Airborne Topographic Mapper (ATM) Project NASA, Goddard Space Flight Center, Wallops Flight Facility Principal Investigator: Bill Krabill (William.B.Krabill@nasa.gov) Description of ATM QFIT Output Data (revised 2009-feb-13 sm) ATM data is generally distributed in the output format of the processing program, qfit, which combines airborne laser ranging data and aircraft attitude from the INS with positioning information from a processed kinematic differential GPS trajectory. Qfit output files, which usually have names ending in a .qi extension, are organized as 32-bit (4-byte) binary words, equivalent to a C or IDL long integer, which are scaled to retain the precision of the measurements. The format and the scaling factors are presented below. The qfit program is run on an Apple PowerPC processor (and formerly Sun/Motorola). Accordingly, the output is written in a big-endian format and must be byte-swapped to be read with a PC (Intel processor) which uses a little-endian format to store 32-bit integers. The files are organized into fixed-length logical records. The beginning of the file contains a header of one or more records followed by a data segment, in which there is one record per laser shot. It is not necessary to interpret the header to use the laser data. The first word of the header (and the file) is a 32-bit binary integer giving the number of bytes in each logical record. Commonly qfit files have 12 words per record and this integer will be the number 48. The remainder of the initial logical record is padded with blank bytes (in this case 44 blank bytes). 10-word and 14-word formats have also been used, as described below. The remainder of the header is generally a series of logical records containing the processing history of the file. In these logical records, the initial word contains a 32-bit binary integer with a value between -9000000 and -9000008. The remaining bytes in each header record is filled with a string of ascii characters containing information on file processing history. In this case, the byte offset (as a longword integer) from the start of file to the start of laser data will be the second word of the second record of the header. (Note: The header records can be removed by eliminating records that begin with a negative value since the first word of records in the data segment is always a positive number.) In the data segment of the file, the information contained in words 1-11 of the output record pertains to the laser pulse, its footprint, and aircraft attitude. The last word of each record is always the GPS time of day when the laser measurement was acquired. Prior to 2008 surveys, the GPS trajectory was edited to restrict PDOP<9 in order to limit GPS errors to be less than roughly 5cm. The output survey data would therefore have occasional gaps where the PDOP>9. Some applications of ATM data have less stringent accuracy requirements that would be better served by preserving the data in these gaps. Starting in 2008, the PDOP limit was changed to 20, which could allow occasional GPS errors up to about 15cm. The PDOP value is carried in the qfit output and can be used to edit data for applications requiring greater precision. Any file in the 10-word format, or files in the 12-word format processed prior to January 2009, will have PDOP limited <9. The three data formats are described below. The format is designated by the logical record length given in the first word of the data file. The qi 12-word format (in use since 2006): Word # Content 1 Relative Time (msec from start of data file) 2 Laser Spot Latitude (degrees X 1,000,000) 3 Laser Spot Longitude (degrees X 1,000,000) 4 Elevation (millimeters) 5 Start Pulse Signal Strength (relative) 6 Reflected Laser Signal Strength (relative) 7 Scan Azimuth (degrees X 1,000) 8 Pitch (degrees X 1,000) 9 Roll (degrees X 1,000) 10 GPS PDOP (dilution of precision) (X 10) 11 Laser received pulse width (digitizer samples) 12 GPS Time packed (example: 153320100 = 15h 33m 20s 100ms) 10-word format (used prior to 2006): Word # Content 1 Relative Time (msec from start of data file) 2 Laser Spot Latitude (degrees X 1,000,000) 3 Laser Spot Longitude (degrees X 1,000,000) 4 Elevation (millimeters) 5 Start Pulse Signal Strength (relative) 6 Reflected Laser Signal Strength (relative) 7 Scan Azimuth (degrees X 1,000) 8 Pitch (degrees X 1,000) 9 Roll (degrees X 1,000) 10 GPS Time packed (example: 153320100 = 15h 33m 20s 100ms) Between 1997 and 2004 some ATM surveys included a separate sensor to measure passive brightness. In the 14-word format, words 10-13 pertain to the passive brightness signal, which is essentially a relative measure of radiance reflected from the earth's surface within the vicinity of the laser pulse. The horizontal position of the passive footprint is determined relative to the laser footprint by a delay formulated during ground testing at Wallops. The elevation of the footprint is synthesized from surrounding laser elevation data. NOTE: The passive data is not calibrated and its use, if any, should be qualitative in nature. It may aid the interpretation of terrain features. The measurement capability was engineered into the ATM sensors to aid in the identification of the water/beach interface acquired with the instrument in coastal mapping applications. 14-word format: Word # Content 1 Relative Time (msec from start of data file) 2 Laser Spot Latitude (degrees X 1,000,000) 3 Laser Spot Longitude (degrees X 1,000,000) 4 Elevation (millimeters) 5 Start Pulse Signal Strength (relative) 6 Reflected Laser Signal Strength (relative) 7 Scan Azimuth (degrees X 1,000) 8 Pitch (degrees X 1,000) 9 Roll (degrees X 1,000) 10 Passive Signal (relative) 11 Passive Footprint Latitude (degrees X 1,000,000) 12 Passive Footprint Longitude (degrees X 1,000,000) 13 Passive Footprint Synthesized Elevation (millimeters) 14 GPS Time packed (example: 153320100 = 15h 33m 20s 100ms)