*** NCAR RAF ***
Differential GPS data
G-V aircraft, START-08 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 GV aircraft with respect to the Inertial Reference Unit (IRU) and the static pressure sensor is as follows:

DGPS antenna 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 EGM96 geoid (based on WGS84 ellipsoid) 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 ±1 m when pitch reaches its extremes at about ±10 deg and roll at ±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.

If comparing the latitude and longitude positions to those produced by the IRU one needs to account also for the yaw of the aircraft. This dataset does not include DGPS position normalized to the IRU location but it can be calculated by individual researches as necessary by utilizing the attitude data along with the relative sensor location that can be provided by special request.

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 regular GPS and DGPS data.

While for most of the flights the DGPS accuracy is on average ±10-15 cm, the data suffered some degradation of quality on several northbound flights. The reason for this is the lack of readily available ground-based reference station data for Canada and very sparse spacial coverage by reference stations in Canada. For flights that were especially affected and overflown a reference station, the data were obtained from the Canadian authorities that helped improve DGPS data quality. For flights whose DGPS accuracy remained within ±25 cm, they were not. If the ±25 cm accuracy is insufficient for your research, individual flight accuracy can possibly be improved upon special request.

DGPS data were time shifted by -0.4 s in order to provide the best possible correlation with the standard GPS data.

Variable definitions:

ALTDG            DGPS antenna altitude (MSL, EGM96) [m]
ALTXDG           DGPS altitude PSX reference (MSL, EGM96) [m]
LATDG            DGPS latitude [degree_N]
LONDG            DGPS longitude [degree_E]
VEWDG            DGPS Ground Speed Vector, East Component [m/s]
VNSDG            DGPS Ground Speed Vector, North Component [m/s]
VSPDDG           DGPS vertical velocity [m/s]


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