Imaging SAR is of limited utility at these scales except for the 

 identification and location of oceanic fronts at water mass boundaries. There 

 is not a great deal of experience concerning the detection threshold contrast 

 in properties across such a boundary, though some indication of this might be 

 extractable from existing SAR imagery. 



(b) Often embedded in or adjacent to these large features are medium- 

 scale, low-frequency structures with horizontal scales of 5-100 km and 

 temporal scales of 1-5 days. These are less well-known than the larger scale 

 motions but are only semi-geostrophic, are subject to horizontal advection by 

 larger scales, and could be rapidly evolving and difficult to measure with 

 passes repeated at time intervals larger than 5 days. These features have 

 been observed in SAR imagery in the coastal zone and as smaller scale, 

 filamented structures embedded in larger scale currents, and their geometrical 

 features could be mapped by repeated observation. They can also be detected 

 by IR. They could be measured with SARD or by altimetry, though the water 

 velocities involved (5-50 cm) and their relatively small scales put them near 

 the limits of resolution, 



(c) Medium-scale tidal motions form the dominant current signal in 

 coastal areas. Horizontal scales depend on topography and are 

 characteristically 5-50 km. Such motions are predominantly barotropic 

 (unrelated to the density field) and extend throughout the water column, with 

 vertical surface displacements of 1-10 m and horizontal currents of 10-200 cm. 

 They produce streaming and rifts related to bottom topography and give notable 

 SAR imagery, in particular, locations such as the Nantucket Shoals and the 

 Southern North Sea. For the measurement of these motions by remote sensing, 

 altimetry would be preferred as an operational tool, or SARD if it becomes 

 available. 



(d) Small-scale motions (50 m-5 km) include internal waves already 

 described, as well as, possibly, wind-wave generated Langmuir cells. The 

 latter are close to the limit of resolution of SAR and may be best detected 

 optically (as they have been in the past). 



(e) Estuarine flows are of considerable significance in fields from 

 coastal engineering to marine biology. Questions of sediment transports, 

 storm surges, river discharges, and tidal exchanges (both patterns and 

 velocities) are of both research and operational interest. Interesting 

 patterns can be discerned in SAR imagery and altimetry may be useful, although 

 it may not offer the clear advantages over traditional methods that remote 

 sensing does offer offshore. SARD would be extraordinarily useful in 

 delineating the often complex current patterns; IR has demonstrated the 

 existence of biologically important estuary fronts. 



If SARD becomes operational, there would be great oceanographic interest 

 in applications to all of these areas except possibly (d), and the need would 

 be continuing. Unique SARD capabilities include the ability to measure wind 

 driven and geostrophic motions and the capability of medium-scale mapping. In 

 addition to general coverage, it would also be of utility to special-purpose, 

 local oceanographic studies. 



4-4 



