参考：blewitt basics of gps.pdf

https://pan.baidu.com/s/1ceMsIE0SMVk3_wzXc-QIEg

The Navigation Message can be found on the L1 channel, being transmitted at a very slow rate
of 50 bps. It is a 1500 bit sequence, and therefore takes 30 seconds to transmit. The
Navigation Message includes information on the Broadcast Ephemeris (satellite orbital
parameters), satellite clock corrections, almanac data (a crude ephemeris for all satellites),
ionosphere information, and satellite health status.