public static function invlatiso($eccent, $ts)
    {
        $phi = static::fL(1.0, $ts);
        $Iphi = 0.0;
        $con = 0.0;

        do {
            $Iphi = $phi;
            $con = $eccent * sin($Iphi);
            $phi = static::fL(exp($eccent * log((1.0 + $con) / (1.0 - $con)) / 2.0 ), $ts);
        } while(abs($phi - $Iphi) > 1.0e-12);

        return $phi;
    }