# US 4,396,918 A · Low-frequency radio navigation system > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 4,396,918 A - Document ID: US4396918A - Status: Granted - Kind: A - Application number: 06/241,154 - Filed: 1981-03-06 - Granted: 1983-08-02 - Earliest claimed: 1981-03-06 - Assignees: The United States of America as represented by the Administrator of the National Aeronautics and Space Administration - Inventors: David E. Wallis - Web page: https://patentgrep.com/patent/US4396918A ## CPC - G01S 1/30: the synchronised signals being continuous waves or intermittent trains of continuous waves, the intermittency not being for the purpose of determining direction or position line and the transit times being compared by measuring the phase difference - G01S: RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES ## Abstract A method of CW navigation is provided using four transmitters (TR1-TR4) operating at sufficiently low frequencies to assure essentially pure groundwave operation. The transmitters are keyed to transmit constant bursts (1/4 sec) in a time-multiplexed pattern with phase modulation of at least one for identification of the transmitters with the ability to identify the absolute phase of the modulated transmitter(s) and the ability to modulate low-rate data for transmission. The transmitters are optimally positioned to provide groundwave coverage over a service region of about 50.times.50 km for the frequencies selected in the range of 200-500 kHz, but their locations are not critical because of the beneficial effect of overdetermination of position of a receiver (R) made possible by the fourth transmitter. Four frequencies are used, at least two of which are selected to provide optimal lane resolution. All transmitters are synchronized to an average phase as received by a monitor receiver (M). ## Claims ### Claim 1 A method of low-frequency radio navigation comprising the steps of transmitting from each of at least three spatially displaced transmitters at known locations a signal of at least one frequency selected from a range of about 200 kHz to about 500 kHz to assure operation with essentially pure ground wave, each transmitter being keyed on for an equal and constant burst length in a predetermined time-division multiplexed pattern, and at least one transmitter being biphase modulated in a predetermined order of repetitions of said pattern to identify said one transmitter, and therefore all transmitters from the multiplex pattern, phase synchronizing the transmitted signals from all transmitters to a common value of phase as received by a monitor at a known location notwithstanding the biphase modulation of one transmitter at predetermined intervals identification of all transmitters, thereby creating a region of zero error within the service area, receiving signals from said transmitters at an unknown position, and at said unknown position identifying the absolute phase of said biphase modulated signal in the multiplex pattern from the predetermined order of the signals in the pattern, measuring the phase of each signal received at said unknown position from each transmitter, and from said phase measurements determining said unknown position by comparing measured phases with predetermined phases for all signals from all transmitters to determine the maximum likelihood of position. ### Claim 2 (depends on 1) A method as defined in claim 1 wherein said monitor is positioned within an area bounded by said transmitters to divide any error due to variations in propagation paths from the transmitters. ### Claim 3 (depends on 1) A method as defined by claim 1 wherein the average phase of signals received from all transmitters by said monitor is used as the common value of phase for phase synchronization of all transmitters. ### Claim 4 (depends on 1) A method as defined in claim 1 wherein N transmitters are employed with as many frequencies as there are transmitters cyclically permuted by frequency and transmitter, where N is an integer greater than 2, whereby every transmitter is transmitting at all times on a different frequency, and all change frequency in unison in N time slots to produce a cyclic pattern of transmission from all transmitters. ### Claim 5 (depends on 1) A method as defined in claim 1 wherein N transmitters are employed with up to N frequencies cyclically permuted by frequency and transmitter in a basic pattern and all transmitters are biphase modulated in a predetermined order of repetitions of said patterns such that the number of inverted frequency patterns is equal to the number of uninverted frequency patterns for identification of transmitters and determination of absolute phase with interference suppression by allowing any interference to be integrated out of phase measurements. ### Claim 6 (depends on 1) A method as defined by claim 1 wherein signal transmission from at least one transmitter is biphase modulated with low-rate data for communications of short messages and said transmitter biphase modulated for communication is other than said designated one modulated for transmitter identification. ### Claim 7 (depends on 1) A method as defined by claim 1 wherein said signal transmission from said one transmitter biphase modulated for identification is modulated with low-rate data for communications of short messages during intervals between biphase modulation for transmitter identification. ### Claim 8 (depends on 1) A method as defined by claim 1 wherein the signal transmission from at least one transmitter is biphase modulated with low-rate data for communication of short messages and the data code for communication is a synchronizable code with an equal number of biphase modulated periods representing encoded binary 1 and 0 digits, thereby to allow interference from communications to be integrated out of phase measurements for navigation.