在 WM 平台上开发 GPS 导航系统

核心提示：101 Case "*"c102 ' Stop processing before the asterisk103 Exit For104 Case Else105 ' Is this the first value for the checksum?106 If Checksum

101 Case "*"c

102 ' Stop processing before the asterisk

103 Exit For

104 Case Else

105 ' Is this the first value for the checksum?

106 If Checksum = 0 Then

107 ' Yes. Set the checksum to the value

108 Checksum = Convert.ToByte(Character)

109 Else

110 ' No. XOR the checksum with this character's value

111 Checksum = Checksum Xor Convert.ToByte(Character)

112 End If

113 End Select

114 Next

115 ' Return the checksum formatted as a two-character hexadecimal

116 Return Checksum.ToString("X2")

117 End Function

118 End Class

119

目前是否有 Fix

$ GPRMC语句包含一个值，显示“Fix”是否已经获得。当至少三颗卫星的信号强度足以与计算您的位置有关一个修复程序时，Fix将会获得。如果至少有四颗卫星都参与，高度也成为众所周知的。$ GPRMC语句的第三个字是两个字母中的一个：“A”为“积极”，其中获得一个Fix，或“V”为“无效”目前没有Fix存在。列表1-6包括代码审查这一字母和修复情况的报告。

代码片段：

1 *******************************************************

2

3 '** Listing 1-6. Extracting satellite fix status

4

5 '*******************************************************

6

7 Public Class NmeaInterpreter

8

9 ' Raised when the current location has changed

10 Public Event PositionReceived(ByVal latitude As String, _

11 ByVal longitude As String)

12 Public Event DateTimeChanged(ByVal dateTime As DateTime)

13 Public Event BearingReceived(ByVal bearing As Double)

14 Public Event SpeedReceived(ByVal speed As Double)

15 Public Event SpeedLimitReached()

16 Public Event FixObtained()

17 Public Event FixLost()

18 ' Processes information from the GPS receiver