README File for the NSF Arctic Data Set titled ``Airborne Laser Altimeter, Global Positioning System (GPS), Inertial Measurement Unit (IMU) and Imagery Campaign of the Surging Negribreen Glacier, Svalbard, in July 2017 and July 2018''


Below is a description of the data format and type for each instrument.


(1) Laser data (laser)

Instrument and Setup: LaserTech Universal Laser System (ULS). Operates at 905nm (Near-
Infrared) with a configurable pulse-repetition frequency (PRF), set to 4000Hz with 10-point aver-
aging to yield an effective PRF of 400 Hz. The ULS was mounted on the helicopter skids as seen in the associated photographs. A user manual for the instrument is freely available online at http://entek.io.s3-website-us-west-2.amazonaws.com/documentation/laser_sensors/ULS_UM.pdf

Data types: (A) raw data and (B) error logs. Note that sometimes these files are split into two parts due to having to restart the data logging mid-flight.

Data format: Each data type is given as a .dat file, which are structured as columned data with either a space as a delimiter (raw laser data) or a comma delimiter (error logfile). 
  (A) Raw laser data columns
      (Column 1) SYS_TIME (seconds, float): System time of the linux computer used for data logging during the flights. This time is used to sync with the button GPS that also uses SYS_TIME. Note that SYS_TIME is not equivalent to UNIX time (i.e., number od seconds since January 1st, 1970 at 00:00:00 UTC) since this computer is rarely connected to the internet, which allows drift in its internal clock. This time is linked to the imagery time by taking a picture with each camera of the computer screen that is printing the SYS_TIME. Similarly, an image is taken of the computer displaying the laser/GPS SYS_TIME side-by-side with the computer screen displaying the IMU reference time. (See the section on gps_unavco for syncing with Kinematic GPS (UTC time) with the SYS_TIME).
      (Column 2) RANGE (meters,  float) The distance from the ULS to the surface signal. Typically ranges greater than 500m result in erroneous data.
      (Column 3) AMP (nanoseconds, float): The return intensity measured by the nanoseconds of pulse width in the return signal. This value is an indicator of the signal strength.
  (B) Error Logfile data columns
      (Column 1) SYS_TIME (seconds, float): Same as SYS_TIME for the regular laser data.
      (Column 2) Error Number Flag (string): Simply a string stating there was an error. Common for all entires (rows).
      (Column 3) Error Number (int): The identifier integer for the type of error the laser is receiving. An error number table is contained on the ULS User Manual whose link is given above. Common error values are: 1 - Invalid Command, 4 - Target not found (no return signal) and 5- Average Not Filled (not a  strong enough return signal).
      
(2) Button GPS data (gps_button)

Instrument and Setup: A low cost, plug-and-play type used for making quick, low-accuracy plots that operates at 1 Hz and is connected to the linux data logging computer via USB. The exact model is unknown, but it is similar to the GlobalSat BU-353-S4 USB GPS Receiver.The button gps was placed on the dash of the helicopter for optimal signal retrieval.

Data types: Logged GPS data. Similar to the laser data, this data may also be split into two parts due to having to restart the data logging mid-flight.

Data format: The data type is given as a .dat file, which are structured as columned data with a space as a delimiter.
    (Column 1) SYS_TIME (seconds, float): The same SYS_TIME as the laser data since the button gps data and the laser data were logged on the same computer during collection.
    (Column 2) GPS_TIME (seconds, float): This is not the full GPS time as the instrument only count up to a certain maximum time before it rolls over and begins again at zero. We therefore do not use this time and rely on the SYS_TIME instead.
    (Column 3) LAT (float): Latitude of the button GPS.
    (Column 4) LON (float): Longitude of the button GPS.
    (Column 5) SATS (int): Number of satellites used to produce to geolocation estimate.
    (Column 6) ALT (meters, float): Altitude above mean sea-level of the button GPS.
    (Column 7) GEOID (meters, float): Height above the reference ellipsoid.
    
