—— ; z 
of the CZCS detectors have remained the same as it was before the launch of 
NIMBUS-7 satellite and this assumption may not be true either. 
In addition to the above discussion on the inflight calibration quality, 
there is another type of loss in the sensitivity of the CZCS detectors which 
does not come within the domain of the inflight calibration. This sensitivity 
loss could be due to a deterioration of the material which is not used in the 
| path of calibration radiance and which is used in the path of the radiation 
a t reaching the CZCS detectors from the Earth (113. 
a ATMOSPHERIC CORRECTION QUALITY 
25 The atmospheric correction algorithm which is being used at present (11) 
00 is based on two simplifying assumptions which were suggested by Gordon (4). 
00 These assumptions are (a) the aerosol path radiance may be. evaluated by assuming 
00 no Rayleigh scattering, i.e. the Rayleigh optical thickness (or depth) TR = Q, 
00 and (b) the Rayleigh path radiance may be evaluated by assuming no aerosol 
00 i.e. the aerosol optical thickness (or depth) tp += 0. Strictly speaking, these 
00 assumptions cannot be satisfactory for the quantitative analysis of the CZCS data 
00 because for a real atmosphere the aerosol path radiance is attenuated by the 
00 Rayleigh atmosphere and the Rayleigh path radiance is attenuated by the aerosol 
90 medium. Therefore, an algorithm based on the assumptions (a) and (b) should 
45 underestimate the water-leaving radiance. 
45 It is not difficult to derive better expressions for the Rayleigh and 
05 aerosol path radiances by ignoring the assumptions (a) and (by. A complete 
45 report on this subject is being prepared. Here, we shall present a typical 
00 example in which we shall compare the water-leaving radiances obtained from two 
10 atmospheric correction algorithms; the OLD algorithm which utilizes the assumpt- 
25 ions (a) and (b), and a NEW algorithm which does not use these assumptions. 
75 If the aerosol path radiance Lpa (A) is evaluated using the assumption 
60 (a) then it can be shown that the aerosol path radiances at two wavelengths A1 
00 and Ap are related through 
eo Lp GO) 0 904:33) LO) (1) 
59) 
00 where the quantity a(Aj,\p) depends, apart from many other factors, on the 
10 aerosol optical thicknesses at wavelengths Aj and A5. The detailed nature of 
00 a(11,35) may be found in (6,11,14,18-20). Unfortunately, the aerosol optical 
‚00 thickness is a highly variable quantity and cannot be known a priori. In the 
65 case of clear water, Gordon et al. (18) and Gordon and Clark (20) have evaluated 
05 404,32) by measuring the upwelling radiance at one position in the CZCS image 
25 from a ship. This value of a(11,32) was then used for the clear water region 
. 00 in the CZCS image. Such a procedure of evaluating the quantity a(A41,32) have 
,59) some limitations. For instance, an observation from a ship is almost a point 
.00 observation whereas the satellite data is an average over the instantaneous 
.20 field of view corresponding to one picture element. The nature of the sea- 
,59) surface as seen by a remote sensor should be quite different from the nature of 
.95 the sea-surface seen by a detector on a ship. The ship data should be 
influenced by the wave heights, wave motion, ripples and by the very presence of 
the ship in the vicinity of the point of observation whereas the satellite data 
are averaged out over many waves and ripples. The presente of a ship within 
deviation the instantaneous field of view should not affect the satellite data severely. 
ounts to Therefore, it is rather difficult to grasp the idea that the use of the sattelite 
the data and in-situ ship data will result in a reasonably good value of a(11,22) 
s'active which can be used in the extended regions of clear water. Most importantly, 
anges in such a procedure of measuring as 32) for all scenes of interest is not possible. 
he exact It is apparent, therefore, that the main difficulty in the remote measurement of 
libration water colour lies in finding the best value of the aerosol optical thickness 
Ihe use (or depth). In the absence of the in-situ measurements, one may have to depend 
ce on an expression of the form 
alibrat- pj: ar Ob (2) 
quality A 
701 
m 
P 
P 
" t pm — " Ss pr = ‘ 
sea er SE AAA EEE AGE