PRO trsk3,xp,yp,zp,alpha,az,el,rng

; Modelled on subroutine TRSK3.FOR in GEOLB.FOR
; Distances in km
; Angle computations in radians, input/output in degrees

rng=SQRT(xp*xp+yp*yp+zp*zp)
az=alpha*!DTOR-ATAN(yp,xp)
negaz=WHERE(az LT 0.,nnegaz)
IF nnegaz GT 0 THEN REPEAT BEGIN
    az(negaz)=az(negaz)+2.0D0*!PI
    negaz=WHERE(az LT 0.,nnegaz)
    ENDREP UNTIL nnegaz EQ 0
toobig=WHERE(az GT 2.D0*!PI,ntoobig)
IF ntoobig GT 0 THEN REPEAT BEGIN
    az(toobig)=az(toobig)-2.0D0*!PI
    toobig=WHERE(az GT 2.D0*!PI,ntoobig)
    ENDREP UNTIL ntoobig EQ 0
el=ATAN(zp,SQRT(xp*xp+yp*yp))*!RADEG
az=az*!RADEG
RETURN
END