To help us facilitate NGS' GRAV-D (Gravity for the Redefinition of the American Vertical Datum) GPS processing method development, we invited interested researchers and companies all over the world to compute and submit position solutions from samples of actual GRAV-D data. A number of kinematic techniques and methods suited for long distance flights have emerged over the past few years. Given the challenging nature of the GRAV-D flights (long distances, high altitudes, varying tropospheric regimes) and the importance of precise kinematic positioning to GRAV-D airborne gravity products, we believe that applying these techniques to such a common data set would be of great interest to the kinematic community.
A new call and new data for the Challenege flights is available until April 1, 2013. This second data release includes data from a different GPS receiver in the aircraft, as well as IMU (inertial measurement unit) data for use in tighly- or loosely-coupled GPS + IMU processing.
Data can be downloaded from our FTP site.
In Fall 2011, gravity processing using the Challenge positions was complete. By December 2011, advanced analysis was done and the results were presented at the American Geophysical Union Meeting.
Dr. Theresa Diehl's AGU Poster on the Gravity Results and additional materials
The analysis included several tests. A summary of the early findings is published in a technical blog for the GPS World website.
The first test was a comparison of each individual gravity result to the average of all results (called the ensemble), testing the precision of the gravity results. By then comparing the individual and ensemble gravity results to the results of a global model (EGM08), accuracy of the gravity results was also tested.
The second test compared NGS' best gravity result to the other Challenge results, the major difference being the inclusion of inertial measurement unit (IMU) data in the NGS gravity result. The NGS results is a noticably better match to the global model than the GPS-only results of the Challege participants. This underscored the need to release the IMU data as part of the Challenge and to always fly a working IMU on the GRAV-D aircraft.
The third test evaluated the repeatability of the gravity solutions, since one line of data was flown twice, each pass on a different flight. The software producing the highest correlation between the two passes is considered the most reliable and therefore the "best".
The fourth test broke down the corrections that are derived from the GPS and then applied to the raw gravity to produce the final gravity results. Some corrections has little effect on the final gravity, but others had significant affects (such as the offlevel correction).
A series of white papers detailing the gravity analysis will be posted here as they are completed. These white papers will become the basis for a peer-reviewed journal article about the Challenge results.
First analysis of the position solutions provided by Challenge participants was completed by Dr. Gerry Mader and a summary is published in a technical blog for the GPS World website.
Originally issued in August 2010
The National Geodetic Survey (NGS) is conducting a 12-year project to re-define the vertical datum of the US. This project, referred to as GRAV-D (Gravity for the Redefinition of the American Vertical Datum), is accomplished by flying airborne gravity missions over the continental and coastal US. Gravity solutions require accurate sensor positioning and accelerations, which in turn requires precise and accurate GPS solutions for the aircraft.
To help us facilitate our software and method development, we invite interested researchers and practitioners to compute and submit solutions from samples of actual GRAV-D data. A number of kinematic techniques and methods suited for long distance flights have emerged over the past few years. Given the challenging nature of these flights (long distances, high altitudes, varying tropospheric regimes) and the importance to GRAV-D, we believe applying these techniques to such a common data set would also be of great interest to the kinematic community.
This is a strictly voluntary exercise for those interested in such a comparison and we will share our results with the participants. We are also interested in possibly co-authoring a publication with the participants on the topic if results are significant.
Additional information, including a description of the test as
well as the test data set, is available at the following FTP site:
Please read through the test description and email Gerry Mader (email@example.com) if you plan to participate or if you have any questions. Also, please feel free to forward this announcement to anyone or group who you think might be interested but might not be on this mailing list. We look forward to your participation!
National Geodetic Survey