## DESCRIPTION

i.atcorr performs atmospheric correction on the input raster using the 6S algorithm (Second Simulation of Satellite Signal in the Solar Spectrum). A detailed algorithm description is available at the Land Surface Reflectance Science Computing Facility website and Mauro A. Homem Antunes website about his 6s version.

Current region settings are ignored. The region is adjusted to cover the input raster before the atmospheric correction is performed. This should not affect the user's current region settings.

Because using a continuous elevation ialt or visibility ivis map makes execution time much longer, it is advised to use categorized maps instead, in conjuction with flag -o. This flag tells i.atcorr to try and speedup calculations. However, this option under certain conditions can make execution time longer.

If flag -r is used, the input data are treated as reflectance. Otherwise, the input data are treated as radiance values and are converted to reflectance at the i.atcorr runtime. The output data are always reflectance.

An example 6s parameters icnd file for i.atcorr:

    8                            - geometrical conditions=Landsat ETM+
    2 19 13.00 -47.410 -20.234   - month day hh.ddd longitude lattitude ("hh.ddd" is a decimal hour GMT)
    1                            - atmospheric mode=tropical
    1                            - aerosols model=continental
    15                           - visibility [km] (aerosol model concentration)
    -.600                        - target at 600m above sea level
    -1000                        - sensor on board a satellite
    64                           - 4th band of ETM+ Landsat 7

## REMAINING DOCUMENTATION ISSUES

1. Using the target elevation and visibility parameters in the icnd file overrides ialt and ivis input rasters. It is not clear what to do to force i.atcorr to use the rasters instead though.

2. The "example 6s parameters file" explains that "-.600" in line 6 means "target at 600 m ASL". However, in the section E of "6S CODE PARAMETER CHOICES" it reads: "xps <=0. means the target is at the sea level". This is contrary.

3. In section E, I'm not sure if the "-100< xpp <0" shouldn't actually be "-1000< xpp <0". ?

4. It is not explained what is the "iaer" parameter that section D refers to.

5. Section D's "Aerosol model concentration" title could use a better wording I suppose. The current one seems to mean "the concentration of the model of the aerosol". Should it be "Aerosol concentration model"?

6. It should be explained under what circumstances the use of categorized maps in conjuction with flag -o can slow down the calculations instead of speeding them up.

7. "This should not affect the user's current region settings" sounds ambigious.

## 6S CODE PARAMETER CHOICES

### A. Geometrical conditions:

Code | Description | Details
1 | meteosat observation | enter month,day,decimal hour (universal time-hh.ddd) n. of column,n. of line.(full scale 5000*2500)
2 | goes east observation | enter month,day,decimal hour (universal time-hh.ddd) n. of column,n. of line.(full scale 17000*12000)c
3 | goes west observation | enter month,day,decimal hour (universal time-hh.ddd) n. of column,n. of line.(full scale 17000*12000)
4 | avhrr (PM noaa) | enter month,day,decimal hour (universal time-hh.ddd) n. of column(1-2048),xlonan,hna give long.(xlonan) and overpass hour (hna) at the ascendant node at equator
5 | avhrr (AM noaa) | enter month,day,decimal hour (universal time-hh.ddd) n. of column(1-2048),xlonan,hna give long.(xlonan) and overpass hour (hna) at the ascendant node at equator
6 | hrv (spot) | enter month,day,hh.ddd,long.,lat. *
7 | tm (landsat) | enter month,day,hh.ddd,long.,lat. *
8 | etm+ (landsat7) | enter month,day,hh.ddd,long.,lat. *

> * NOTE: for hrv, tm and etm+ experiments, longitude and lattitude are the coordinates of the scene center. Lattitude must be >0 for northern hemisphere and <0 for southern. Longitude must be >0 for eastern hemisphere and <0 for western.

### B. Atmospheric model

Code | Meaning
0 | no gaseous absorption
1 | tropical
2 | midlatitude summer
3 | midlatitude winter
4 | subarctic summer
5 | subarctic winter
6 | us standard 62

7

Define your own atmospheric model as a set of the following 5 parameters per each measurement:
altitude [km]
pressure [mb]
temperature [k]
h2o density [g/m3]
o3 density [g/m3]
For example: there is one radiosonde measurement for each altitude of 0-25km at a step of 1km, one measurment for each altitude of 25-50km at a step of 5km, and two single measurements for altitudes 70km and 100km. This makes 34 measurments. In that case, there are 34*5 values to input.

