TIME CODE

CCSDS Recommended Time Codes
Management Council of the Consultative Committee for Space Data Systems (CCSDS) has four recommended time code formats (One "unsegmented" and three "segmented" codes) which use the international standard second as the fundamental unit of time. An unsegmented time code is a pure binary count of time units and fractional time units from a starting time called the "epoch". A segmented time code is one in which the count of time units and fractional time units is accumulated in two or more cascaded counters which count modulo of various bases and start from the epoch.

Internet Time Code Protocols
Internet time code protocols are defined by a series of documents called Request for Comments, or RFCs. These documents are available on-line from several sites on the Internet. The protocols supported by the NIST Internet Time Service are:
Network Time Protocol (RFC-1305) - the most commonly used Internet time code protocol, and the one that provides the best performance.
Daytime Protocol (RFC-867) - This time code protocol is widely used by small computers running MS-DOS and similar operating systems
Time Protocol (RFC-868) - This simple time code protocol is now used by only about 1% of ITS customers.

IRIG Standards for Time Codes
The Timing Committee of the Telecommunications And Timing Group of the Range Commanders Council has prepared a document for IRIG Serial Time Code Formats.

The need for IRIG standards is that standardization of time codes allows equipment to be synchronized to a known reference time. This also allows facilities at geographically separated locations to synchronize to a known IRIG time code source. In practice, many labs are equipped with devices that create IRIG time code from GPS Satellite time and distribute IRIG time code to other devices for synchronization.

IRIG Family of Time Codes
The IRIG standard consists of a family of rate scaled IRIG serial time codes with IRIG formats containing up to three coded expressions or words. The first IRIG word of the IRIG time code frame is time of year in binary coded decimal (BCD) notation in days, hours, minutes, seconds, and fractions of seconds depending on the IRIG code frame rate. The second IRIG word is a set of bits reserved for encoding of various control, identification, and other special purpose functions. The third IRIG word is seconds of day weighted in straight binary seconds (SBS) notation.

Symmetricom’s Use of Time Code Formats
Symmetricom Time and Frequency Solutions use a variety of Time Code Formats, including IRIG Time Code and others. Almost the entire line of Symmetricom’s time and frequency solutions use one or multiple time code formats, including IRIG.

For example, Symmetricom’s XLi time and frequency solution’s available time code format menu contains IRIG A, IRIG B, IRIG E, IRIG G, IRIG H, XR3/2137, and NASA 36.