Real-time Gulf Stream Monitoring from ERS-1 altimetry

F C Vossepoel (Delft Institute for Earth-Oriented Space Research,
      Delft University of Technology, Kluyverweg 1, 2629 HS Delft, 
      The Netherlands; 31-15-278-1483; e-mail: F.Vossepoel@lr.tudelft.nl)
	  
J L Lillibridge (NOAA Geosciences Laboratory, SSMC-IV # 8342,
      1305 East-West Highway, Silver Spring, MD  20910; 301-713-2857; 
      e-mail: johnl@bigbird.grdl.noaa.gov)
	  
Since November 1995, NOAA & DUT have been generating real-time altimetry data
from ERS-1, in the form of RGDRs. These are based on the ERS-1 Fast-Delivery
altimeter data disseminated by the European Space Agency. The data arrive at
NOAA within 6 hours of acquisition, but at that point contain only crude
geophysical corrections and inadequate orbit information to transform the
altimeter range into sea surface height.

The most important enhancement to the data is the application of the Delft
JGM-3 orbits. Because of delays in obtaining tracking data, the computed
orbits are not available for 7-10 days. We utilize a predicted orbit phase,
which is a 6.5 day extension to each orbital solution. The predicted orbit
remains accurate at the sub-meter level, based on crossover differences.

NOAA then provides the following geophysical corrections:  ocean+load tides
from the Univ. of Texas CSR 3.0 tide model; solid earth tide; ionosphere
correction from the IRI90 model; wet and dry troposphere corrections based on
NMC model grids; and a wet troposphere correction from the SSM/I sensor
onboard the DMSP satellites.

The sea surface heights are combined with these geophysical corrections and a
surface fitting technique is applied to reduce the residual orbit error. This
yields sea surface height anomalies relative to the mean sea surface. The
anomalies are averaged into bins and the relative dynamic topography is
determined by interpolation.  A semi-permanent "mean" dynamic topography is
added to obtain an estimate of the total dynamic topography.

The geostrophic velocities computed from this dynamic topography are then used
to study the behaviour of the Gulf Stream. The immediate availability of the
RGDRs makes it possible to study eddy velocities and Gulf Stream meandering on
a daily basis, and in all weather conditions.  A comparison of velocity field
nowcasts with hindcasts based on the operational ( non-predicted ) orbits
demonstrates the reliability of the real-time altimeter product.  Gulf Stream
path and ring locations in the flow fields coincide with those seen in sea
surface temperature imagery. The RGDRs provide an opportunity to assimilate
reliable real time altimeter data into forecast models of the Gulf Stream.