AGO Data Formats

The AGO instrument data is recorded at each site on magneto-optical disks.  This data is reorganized by instrument and day for distribution to the AGO Investigators on CD-ROM.  The Augsburg College AGO homepage details the general format for the data CD-ROMs.  Note that the files on CD are written in a big-endian data format.

Narrowband Data Format

The VLF narrowband data before 1998 can be found on channels 9 - 15 and from 1998 on, can be found on channels 9 - 15, 19 and 20.  The description of the channels can be found in this table.  The narrowband channels have a straightforward data format which follows directly from the AGO header.  The header field data_rate will either be 4 which corresponds to 1 Hz or 5 which corresponds to 2 Hz. The field data_size will always be 12-bits.  All narrowband data is unsigned.  The calibration tables show the correspondence between the digital output values and the electric field measured at the antenna.

Broadband Data Format

The broadband data format is somewhat more complicated due to the synoptic nature of the recordings.  The broadband data can be found on channel 16.  Note, the date_rate field of the AGO Header is incorrect.  The correct rate is 2.5kHz.  In addition to the AGO headers every 2048 bytes, the beginning of a broadband snapshot is indicated by an additional VLF Header.  The VLF Headers are written in a little-endian format, so byte swapping must be performed on the multi-byte fields in order to correctly read header data.   The beginning of a VLF Header is signified by the 21 byte string '=> AGO VLF BB Data <=' and then a null  byte.  The following table describes the remainder of the VLF Header fields.
 
Data Fields of the VLF Header
Field Size Description
majorVersion byte Major version number of the broadband processing software
minorVersion byte Minor version number of the broadband processing software
time dbl word Time of day in milliseconds since midnights as received from the AGO data acquistion unit
day word Day of the year, 1-365 (or 366 in a leap year) as received from the AGO data acquistion unit
year word Last two digits of the year as received from the AGO data acquistion unit
rtcTime dbl word Time of day in milliseconds since midnight according the real-time clock in the broadband processing hardware
rtcDay word Day of the year, 1-365 (or 366 in a leap year) according the real-time clock in the broadband processing hardware
rtcYear word Last two digits of the year according the real-time clock in the broadband processing hardware
recSize dbl word  
offsetTime dbl word  
mixerFrequency word This field records the frequency used in the complex bandshift during the processing.  The value indicates the center of the 2 kHz band, thus a value of 2000 indicates that the data recorded is in the range of 1-3 kHz.
sampleTime dbl word  
sampleBits word  
sampleWordSize word  
agcBits word Multiply this quantity by 6 in order to get the value of the automatic gain control in decibels during this recording. 
errors word Number of errors which occured during snapshot.
 NOTE: A word refers to a 16-bit quantity and a dbl word refers to a 32-bit quantity.

Thus, the total length of the VLF Header is 62 bytes ( 21 bytes of the beginning string, 1 null byte, plus 40 bytes of header fields).  However, if any errors occured during the snapshot or subsequent processing ( errors > 0 ), there is an additional Error Record which follows the VLF Header.
 
Data Fields in the Error Record

Following the VLF Header is the data from the broadband snapshot.  The data is read as eight-bit signed quantities.  Each data point consists of an 8-bit real component and 8-bit imaginary component which are read consequectively.  In addition, every other data point must be multiplied by the quantity -1.  Therefore, the broadband looks like this:
 
real( dataPoint1)
imag( dataPoint1)
- real( dataPoint2)
- imag( dataPoint2)
real( dataPoint3)
....
The broadband data continues until the next VLF Header is reached.

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