double cpa_calc_ownship_cog = CogTrial;
    double cpa_calc_target_cog = ptarget->COG;

//    Ownship is not reporting valid SOG, so no way to calculate CPA
    if( wxIsNaN(SogTrial) || ( SogTrial > 102.2 ) ) {
        ptarget->bCPA_Valid = false;
        return;
    }

//    Ownship is maybe anchored and not reporting COG
    if( wxIsNaN(CogTrial) || CogTrial == 360.0 ) {
        if( SogTrial < .01 ) cpa_calc_ownship_cog = 0.;          // substitute value
                                                             // for the case where SOG ~= 0, and COG is unknown.
        else {
            ptarget->bCPA_Valid = false;
            return;
        }
    }

//    Target is maybe anchored and not reporting COG
    if( ptarget->COG == 360.0 ) {
        if( ptarget->SOG > 102.2 ) {
            ptarget->bCPA_Valid = false;
            return;
        } else if( ptarget->SOG < .01 ) cpa_calc_target_cog = 0.;           // substitute value
                                                                            // for the case where SOG ~= 0, and COG is unknown.
        else {
            ptarget->bCPA_Valid = false;
            return;
        }
    }

    //    Express the SOGs as meters per hour
    double v0 = SogTrial * 1852.;
    double v1 = ptarget->SOG * 1852.;

    if( ( v0 < 1e-6 ) && ( v1 < 1e-6 ) ) {
        ptarget->TCPA = 0.;
        ptarget->CPA = 0.;

        ptarget->bCPA_Valid = false;
    } else {
        //    Calculate the TCPA first

        //    Working on a Reduced Lat/Lon orthogonal plotting sheet....
        //    Get easting/northing to target,  in meters

        double east1 = ( ptarget->Lon - LonTrial ) * 60 * 1852;
        double north1 = ( ptarget->Lat - LatTrial ) * 60 * 1852;

        double east = east1 * ( cos( LatTrial * PI / 180. ) );
        
        double north = north1;

        //    Convert COGs trigonometry to standard unit circle
        double cosa = cos( ( 90. - cpa_calc_ownship_cog ) * PI / 180. );
        double sina = sin( ( 90. - cpa_calc_ownship_cog ) * PI / 180. );
        double cosb = cos( ( 90. - cpa_calc_target_cog ) * PI / 180. );
        double sinb = sin( ( 90. - cpa_calc_target_cog ) * PI / 180. );

        //    These will be useful
        double fc = ( v0 * cosa ) - ( v1 * cosb );
        double fs = ( v0 * sina ) - ( v1 * sinb );

        double d = ( fc * fc ) + ( fs * fs );
        double tcpa;

        // the tracks are almost parallel
        if( fabs( d ) < 1e-6 ) tcpa = 0.;
        else
            //    Here is the equation for t, which will be in hours
            tcpa = ( ( fc * east ) + ( fs * north ) ) / d;

        //    Convert to minutes
        ptarget->TCPA = tcpa * 60.;

        //    Calculate CPA
        //    Using TCPA, predict ownship and target positions

        double OwnshipLatCPA, OwnshipLonCPA, TargetLatCPA, TargetLonCPA;

        ll_gc_ll( LatTrial, LonTrial, cpa_calc_ownship_cog, SogTrial * tcpa, &OwnshipLatCPA, &OwnshipLonCPA );
        ll_gc_ll( ptarget->Lat, ptarget->Lon, cpa_calc_target_cog, ptarget->SOG * tcpa,
                &TargetLatCPA, &TargetLonCPA );

        //   And compute the distance
        ptarget->CPA = DistGreatCircle( OwnshipLatCPA, OwnshipLonCPA, TargetLatCPA, TargetLonCPA );
        ptarget->bCPA_Valid = true;

        if( ptarget->TCPA < 0 ) ptarget->bCPA_Valid = false;
    }
}