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。

图1
根据手册的规范定义，抽取想要的信息。如下代码：

        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;
        }

代码2
从ParseGPRMC看，传递的参数是一个string的数组，分析要做的事情就是把数组的元素根据手册翻译成需要的信息，例如字段1为UTC的日期信息，字段9为UTC的时间信息。字段2为fix信息，就是GPS是否完成了初始化的信息。字段3，4为经度，字段5，6为维度。字段7为速度。字段8为角度。

        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;
        }

代码3
分析ParseGPGGA和分析ParseGPRMC一样，从数组抽取信息，字段6为fix类型，这个参数表示使用了那些辅佐卫星或者地面信号站来提高GPS的精度。SPS为普通类型，DSPS使用了DGPS 地面信号站fix，DSPS使用了WAAS位置卫星fix（只是用在美国），PPS使用了EGNOS位置卫星fix（只是用在欧洲），RTK使用了MSAS位置卫星fix（只是用在亚洲）。字段7为使用卫星的数量。字段8为水平精度。字段9为海拔。

        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;
        }

代码4
从ParseGPGSA看，字段1为fix的状态，手工fix或者自动fix。字段2为fix的方法，2D或者3D。字段3到14共12个字段分别为在使用卫星的信息。字段15为位置精度信息。字段16为水平精度信息。字段17为垂直精度信息。

        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;
        }

代码5
从ParseGPGSA看，这个比较特别，他把在使用的卫星信息分开多条语句output。如下：

$GPGSV,3,1,12,03,43,246,46,06,57,263,52,09,10,090,00,14,29,357,41*71
$GPGSV,3,2,12,15,12,140,00,16,10,307,00,18,59,140,00,19,20,224,00*75
$GPGSV,3,3,12,21,48,089,00,22,69,265,36,24,09,076,00,34,00,000,00*76

字段1为一共分开多少条语句。字段2为当前语句的序号。字段3为在使用的卫星的数量。后面字段分别表示三个不同卫星的信息，取其中一个卫星来解释，字段4为卫星的ID，字段5为太空海拔，字段6为角度，字段7为信号强弱。

对于厂商的私有NMEA data也是同样的方法进行分析，根据文档的描述进行分析。下面为整个类的代码。

 

    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