at a rate f.sub.r to determine which particular correction PRS code produces the greatest correlation signal, where f.sub.r >>f.sub.c.

Abstract: A system for digitally detecting the energy content of a received analog signal of frequency f.sub.c. The system comprises signal means for generating sampling pulses of repetition rate 4 f.sub.c, a first two level signal of frequency 2 f.sub.c, whose successive level transitions occur near successive sampling pulses, a second two level signal whose successive level transitions occur near successive level transition of a first polarity of said first two level signal, and first and second interleaved trains of clock pulses each having a repetition rate 2 f.sub.c, and with each clock pulse occurring between the level transitions of successive half cycles of said first two level signal. Also provided are signal means responsive to said sampling pulses to obtain alternate I and Q samplings of said analog signal with each sampling comprising a magnitude component and a sign component.

Abstract: A system for on-line detection of distortion in a received digital signal which is phase modulated at a data rate f.sub.d and which has a suppressed carrier frequency f.sub.c and without interrupting the signal. The system comprises means for multiplying a received digital signal by itself N times to produce a resultant signal comprising a reconstructed carrier signal of frequency Nf.sub.c and phase .phi..sub.1 and distortion produced upper and lower sidebands of frequencies f.sub.u =Nf.sub.c +f.sub.d and f.sub.l =Nf.sub.c -f.sub.d and also means for generating I and Q signals of frequency Nf.sub.c and of phase .phi..sub.1 and .phi..sub.1 .+-.90.degree., respectively. Further provided are means for generating a clock signal of frequency f.sub.d and a control means for mixing said I and Q signals with said clock signal to produce I/D and Q/D signals, respectively, where I/D represents the product of the I signal and said clock signal f.sub.

Abstract: A data communications system for transmitting and receiving 64 different audio-frequency signals representing 64 respective alpha-numeric code symbols. A synchronizing signal consists of three time-spaced bursts where each burst consists of three audio-frequency signals. A following information signal consists of a three time-spaced message bursts where each burst consists of twenty audio-frequency signals, all three of the message bursts containing the same audio-frequency signals but in different orders. A receiver includes synchronizer means to recognize the predetermined synchronizing signal, and decoder means enabled and synchronized by the synchronizer means to decode the information signal. The audio-frequency signals may be 64 different fixed-frequency tones, or may be 64 differently-changing signals such as may be produced by unidirectional phase shift keying by 64 different pseudo-random number codes.

Abstract: A means and method for fast determination of the frequency of an input signal comprising mean for digitizing and storing the digitized bits of the input signal in an input register of N bits at a bite rate f.sub.c. Other means generate a series of digitized reference signals whose frequencies are represented by patterns of binary 1's and 0's and spaced apart by predetermined frequency intervals. At least one digitized reference signal is clocked into a reference signal register during each time period 1/f.sub.c. The digitized reference signals are successively compared to the digitized input signals to produce output signals whose magnitudes are proportional to the degree of correlation therebetween. Further, means are provided to determine the largest output signal and the frequency of the reference signal which produced said largest output signal.

Abstract: A demodulator for demodulating an encoded signal comprising first and second predetermined patterns of contiguous signal segments of frequencies f.sub.1 and f.sub.2. A first signal processing means comprises first correlation means including a reference signal which is correlative with the signal segments of frequency f.sub.1 of said first and second patterns to produce a first and second output signals when correlation occurs. A second signal processing means comprising second correlation means including a reference signal which is correlative with the signal segments of frequency f.sub.2 of said first and second patterns to produce third and fourth output signals when correlation occurs. The said first and third output signals are combined to produce a first resultant signal indicating correlation with said first pattern of signal segments and said second and fourth output signals are combined to produce a second resultant signal indicating correlation with said second pattern of signal segments.

Abstract: A range finding method and system comprising an interrogator and a transponder. The interrogator generates a signal with a first portion comprising a carrier signal amplitude modulated by a first two-level iterative pseudo random sequence (PRS) whose level changes coincide with level changes of a first two-level tone of frequency f.sub.T, which phase modulates said first portion. A second portion of the signal is phase modulated by the product of the PRS and said first tone. The transponder tracks the received interrogation signal and then generates and transmits a responsive signal having first and second portions comprised of a carrier signal phase modulated by a second two-level tone phase synchronized with the received first tone. The first portion is further amplitude modulated by a second iterative PRS phase synchronized with the received PRS signal, and the second portion is further phase modulated by the second, phase synchronized PRS.

Abstract: A means for reducing side lobe signals and noise signals generated in a correlator in N chip positions for correlating a received input signal with a reference signal. The coincidence and non-coincidence indicating signals of first and second halves of the chip positions of the correlator are added together separately and then subtracted one from the other, with the resulting difference signal then being rectified. Such rectified difference signal which contains the side lobe signals and the noise signals but not the desired correlator indicating signal, is subtracted from the total summed and rectified signals generated by all of the N chip positions of said correlator to produce a resultant signal with much of the side lobe signals and the noise signals cancelled out.