在Windows Mobile和Wince(Windows Embedded CE)下使用.NET Compact Framework进行GPS NMEA data数据分析的开发

提供GPS功能的Wince和Windows Mobile都需要一个GPS接收器（GPS Receiver）。GPS receiver就像一个收音机，他从太空中各个GPS卫星（Satellites）接收信号，通过自身的算法（一般在Firmware里面）计算出位置等信息，然后以NMEA data的格式输出。GPS receiver就是接收卫星信号转换成NMEA data的设备。

进行GPS的开发需要从GPS receiver取出NMEA data，分析出关心的数据。关心的数据包括经度（Longitude），维度（Latitude）和海拔（Altitude）等等。在Windows Mobile 5以上MS提供了GPS Intermediate Driver，开发人员不再需要自己分析NMEA data了。但是Wince5以及以下版本不提供GPS Intermediate Driver，还是需要自己分析NMEA data来取出关心的信息。本文讲述如何使用C#进行NMEA data的分析。第一眼看，分析NMEA有自己做轮子之嫌，其实了解NMEA的分析也是有好处的，由于各个生产GPS receiver的厂商在硬件工艺和算法的不一样，各个厂商都提供自己扩展的NMEA data，这些数据GPS Intermediate Driver是不支持的，需要自己分析。

NMEA 全称NMEA 0183，是电子与数据的通信规范，也就是协议。实现该协议的设备输出这种规范的数据，其他应用就可以基于这协议分析出相关的数据。NMEA开始用在航海设备上，现在广泛用在GPS设备上，这就是为什么NMEA的原始速度使用Knot（海里/小时）表示。下面是一段GPS NMEA data的范例

$GPRMC,000006,A,3754.6240,S,14509.7720,E,010.8,313.1,010108,011.8,E*6A
$GPGGA,201033,3754.6240,S,14509.7720,E,1,05,1.7,91.1,M,-1.1,M,,*75
$GPGSA,A,3,,05,10,,,,21,,29,30,,,2.9,1.7,1.3*32
$GPGSV,3,3,12,29,74,163,41,30,53,337,40,31,09,266,00,37,00,000,00*78
$PGRME,6.3,M,11.9,M,13.5,M*25
$PGRMB,0.0,200,,,,K,,N,W*28
$PGRMM,WGS 84*06

GPS NMEA data有以下特点：
* 每一条NMEA data的数据都是以dollar符号开头。
* 从第二个字符开始的前2个字符表示发送者（talker）和接着3个字符表示数据（message）。其中上面的talker中，GP表示通用的GPS NMEA data，而PG为特定厂商的NMEA data。
* 所有数据字段（data fields）都是使用逗号隔开(comma-delimited)。
* 最后一个数据段接着一个星号(asterisk)。
* 星号后面是两位数字的校正码(checksum)，checksum的计算方法是或计算在 '$' 和 '*'之间的所有字符。
* 最后以回车换行(<CR><LF>)结尾。

有了上述规范，开发NMEA的分析器就变得十分简单，分析流程是：先接收一条NMEA语句（NMEA sentence），然后检查语句格式，检查checksum，然后再根据talker和message进行分发，使用不同的算法进行分析。下面为核心分析流程。

public bool Parse(string sentence)
{
string rawData = sentence;
try
{
if (!IsValid(sentence))
{
return false;
}

sentence = sentence.Substring(1, sentence.IndexOf('*') - 1);
string[] Words = Getwords(sentence);
switch (Words[0])
{
case "GPRMC":
return ParseGPRMC(Words);
case "GPGGA":
return ParseGPGGA(Words);
case "GPGSA":
return ParseGPGSA(Words);
case "GPGSV":
return ParseGPGSV(Words);
default:
return false;
}
}
catch (Exception e)
{
Console.WriteLine(e.Message + rawData);
return false;
}
}
代码1
Parse为分析接口，所有从GPS Receiver接收到NMEA data全部调用这个接口进行分析。
IsValid检验该NMEA sentence是否有效。
Checksum进行Checksum运算，检验校验码是否有效。

接下来，讲述关键语句的分析。进行语句的分析，需要一个NMEA的规范手册，这个手册可以从GPS厂商下载，例如从Garmin下载 NMEA手册
在该手册的第24页可以看到GPRMC的协议定义。

