XLi Time & Frequency System 289
XLi-man, Issue 8, 6/17/2008, Rev. H
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NASA 36
Introduction
The NASA 36 time code is similar to the previously mentioned IRIG codes. The NASA 36 code frame
also contains 100 bit elements like the IRIG codes. In the strict sense, NASA 36 encodes Coordinated
Universal Time (UTC) in 24 hour format and not local time. Nonetheless, this instrument can encode
UTC or local time in either 24 or 12 hour formats.
NASA 36 Code Format
The level shifted, pulse width modulated, serial format of NASA 36 is divided into two segments. The
first segment encodes time of year in binary coded decimal (BCD) notation. The second segment
encodes control functions (unused on Model XLi).
The two code segments are contained within one "frame". The frame length for NASA 36 is 1 second
long and contains 100 "elements" (pulses) each of which start every 10 milliseconds.
An element may represent either a binary zero, a binary one, a reference marker or a position identifier.
A zero is 2 ms, a one is 6 ms, a position identifier is 6 ms. A reference marker is 5 consecutive position
identifiers. A reference marker locates the beginning of each frame.
XLi NASA 36 Time Code Input/Output
The XLi generates and decodes the following NASA 36 time codes:
NASA 36 (AM):1 kHz sine wave amplitude modulated BCD
NASA 36 (DC):DC level shift, pulse width coded BCD
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