User's Guide

NAVIGATE

Update attitude and altitude information for the AVHRR model

Function:

Update attitude and altitude information for the AVHRR model. Attitude (roll, pitch, yaw) and altitude corrections are calculated using a Least Median of Squares (LMS) fit of the given tie point pairs.

Parameters:

INTL
Input tie point location file. This file contains verification point pairs.

PRINT("TERM")
Output destination. The destination of the output.


  = TERM:      Terminal.  Output is sent to the user's 
               terminal.
  = LP:        Line printer.  Output is sent to the 
               printer defined by $PRINTER.
  = Filename:  User-supplied filename.  Output is sent 
               to the user-supplied file with the ex-
               tension ";prt".

OUTPLOT(--)
Output plot file. Name of the plot file containing the tie point id, the X and Y coordinates in the search image, and residual errors. If not specified, a plot file will not be created.

DBFILE(--)
Database file. Name of the database file containing statistical information (see User Note for file format). If not specified, a database file will not be created.

SAREA(--)
Study area. Specifies the area of interest defined by the 12 regions of the global GOODES projection. This information is only used to log the study area into DBFILE. If defaulted, the study area field in DBFILE will be left blank.

Examples:

  1. LAS> navigate intl=check outplot=plotfile dbfile=datafile sarea=3

    Attitude and altitude corrections will be calculated from the tie point pairs in CHECK;TPL. The updates will be stored in the associated ADDR (AVHRR Data Descriptor Record) file of the search image named in INTL. A report of residuals calculated from the updated AVHRR model will be sent to the user's terminal. The plot file will be written to "plotfile;plt" and the database file will be written to "datafile;dat" (see User Notes). The study area "3" will be written to "datafile;dat".

Description/Algorithm:

All active points are read from INTL. If there are less than eight active points in INTL, no LMS fit can be calculated and processing will terminate. The points are grouped five at a time to calculate a plane. The plane that has the smallest median residual when applied to all the points, whose coefficients fall within reasonable bounds, and whose set of points are well-distributed around the image is selected. A residual to this plane for each point is then calculated. Those points whose residual is less than 0.8 are used to calculate the plane coefficients which are converted to roll, pitch, yaw, and altitude coefficients. If there are more than 25 points in INTL, the points will be divided into groups, and the points within each group will be taken five at a time to calculate a plane. The plane that has the most points with a residual less than 0.8 is used to select points for the final plane. These coefficients update the model and are stored in the image's ADDR file.

A report is then generated showing the accuracy of the fit and is sent to the destination specified by PRINT. X and Y residuals are calculated by subtracting the coordinates calculated by the updated model from the observed coordinates. The Root Mean Squared Error (RMSE) is then calculated by:

   RMSE = [((Xres * Xres) + (Yres * Yres)) / Npoints ]^(1/2)
An optional plot file may be generated showing the accuracy of the fit and is sent to the destination specified by OUTPLOT. An optional database file may be generated and is sent to the destination specified by DBFILE.

Nonfatal Error Messages:

  1. [navigate-many] Only the first <XXX> points will be used

    A maximum of <XXX> points may be used to calculate the LMS fit. EDITCORR may be used to reduce the number of active points in INTL.

  2. [navigate-noplane] Group <XXX> does not pass constraints

    When there are many tie points in INTL, the points will be split into groups, and a correction plane will be calculated for each group. The plane from each group is considered in producing the final coefficients. Some groups, however, may not produce a valid plane because of point distribution or a bad fit. Coefficients will be calculated from the planes of other groups.

  3. [navigate-many] Only the first <XXX> points will be used

    NAVIGATE will use a maximum of <XXX> tie points for calculating correction coefficients. If some tie points near the end of INTL are required to be used in the fit, the INTL may be edited before running NAVIGATE.

  4. [navigate-warn] Warning: median value is greater than 1 pixel

    The median residual of the LMS fit was greater than 1. This indicates that more than half of the points do not agree with the fit. The resulting attitude and altitude coefficients should be verified to ensure they are in a reasonable range.

Fatal Error Messages:

  1. [navigate-fatal] Fatal error encountered

    The message displayed immediately preceding this message is the specific error encountered. Processing is terminated.

  2. [navigate-noplane] No planes were found that pass the constraints

    No set of five points were found to create a plane which passed the constraints of distribution of points and maximum rates.

  3. [navigate-npts] Not enough points to do LMS fit

    At least eight active points must be present in INTL to calculate an LMS fit. Retry with a larger set of tie points.

  4. [navigate-open] Error opening <XXXXX> file

    An error occurred opening the specified output file. Check the permissions on the current directory.

  5. [navigate-queue] Queue error

    An error occurred splitting the tie points into groups. NAVIGATE uses a queue to read the points from INTL and divide them up among groups. Contact the system administrator.

  6. [navigate-toofew] Too few points used in final fit

    60% of the active points in INTL must agree with the LMS fit. Edit the points using EDITCORR so that points that obviously do not fit will not be used in the calculation.

User Notes:

  1. All active points in INTL are grouped five-at-a-time to calculate a plane. If there are more than 44 points, over one million planes will be calculated. In order to reduce processing time, fewer than 44 points should be input to NAVIGATE.

  2. The format of the plot file specified by OUTPLOT is as follows:

    
        Number of points used in the fit
        Point ID 1    Sample    Line    Line residual    Sample residual    Active
        Point ID 2    Sample    Line    Line residual    Sample residual    Active
        Point ID 3    Sample    Line    Line residual    Sample residual    Active
        Point ID 4    Sample    Line    Line residual    Sample residual    Active
    		    .                                  .
    		    .                                  .
    		    .                                  .
        Total root mean squared error (RMSE)
    
    
    The line residual consists of the difference between the computed and observed value in the line direction. The sample residual consists of the difference between the computed and observed value in the sample direction. The active flag indicates which control points are active.

  3. An example of the database file specified by DBFILE follows:

    
        Scene   ID:  ah11122491203247
        Study area:  3
        Start Time:  20:32:47.306
        End   Time:  20:43:10.139
                NN:  53D 32M 57S  -100D 48M 04S
                SN:  17D 37M 04S   -88D 40M 12S
             Orbit:  16738
              Roll:  -0.000456
             Droll:  -0.000013
             Pitch:  -0.000899
            Dpitch:  -0.000024
               Yaw:  0.005335
         Daltitude:  -2977.072259
        # WVS pts :  0
        # DCW pts :  382
    
    
    NN is the latitude and longitude of the north-most nadir pixel. SN is the latitude and longitude of the south-most nadir pixel. "Droll" is the roll rate of change across the scene. "Dpitch" is the pitch rate of change across the scene. "Daltitude" is a correction applied to the estimated satellite altitude. The number of WVS points used in the fit as well as the number of DCW points used in the fit are also stored.