93 
conjugate of the other. The magnitude of each pixel in this interferogram is a measure of 
the target pixel’s radar backscatter and its phase, O, is a measure of the target’s radial 
velocity. The reason for this is that the doppler processed image from the second antenna 
represents the same scene as shown in the image from the first antenna but at a time which 
is later by the period taken for the aircraft (and thus the phase centre of the antenna) to 
travel the distance between the two antennas, t-dlV. In that time a target with radial 
velocity, v, will have moved a distance vt parallel to the zero doppler direction and thus 
its phase will have changed by 4 nvtlk, where A is the wavelength of the radar (5.656 
cm in the case of the CCRS along track InSAR). The phase of the interferogram can thus 
be directly translated to radial velocity using v -VX O / 2dn , where the reduction by a 
factor of two is due to the fact that the same transmit antenna is being used for both 
InSAR antennas and so the effective difference between the transmit/receive zero doppler 
positions is only d / 2. The necessity of exactly registering both images prior to combining 
them in an interferogram adds some complexity, but is not a problem provided the physical 
separation and electronic path lengths associated with each antenna are known. If the 
target becomes decorrelated over the time interval, t , however, there will be a problem 
because the two images will not be phase coherent and the interferogram will become 
meaningless. This is not a problem for the CCRS InSAR when viewing typical ocean 
targets, since t - dIV is typically around 5 ms whereas the ocean decorrelation time at 
C-band is expected to be between 50 and 100 ms. 
The first test of the CCRS along track InSAR took place on January 12, 1994 and 
demonstrated that the radial velocities determined by the InSAR system were accurate to 
within approximately 5 cm/s even though the InSAR antennas were stuck at their steering 
limits due to high cross winds [A. Laurence Gray et al., 1994]. 
2.2 CHS SURVEY SHIP 
The Canadian Hydrographic Service of the Department of Fisheries and Oceans provided 
the survey ship NSC Frederick G. Creed to take part in the June, 1994 experiment. It was 
equipped with a SIMRAD EM 1000 multibeam sonar, which had 60 separate beams, each 
of 3.3° by 3.3°. These beams were spaced 2.5° apart in an angular sector covering +75° to 
-75° from the vertical so that as the ship moved they could be used in a pushbroom 
imaging mode. The insonification frequency was 95 kHz using a 0.2 ms CW pulse. 
In standard survey mode the ship would cruise at 12-16 knots, covering a swath of a few 
hundred meters on the ocean bottom. The size of the swath varied because the bottom 
footprint of each sonar beam varied from 2-5 m at the depths where the surveys were 
being conducted. A typical survey of the Scots Bay dune field required approximately 
twelve hours. Since this corresponds to a full tidal cycle in which the water level could 
change by as much as 10 m, it was necessary to reference the bottom topography to 
depths below the WGS-84 ellipsoid rather than below the water surface. This was 
accomplished using differentially processed GPS data from a receiver on board the ship, 
which also allowed the x-y coordinates of the bottom topographical features to be 
converted to an accurate UTM map.