Method for decoding and error correcting data of tactical air navigation and distance measuring equipment signals

• US 5,929,783 A
• Filed: 07/30/1997
• Issued: 07/27/1999
• Est. Priority Date: 09/11/1995
• Status: Expired due to Fees

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: King, Dennis D., Brabender, Jeffery W.
• Primary Examiner(s): Horabik, Michael
• Assistant Examiner(s): WONG, ALBERT KANG
• Application Number:
  US08/902,959
• Time in Patent Office: 727 Days
• Field of Search: 340/870.05, 371/36, 371/31, 371/69.1, 371/70, 375/267, 342/47, 342/357
• US Class Current: 340/870.05
• CPC Class Codes: G01S 13/765 with exchange of information between interrogator and responder
• CPC Class Codes: G01S 13/785 Distance Measuring Equipment [DME] systems
• CPC Class Codes: G01S 19/41 Differential correction, e.g. DGPS [differential GPS]
• CPC Class Codes: G01S 5/009 Transmission of differential positioning data to mobile
• CPC Class Codes: H04L 25/4902 Pulse width modulation; Pulse position modulation

Abstract

A method and apparatus for encoding and decoding data on navigation signal pulse pairs utilizes pulse position modulation (PPM). The pulse interval between a first pulse and a second pulse of the pulse pair is varied depending on whether the logic value of the data to be encoded is a logic "0" or a logic "1". A tri-graph encoding process converts the raw input data into encoded input data. Each input data bit is encoded into three bits with each bit having the same logical value as the input data bit. The encoded input data is then encoded on the navigation signal using a pulse position modulation (PPM) technique. A decoder recovers the encoded data and provides an error correction process for high data integrity that corrects certain bit errors that may occur during the transmission and reception of the data on the navigation signal. The error correction process detects and corrects errors such as a missing bit, an extra bit or a bit error.

Claims

1. A method for detecting an error in an encoded data bit stream encoded according to a tri-graph encoding process, comprising:

compiling one or more groups of three data bits from an encoded data bit stream, each group of three data bits comprising a first, a second, and a last data bit;

detecting when the compiling of data bits results in a data bit error in a group of three data bits; and

generating, in response to a group of three data bits and the detection of a data bit error, an output data bit representing an error detection, the data bit having logic "1" when two data bits of a group of three data bits equals a first logic level and having logic "0" when two data bits of a group of three data bits equals a second logic level.

2. A method in accordance with claim 1 further comprising:

detecting when the compiling of data bits results in an extra data bit error in a group of three data bits; and

generating, in response to a group of three data bits and the detection of an extra data bit error, an output data bit representing an error detection, the data bit having a logic "1" when each data bit of a group of three data bits equals a first logic level and having a logic "0" when each data bit of a group of three data bits equals a second logic level.

3. A method in accordance with claim 1 further comprising the step of:

generating in response to a group of three data bits an output data bit having a logic "1" when said group of three data bits has a logic pattern "101" and an output data bit having a logic "0" when said group of three data bits has a logic pattern "010".

17. A method in accordance with claim 2 further comprising:

counting the number of detections of missing data bit errors in the encoded data bit stream;

compiling a next group of three data bits when the number of detections of missing data bit errors reaches a first predetermined value, the next group of three data bits comprising;

the last data bit from a most recent group of three data bits in which a missing data bit error was detected;

the next two data bits in the encoded data bit stream following said last data bit;

counting the number of detections of extra data bit errors in the encoded data bit stream;

compiling a next group of three data bits when the number of detections of extra data bit errors reaches a second predetermined value, the next group of three data bits comprising three consecutive data bits in the encoded data bit stream that immediately follow a single data bit in the encoded data bit stream, the single data bit immediately following a most recent group of three data bits in which an extra data bit error was detected.

