PRO polster,alon,alat,x1,y1,xlam,slat ; ; COMPUTES THE POLAR STEROGRAPHIC TRANSFORMATION FOR A LON,LAT ; INPUT OF (ALON,ALAT) WITH REFERENCE ORIGIN LON,LAT=(XLAM,SLAT). ; OUTPUT IS (X1,Y1) IN KM ; ; ALGORITHM IS THE SAME AS USED FOR PROCESSING ERS-1 SAR IMAGES ; AS RECEIVED FROM M. DRINKWATER (1994) ; E2=0.006693883D0 RE=6378.273D0 DTR=3.141592654/180.0 E=SQRT(E2) IF SLAT LT 0.0 THEN BEGIN SN=-1.0 RLAT=-ALAT ENDIF ELSE BEGIN SN=1.0 RLAT=ALAT ENDELSE T=((1.D0-E*SIN(RLAT*DTR))/(1.D0+E*SIN(RLAT*DTR)))^(E*0.5D0) TY=TAN(DTR*(45.0D0-0.5D0*RLAT))/T IF SLAT LT 0.0 THEN RLAT=-SLAT ELSE RLAT=SLAT IF ABS(SLAT) EQ 90.0 THEN PRINT,"*** Invalid reference latitude in polster ***",SLAT T=((1.D0-E*SIN(DTR*RLAT))/(1.D0+E*SIN(DTR*RLAT)))^(E*0.5D0) TX=TAN(DTR*(45.0D0-0.5D0*RLAT))/T CM=COS(DTR*RLAT)/SQRT(1.D0+E2*SIN(DTR*RLAT)^2) RHO=RE*CM*TY/TX X1= (SN*SIN(DTR*(SN*ALON-XLAM)))*RHO Y1=-(SN*COS(DTR*(SN*ALON-XLAM)))*RHO RETURN END