CASES-99 Long-EZ DATA SET Version 1.0 Richard M. Eckman NOAA/ARL Field Research Division 1750 Foote Drive Idaho Falls, ID 83401 Office: (208) 526-2740 Fax: (208) 526-2549 Email: richard.eckman@noaa.gov Aircraft Sensors The GPS receivers onboard the aircraft measure the aircraft velocity and position in earth coordinates, with the x axis pointing east, the y axis pointing north, and the z axis pointing up. Other sensors use an aircraft coordinate system. From the pilot's perspective, the aircraft x axis extends forward in the direction the aircraft is pointing, the y axis extends to the left parallel to the wings, and the z axis points up perpendicular to the wings. The aircraft has 3 attitude angles: pitch, roll, and heading. Pitch is a rotation relative to the aircraft y axis, roll is relative to the x axis, and heading is relative to the z axis. Some sensors are located in a probe extending from the aircraft's nose. Other are located behind the pilot near the aircraft's center of gravity (CG). Sensor Units Description ==================================================== Alt m Altitude MSL from GPS Ax m/s2 Probe acceleration along aircraft x axis Ay m/s2 Probe acceleration along aircraft y axis Az m/s2 Probe acceleration along aircraft z axis Arol rad/s2 Angular acceleration about aircraft x axis Axb m/s2 CG acceleration along aircraft x axis Ayb m/s2 CG acceleration along aircraft y axis Azb m/s2 CG acceleration along aircraft z axis F_H2O g/m3 Water vapor density from IRGA IR_DN C Infrared temperature from downward pointing sensor IR_UP C Infrared temperature from upward pointing sensor LRange m Laser altitude AGL Lat deg Latitude from GPS in degrees Lon deg Longitude from GPS in degrees Net watt/m2 Net radiation Nu m/s Aircraft east-west velocity component from GPS Nv m/s Aircraft north-south velocity component from GPS Nw m/s Aircraft vertical velocity component from GPS PAR_DN umole/m2/s Photosynthetically active radiation, downward pointing sensor PAR_UP umole/m2/s Photosynthetically active radiation, upward pointing sensor Pr mbar Reference pressure used in deriving static pressure Px mbar Dynamic pressure in center hole on pressure sphere Py mbar Differential pressure on pressure sphere along y axis Pz mbar Differential pressure on pressure sphere along z axis Tbar C Reference temperature sensor for calibrating Tp1 & Tp2 THdg deg Aircraft GPS heading TPitch deg Aircraft GPS pitch TRoll deg Aircraft GPS roll Tdew C Dew point Tp1 C Microbead temperature sensor 1 in probe Tp2 C Microbead temperature sensor 2 in probe (backup to 1) Data Set Variables Some of the variables in the Long-EZ data set, such as temperature, are measured directly. Others, particularly the ambient wind vector, are derived from the raw measurements. The variables are either "fast" variables sampled at 50 Hz or "slow" variables sampled at 1 Hz. Variable Units Origin Rate (Hz) Description ==================================================== U m/s derived 50 East-west ambient wind component V m/s derived 50 North-south ambient wind component W m/s derived 50 Vertical ambient velocity component Tp1 C direct 50 Probe temperature sensor 1 Tp2 C direct 50 Probe temperature sensor 2 RhoD g/m3 derived 50 Dry air density F_H2O g/m3 direct 50 Water vapor density Ps mbar derived 50 Static pressure LRange m direct 50 Laser altitude AGL Lat deg direct 1 Latitude Lon deg direct 1 Longitude Alt m direct 1 Altitude MSL PAR_UP umole/m2/s direct 1 Photosynthetically active radiation, upward pointing sensor PAR_DN umole/m2/s direct 1 Photosynthetically active radiation, downward pointing sensor Net watt/m2 direct 1 Net radiation IRT_DN C direct 1 Infrared temperature, downward pointing sensor Tdew C direct 1 Dew point GndSpd m/s direct 1 Aircraft ground speed AirSpd m/s derived 1 Aircraft air speed UTCSec s direct 1 Seconds since 0000 UTC Sunday UTCTime hhmmss direct 1 UTC clock time in hhmmss Dataflag - derived 1 Data quality flag Data Format There are two types of files available for each Long-EZ flight. The first type is a binary file with a .ncp file-name extension, and the second is an ASCII file with a .mkc extension. The ncp file is a NetCDF file that contains the variables described above. It is designed to be self-describing, in that variable names, units and other information are included in the file as NetCDF attributes. Most of the variables are stored as short integers to save space. These shorts must be multiplied by the "scale_factor" attribute and then added to the "add_offset" attribute to get the data in the units listed. The primary dimension in the ncp file is the "Scan" dimension, which represents 1 second of data. The size of the Scan dimension therefore equals the total number of seconds of data in the file. All the 1 Hz variables are one-dimensional in Scan. All the 50 Hz variables are two-dimensional, with