*-- Author :  R. Henderson
      SUBROUTINE FPPPTZ(PERSEG,Z)
C                                                                                                             
C---  Propagates planars error matrix from z=0 to z=z                                                         
C                                                                                                             
      DOUBLE PRECISION COVAR,PERSEG                                     
C---                                                                                                          
      DIMENSION PERSEG(4,4)                                             
      DIMENSION COVAR(4,4) , DERIV(4,4) , WM(4,4)                       
C                                                                                                             
C---  Construct the derivative matrix                                                                         
C                                                                                                             
      CALL VZERO(DERIV,16)                                                                             
      DO 10 I = 1,4                                                     
      DERIV(I,I) = 1.0                                                  
   10 CONTINUE                                                          
      DERIV(1,3) = Z                                                    
      DERIV(2,4) = Z                                                    
C                                                                                                             
C---  Propagate PERSEG FROM Z=0 TO Z                                                                          
C                                                                                                             
      CALL VZERO(WM,16)                                                                                
C                                                                                                             
      DO 24 I = 1,4                                                     
        DO 25 J = 1,4                                                   
         DO 26 K = 1,4                                                  
         WM(I,J) = WM(I,J) + PERSEG(I,K) * DERIV(J,K)                   
   26    CONTINUE                                                       
   25   CONTINUE                                                        
   24 CONTINUE                                                          
C---                                                                                                          
      CALL VZERO(COVAR,32)                                                                             
      DO 54 M = 1,4                                                     
        DO 55 K = 1,4                                                   
         DO 56 I = 1,4                                                  
         COVAR(M,K) = COVAR(M,K) + DERIV(M,I) * WM(I,K)                 
   56    CONTINUE                                                       
   55   CONTINUE                                                        
   54 CONTINUE                                                          
C---                                                                                                          
      CALL UCOPY(COVAR,PERSEG,32)                                                                      
C---                                                                                                          
      RETURN                                                            
      END                                                               
*