FAA IRIG B Code Description
SecureSync can be configured to provide IRIG timing, reflecting UTC or local time, with or without daylight saving time corrections. Below is a detailed description of the FAA modified IRIG B code. The FAA modified the IRIG B code by including satellite lock status and time error flags in the Control Function Field. The error flags provide an inaccuracy estimate based on the time elapsed since loss of GPS lock. In addition, the Straight Binary Seconds (SBS) data was removed from the data stream. The SBS time is the number of seconds elapsed since midnight.
FAA IRIG B OUTPUT
The FAA IRIG B code contains the Binary Coded Decimal (BCD) time of year and a Control Function (CF) field containing satellite lock status and time error flags. With the exception of the position identifiers, all remaining code elements are set to a binary 0. Figure A-1 illustrates the FAA IRIG B data structure. The BCD time of year provides the day of the year, 001-366, and the time of day including seconds. The hour of the day is expressed in 24-hour format.
FAA IRIG B General Description
- Time frame: 1.0 seconds
- Pulse rates:
- Element rate: 100 per second
- Position identifier rate: 10 per second
- Reference marker rate: 1 per second
- Element identification: The "on time" reference point for all elements is the pulse leading edge.
- Index marker (Binary 0 or uncoded element): 2 millisecond duration
- Code digit (Binary 1): 5 millisecond duration
- Position identifier: 8 millisecond duration
- Reference marker, 1 per second. The reference marker appears as two consecutive position identifiers. The second position identifier marks the on-time point for the succeeding code word.
- Resolution: 10 milliseconds
- Code word structure:
- BCD: Word seconds digits begin at index count 1. Binary coded elements occur between position identifier elements P0 and P5 (7 for seconds, 7 for minutes, 6 for hours, and 10 for days) until the code word is complete. An index marker occurs between decimal digits in each group to provide separation for visual resolution. Least significant digit occurs first.
- CF: IRIG formats reserve a set of elements known as Control Functions (CF) for the encoding of various control, identification, or other special purpose functions. IRIG B has 27 Control Functions located between elements 50 and 78. The FAA IRIG B code uses five of the Control Function elements to encode satellite lock status and time error flags. For a description of the status and error flag implementation, refer to the table and the paragraphs below.
Element 53 (530 ms) is the time sync status bit. Element 53 is a Binary 1 when the receiver locked to GPS, and a Binary 0 when the receiver is not locked to GPS.
Element 55 (550 ms) is the ±1.0 millisecond error flag. Element 55 is set to Binary 1 when the expected time error is within +/- 1.0 millisecond, and a Binary 0 during all other conditions of operation.
Element 56 (560 ms) is the ±5.0 millisecond error flag. Element 56 is set to Binary 1 when the expected time error is within +/- 5.0 milliseconds. and a Binary 0 during all other conditions of operation.
Element 57 (570 ms) is the ±50 millisecond error flag. Element 57 is set to Binary 1 when the expected time error is within +/- 50 milliseconds, and a Binary 0 during all other conditions of operation.
Element 58 (580 ms) is the ±500 millisecond error flag. Element 58 is set to Binary 1 when the expected time error is within ±500 milliseconds, and a Binary 0 during all other conditions of operation.
FAA Time Error Indicators
|Time Since Loss of Lock||Status/Error||Lock Indicator||±1ms||±5ms||±50 ms||±500 ms|
|N/A||Locked Error < 2μs||1||0||0||0||0|
|< 00:16:40||Unlocked Error < 1ms||0||1||0||0||0|
|00:16:41 to 01:23:39||Unlocked Error < 5ms||0||0||1||0||0|
|01:23:40 to 13:53:19||Unlocked Error < 50 ms||0||0||0||1||0|
|13:53:20 to 5 days 18:53:19||Unlocked Error < 500 ms||0||0||0||0||1|
|>5 days 18:53:20||Unlocked Error Unknown||0||0||0||0||0|
FAA modified IRIG B
The beginning of each 1.0 second time frame is identified by two consecutive 8.0 ms elements (P0 and P8). The leading edge of the second 8.0 ms element (PR) is the "on time" reference point for the succeeding time code. 10 pps position identifiers P0, P1, ..... P8 (8.0 ms duration) occur 10 ms before 10 pps "on time" and refer to the leading edge of the succeeding element.
The time code word and the control functions presented during the time frame are pulse-width coded. The binary "zero" and index markers have a duration of 2.0 ms, and the binary "one" has a duration of 5.0 ms. The leading edge is the 100 pps "on time" reference point for all elements.
The binary coded decimal (BCD) time-of-year code word consists of 30 digits beginning at index count 1. The binary coded subword elements occur between position identifiers P0 and P5 (7 for seconds; 7 for minutes; 6 for hours; 10 for days) until the code word is complete. An index marker occurs between the decimal digits in each subword to provide separation for visual resolution. The least significant digit occurs first. The BCD code recycles yearly.
Twenty-seven control functions occur between position identifiers P5 and P8. FAA uses this field to communicate satellite lock status and time error and indicators. The first flag element is at 530 ms which indicates satellite lock. The ±1ms error flag occurs at 550 ms. The ±5ms error flag occurs at 560 ms. The ±50 ms error flag occurs at 570 ms. The ±500 ms error flag occurs at 580 ms.
The straight binary (SB) time-of-day code word normally found between position identifiers P8 and P0 is eliminated for FAA IRIG B. All elements between position identifiers P8 and P0 are set to Binary 0.