GNSS Estimates of Vertical Land Motion at Tide Gauges: Brief History, Present Status and Current Challenges

Abstract:

Precise estimates of sea level change from tide gauges require precise knowledge of the vertical land motion at the tide gauges, as these signals can be comparable in magnitude. This is also true for estimates from satellite altimeters because the tide gauge data are needed to assess the temporal stability of the altimetric heights. The presentation will begin with a very brief review of early efforts to apply GNSS data to this problem. We will then present the current status of efforts (with some emphasis on the TIGA program) aimed at monitoring vertical motions at the global GLOSS sea level network and at the set of gauges that are deemed most useful for estimating the stability of satellite altimeters. This summary will document a tremendous amount of progress over the past decade or so, and continued progress seems likely. There are, however, two current challenges that we argue need to be addressed. First, completing the global network means expanding the network into much more challenging parts of the world. The GLOSS program has long experience with this problem while trying to complete the tide gauge network, and that experience should be very useful to the groups attempting to complete the GNSS network. Second, we will present an analysis based on the calculations used to estimate satellite altimeter stability that indicates that the uncertainties in the reference frame used for the GNSS data are possibly the dominant error when attempting to measure global sea level change. The scale rate error is of primary concern, with the error in the z-rate being a minor, but not insignificant, contributor. We will make this point via a set of simulations using varying levels of the scale rate and z-rate uncertainties; i.e., by doing a sensitivity analysis.