Ground Penetrating Radar (GPR) Navigation Data Collected from Dauphin Island, Alabama in April 2013

Navigation Acquisition: Location information associated with each GPR line was determined by post-processed differential correction using a base/rover setup. The field setup consisted of two (rover and base station) Ashtech ProFlex Global Navigation Satellite System (GNSS) receivers and antennas. The GNSS base and rover receivers recorded the raw full-carrier-phase positioning signals (L1/L2) from satellites, via Ashtech GNSS antenna, concurrently at 0.2 seconds (s) (5 Hertz [Hz]) intervals throughout the survey. The base station receiver was positioned on the National Geodetic Survey (NGS) published reference point NGS PID# BH1755 (Station ID: 8735180), located on the east side of the island, at the entrance to Fort Gaines. The vertical offset between the GNSS antenna and the NGS benchmark was 2.476 m. The rover GPS was mounted on a fixed frame above the GPR unit. The offset between the rover GNSS antenna (located on the GPR unit) and the ground was 1.915 m. Uncorrected positions were output from the rover GPS to the GPR as a National Marine Electronics Association (NMEA) GGA string at a 9600 baud rate.

Navigation processing: The coordinate values for the GNSS base station were checked using the time-weighted average of values obtained from the NGS On-Line Positioning User Service (OPUS). The average of the coordinates from all the occupations collected at the base during the survey did not differ significantly from the NGS published coordinates; therefore, the published coordinates were used for post-processing. The kinematic GNSS data from the GPR were post-processed using the concurrent GNSS data from the base station. The raw base station and rover data were imported into NovAtel GrafNav version 8.4 software and were post-processed by differential correction. During processing, steps were taken to ensure that the trajectories between the base and rover were clean, resulting in fixed positions. By analyzing the graphs, trajectory maps, and processing logs that GrafNav produces for each GNSS session, data from satellites flagged by the program as having poor health or satellite time segments that had cycle slips could be excluded, or the satellite elevation mask angle could be adjusted to improve the position solutions. The final, differentially corrected, precise DGPS positions were computed at 1-s intervals for each roving GNSS session and exported in American Standard Code for Information Interchange (ASCII) text format (as a GGA string), which replaced the uncorrected real-time rover positions recorded during acquisition. The GPS data were acquired, processed, and exported in the World Geodetic System of 1984 (WGS84) (G1150) geodetic datum. During acquisition of lines 66-116, the signal between the GPS rover unit and multiple satellites was interrupted or lost, most likely due to dense tree canopy within the survey area. This lack of communication with the satellites caused either incorrect values to be recorded or missing/incomplete GPS data to be collected. For lines missing post-processed GPS and (or) elevation data, elevation and location data were extracted from a topographic light detection and ranging (lidar) survey, which was contracted by the USGS in July 2013 and used instead. In cases where the DGPS data were incomplete or corrupted and lidar data were unavailable, non-elevation corrected profiles have been provided. Lines with post-processed GPS and elevation data issues are noted in this metadata file.

Added keywords section with USGS persistent identifier as theme keyword.