NmeaParser
public class NmeaParser
{
public struct Coordinate
{
public int Hours;
public int Minutes;
public double Seconds;
}

public enum FixStatus
{
NotSet,
Obtained, //A
Lost //V
}

public enum FixMode
{
Auto, //A
Manual
}

public enum FixMethod
{
NotSet,
Fix2D,
Fix3D
}

public enum DifferentialGpsType
{
NotSet,
SPS,
DSPS,
PPS,
RTK
}

public class GpsSatellite
{
public int PRC { get; set; }
public int Elevation { get; set; }
public int Azimuth { get; set; }
public int SNR { get; set; }
public bool InUsed { get; set; }
public bool InView { get; set; }
public bool NotTracking { get; set; }
}

private static readonly CultureInfo NmeaCultureInfo = new CultureInfo("en-US");
private static readonly decimal KMpHPerKnot = decimal.Parse("1.852", NmeaCultureInfo);

private Coordinate latitude;
private Coordinate longitude;
private decimal altitude = 0;

private DateTime utcDateTime;
private decimal velocity = 0;
private decimal azimuth = 0;

private FixStatus fixStatus;
private DifferentialGpsType differentialGpsType;
private FixMode fixMode;
private FixMethod fixMethod;

private int satellitesInView;
private int satellitesInUsed;
private readonly Dictionary<int, GpsSatellite> satellites;

private decimal horizontalDilutionOfPrecision = 50;
private decimal positionDilutionOfPrecision = 50;
private decimal verticalDilutionOfPrecision = 50;

public NmeaParser()
{
satellites = new Dictionary<int, GpsSatellite>();
}

public bool Parse(string sentence)
{
string rawData = sentence;
try
{
if (!IsValid(sentence))
{
return false;
}

sentence = sentence.Substring(1, sentence.IndexOf('*') - 1);
string[] Words = Getwords(sentence);
switch (Words[0])
{
case "GPRMC":
return ParseGPRMC(Words);
case "GPGGA":
return ParseGPGGA(Words);
case "GPGSA":
return ParseGPGSA(Words);
case "GPGSV":
return ParseGPGSV(Words);
default:
return false;
}
}
catch (Exception e)
{
Console.WriteLine(e.Message + rawData);
return false;
}
}

private bool IsValid(string sentence)
{
// GPS data can't be zero length
if (sentence.Length == 0)
{
return false;
}

// first character must be a $
if (sentence[0] != '$')
{
return false;
}

// GPS data can't be longer than 82 character
if (sentence.Length > 82)
{
return false;
}

try
{
string checksum = sentence.Substring(sentence.IndexOf('*') + 1);
return Checksum(sentence, checksum);
}
catch (Exception e)
{
Console.WriteLine("Checksum failure. " + e.Message);
return false;
}
}

private bool Checksum(string sentence, string checksumStr)
{
int checksum = 0;
int length = sentence.IndexOf('*') - 1;
// go from first character upto last *
for (int i = 1; i <= length; ++i)
{
checksum = checksum ^ Convert.ToByte(sentence[i]);
}

return (checksum.ToString("X2") == checksumStr);
}

// Divides a sentence into individual Words
private static string[] Getwords(string sentence)
{
return sentence.Split(',');
}

private bool ParseGPRMC(string[] Words)
{
if (Words[1].Length > 0 & Words[9].Length > 0)
{
int UtcHours = Convert.ToInt32(Words[1].Substring(0, 2));
int UtcMinutes = Convert.ToInt32(Words[1].Substring(2, 2));
int UtcSeconds = Convert.ToInt32(Words[1].Substring(4, 2));
int UtcMilliseconds = 0;

// Extract milliseconds if it is available
if (Words[1].Length > 7)
{
UtcMilliseconds = Convert.ToInt32(Words[1].Substring(7));
}

int UtcDay = Convert.ToInt32(Words[9].Substring(0, 2));
int UtcMonth = Convert.ToInt32(Words[9].Substring(2, 2));
// available for this century
int UtcYear = Convert.ToInt32(Words[9].Substring(4, 2)) + 2000;

utcDateTime = new DateTime(UtcYear, UtcMonth, UtcDay, UtcHours, UtcMinutes, UtcSeconds, UtcMilliseconds);
}

fixStatus = (Words[2][0] == 'A') ? FixStatus.Obtained : FixStatus.Lost;

if (Words[3].Length > 0 & Words[4].Length == 1 & Words[5].Length > 0 & Words[6].Length == 1)
{
latitude.Hours = int.Parse(Words[3].Substring(0, 2));
latitude.Minutes = int.Parse(Words[3].Substring(2, 2));
latitude.Seconds = Math.Round(double.Parse(Words[3].Substring(5, 4)) * 6 / 1000.0, 3);
if ("S" == Words[4])
{
latitude.Hours = -latitude.Hours;
}