18. A method in accordance with claim 17 further comprising:

resetting the count of the number of detections of missing data bit errors to an initial value upon detection of an extra data bit error; and

resetting the count of the number of detections of extra data bit errors to an initial value upon detection of a missing data bit error.

19. A method in accordance with claim 17 further comprising:

generating, in response to a group of three data bits, an output data bit having logic "1" when a group of three data bits has a logic pattern "101" and an output data bit having logic "0" when a group of three data bits has a logic pattern "010";

resetting the count of the number of detections of missing data bit errors to an initial value when a group of three data bits has a logic pattern "101" or a logic pattern "010"; and

resetting the count of the number of detections of extra data bit errors to an initial value when a group of three data bits has a logic pattern "101" or a logic pattern "010".

20. A method in accordance with claim 19 wherein the first predetermined value equals three and the second predetermined value equals three.

4. A method for decoding and detecting errors in an encoded data bit stream, comprising:

compiling sequential groups of "n" data bits from the encoded data bit stream, the integer "n" equaling three or more;

generating, in response to a group of "n" data bits, an output data bit having a logic "1" when each data bit of a group of "n" data bits equals a first logic level and an output data bit having a logic "0" when each data bit of a group of "n" data bits equals a second logic level;

detecting when the compiling of data bits results in a data bit error in a group of "n" data bits; and

generating, in response to a group of "n" data bits and the detection of a data bit error, an output data bit representing an error detection, the data bit having logic "1" when a majority of a group of "n" data bits equals a first logic level and having logic "0" when a majority of a group of "n" data bits equals a second logic level.

5. A method in accordance with claim 4 further comprising:

detecting when the compiling of data bits results in a missing data bit error in each of a plurality of groups of "n" data bits;

compiling, in response to a predetermined number of detections of missing data bit errors, a next group of "n" data bits, the next group comprising;

a last bit from a group of "n" data bits having a most recently detected missing data bit; and

a sub-group of "n-1" consecutive data bits that immediately follow the group of "n" data bits with the most recently detected missing data bit in the encoded data bit stream.

6. A method in accordance with claim 5 further comprising generating an output data bit representing an error detection, the data bit having a logic "1" when each data bit of the next group of "n" data bits equals a first logic level and having a logic "0" when each data bit of the next group of "n" data bits equals a second logic level.

7. A method in accordance with claim 6 wherein the predetermined number of detections of missing data bit errors equals three.

8. A method in accordance with claim 4 further comprising:

detecting when the compiling of data results in an extra data bit error in a group of "n" data bits; and

generating, in response to a group of "n" data bits and the detection of an extra data bit error, an output data bit representing an error detection, the data bit having logic "1" when a majority of a group of "n" data bits equals a first logic level and having logic "0" when a majority of a group of "n" data bits equals a second logic level.

9. A method in accordance with claim 4 further comprising:

detecting when the compiling of data bits results in a missing data bit error in each of a plurality of groups of "n" data bits;

compiling, in response to a predetermined number of detections of missing data bit errors, a next group of "n" data bits, the next group comprising;

a last bit from a group of "n" data bits having a most recently detected missing data bit error; and

a sub-group of "n-1" consecutive data bits that immediately follow the group of "n" data bits having the most recently detected missing data bit error in the encoded data bit stream;

detecting when compiling the data bits results an extra data bit error in each of a plurality of groups of "n" data bits;

compiling, in response to a predetermined number of detections of extra data bit errors, a next group of "n" data bits, comprising the steps of;

compiling "n+1" consecutive data bits that immediately follow a group of "n" data bits having a most recently detected extra data bit error in the encoded data bit stream; and

compiling the next group of "n" data bits to include a last "n" consecutive data bits from the "n+1" consecutive data bits.

