There are over 20 commands which can be sent between the MPS and the 
PC.ranging from a request to send up the current status of all switches, 
to the dispatch and installation of a new exterior orientation transforma- 
tion matrix. The command option allows direct interaction with the MPS to 
check each command, and functions inside the MPS itself. Note that as the 
connexion with the MPS is a RS232 serial line running at 9600 baud, many 
functions can be checked by simply connecting a computer terminal to this 
line and using the keyboard and screen to send and receive data. The test 
program simply provides more dynamic control of the process. 
Job data and the orientation parameters are stored in a work file in 
binary format. This allows for rapid access, maintains precision, and 
minimizes the storage requirements. Option 5 allows a quick look at the 
contents of this file. 
(ii) Calibration software 
This software is used to measure the systematic errors in the 
drive systems, and store corrections for these errors in a table. Small 
systematic errors can be caused by a variety of causes including small 
variations in pitch of the lead screws, minor irregularities in straightness 
of the guide rods, or non-orthogonality of the X and Y axis systems. It is 
more effective to measure the effects of these factors and apply corrections, 
rather than rely solely on mechanical perfection. 
  
If the results of the tests indicate that the corrections are irregular, 
or above the expected size, then it becomes necessary to find and replace 
the faulty component. The software can therefore be used as a check on the 
mechanical performance as well as providing fine tuning on the system. 
A standard test grid with 23 evenly spaced crosses, and four fiducial 
marks is placed in each photo carriage. Under program control the MPS 
moves to each point in turn and waits for the operator to accurately posi- 
tion the floating mark on the selected cross. Each cross is measured four 
times with the mark being approached from a different direction for each 
measurement. If the four readings for a selected point are within a set 
tolerence, a mean is taken, and the MPS proceeds to the next mark. 
After all points have been observed, affine transformation parameters 
are computed by a least squares procedure, and residuals after transforma- 
tion computed. It is possible at this stage to select a more suitable value 
for the lead screw pitch on each axis, and store the results of the measure- 
ments. 
The correction table is constructed on a 16 by 16 regular mesh from 
the residuals computed. 
Following a calibration, the grid plate is reobserved by carrying out 
an interior orientation on the 'fiducials', before visiting each mesh point 
in turn and observing its co-ordinates using MPS80. If the calibration is 
correct, the systematic errors will have been eliminated. 
(iii) Verification Checks in Orientation Routines 
The orientation routines can either operate from data observed 
by the MPS, or use data from a data file. This is useful in three types of 
situations: 
  
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