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Poster Session
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Remote determination of the sea-surface chlorophyll a concentration along the coastal water by FY-1 satellite
FY-1 Data Processing
- FY-1 Satellite System and Senior.
FY-1 satellite is in a relatively low orbit around the Earth. Its altitude is 900 km approximately. Table 1 lists the main orbital parameters. A main sensor aboard the FY-1 is a Very-High Resolution Scan Radiometer (VHRSR) It has 5 spectral channels, including 2 ocean channels in the blue-green region. Table 2 lists the wavelength region and the use of VHRSR.
Table. 1 FY-1 satellite orbital parameters
| Altitude |
h=901 km |
| Orbital Inclination |
i=99° |
| Orbital Period |
T=102.86 min |
| Eccentricity |
e=0.005 |
Table. 2 VHRSR's vavelength region and the use
| Channel |
Wavelength Region (µm) |
Use |
| 1 |
0.58-0.68 |
Daytime cloud and surface |
| 2 |
0.725-1.1 |
Daytime cloud image and water,ice,snow and vegetation observation |
| 3 |
0.48-0.53 |
Ocean colour image |
| 4 |
0.53-0.58 |
Ocean colour image |
| 5 |
10.5-12.5 |
Diurnal cloud image, surface observation and sea surface temperature |
- Defintion of Specfral Equivalent Albedo
The VHRSR four visible channels data are related to the spectral equivalent albedoes. Generally, the spectral equivalent albedo is defined as a ratio of the object upwelling irradiance and the downwelling solar irradiance of the top of the atmospheres. (7) In fact, most of the objects on the earth surface do not have an ability of the visible emission. They only reflect or scatter sun light. So that, we have to assume that each object on the earth surface is an equivalent source of visible mission. According to above definition, the spectral equivalent albedo As can be written as:
As = Es / Eo ----------------(1)
Where Es is the upwelling object irradiance reached to sensor. Eo is the downwelling solar irradiance on the top of the atmosphere. in our study, the Es is divided into two components: (1) the contribution from the scattering of solr irradiance by the atmospheric constituents, and (2) the contribution form the irradiance which is diffusely reflected by the object on the earth's surface. The contribution from (1) is usually referred to as the path irradiance, which will be denoted by Ea. Let Ew denote the contribution from (2) at the earth's surface, then the irradiance reaching the sensor is Ew x Ta, where Ta is the diffuse transmittance as given by Gordon et al.
(8) Therefore
Es = Ew * Ta + Ea -----------------(2)
Then, Equation (1) can be rewritten as:
As = Ew / Eo * Ta
+ Ea / Eo = Aw * Ta
+ A a ---------------(3)
Where Aw = Ew / Eo and Aa = Ea / Eo are called the spectral equivalent abed of the object and atmosphere, respectively.
- Estimate the Atmospheric Equivalent Albedoes of Two Ocean Channels
The percentage spectral equivalent albedo As, which can be calculated from the VHRSR visible channel digital number DN has the form
As = G * DN + I -------------(4)
Where G is the perecntage spectreal albed per count and I is the percentage intercept albedo.
For remote sensing ocean colour experiment, the sea-surface equivalent albedo can be estimated from equation (3) if the transmittance and the equivalent albedo of atmosphere are known. Unfortunately, we could not directly obtain the transmittance and the equivalent albedoes of atmosphere for the visible channels. But we well known that the infrared radiance was almost absorbed by water body, especially, by the open seawater. We shall assume that determined equivalent albedo by channel 2 on the orbit equate to its atmospheric equivalent albedo approximately, i.e
As (2) = Aa (2) ------------(5)
On the other hand, according to Duntly's method (9) of measuring atmospheric parameter from ground, we measured the atmospheric transmittance and path radiances using a four visible channels radiometer in which the center wavelength of each channel is the same as VHRSR's. A
relationship of atmospheric path radiances between channel 2 and other channel is showed as.
La (1) / La(2) = (l1 / l2)B ---------------(6)
Where l1 is the centre wavelength of two channels and B=3.1665. Using above result and La = Ea/p , As = Ea/Eo,
the atmospheric equivalent albedoes of two ocean channels could be estimated.
Aa(3) = As(2) * Eo(2) / E0(3)
* ( l3 / l2 )B
and
Aa(4) = As(2) * ( Eo(2) / Eo(4) ) *
( l4 / l2 )B ------------(7)
Where Eo(2), Eo(3), Eo(4) are the solar irradiances on the top of the atmosphere of
VHRSR's channe 1,2,3,4 respectively. Its unit is mw/cm2 their values are given as following:
Eq....8
Where Eo (l) is the recommended value of the average solar irradiance on the top of atmosphere.
(10) Dl is the wavelength width band of each channel.
f(l) is the relative response function of the VHRSR. Fig. 1 shows variation of the wavelength of four visible channels. (11)
Fig. 1 Relative spectral response curves of VHRSR
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