Time Sync with GPS

Time Sync with GPS

1. 設定 enable/disable 放在 Settings.system 中
2. 應該由 Location Service 讀取內容，負責動作。

Location Service 和 hardware gps 的分層負責就因人而異了。

可以增加　interface (?)，讓 service 告知 gps driver 收到 gps time nmea data 時，set system time。
也可以 service 自己作，當取得 nmea time data 時，依照 Setting 內容決定。

要sync system time 的時機：

1. 改 Auto Sync 設定時
2. GPS lock 時

所以要寫在兩個地方，互相 check 對方。

但是要確認 gps driver , check lock 的時間，要比 server 啟動, check system setting 的後面才行。
否則就會不同步。

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直接把 GPS NMEA Channel cat 出來.. (只列出 GPRMC)..並且修改 location data.. (所以 checksum 是錯的) $GPRMC,080058.000,A,2502.9061,N,14122.4832,E,0.00,0.00,090212,,,A*6x $GPRMC,080100.000,A,2502.9061,N,14122.4832,E,0.00,0.00,090212,,,A*6x $GPRMC,080101.000,A,2502.9061,N,14122.4832,E,0.00,0.00,090212,,,A*6x $GPRMC,080102.000,A,2502.9061,N,14122.4832,E,0.00,0.00,090212,,,A*6x GPRMC format : (http://aprs.gids.nl/nmea/#rmc)

eg1. $GPRMC,081836,A,3751.65,S,14507.36,E,000.0,360.0,130998,011.3,E*62
eg2. $GPRMC,225446,A,4916.45,N,12311.12,W,000.5,054.7,191194,020.3,E*68


           225446       Time of fix 22:54:46 UTC
           A            Navigation receiver warning A = OK, V = warning
           4916.45,N    Latitude 49 deg. 16.45 min North
           12311.12,W   Longitude 123 deg. 11.12 min West
           000.5        Speed over ground, Knots
           054.7        Course Made Good, True
           191194       Date of fix  19 November 1994
           020.3,E      Magnetic variation 20.3 deg East
           *68          mandatory checksum

GPS FIX 的資料是在 $GPGGA - Global Positioning System Fix Data (http://aprs.gids.nl/nmea/)

eg2. $--GGA,hhmmss.ss,llll.ll,a,yyyyy.yy,a,x,xx,x.x,x.x,M,x.x,M,x.x,xxxx

hhmmss.ss = UTC of position 
llll.ll = latitude of position
a = N or S
yyyyy.yy = Longitude of position
a = E or W 
x = GPS Quality indicator (0=no fix, 1=GPS fix, 2=Dif. GPS fix) 
xx = number of satellites in use 
x.x = horizontal dilution of precision 
x.x = Antenna altitude above mean-sea-level
M = units of antenna altitude, meters 
x.x = Geoidal separation
M = units of geoidal separation, meters 
x.x = Age of Differential GPS data (seconds) 
xxxx = Differential reference station ID 

其中 'GPS Quality indicator' 就是 FIX 的狀態..
實際的資料： (pull-out gps antenna): $GPGGA,000935.066,0000.0000,N,00000.0000,E,0,00,,0.0,M,0.0,M,,0000*4C $GPGGA,000936.064,0000.0000,N,00000.0000,E,0,00,,0.0,M,0.0,M,,0000*4D $GPGGA,000938.066,0000.0000,N,00000.0000,E,0,00,,0.0,M,0.0,M,,0000*41 $GPGGA,000940.075,0000.0000,N,00000.0000,E,0,00,,0.0,M,0.0,M,,0000*4C $GPGGA,000941.065,0000.0000,N,00000.0000,E,0,00,,0.0,M,0.0,M,,0000*4C