Control file

For convenience, we will consider the control file as consisting of a number of lines, each containing one of more data values. In practice, the program simply reads data values one after another, so they may be entered from a keyboard or may be concatenated into a single string, provided each is separated for the next by one or more spaces. Most data values are mandatory, but some are optional and depend upon the specific choices made by the user, so not all 6S control files will contain the same number of data values.

Line 1 : This line contains a single number indicating whether 6S should calculate the view and illumination geometry for a specific satellite observation, or use the data provided by the user. If one of the pre-set satellite platforms is chosen, the user has to enter the exact time and date of sensing, and 6S then calculates the view and illumination position based on the orbital geometry of the selected platform.
0 = Base calculations on view/illumination geometry entered by the user.
1 = Meteosat
2 = GOES East
3 = GOES West
4 = NOAA AVHRR (pm observation)
5 = NOAA AVHRR (am observation)
6 = SPOT HRV
7 = Landsat TM
(Mauro Antunes version of 6S has an additional option: 8 = Landsat 7 ETM+)

Line 2. If a value of 0 is entered in Line 1, then this line contains six numbers describing the position of the Sun in the sky at the time of sensing and the viewing angles:
1. Solar zenith angle (degrees)
2. Solar azimuth angle (degrees)
3. View zenith angle (degrees)
4. View azimuth angle (degrees)
5. Month of year (integer)
6. Day of month (integer)
Example : 40.0 100.0 45.0 50.0 7 23

OR

Line 2. If a particular satellite is chosen in Line 1, then this line should contain the following values:
1. Month of observation
2. Day of observation
3. Time of observation (Universal Time, expressed as decimal hours : hh.ddd)
4. Column number / Longitude of pixel (see 6S code for details)
5. Row number / Latitude of pixel (see 6S code for details)
(Mauro Antunes version of 6S works on whole images, the original 6S code provides results for individual pixels)

Line 3. This line contains a single number indicating whether one of the predetermined atmosphere models should be used or whether the user's own data should be used to represent the absorption, primarily by water vapour and ozone, of energy passing through the atmosphere.
0 = Assume no gaseous absorption
1 = Tropical atmosphere model
2 = Mid-latitude summer model
3 = Mid-latitude winter model
4 = Sub-arctic summer model
5 = Sub-arctic winter model
6 = Use the US Standard Atmosphere (US62)
7 = Use an atmospheric model based on a radiosonde profile supplied by the user
8 = Use an atmospheric model based on measurements of the amount of water vapour in the atmosphere at the time of sensing and either a measurement or an estimate of the amount of ozone present at the time of sensing. The vertical distribution of water vapour and ozone is taken from the US Standard 62 Atmosphere.

The various 'typical' atmospheres are represented by the following amounts for water vapour and ozone:

Line 4. If the user entered '7' in Line 3, then the following line(s) should contain five values for each radiosonde observation:
1. Altitude (km)
2. Pressure (mb)
3. Temperature (degrees K)
4. Density of water vapour (g/m3)
5. Density of ozone (g/m3)
Example : ??

OR

Line 4. If the user entered '8' in Line 3, then this line contains two values:
1. The amount of water vapour in the atmosphere at the time of sensing, expressed in g/cm2. This value is often derived from a sunphotometer operating on the ground.
2. The amount of ozone in the atmosphere over the site at the time of sensing, expressed in cm-atm (?). This is rarely measured directly at the site, but is estimated from regional data as it is assumed to change slowly through time and across space.
Example : 3.0 0.35

Line 5 : This line contains a single number indicating whether one of the predetermined aerosol models should be used or whether the user's own data should be used to determine the scattering of EMR by aerosols in the atmosphere. A comment is the 6S code states that the aerosol calculations are only valid for visibilities greater than 5km.
0 = No aerosol model
1 = Continental model
2 = Maritime model
3 = Urban model
4 = User's own model, based on the proportions of four types of aerosol (see below)
5 = Shettle model for background deserta aerosol
6 = Biomass burning
7 = Stratospheric model
8 = User's model, based on multimodal log normal distribution (see 6S code for details)
9 = User's model, based on modified gamma distribution (see 6S code for details)
10 = User's model, based on Junge Power-Law distribution (see 6S code for details)
11 = User's model, based on sunphotometer measurements at the time of sensing (see 6S code for details)
12 = Data retrieved from a file (see 6S code for details)

Line 6. If the user enters '4' in Line 5, the the next line should contain four values:
1. Volumetric percentage of dust-like aerosols (as a proportion 0 to 1.0)
2. Volumetric percentage of water soluble aerosols (as a proportion 0 to 1.0)
3. Volumetric percentage of oceanic aerosols (as a proportion 0 to 1.0)
4. Volumetric percentage of soot aerosols (as a proportion 0 to 1.0)
Example (lines 5 and 6):
4
0.25 0.25 0.25 0.25

Line 7. This line contains data on the hazyness of the atmosphere, represented either as the horizontal visibility or the aerosol optical thickness at 550 nm, measured using a sunphotometer.
1. Horizontal visibility (km). If a value of 0 is used, the next number is taken to be the aerosol optical thickness at 550nm (dimensionless). If a value for horizontal visibility is entered then the aerosol optical thickness is computed from a standard atmospheric profile. If you selected 'No aerosol model' in Line 5, enter a value of -1 for horizontal visibility.
2. Aerosol Optical Thickness at 550 nm
Example : 0 0.50

Line 8. Height of the ground within the pixel above sea level (km). This is entered as a negative number in 6S but a positive number in the Mauro Antunes version (?).

Line 9. Height of the sensing platform above the ground (km). This is entered as a negative number, where 0 means that the sensor is at ground level, -1000 means that it is on-board a satellite, and a value between 0 and -100 is the altitude of the platform in km above the target.

Lines 10 and 11. For aircraft observations, additional values must be entered for the water vapour content, ozone content and aerosol optical thickness at 550nm between the aircraft and the surface. Sensible default values will be chosen if these data are not available (see 6S code for details).

Line 12. This gives the details of the spectral band being sensed. The user can choose a band from a number of pre-set values, or can enter their own band, based on a filter function. The full list of pre-set bands is quite long and can be found in the 6S code.

Line 13. In 6S, this is the ground surface type. A value of 0 indicates a homogeneous surface, whereas a value of 1 indicates a non-homogeneous surface.

Line 14. In 6S, if the value on Line 13 indicated a homogeneous surface, the value on this line indicates whether it is has no directionality (0) or has a directional effect (1). If a directional effect is specified then further information on the directional properties of the surface must be supplied (see 6S code for details). Several standard brdf models are provided.

OR

Line 14. If the value on Line 13 indicates a non-homogeneous surface, the values on this line describe its relationship with its surroundings : first, the reflectance of the target, then the reflectance of the surroundings, and finally, the radius of the target (which is assumed to be circular).

Line 15. This contains a single number which determines whether the code should calculate the surface reflectance from a radiance value provided by the user (units : W/m2/str/micrometre) , or from a value of apparent reflectance outside the Earth's atmosphere (sometimes called the TOA reflectance). A value greater than zero indicates the former, while a value between 0 and -1 indicates the latter. A value of -1 indicates that no atmospheric correction is to be performed.

An example of a 6S control file