Differential GPS (DGPS) data are time series of three-dimensional airplane coordinates that provide increased accuracy and precision compared to standard GPS measurements.  DGPS data were collected using a Sensor Systems model # S67-1575-76 L1/L2 antenna and NovAtel OEM4 dual-frequency (L1/L2) receiver.  The DGPS antenna on the GV aircraft is located 0.57 m left, 3.25 m aft and 1.25 m up from the static pressure sensor.

Raw measured GPS antenna height referenced to the NAD83 geoid is given as ALTDG.  If compared to the Pressure Altitude, this value should be adjusted by accounting for the aircraft attitude values (pitch and roll), which affect the relative positions of sensors.  Significant effect on relative height is caused by both pitch and roll changes and can be as high as &plusm;1 m when pitch reaches its extremes at about &plusm;10 deg and roll at &plusm;40 deg.  In the provided dataset, ALTXDG is the DGPS height corrected for pitch and roll normalized to the location of the static pressure sensor.

Horizontal coordinates are not attitude corrected in the data files.  Note that DGPS antenna is the same that the standard research GPS is using, so no correction is needed when comparing GPS and DGPS data.

Note that the sampling rate for the DGPS is 10 sps.  Low rate data are produced by selecting every 10th data point from the raw data set.  High rate (25 sps) data are produced by linear interpolation of the adjacent 10 sps values.  Original 10 sps data that may be more suitable for frequency analyses are also part of the archive.

   Variable definitions:
      ALTDG    DGPS antenna altitude (MSL, NAD83)
      ALTXDG   DGPS altitude PSX reference (MSL, NAD83)
      HOUR     Raw Tape Time Component
      LATDG    DGPS latitude
      LONDG    DGPS longitude
      VEWDG    DGPS Ground Speed Vector, East Component
      VNSDG    DGPS Ground Speed Vector, North Component
      VSPDDG   DGPS vertical velocity