10. A method in accordance with claim 4 further comprising:

detecting a missing data bit error in each of a first plurality of groups of "n" data bits;

compiling a next group of "n" data bits in response to a predetermined number of detections of missing data bit errors, the predetermined number of detections of missing data bit errors occurring without an intervening detection of an extra data bit error, the next group of "n" data bits comprising;

a last bit from a group of "n" data bits having a most recently detected missing data bit error;

a sub-group of "n-1" consecutive data bits that immediately follow the group of "n" data bits having the most recently detected missing data bit error in the encoded data bit stream;

detecting an extra data bit error in each of a plurality of groups of "n" data bits;

compiling the next group of "n" data bits, in response to a predetermined number of detections of extra data bit errors, the predetermined number of detections of an extra data bit error occurring without an intervening detection of a missing data bit error, composing the steps of;

compiling "n+1" consecutive data bits that immediately follow a group of "n" data bits having a most recently detected extra data bit error in the encoded data bit stream; and

compiling the next group of "n" data bits to include a last "n" consecutive data bits from the "n+1" consecutive data bits.

11. A method in accordance with claim 10 further comprising the step of generating an output data bit having a logic "1" when each data bit of the next group of "n" data bits equals a first logic level and an output data bit having a logic "0" when each data bit of the next group of "n" data bits equals a second logic level.

12. A method in accordance with claim 10 wherein:

the predetermined number of detections of missing data bit errors equals three; and

the predetermined number of detections of a extra data bit errors equals three.

13. A method in accordance with claim 9 further comprising:

detecting a bit error other than a missing bit error or an extra bit error; and

generating, in response to a group of "n" data bits and the detection of a bit error other than a missing bit error or an extra bit error, an output data bit representing a bit error, the data bit having logic "1" when a majority of a group of "n" data bits equals a first logic level and having logic "0" when a majority of a group of "n" data bits equals a second logic level.

14. A method in accordance with claim 8 further comprising:

detecting a missing data bit error in each of a first plurality of groups of "n" data bits;

compiling a next group of "n" data bits in response to a predetermined number of detections of missing data bit errors, the predetermined number of detections of missing data bit errors occurring without an intervening detection of a bit error other than a missing bit error, the next group of "n" data bits comprising;

a last bit from a group of "n" data bits having a most recently detected missing data bit error;

a sub-group of "n-1" consecutive data bits that immediately follow the group of "n" data bits having the most recently detected missing data bit error in the encoded data bit stream;

detecting an extra data bit error, in each of a plurality of groups of "n" data bits;

compiling a next group of "n" data bits, in response to a predetermined number of detections of an extra data bit error, the predetermined number of detections of extra data bit errors occurring without an intervening detection of a bit error other than an extra bit error, comprising the steps of;

compiling "n+1" consecutive data bits that immediately follow a group of "n" data bits having a most recently detected extra data bit error in the encoded data bit stream; and

compiling the next group of "n" data bits to include a last "n" consecutive data bits from the "n+1" consecutive data bits.

15. A method for detecting an error in an encoded data bit stream encoded according to a tri-graph encoding process, comprising:

compiling one or more groups of three data bits from said encoded data bit stream, each group of three data bits comprising a first, a second, and a last data bit;

detecting when the compiling of data bits results in the presence of a data bit error in a group of three data bits;

generating, in response to a group of three data bits and the detection of a data bit error, an output data bit representing an error detection, the data bit having logic "1" when two data bits of a group of three data bits equals a first logic level and having logic "0" when two data bits of a group of three data bits equals a second logic level;

counting the number of detections of data bit errors in the encoded data bit stream;

compiling a next group of three data bits when the number of detections of data bit errors reaches a first predetermined value, the next group of three data bits comprising;

the last data bit from a most recent group of three data bits in which a data bit error was detected; and

the next two data bits in the encoded data bit stream following said last data bit.

16. A method in accordance with claim 15 further comprising:

generating, in response to the next group of three data bits, an output data bit representing an error detection, the data bit having a logic "1" when each data bit of the next group of three data bits equals a first logic level and having a logic "0" when each data bit of the next group of three data bits equals a second logic level.