Enhanced time of arrival method

• US 5,764,686 A
• Filed: 06/06/1995
• Issued: 06/09/1998
• Est. Priority Date: 05/05/1994
• Status: Expired due to Term

• Current Assignee: Globalstar Incorporated
• Original Assignee: Sanconix Inc.
• Inventors: Souvestre, John R., Sanderford, H. Britton
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Roundtree, Joseph
• Application Number:
  US08/470,768
• Time in Patent Office: 1,099 Days
• Field of Search: 375/200, 375/206, 375/342, 375/343, 375/346, 375/208, 375/209, 375/365, 342/346, 342/350, 342/450, 342/463, 455/33.1, 364/550, 364/579
• US Class Current: 375/149
• CPC Class Codes: G01S 3/043 Receivers
• CPC Class Codes: G01S 3/50 the waves arriving at the antennas being pulse modulated and the time difference of their arrival being measured
• CPC Class Codes: H04B 17/318 Received signal strength

Abstract

A method for improving a radio location system based on time-of-arrival. Time-of-arrival radio location systems are limited in ultimate accuracy by signal-to-noise ratio and by the time varying multipath environment in which they must operate. The present invention teaches techniques which maintain a high signal-to-noise ratio while identifying a feature of the received signal which is least affected by multipath. The technique uses correlation peak/envelope information to estimate the leading edge of the correlation function, then enhances discrete samples at the leading edge of the correlation function to yield high signal-to-noise ratio readings. The present invention can reduce required transmitted bandwidth, increase system resolution and accuracy by twenty to one, and maintain high message traffic throughput.

Claims

1. A method for improving accuracy of a radio location system based on time-of-arrival using direct sequence spread spectrum to time stamp a signal received as a radio broadcast, the method comprising the steps of:

determining a time position of a correlation peak;

estimating, using the correlation peak, a leading edge of a correlation function;

moving a chip code position backward by less than one chip time;

moving the chip code position backward by subchip increments until a correlation envelope decreases by at least one of a preset margin and an auto-correlation noise floor; and

time stamping the chip code position to represent time-of-arrival of the signal.

2. The method as set forth in claim 1, further comprising after the step of moving a chip code position backward, the steps of:

moving the chip code position at least one of forward and backward;

measuring a resulting change in at least one of signal strength, quieting output, and lock detect, as a first reading;

moving the chip code position by at least one of forward and backward;

measuring a resulting change in at least one of signal strength, quieting output, and lock detect, as a second reading; and

using the first reading and the second reading as table pointers, to a result in a look-up table of best-fit results providing a best-fit time-of-arrival.

3. The method as set forth in claim 2, further comprising the steps of:

transferring the first reading and at least one of a multiplicity of second reading to a central processing hub; and

combining, at the central processing hub, the first reading and at least one of the multiplicity of second readings with readings from other receivers for increasing accuracy in time-of-arrival readings.

7. The method as set forth in claim 1, further comprising after the step of determining a time position of a correlation peak, the step of reducing bandwidth to enhance signal-to-noise ratio.

4. A method for improving a radio location system based time-of-arrival, using spread spectrum to time stamp a received radio broadcast, the method comprising the steps of:

receiving a signal string having a signal of interest;

enhancing or delaying a chip code position until said signal of interest is received

decrementing in phase a referenced code position until one or more of the samples descends to the noise floor, forming a delay spread profile having a leading edge;

moving a chip code position backwards by less than one chip time;

moving the chip code position at least one of forward and backward;

measuring a resulting change in at least one of signal strength, quieting output, and lock detect, as a first reading;

moving the chip code position by at least one of forward and backward;

measuring a resulting change in at least one of signal strength, quieting output, and lock detect, as a second reading, and

calculating, using the first reading and the second reading, a slope for increasing the resolution of a time-of-flight radio location system.

5. The method as set forth in claim 4, further comprising the step of:

using the first reading and the second reading as table pointers, to a result in a look-up table of best-fit results providing a best-fit time-of-arrival.

6. The method as set forth in claim 5, further comprising the steps of;

transforming the first reading and at least one of a multiplicity of second readings to a central processing hub; and

combining, at the central processing hub, the first reading and at least one of the multiplicity of second readings with readings from other receivers for increasing accuracy in time-of-arrival readings.