Scan as their first dimension and 50HzData as their second. A third dimension called TimeChars appears in the ncp file, but it is only used in writing the hhmmss characters for the UTCTime variable. The ASCII mkc file is basically a description of the route taken during the flight. Each flight largely consists of a series of legs flown between two waypoints. When a leg is started, the pilot toggles a switch that writes a line in the mkr file. The switch is toggled again at the end of the leg. This creates a second line in the mkc file. The two lines together form a pair that looks like: NSA -1 3250 11:41:32 128492 # 37 39.7 -96 49.2 2200' 0 3521 11:46:03 128763 # 37 31.9 -96 49.0 The first line starts with a 3-character description of the leg. It is followed by a -1 that indicates this is the start of a marker pair. The third field 3250 is the scan number in the ncp file (starting from 0), the fourth field is the UTC time in hh:mm:ss, and the fifth field is the number of seconds since 0000 UTC Sunday. The fields after the # are optional. In this case they are the latitude, longitude, and altitude at the time the switch was toggled. The second line in the marker pair has no description field. The first field is a 0 to indicate the end of a marker pair. The following fields are the same as in the first line of the marker pair. The 3-character description fields have the following values: EWA Flight leg between waypoints EW1 and EW2 EWB Flight leg between waypoints EW3 and EW4 NSA Flight leg between waypoints NS3 and NS4 TCB Slant ascent from takeoff PRF Slant profile TRA Transit to new leg RTN Return to airport BAD Bad or erronious marker SPD Speed calibration WBX Wind box calibration CIR Wind circle calibration YAW Yaw calibration SLP Slip calibration XXX Default fill value for description The waypoints were located at the following locations: Waypoint Latitude Longitude =============================================== EW1 37d 38.50m -96d 49.00m EW2 37d 38.60m -96d 41.20m EW3 37d 36.55m -96d 49.65m EW4 37d 34.93m -96d 40.20m NS3 37d 32.00m -96d 49.00m NS4 37d 38.00m -96d 49.00m =============================================== The mkc file also contains diagnostic error messages when problems are detected during the flight. If all goes well, there is a single ncp file and a single mkc file for each Long-EZ flight. On some flights, however, the data acquisition system failed and was restarted. This results in two sets of files for the flight. On the 19991021 flight, there were two system failures, so there are three sets of files. Quality Control The data are passed through a series of quality-control tests during processing. These tests check whether the values for each variable fall within a physically realistic range. Also, checks are made for rapid changes in a variable that are associated with noise spikes or missing data. The results of these tests are reflected in the Dataflag variable. This flag is a 32-bit integer, with each bit indicating a particular problem with a variable. The bit meanings are as follows: Problem Bit value in hexidecimal ================================================================= U and V bad 0x1 W bad 0x2 Probe temperature 1 bad 0x4 Probe temperature 2 bad 0x8 Fast CO_2 bad 0x10 Fast H_2O bad 0x20 Static pressure bad 0x40 Latitude bad 0x80 Longitude bad 0x100 Altitude bad 0x200 PAR up bad 0x400 PAR down bad 0x800 Net radiation bad 0x1000 Downward IR temperature bad 0x2000 Licor CO_2 bad 0x4000 Dew point bad 0x8000 Air Speed bad 0x10000 U,V,W affected by long GPS gap 0x20000 Latitude affected by GPS gap 0x40000 Longitude affected by GPS gap 0x80000 Altitude affected by GPS gap 0x100000 No diff.corrections, position 0x200000 No diff.corrections, velocity 0x400000 Major timing problem 0x800000 Minor timing problem 0x1000000 Laser altitude bad 0x2000000 Most of the flags simply indicate that a data value is bad. The flags related to a GPS gap represent periods in which a long gap existed in the GPS data. These flags are set if the gap was greater than 4 s for the GPS positions and velocities, and 2 s for the GPS attitude angles. The longer these gaps are, the more suspect the velocity data become. Two of the flags are related to differential corrections in the GPS positions and velocities. If no differential corrections are available, the velocity data are much less accurate and precise. Note that the Dataflag is a 50 Hz variable. For problems in the 1 Hz variables, all 50 values are flagged within the second. The NetCDF variable also have valid_min, valid_max, and _FillValue attributes. These are the same data type (usually short) as the variable itself. For most variables, the valid range excludes the smallest and largest allowable integers (e.g., -32767 and 32767 for a short); these are reserved both for fill values or for data overflows or underflows.