About the Aircraft Data Format

Aircraft Data Structure

Aircraft Data TAPE_STRUCTURE

The on board computers of the research aircraft record data onto magnetic tape. Data from the sensors are collected and organized into AIRCRAFT_DATA_BLOCKS (ADBs). There are one or more ADBs on an aircraft data tape.

AIRCRAFT_DATA_BLOCK

Each AIRCRAFT_DATA_BLOCK contains one or more DIRECTORYs and one or more DATA_SETs.

The user often thinks of the data in terms of its MNEMONIC or TAG number.

A DATA_SET consists of one or more SAMPLEs.

A SAMPLE consists of one or more bytes of information. The information and the structure of the SAMPLE are defined either by the sensor or the program creating the SAMPLE.

The structure of a SAMPLE is defined as a set of FIELDS.

A FIELD is a part of a SAMPLE, usually representing one data value. A FIELD could be a set of bits within a word, a set of bytes, or a set of words.

Each DIRECTORY consists of 8 byte entries. Each entry is organized as follows:

Offset Data Type Information

0 Word TAG - unique number for sensor or DATA_SET 2 Word Pointer to DATA_SET (offset into ADB in bytes) 4 Word Total number of bytes in DATA_SET 6 Byte Maximum number of SAMPLEs. This number can range from 1 to 254. 7 Byte Size (in bytes) of each SAMPLE. This number can range from 1 to 245. This number is set to 255 if a SAMPLE contains more than 254 bytes.

The directory is terminated by an entry containing the tag EXDTAG or LSTTAG. EXDTAG indicates that this directory has been extended and the entry points to the next directory in the block. LSTTAG indicates that this is the last directory in the directory chain. An empty directory entry is marked by NULTAG.

A MNEMONIC, up to 8 characters long, is associated with each particular data type.

Each sensor or type of SAMPLE has a unique TAG assigned to it by which it can be located in the AIRCRAFT_DATA_BLOCK. Since the position of data within an ADB may vary with time, the TAG is the only method of locating data.