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The following program converts from latitude and longitude coordinates to polar stereographic SSM/I grid coordinates to pixel sample lines and indexes. Program I converts from pixel sample and line indexes to polar stereographic SSM/I grid coordinates to latitude and longitude coordinates.

c Convert_11_xy c c This program converts from latitude and longitude c to polar stereographic SSM/I grid coordinates (in km) c and to Leads ARI grid sample,line indexes. This program is a c version of the program written by C.S. Morris, Jet Propulsion c Laboratory, which appears in the NSIDC SSM/I User's c Guide, Section F, "SSMII Polar Grids". c c Program-adapted for Leads ARI grids by F.Fetterer, c NOARL Remote Sensing,Stennis Space Center, March 1991 dimension t (2) real*4 x,y,alat,along real*8 e,e2,cdr,pi character*1 grid c Conversion constant, degrees to radians: cdr=57.29577951 c Radius and eccentricity of earth (Hughes ellipsiod, km): re=6378.273 e2=0.006693883 e=SQRT(e2) pi=3.141592654 c Standard parallel - latitude of no distortion slat=70.0 c Puts 135 deg long. at the top of the grid: xlam=-45. print*,'Type ''P'' for Pacific grid, ''E'' for European' accept*,grid 100 print*,'Type latitude, longitude (9999,9999 to stop)' accept*,alat,along if (alat.eq.9999.) goto 999 if(alat.lt.89.995)goto 250 x=0. y=0. goto 300 250 rlat=alat do 600 II=1,2 if(II.eq.2)rlat=slat 600 t(II)=TAN((pi/4.)-(rlat/(2.*cdr)))/((1.-e*SIND(rlat))/ & (1.+e*SIND(rlat)))**(e/2.) cm=COSD(slat)/SQRT(1.-e2*(SIND(slat)**2)) rho-re*cm*t(1)/t(2) x=rho*SIND(along-xlam) y=rho*COSD(along-xlam) c Transform sample, line coordinates to SSMI grid coordinates 300 if(grid.eq.'P'.or.grid.eq.'p')then rline=1975-y sample=x+2250 else rline=1300-y sample=x end if print*,' Sample Line SSMI x SSMI y Lat & Long' write (6,310)sample,rline,x,y,alat,along print* goto 100 310 format(4f1O.2,2f1O.4) 999 continue end

The program is taken from:

**Fetterer and Hawkins. 1991**. An AVHRR data set for the Arctic leads ARI. *Naval Oceanographic and Atmospheric Research Laboratory, Technical Note 118*. 61
p.