8 | Define your own atmospheric model providing values of the water vapor and ozone content: uw [g/cm2] uo3 [cm-atm] The profile is taken from us62.

### C. Aerosols model

Code | Meaning | Details
0 | no aerosols |
1 | continental model |
2 | maritime model |
3 | urban model |
4 | shettle model for background desert aerosol |
5 | biomass burning |
6 | stratospheric model |

7

define your own model

Enter the volumic percentage of each component:
c(1) = volumic % of dust-like
c(2) = volumic % of water-soluble
c(3) = volumic % of oceanic
c(4) = volumic % of soot
All values between 0 and 1.

8 | define your own model | Size distribution function: Multimodal Log Normal (up to 4 modes).
9 | define your own model | Size distribution function: Modified gamma.
10 | define your own model | Size distribution function: Junge Power-Law.

11

define your own model

Sun-photometer measurements, 50 values max, entered as:
r and d V / d (logr)
where r is the radius [micron], V is the volume, d V / d (logr) [cm3/cm2/micron].
Followed by:
nr and ni for each wavelength
where nr and ni are respectively the real and imaginary part of the refractive index.

### D. Aerosol model concentration (visibility)

> If you have an estimate of the meteorological parameter visibility v, enter directly the value of v [km] (the aerosol optical depth will be computed from a standard aerosol profile).

> If you have an estimate of aerosol optical depth, enter v=0 for the visibility and enter the aerosol optical depth at 550nm.

> NOTE: if iaer=0, enter v=-1.

### E. Target altitude (xps), sensor platform (xpp)

> xps <=0 means the target is at the sea level.
> xps >0 means you know the altitude of the target expressed in km, and you put that value as xps.
> xpp=-1000 means that the sensor is on board a satellite.
> xpp=0 means that the sensor is at the ground level.
> -100<xpp<0 means you know the altitude of the sensor expressed in kilometers; this altitude is relative to the target altitude.

> For aircraft simulations only (xpp is neither 0 nor -1000):

> > puw,po3 (water vapor content,ozone content between the aircraft and the surface)
> > taerp (the aerosol optical thickness at 550nm between the aircraft and the surface)

> > If these data are not available, enter negative values for all of them. puw,po3 will then be interpolated from the us62 standard profile according to the values at the ground level. taerp will be computed according to a 2km exponential profile for aerosol.

### F. Sensor band

There are two possibilities: either define your own spectral conditions (codes -2, -1, 0, or 1) or choose a code indicating the band of one of the pre-defined satellites.

Define your own spectral conditions:

Code | Meaning
-2 | Enter wlinf, wlsup. The filter function will be equal to 1 over the whole band (as iwave=0) but step by step output will be printed.
-1 | Enter wl (monochr. cond, gaseous absorption is included).
0 | Enter wlinf, wlsup. The filter function will be equal to 1over the whole band.
1 | Enter wlinf, wlsup and user's filter function s(lambda) by step of 0.0025 micrometer.

Pre-defined satellite bands:

