'Extracts the information regarding which satellites were used in
obtaining the GPS fix from a GSA sentence.
Precondition: A GSA sentence was fired. Postcondition: The current
sentence data (C{self._sentenceData} will contain a set of the
currently used PRNs (under the key C{_usedPRNs}.'

def _fixGSA(self):

    self._sentenceData['_usedPRNs'] = set()
    for key in (('usedSatellitePRN_%d' % (x,)) for x in range(12)):
        prn = getattr(self.currentSentence, key, None)
        if (prn is not None):
            self._sentenceData['_usedPRNs'].add(int(prn))
