navigate - Autonavigation (attitude correction) of satellite images

navigateb - version of navigate to work with 8-bit AVHRR images.

SYNOPSIS

navigate   [ parameter=value ... ] [ inputfile ]
navigateb  [ parameter=value ... ] [ inputfile ]

Parameters are: variable, box_file, coast_file, mincorr, clear, min_good_boxes, max_coast_pts, ch2_delta, ch2_max, ch4_delta, min_ch4_temp, ch45_delta, ch45_maxdelta, ch34_delta.

DESCRIPTION

navigate is an implementation of the algorithm created at SeaSpace. Since areas with too many clouds cannot be used for navigation, cloud screening is also performed using some tests utilized in nitpix. See nitpix for details on the cloud-screening algorithms and a list of references.

The following additional tests are used in navigate program to distinguish between land and water:

            day time:  vegetation index (ch2-ch1)/(ch2+ch1)
                       > -0.1 for Land,
                       < -0.1 for Sea.
            night time: (ch4 - ch5)
                       < ch45_delta for Land,
                       > ch45_delta for Sea,
                       > ch45_maxdelta for Clouds.


PARAMETERS

variable

Specifies the name of the image variable(s). The default answer avhrr should be accepted for calibrated AVHRR image. Otherwise, it will specify the byte image classified with class function where Land and Sea must be coded as 0 and 1 or visa versa. The valid answer is avhrr or the actual image variable name. The default is avhrr.

box_file

Name of the input file, specifying selected boxes as parallel (i.e., having the same size) arrays of variables x, y, delta_x, delta_y, created either manually as xvu->select->locations->boxes->(X/Y), or with navboxes function.

coast_file

These are the names of files used to generate the coastline. The default is wdb2.cil.

mincorr

Minimum required correlation between the coastline and the actual image, in percents. Empirically found range of values that seem to work reasonably well is between 70 for a moderately cloudy image and 90 for an almost cloud-free image. The valid answer is any integer number between 0 and 100. The default is 90.

clear

If specified as yes, then cloud screening will not be performed and all pixels will be classified as either Land or Water. The valid answer is yes or no. The default is no.

min_good_boxes

Minimum required number of boxes from the box_file file that can be used for navigation. The valid answer is any positive integer number. The default is 1.

max_coast_pts

Maximum allowed number of coastline points in any box selected for navigation. This parameter is to protect the algorithm from too long coastlines. The valid answer is any integer number less then 5000. The default is 4000.

max_cloud_perc

Maximum allowed percent of clouds in an image window. The valid answer is any integer number between 0 and 100. The default is 1.

ch4_delta - CURRENTLY NOT USED

This is the user-specified maximum value for the following test:

        MAX | avhrr_ch4 - center avhrr_ch4 | > user-specified value

Within each box the absolute value of each (channel 4 value - the center channel 4 value) is computed. If it exceeds the user-specified value, the data element is assumed to be cloudy.

The valid range is [positive numbers]. The default is 0.3.

ch2_max

This is the user-specified maximum value for the following test;

        CENTER (avhrr_ch2) > user-specified value

If, within each box, the center channel 2 value exceeds the user specified value, the data element is assumed to be cloudy. This test is only used for daytime data.

The valid range is [positive numbers]. The default is 15..

ch2_delta

This is the user-specified maximum value for the following test:

        MAX | avhrr_ch2 - center avhrr_ch2 | > user-specified value

Within each box the absolute value of (each channel 2 value - the center channel 2 value) is computed. If it exceeds the user-specified value the pixel, is assumed to be cloudy. This test is only used on daytime data.

The valid range is [positive numbers]. The default is 2..

ch34_delta - CURRENTLY NOT USED

This is the user-specified maximum value for the following test:

       AVERAGE | avhrr_ch3 - avhrr_ch4 | > user-specified value

Within each box the average of (each channel 3 value - its corresponding channel 4 value) is computed. If it exceeds the user-specified value the pixel is assumed to be cloudy. This test is only used on nighttime data.

The valid range is [any number]. The default is 0.

min_ch4_temp

Minimum allowable channel 4 brightness temperature in degrees Celsius. Pixels with ch4 brightness temperature less than this minimum will be classified as covered by high cloud and will not be used in further computations.

The default value is 0 degrees Celsius.

ch45_delta

This is the user-specified maximum value for the following test:

     ( avhrr_ch4 - avhrr_ch5 ) > user-specified value

If this condition is true, then the pixel is classified as sea, otherwise as land. However, if this difference is too big (greater than ch45_maxdelta), then the pixel is unclassified.

The default value is 1.5 degrees.

ch45_maxdelta

This is the user-specified maximum value for the following test:

     ( avhrr_ch4 - avhrr_ch5 ) > user-specified value

If this condition is true, then the pixel is uclassified.

The default value is 2.5 degrees.

EXAMPLES

Navigate AVHRR calibrated image.

[1] % navigate n12.93300.1546.avhrr
variable       : char(255) ? [avhrr]
box_file       : char(255) ? n12.bigbox
coast_file     : char(255) ? [wdb2.cil]
mincorr        : int       ? [90] 80
clear          : char(  9) ? [no]
min_good_boxes : int       ? [1] 3
max_cloud_perc : int       ? [1]
ch2_delta      : real      ? [0.25]
ch2_max        : real      ? [3]
min_ch4_temp   : real      ? [0]
ch45_delta     : real      ? [1.5]
ch45_maxdelta  : real      ? [2.5]
n12.93300.1546.avhrr: Old Attitude: (0.0 0.0 0.0)
n12.93300.1546.avhrr: 7 out of 40 boxes used ...
n12.93300.1546.avhrr: New Attitude: (0.45 -0.20 0.67)

Navigate image classified with class function. Boxes were selected from xvu or with navboxes function. IMPORTANT: land and sea must be coded as 0 and 1 or visa versa, otherwise navigate will not work properly.

[2] % audit n11.cls
n11.cls  History  Page 1

class include_vars=avhrr_ch3 training_sets=n11.land n11.ocean
n11.cloud class_codes=1 0 2 unclass_code=3 reject_level=0 0 0

n11.93321.1211.avhrr

   hrptin hrpt_types=avhrr on_pass_disk=yes pass_number=4
   max_time_adj=0 skip_frames=0 hrpt_format=auto channels=3
   fix_missing=yes sharpen=y calibrate=n geo_correct=no
   delta_line=1 delta_sample=1 use_master=yes

[3] % navigate n11.cls
variable       : char(255) ? [avhrr] class
box_file       : char(255) ? n11.boxes
coast_file     : char(255) ? [wdb2.cil]
mincorr        : int       ? [90] 80
min_good_boxes : int       ? [1] 2
n11.cls: Old Attitude: (0.0 0.0 0.0)
n11.cls: 3 out of 4 boxes used ...
n11.cls: New Attitude: (0.26 -0.00 0.17)

SEE ALSO

navboxes,class,avhrr,nitpix,satvis,hrptin,avin,avcal.


Last Update: $Date: 1998/05/29 18:44:48 $