96 
the absolute depth, but transects A and C do show that minima in the velocity correspond 
to minima in the depth (i.e. shallower areas). 
Another more qualitative comparison of the radar and sonar results is shown in Figure 6. 
This figure shows a combined radar intensity and velocity image (where the radial velocity 
has been mapped as colour ranging from 0.5 m/s S (red) through zero (yellow) to 1.5 m/s 
N (dark green)) contrasted with a simulated sun-illuminated view of the dune field created 
from the side scan sonar DTM. Many of the major dune features appear in both the radar 
and sonar images. The sonar image does give more detailed information than the radar and 
provides accurate depth information, but it is important to remember that it required a ship 
to travel to the dune field and then spend approximately 12 hours of sonar scanning to 
produce this image. The corresponding radar image was acquired in less than a minute. 
The InSAR detected many other known subsurface features during this mission including 
the Cape d’Or - Cape Spencer dune field and a large shoal off Cape d’Or. In most cases, 
the objects appeared in both the magnitude and velocity images, but in some cases debris 
on the water or other non-topographic factors made it difficult to observe a given feature 
in the magnitude image. The velocity image was not subject to degradation by these 
effects. One of the most exciting features imaged during the 1994 mission was a small 
dune field just SW of Advocate Harbour, which produced a similar signature of intensity 
and velocity modulations as the Scots Bay dune field. This dune field did not exist on any 
existing Canadian Hydrographic Services maps and gave a perfect example of how the 
ability of the airborne SAR to cover very large areas allowed it to locate subsurface 
features which had previously gone undetected. Verification of the location of the dune 
field was provided by local fisherman who were familiar with the shoals and dunes in the 
immediate vicinity of Advocate Harbour and a sonar based survey is now planned for 
1996 to produce a detailed topographic map of this dune field. 
5. CONCLUSIONS 
This study has revealed several things about the CCRS airborne along track InSAR as a 
tool for studying ocean currents and subsurface features. It can produce high resolution, 
geocoded, calibrated velocity images which can be combined to produce accurate current 
velocity maps for currents which are stable over the time between passes (approximately 
twenty minutes under typical flight circumstances). Since there is currently no simple way 
of producing a high resolution current map over a large area, this may be a useful tool for 
examining the current patterns in river deltas, harbours, or other areas where a detailed 
knowledge of the currents will assist in predicting safe shipping paths or in environmental 
monitoring of erosion and sedimentation processes. The along track InSAR can also be 
used to detect subsurface features, especially those in depths less than 30 m which are 
more likely to pose hazards to shipping. The depth to height modulation ratio of 
detectable features is limited to somewhere between 5 and 10 and a lack of currents or 
surface wind will also adversely affect the performance of the InSAR. This means that ship 
based sonar surveys are reliable at deeper depths and can produce more detailed maps of 
subsurface features than airborne InSAR surveys. Ship based surveys, however, are very