*** NCAR RAF ***
Differential GPS data
C-130 aircraft, RICO project

Differential GPS (DGPS) data are time series of three-dimensional airplane coordinates that provide increased accuracy and precision compared to a 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 location of the DGPS antenna on the C-130 aircraft with respect to the Inertial Reference Unit (IRU) and the static pressure sensor is as follows:

DGPS antenna is located 0.89 m left, 10.01 m aft and 3.38 m up from the IRU.
DGPS antenna is located 0.71 m right, 9.83 m aft and 3.10 m up from the static pressure sensor.

Raw measured GPS antenna height referenced to the EGM96 geoid is given as H-MSL. 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. Most significant effect on relative height is caused by pitch changes and can be as high as 1.3 m when pitch reaches its extremes at about ±10 deg. In the provided dataset, H-CORR is the DGPS height corrected for pitch and roll normalized to the location of the static pressure sensor.

If comparing the latitude and longitude positions to those produced by the IRU one needs to account also for the yaw of the aircraft. 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.

Variable definitions:

UTCSECS		Seconds since midnight on flight day, UTC
DGPSLAT		Latitude, fractional degree
DGPSLONG	Longitude, fractional degree
SDHORIZ		Standard deviation, horizontal position, m.
H-MSL		Height above mean sea level (EGM96 geoid), m.
SDHGHT		Standard deviation of height measurement, m.
COG		Course over ground (track angle), deg.
VEAST		Aircraft velocity, east vector, m/s.
SD-VE		Standard deviation of east velocity, m/s.
VNORTH		Aircraft velocity, north vector, m/s.
SD-VN		Standard deviation, velocity, north vector, m/s.
VUP		Aircraft velocity, upward vector, m/s.
SD-VUP		Standard deviation, velocity, upward vector, m/s.
H-CORR		H-MSL corrected to PSX location, m.

If you have any questions about this data set, please contact Pavel Romashkin (pavel@ucar.edu).