Constant ratio coding for multipath rejection and ECCM enhancement

US5245659A · kind A · utility

Assignee

• The United States of America, as represented by the Director, National Security Agency · US

Inventors

• Robert L. Ward · Houston, US
• Richard W. Marsh · Silver Spring, US
• Ronald M. Benincasa · Pasadena, US

Key dates

• Filing date: Nov 17, 1978
• Grant date: Sep 14, 1993
• Expiry date: Nov 17, 1998

Classification

• Technology area (CPC H): Electricity
• CPC primary: H04L9/0662
• WIPO field: Measurement
• WIPO sector: Instruments

Abstract

A method is disclosed for encoding signals for transmission to provide signals which simultaneously have specifiable autocorrelation properties and low predictability. To enhance the accuracy of time of arrival measurements in the presence of either noise or multipath due to skywaves in systems such as LORAN, the coding method provides for low autocorrelation function values at time offsets approximately equal to the expected delay between direct waves and skywaves. This is accomplished by constructing the codeword which is used to encode the signal, from an Nth difference constant ratio codeword, where the expected time delay is equal to N-bits. The constant ratio codewords and the transmitted codewords are both pseudo-randomly generated, resulting in a transmitted signal having very low predictability to an unintended recipient, thereby considerably reducing the effectiveness of attempts at jamming or other electronic countermeasures attacks.