(3) IMU data (imu)

Instrument and Setup: LORD 3DM-GX5-15 Vertical Reference Unit. Inertial Measurement Device (IMU) data record the attitude of the helicopter during the flights. These data are collected to allow correction for attitude and vibration. The IMU was tapped to the floor, surrounded by insulating foam to prevent high-frequency noise, below the left rear passenger seat as seen in the associated photographs. The IMU was connected via USB to a windows laptop for data logging (a different computer than that used for the laser and button GPS since the logging software required a windows OS).  The positive X direction points toward the from of the aircraft, the positive Y direction points to the right (starboard) of the aircraft and the positive Z direction points downwards. The IMU was positioned initially (when the helicopter was on level ground) so there was no X or Y tilt. Note that the IMU was not functional for flight1 in 2017. One is encourage to reference the user manual for a full understanding of the IMU data. It is available at: https://www.microstrain.com/sites/default/files/3dm-gx5-15_user_manual_8500-0061_0.pdf

Data types: (A) IMU data file (CSV) and (B) Instrument Settings (.ini). Again, these data may be split into parts due to having to reset the instrument and/or computer during the flight.
  (A) IMU data columns: There are 75 columns in these files. We detail only the salient data columns here and we encourage the use of the user manual for explanations of the additional columns.
      (Column 3) GPS TOW (seconds, float): GPS time that rolls over and resets at zero. This is the time that the IMU logger displays in the logging software and should be used to time-sync with the laser and gps data using the photographs of the two data logging screens which are in the imu folder.
      (Column 4,5,6) X,Y,Z Accel (float,g): This is a vector quantifying the direction and magnitude of the acceleration that the 3DM-GX5-15 is exposed to. This quantity is fully temperature compensated and scaled into physical units of g (1 g = 9.80665 m/sec^2). It is expressed in terms of the 3DM-GX5-15's local coordinate system.
      (Column 7,8,9) X,Y,Z Gyro (float, radians/second): his is a vector quantifying the rate of rotation (angular rate) of the 3DM-GX5-15. This quantity is fully temperature compensated and scaled into units of radians/second. It is expressed in terms of the 3DM-GX5-15’s local coordinate system.
      (Column 10 11 12) X,Y,Z Delta Theta (float, radians): This is a vector which gives the time integral of angular rate over the interval set by the IMU message format command. It is expressed in terms of the 3DM-GX5-15's local coordinate system in units of radians.
      (Column 13,14,15) X,Y,Z Delta Vel (float,g*seconds): Delta velocity vector. This is a vector which gives the time integral of specific acceleration over the interval set by the IMU message format command. It is expressed in terms of the 3DM-GX5-15's local coordinate system in units of g*second where g is the standard gravitational constant. To convert Delta Velocity into the more conventional units of m/sec, simply multiply by the standard gravitational constant, 9.80665 m/sec^2.
      (Column 16) Pressure (float, milliBar): Scaled Ambient Pressure. This is a scalar which gives the instantaneous ambient pressure reading. This quantity is fully temperature compensated and scaled into units of milliBar.
      (Column 17-25) 3x3 Orientation Matrix M11,M12,M13,M21,M22,M23,M31,M32,M33 (float): This is a nine component coordinate transformation matrix which describes the orientation of the 3DM-GX5 with respect to the fixed earth coordinate system.
      (Column 26,27,28) Euler Angles Roll,Pitch,Yaw (float, radians): This is a three component vector containing the Roll, Pitch and Yaw angles in radians. It is computed by the INS from the orientation quaternion Q.
      (Column 29-32) Quaternion q1,q2,q3,q4 (float): Estimated Orientation in quaternion form. This is a four component quaternion which describes the orientation of the 3DM-GX5 with respect to the fixed earth coordinate system.
      (Column 33,34,35) North X,Y,Z (float, Gauss): Gyro stabilized estimated vector for geomagnetic vector.This is a vector which represents the complementary filter's best estimate of the geomagnetic field direction (magnetic north). In the absence of magnetic interference, it should be equal to Magnetometer. When transient magnetic interference is present, Magnetometer will be subject to transient (possibly large) errors. The IMU/AHRS complementary filter computes Stabilized North which is its estimate of the geomagnetic field vector only, even thought the system may be exposed to transient magnetic interference. Note that sustained magnetic interference cannot be adequately compensated for by the complementary filter.
      (Column 36,37,38) Up X,Y,Z (float, G): Gyro stabilized estimated vector for the gravity vector. This is a vector which represents the IMU/AHRS complementary filter's best estimate of the vertical direction. Under stationary conditions, it should be equal to Accel. In dynamic conditions, Accel will be sensitive to both gravitational acceleration as well as linear acceleration. The Complementary filter computes Stab Accel which is its estimate of the gravitation acceleration only, even thought the system may be exposed to significant linear acceleration.
  (B) Instrument settings file. One should reference the user manual to decode this file. It consists of listing the setting commands and replay descriptions which need tables in the user manual to decode.
  
