Guralp Systems CMG-DM24 - Manual (Page 60)

Operator’s Guide DM-24 Digitiser
Issue G January 2003 58
DATA BLOCK STRUCTURE
Block Header
All elements of the GCF data block can be considered as long words (4 bytes or 32
bits).
The GCF block header consists of 4 elements (16 bytes) as shown
System-ID
32 bit number (base 36) - see below
Stream-ID 32 bit number (base 36) - see below
Date/Time 15 bit Day number 17 bit Second number
Format R.F.U. s/sec compression block size
The first element contains the unique identifier for the system from which the data
came. This is usually a six character string encoded in base 36 i.e. each character can be
extracted by taking the number modulo 36 and converting to the characters 0 - 9 and A -
Z. Only positive integers are allowed (most significant bit=0) limiting the range of
values to 0 - 7FFF,FFFF in hex (2,147,483,654 decimal) or ZIK0ZJ in base 36. This
field can be set to any convenient unique identification number by the user, but units are
shipped from the factory with this set to the factory Works Order number e.g. WO1234.
The second element identifies the stream of data i.e. source as horizontal or vertical
sensor and its origin from the digitiser. This again uses base 36 encoding, but this field
should not be changed as it is dynamically set according to the configuration of the
digitiser.
This is best viewed in its base 36 form i.e. as 6 alphanumerics. The 4 most significant
characters are the instrument serial number (decimal) and the 2 least significant
characters encode the sensor component and digitiser output.
Stream-ID
digit 4 digit 3 digit 2 digit 1 Z,N or E see text
serial number comp source
The Guralp CMG-DM24 digitisers can output upto 4 data rates per component
simultaneously.
The third header element contains the date/time information as a 15 bit day number
packed with a 17 bit second number. The second number is the time since midnight -
maximum 86,399 normally but possibly 86,400 in the case of a ‘leap second’. The day
number increments on the roll-over of the seconds count at midnight. The origin of the
day number (day zero) was 17th November 1989. Thus the date/time can be uniquely
decoded for the next 80 years.
The final header element defines the format of the data in the block, i.e. compression,
sample rate, size etc.
The most significant (first) byte is currently unused and is set to zero.