Towards 1 MilliGal Global Marine Gravity Accuracy from CryoSat, Envisat, and Jason-1
Sandwell, David1; Garcia, Emmanuel1; Soofi, Khalid2; Wessel, Paul3; Smith, Walter4
1Scripps Institution of Oceanography, UNITED STATES; 2ConocoPhillips Co., UNITED STATES; 3University of Hawaii, UNITED STATES; 4NOAA, UNITED STATES
Marine gravity from satellite altimetry has become a primary tool for investigating the tectonics of the remote ocean basins as well as unexplored continental margins. Gravity field accuracy depends on four factors: spatial track density (especially near coastlines); altimeter range precision; diverse track orientation; and the accuracy of the coastal tide models. Recently three new non-repeat altimeter data sets are becoming available. (1) CryoSat has been collecting data over the world’s oceans for the past 31 months. (2) In October 2010, the Envisat radar altimeter was placed in a new partly drifting phase orbit to conserve fuel. Although the spacecraft failed in April 2012, it was able to collect 18 months of data along this new ground track. (3) In May of 2012, the Jason-1 radar altimeter was maneuvered from its 10-day repeat track to a 406 day repeat track to begin its new geodetic mission phase. The accuracy of the global marine gravity field derived from these data has been established using high accuracy shipboard gravity from the Gulf of Mexico (GOM) and the Canadian Arctic. The results of these and other comparisons provide the following conclusions:
- New altimeter data from CryoSat, Envisat, and Jason-1 have 1.3 times better range precision than the older data from Geosat and ERS-1. In addition the newer satellites contribute 60 months of new data compared with the 31 months of data provided by the older satellites. These two improvements provide nearly a factor of 2 improvement in gravity accuracy at lower latitudes and a factor of 2-3 improvement in Arctic and Antarctic regions where seasonal ice cover has prevented high precision altimeter measurements.
- Most of the improvement in the altimeter-derived gravity field occurs in the 14 to 40 km wavelength band, which is of interest for investigation of structures as small as 7 km.
- The current version of the altimeter-derived gravity field has an accuracy of 1.7 mGal in the Gulf of Mexico and 2.4 mGal in the Canadian Arctic. Unlike terrestrial gravity where coverage is uneven, these accuracies are available over ALL marine areas and large inland bodies of water so this gravity provides an important resource for exploration of remote sedimentary basins.
- The altimeter derived gravity field is about 2 times more accurate than the shipboard gravity collected by the academic institutions. However, some shipboard data, more carefully collected by industry (e.g., EDCON), is three times better than the altimeter-derived gravity.
- Jason-1 has completed only 1/2 of its 406 geodetic phase as this paper is being prepared. Data to be collected between February and July 2013 will provide an additional improvement in the E-W component of gravity. Moreover, CryoSat has enough capacity to collect altimetry data for another 5-7 years. This will provides significant improvement in gravity accuracy, especially over ice-covered ocean regions.
- New global gravity models derived from GRACE and GOCE will improve the accuracy of the gravity models at longer wavelengths (i.e. > 200 km).