Code | Meaning
2 | meteosat vis band (0.350-1.110)
3 | goes east band vis (0.490-0.900)
4 | goes west band vis (0.490-0.900)
5 | avhrr (noaa6) band 1 (0.550-0.750)
6 | avhrr (noaa6) band 2 (0.690-1.120)
7 | avhrr (noaa7) band 1 (0.500-0.800)
8 | avhrr (noaa7) band 2 (0.640-1.170)
9 | avhrr (noaa8) band 1 (0.540-1.010)
10 | avhrr (noaa8) band 2 (0.680-1.120)
11 | avhrr (noaa9) band 1 (0.530-0.810)
12 | avhrr (noaa9) band 1 (0.680-1.170)
13 | avhrr (noaa10) band 1 (0.530-0.780)
14 | avhrr (noaa10) band 2 (0.600-1.190)
15 | avhrr (noaa11) band 1 (0.540-0.820)
16 | avhrr (noaa11) band 2 (0.600-1.120)
17 | hrv1 (spot1) band 1 (0.470-0.650)
18 | hrv1 (spot1) band 2 (0.600-0.720)
19 | hrv1 (spot1) band 3 (0.730-0.930)
20 | hrv1 (spot1) band pan (0.470-0.790)
21 | hrv2 (spot1) band 1 (0.470-0.650)
22 | hrv2 (spot1) band 2 (0.590-0.730)
23 | hrv2 (spot1) band 3 (0.740-0.940)
24 | hrv2 (spot1) band pan (0.470-0.790)
25 | tm (landsat5) band 1 (0.430-0.560)
26 | tm (landsat5) band 2 (0.500-0.650)
27 | tm (landsat5) band 3 (0.580-0.740)
28 | tm (landsat5) band 4 (0.730-0.950)
29 | tm (landsat5) band 5 (1.5025-1.890)
30 | tm (landsat5) band 7 (1.950-2.410)
31 | mss (landsat5) band 1 (0.475-0.640)
32 | mss (landsat5) band 2 (0.580-0.750)
33 | mss (landsat5) band 3 (0.655-0.855)
34 | mss (landsat5) band 4 (0.785-1.100)
35 | MAS (ER2) band 1 (0.5025-0.5875)
36 | MAS (ER2) band 2 (0.6075-0.7000)
37 | MAS (ER2) band 3 (0.8300-0.9125)
38 | MAS (ER2) band 4 (0.9000-0.9975)
39 | MAS (ER2) band 5 (1.8200-1.9575)
40 | MAS (ER2) band 6 (2.0950-2.1925)
41 | MAS (ER2) band 7 (3.5800-3.8700)
42 | MODIS band 1 (0.6100-0.6850)
43 | MODIS band 2 (0.8200-0.9025)
44 | MODIS band 3 (0.4500-0.4825)
45 | MODIS band 4 (0.5400-0.5700)
46 | MODIS band 5 (1.2150-1.2700)
47 | MODIS band 6 (1.6000-1.6650)
48 | MODIS band 7 (2.0575-2.1825)
49 | avhrr (noaa12) band 1 (0.500-1.000)
50 | avhrr (noaa12) band 2 (0.650-1.120)
51 | avhrr (noaa14) band 1 (0.500-1.110)
52 | avhrr (noaa14) band 2 (0.680-1.100)
53 | POLDER band 1 (0.4125-0.4775)
54 | POLDER band 2 (non polar) (0.4100-0.5225)
55 | POLDER band 3 (non polar) (0.5325-0.5950)
56 | POLDER band 4 P1 (0.6300-0.7025)
57 | POLDER band 5 (non polar) (0.7450-0.7800)
58 | POLDER band 6 (non polar) (0.7000-0.8300)
59 | POLDER band 7 P1 (0.8100-0.9200)
60 | POLDER band 8 (non polar) (0.8650-0.9400)
61 | etm+ (landsat7) band 1 (0.435-0.520)
62 | etm+ (landsat7) band 2 (0.506-0.621)
63 | etm+ (landsat7) band 3 (0.622-0.702)
64 | etm+ (landsat7) band 4 (0.751-0.911)
65 | etm+ (landsat7) band 5 (1.512-1.792)
66 | etm+ (landsat7) band 7 (2.020-2.380)
67 | etm+ (landsat7) band 8 (0.504-0.909)

## AUTHORS

Original version of the program for GRASS 5:
Christo Zietsman, 13422863(at)sun.ac.za

Code clean-up and port to GRASS 6.3, 15.12.2006:
Yann Chemin, ychemin(at)gmail.com

## REFERENCES

Vermote, E.F., Tanre, D., Deuze, J.L., Herman, M., and Morcrette, J.J., 1997, Second simulation of the satellite signal in the solar spectrum, 6S: An overview., IEEE Trans. Geosc. and Remote Sens. 35(3):675-686.

6s manual at the 6s homepage of the Land Surface Reflectance Science Computing Facility

Mauro A. Homem Antunes website about his 6s version

Last changed: $Date: 2008-04-20 02:37:10 -0500 (Вс, 20 апр 2008) $