longitude.Hours = int.Parse(Words[5].Substring(0, 3));
longitude.Minutes = int.Parse(Words[5].Substring(3, 2));
longitude.Seconds = Math.Round(double.Parse(Words[5].Substring(6, 4)) * 6 / 1000.0, 3);
if ("W" == Words[6])
{
longitude.Hours = -longitude.Hours;
}
}

if (Words[8].Length > 0)
{
azimuth = decimal.Parse(Words[8], NmeaCultureInfo);
}

if (Words[7].Length > 0)
{
velocity = decimal.Parse(Words[7], NmeaCultureInfo) * KMpHPerKnot;
}
return true;
}

private bool ParseGPGGA(string[] Words)
{
if (Words[6].Length > 0)
{
switch (Convert.ToInt32(Words[6]))
{
case 0:
differentialGpsType = DifferentialGpsType.NotSet;
break;
case 1:
differentialGpsType = DifferentialGpsType.SPS;
break;
case 2:
differentialGpsType = DifferentialGpsType.DSPS;
break;
case 3:
differentialGpsType = DifferentialGpsType.PPS;
break;
case 4:
differentialGpsType = DifferentialGpsType.RTK;
break;
default:
differentialGpsType = DifferentialGpsType.NotSet;
break;
}
}

if (Words[7].Length > 0)
{
satellitesInUsed = Convert.ToInt32(Words[7]);
}

if (Words[8].Length > 0)
{
horizontalDilutionOfPrecision = Convert.ToDecimal(Words[8]);
}

if (Words[9].Length > 0)
{
altitude = Convert.ToDecimal(Words[9]);
}
return true;
}

private bool ParseGPGSA(string[] Words)
{
if (Words[1].Length > 0)
{
fixMode = Words[1][0] == 'A' ? FixMode.Auto : FixMode.Manual;
}

if (Words[2].Length > 0)
{
switch (Convert.ToInt32(Words[2]))
{
case 1:
fixMethod = FixMethod.NotSet;
break;
case 2:
fixMethod = FixMethod.Fix2D;
break;
case 3:
fixMethod = FixMethod.Fix3D;
break;
default:
fixMethod = FixMethod.NotSet;
break;
}
}

foreach (GpsSatellite s in satellites.Values)
{
s.InUsed = false;
}
satellitesInUsed = 0;
for (int i = 0; i < 12; ++i)
{
string id = Words[3 + i];
if (id.Length > 0)
{
int nId = Convert.ToInt32(id);
if (!satellites.ContainsKey(nId))
{
satellites[nId] = new GpsSatellite();
satellites[nId].PRC = nId;
}
satellites[nId].InUsed = true;
++satellitesInUsed;
}
}

if (Words[15].Length > 0)
{
positionDilutionOfPrecision = Convert.ToDecimal(Words[15]);
}

if (Words[16].Length > 0)
{
horizontalDilutionOfPrecision = Convert.ToDecimal(Words[16]);
}

if (Words[17].Length > 0)
{
verticalDilutionOfPrecision = Convert.ToDecimal(Words[17]);
}
return true;
}

private bool ParseGPGSV(string[] Words)
{
int messageNumber = 0;

if (Words[2].Length > 0)
{
messageNumber = Convert.ToInt32(Words[2]);
}
if (Words[3].Length > 0)
{
satellitesInView = Convert.ToInt32(Words[3]);
}

if (messageNumber == 0 || satellitesInView == 0)
{
return false;
}

for (int i = 1; i <= 4; ++i)
{
if ((Words.Length - 1) >= (i * 4 + 3))
{
int nId = 0;
if (Words[i * 4].Length > 0)
{
string id = Words[i * 4];
nId = Convert.ToInt32(id);
if (!satellites.ContainsKey(nId))
{
satellites[nId] = new GpsSatellite();
satellites[nId].PRC = nId;
}
satellites[nId].InView = true;
}

if (Words[i * 4 + 1].Length > 0)
{
satellites[nId].Elevation = Convert.ToInt32(Words[i * 4 + 1]);
}

if (Words[i * 4 + 2].Length > 0)
{
satellites[nId].Azimuth = Convert.ToInt32(Words[i * 4 + 2]);
}

if (Words[i * 4 + 3].Length > 0)
{
satellites[nId].SNR = Convert.ToInt32(Words[i * 4 + 3]);
satellites[nId].NotTracking = false;
}
else
{
satellites[nId].NotTracking = true;
}
}
}
return true;
}
}

参考文献：
http://en.wikipedia.org/wiki/NMEA_0183
GPS Intermediate Driver Reference