US 4,396,918 AGranted

Low-frequency radio navigation system

AssigneeThe United States of America as represented by the Administrator of the National Aeronautics and Space Administration

InventorDavid E. Wallis

Life of this patent

Estimating the term…

patentgrep

Loading patent…

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).

Claims8

Dependent claims

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.

2 more elements

Dependencies are inferred from the claim text.