'Gets a particular dilution of position value.
@param dopType: The type of dilution of position to get. One of
(\'pdop\', \'hdop\', \'vdop\').
@type dopType: C{str}
@return: The DOP if it is known, L{None} otherwise.
@rtype: C{float} or L{None}'

def _getDOP(self, dopType):

    for dopExpression in self._DOP_EXPRESSIONS[dopType]:
        try:
            return dopExpression(self)
        except TypeError:
            continue