(4) Kinematic GPS Data, Base and Rover (gps_unvaco) 2018 only

Instrument and Setup: The GPS Base Station placed on the side of Negribreen (Trimble NetR9) and the GPS Rover, mounted on helicopter skid opposite of the ULS (Septentrio APS-3G). Note there is no base station data for flight 1 in 2018 since we did not make it to Negribreen during this flight.

Data types: (A) Raw base data (.T02), (B) Raw rover data (.sbf,.sbf.A), (C) Processed data (CSV, TXT, XLSX). 
  (A) Raw base data. These data are in a format generated by the Trimble Instrument and must be processed using the Trimble Business Center (TBC) service. Both 15Hz and 1Hz data are available.
  (B) Raw rover data. These sbf files are Septentrio Binary Files. One must use tools from the Septentrio RxTools Suite to convert these files to more common, human-readable formats. Flight 1 contains additional .sbf files generated by the UNAVCO engineer in the folder titled ``additional_sbf".
  (C) Processed data (Flight 2 only). Created by the UNAVCO Engineer as a CSV in .csv, .xlsx and/or .txt format. 10 Hz data.
      (Column 1) Point (string): Data point identifier
      (Column 2) Northing (float): Y coordinate in the UTM coordinate system, Zone 34
      (Column 3) Easting (float): X coordinate in UTM coordinate system, Zone 34
      (Column 4) Elevation (float, meters): Elevation, or altitude, of the GPS above mean sea level.
      (Column 5) Horizontal Quality (string): Useless string always of the value ``Survey".
      (Column 6) Latitude (Local) (float): The local latitude in reference to the local datum (UTM 34).
      (Column 7) Longitude (Local) (float): The local longitude in reference to the local datum (UTM 34).
      (Column 8) Latitude (Global) (float): The  latitude in reference to the global datum (WGS84).
      (Column 9) Longitude (Global) (float): The  longitude in reference to the global datum (WGS84).
      (Column 10) Ellipsoid Height (float, meters): Height above the global reference ellipsoid.
      (Column 11) Start time (Date format): GNSS Vector Observation Start time given as ``M/DD/YYYY H:MM:SS AM/PM"
      
(5) Imagery and Video Imagery

Instruments and setup: (A) (D5100) A handheld Nikon D5100 camera operated by the person in the front passenger seat of the helicopter. This imagery is time stamped. (B) (L120-1)(L120-2) Two handheld Nikon Coolpix cameras (L120-1 amd L120-2) operated by the two rear seat passengers. These images are also timestamped. (C) (GoPro) GoPro Hero 5 Imagery (2 Hz) mounted to the underside of the helicopter (2018 flight 2 only). The GoPro was set to linear mode. (D) (video-aiptek) Aiptek Video Recorder (2017 flight 1 only).  The video file did not record for the whole flight due to subpar battery life. Exact model and specs are unknown. Each of these imagery and video imagery types contain an image of the SYS_TIME given on the linux data logging computer screen, which is used to sync the data to the GPS data.

Data Types: (A) JPG. (B) JPG. (C) JPG. (D) MOV. The directories for the imagery are in their default structure as given by their SIM cards. Some image directories contain a reference image that have plotted the photograph numbers over the GPS track over Negribreen.