*** NCAR RAF ***

Differential GPS data

C-130 aircraft, ICE-L 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 0.89 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 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 ±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 provided earlier in this file.

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.



During ICE-L, the differential GPS suffered a severe degradation of signal quality from RF08 on. The reason for this is unknown but is thought to be RF interference from another instrument onboard the C-130. Data accuracy for RF08-RF12 is reduced with average altitude accuracy of approximetely 0.5-1m. If using that data for highly sensitive calculations, users are advised to request the source DGPS data only for the flights they need to help assess the accuracy of the DGPS altitude data for each individual flight.



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





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