code: function(str, gsa) {
  
        if (gsa.length !== 19 && gsa.length !== 20) {
          throw new Error('Invalid GSA length: ' + str);
        }
  
        /*
         eg1. $GPGSA,A,3,,,,,,16,18,,22,24,,,3.6,2.1,2.2*3C
         eg2. $GPGSA,A,3,19,28,14,18,27,22,31,39,,,,,1.7,1.0,1.3*35
         
         
         1    = Mode:
         M=Manual, forced to operate in 2D or 3D
         A=Automatic, 3D/2D
         2    = Mode:
         1=Fix not available
         2=2D
         3=3D
         3-14 = PRNs of Satellite Vehicles (SVs) used in position fix (null for unused fields)
         15   = PDOP
         16   = HDOP
         17   = VDOP
         (18) = systemID NMEA 4.10
         18   = Checksum
         */
  
        var sats = [];
        for (var i = 3; i < 15; i++) {
  
          if (gsa[i] !== '') {
            sats.push(parseInt(gsa[i], 10));
          }
        }
  
        return {
          'mode': parseGSAMode(gsa[1]),
          'fix': parseGSAFix(gsa[2]),
          'satellites': sats,
          'pdop': parseNumber(gsa[15]),
          'hdop': parseNumber(gsa[16]),
          'vdop': parseNumber(gsa[17]),
          'systemId': gsa.length > 19 ? parseNumber(gsa[18]) : null
        };
      }
docs: GPS DOP and active satellites