少问多问，这样我才能更好地理解。因此，我使用GPS已有一段时间，并且对NMEA格式有所了解。对于我正在处理的项目，我只需要位置数据，因此我一直在筛选出除GPRMC之外的所有消息，并使用Active或Void字符确定我是否有锁。但是我只是获取了一些带有空白字符的数据，但是当我查看记录的数据时，我看到了：
$GPRMC,171959.460,V,,,,,,,060220,,,N*4B
$GPRMC,172000.465,V,,,,,,,060220,,,N*48
$GPRMC,172001.465,V,,,,,,,060220,,,N*49
$GPRMC,172002.465,V,,,,,,,060220,,,N*4A
$GPRMC,172003.465,V,,,,,,,060220,,,N*4B
$GPRMC,172004.465,A,xxxx.xxxx,X,xxxxx.xxxx,X,7.62,270.66,060220,,,A*77
$GPRMC,172005.465,V,,,,,,,060220,,,N*4D
$GPRMC,172006.465,A,xxxx.xxxx,X,xxxxx.xxxx,X,8.48,271.72,060220,,,A*7A
$GPRMC,172007.000,A,xxxx.xxxx,X,xxxxx.xxxx,X,8.13,273.77,060220,,,A*7F
$GPRMC,172008.000,A,xxxx.xxxx,X,xxxxx.xxxx,X,7.98,278.81,060220,,,A*76
$GPRMC,172009.000,A,xxxx.xxxx,X,xxxxx.xxxx,X,8.07,286.60,060220,,,A*74
$GPRMC,172010.000,V,xxxx.xxxx,X,xxxxx.xxxx,X,,,060220,,,N*67
$GPRMC,172011.000,V,xxxx.xxxx,X,xxxxx.xxxx,X,,,060220,,,N*6D
$GPRMC,172012.060,V,xxxx.xxxx,X,xxxxx.xxxx,X,,,060220,,,N*63
$GPRMC,172013.060,V,xxxx.xxxx,X,xxxxx.xxxx,X,,,060220,,,N*6C
$GPRMC,172014.064,V,xxxx.xxxx,X,xxxxx.xxxx,X,,,060220,,,N*6F
$GPRMC,172015.065,V,xxxx.xxxx,X,xxxxx.xxxx,X,,,060220,,,N*69
想知道什么是有效数据与无效数据，以及如果无效的话GPS如何拥有位置数据。