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10楼#
发布于：2005-03-11 09:16
<h3>Final Thoughts</h3>
You should now have a good understanding that an NMEA interpreter isall about extracting words from sentences. You can harness the power ofsatellites to determine your location, synchronize your computer clock,find your direction, watch your speed, and point to a satellite in thesky on a cloudy day. This interpreter will also work with the .NETCompact Framework without any modifications. If sentences were alsostored in a file, the interpreter can be used to play back an entireroad trip. These are all great features, especially considering thesmall size of the class, but is this interpreter ready to drive yourcar? Not quite yet. There is one important topic remaining which isrequired to make GPS applications safe for the real world: precision. <p>GPS devices aredesigned to report any information they find, even if the informationis inaccurate. In fact, information about the current location can beoff as much as half a football field, even when devices are equippedwith the latest DGPS and WAAS correction technologies! Unfortunately,several developers are not aware of this problem. There are somethird-party components out there that are not suitable for commercialapplications that require enforcing a minimum level of precision. Keepthis article handy, however, because in Part Two of this series, I willexplain precision enforcement in detail and take the interpreter evenfurther to make it suitable for professional, high-precisionapplications!</p><p><font><i>(To be continued...)</i></font></p><p>[Please indicate your interest in part two by rating this article.]</p><p> <b>About the Author</b> Jon Person is the author of the award-winning "<a href="http://www.gpsdotnet.com/" target="_blank" >GPS.NET Global Positioning SDK</a>"component and has consulted for military companies and the ForestService on GPS solutions. Jon frequently writes about GPS applicationdevelopment and welcomes all questions and comments atinfo@gpsdotnet.com. </p><p><li><a href="http://www.codeguru.com/dbfiles/get_file/PartOneListings.zip?id=8079;lbl=PARTONELISTINGS_ZIP;ds=20041229" target="_blank" >PartOneListings.zip</a> - Listings for Part One (Visual Basic.NET) </li><li><a href="http://www.codeguru.com/dbfiles/get_file/WritingGPSApplications2_demo_gpsdotnet.zip?id=8079;lbl=WRITINGGPSAPPLICATIONS2_DEMO_GPSDOTNET_ZIP;ds=20041229" target="_blank" >WritingGPSApplications2_demo_gpsdotnet.zip</a> - Demonstration using GPS.NET SDK <p></p></li><!--content_stop--><table border="0" cellpadding="3" width="100%"> <tr> <td align="left" valign="top"> </td></tr></table></p> </p> |
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11楼#
发布于：2005-03-11 09:14
<h3>A World-Class Interpreter</h3>
International readers may have spotted a subtle problem early onthat was not handled in the listings—numbers were being reported in thenumeric format used in the United States! Countries such as Belgium andSwitzerland, which use different formats for numbers, requireadjustments to the interpreter to work at all. Fortunately, the .NETframework includes built-in support for converting numbers amongdifferent cultures, so the changes to the interpreter required arestraightforward. In the interpreter, the only fractional value isspeed, so only one change is necessary. The <b>NmeaCultureInfo</b> variable represents the culture used for numbers within NMEA sentences. The <b>Double.Parse</b>method is then used with this variable to convert speed into themachine's local culture. Listing 1-8 shows the completed interpreter,now ready for use internationally.</p><p><b>Listing 1-8:</b> The completed interpreter, suitable for use anywhere in the world.</p><pre>'************************************************************* '** Listing 1-8. Adding support for international cultures '************************************************************* Imports System.Globalization Public Class NmeaInterpreter ' Represents the EN-US culture, used for numers in NMEA sentences Private NmeaCultureInfo As New CultureInfo("en-US") ' Used to convert knots into miles per hour Private MPHPerKnot As Double = Double.Parse("1.150779", _ NmeaCultureInfo) ' Raised when the current location has changed Public Event PositionReceived(ByVal latitude As String, _ ByVal longitude As String) Public Event DateTimeChanged(ByVal dateTime As DateTime) Public Event BearingReceived(ByVal bearing As Double) Public Event SpeedReceived(ByVal speed As Double) Public Event SpeedLimitReached() Public Event FixObtained() Public Event FixLost() Public Event SatelliteReceived(ByVal pseudoRandomCode As Integer, _ ByVal azimuth As Integer, _ ByVal elevation As Integer, _ ByVal signalToNoiseRatio As Integer) ' Processes information from the GPS receiver Public Function Parse(ByVal sentence As String) As Boolean ' Discard the sentence if its checksum does not match our ' calculated checksum If Not IsValid(sentence) Then Return False ' Look at the first word to decide where to go next Select Case GetWords(sentence)(0) Case "$GPRMC" ' A "Recommended Minimum" sentence was found! Return ParseGPRMC(sentence) Case "$GPGSV" Return ParseGPGSV(sentence) Case Else ' Indicate that the sentence was not recognized Return False End Select End Function ' Divides a sentence into individual words Public Function GetWords(ByVal sentence As String) As String() Return sentence.Split(","c) End Function ' Interprets a $GPRMC message Public Function ParseGPRMC(ByVal sentence As String) As Boolean ' Divide the sentence into words Dim Words() As String = GetWords(sentence) ' Do we have enough values to describe our location? If Words(3) <> "" And Words(4) <> "" And Words(5) <> "" _ And Words(6) <> "" Then ' Yes. Extract latitude and longitude Dim Latitude As String = Words(3).Substring(0, 2) ; "0" ' Append hours Latitude = Latitude ; Words(3).Substring(2) ; """" ' Append minutes Latitude = Latitude ; Words(4) ' Append the hemisphere Dim Longitude As String = Words(5).Substring(0, 3) ; "0" ' Append hours Longitude = Longitude ; Words(5).Substring(3) ; """" ' Append minutes Longitude = Longitude ; Words(6) ' Append the hemisphere ' Notify the calling application of the change RaiseEvent PositionReceived(Latitude, Longitude) End If ' Do we have enough values to parse satellite-derived time? If Words(1) <> "" Then ' Yes. Extract hours, minutes, seconds and milliseconds Dim UtcHours As Integer = CType(Words(1).Substring(0, 2), _ Integer) Dim UtcMinutes As Integer = CType(Words(1).Substring(2, 2), _ Integer) Dim UtcSeconds As Integer = CType(Words(1).Substring(4, 2), _ Integer) Dim UtcMilliseconds As Integer ' Extract milliseconds if it is available If Words(1).Length > 7 Then UtcMilliseconds = _ CType(Words(1).Substring(7), Integer) ' Now build a DateTime object with all values Dim Today As DateTime = System.DateTime.Now.ToUniversalTime Dim SatelliteTime As New System.DateTime(Today.Year, _ Today.Month, Today.Day, UtcHours, UtcMinutes, UtcSeconds, _ UtcMilliseconds) ' Notify of the new time, adjusted to the local time zone RaiseEvent DateTimeChanged(SatelliteTime.ToLocalTime) End If ' Do we have enough information to extract the current speed? If Words(7) <> "" Then ' Yes. Parse the speed and convert it to MPH Dim Speed As Double = Double.Parse(Words(7), NmeaCultureInfo) * _ MPHPerKnot ' Notify of the new speed RaiseEvent SpeedReceived(Speed) ' Are we over the highway speed limit? If Speed > 55 Then RaiseEvent SpeedLimitReached() End If ' Do we have enough information to extract bearing? If Words(8) <> "" Then ' Indicate that the sentence was recognized Dim Bearing As Double = CType(Words(8), Double) RaiseEvent BearingReceived(Bearing) End If ' Does the device currently have a satellite fix? If Words(2) <> "" Then Select Case Words(2) Case "A" RaiseEvent FixObtained() Case "V" RaiseEvent FixLost() End Select End If ' Indicate that the sentence was recognized Return True End Function ' Interprets a "Satellites in View" NMEA sentence Public Function ParseGPGSV(ByVal sentence As String) As Boolean Dim PseudoRandomCode As Integer Dim Azimuth As Integer Dim Elevation As Integer Dim SignalToNoiseRatio As Integer ' Divide the sentence into words Dim Words() As String = GetWords(sentence) ' Each sentence contains four blocks of satellite information. ' Read each block and report each satellite's information Dim Count As Integer For Count = 1 To 4 ' Does the sentence have enough words to analyze? If (Words.Length - 1) >= (Count * 4 + 3) Then ' Yes. Proceed with analyzing the block. Does it contain any ' information? If Words(Count * 4) <> "" And Words(Count * 4 + 1) <> "" _ And Words(Count * 4 + 2) <> "" _ And Words(Count * 4 + 3) <> "" Then ' Yes. Extract satellite information and report it PseudoRandomCode = CType(Words(Count * 4), Integer) Elevation = CType(Words(Count * 4 + 1), Integer) Azimuth = CType(Words(Count * 4 + 2), Integer) SignalToNoiseRatio = CType(Words(Count * 4 + 2), Integer) ' Notify of this satellite's information RaiseEvent SatelliteReceived(PseudoRandomCode, Azimuth, _ Elevation, SignalToNoiseRatio) End If End If Next ' Indicate that the sentence was recognized Return True End Function ' Returns True if a sentence's checksum matches the calculated ' checksum Public Function IsValid(ByVal sentence As String) As Boolean ' Compare the characters after the asterisk to the calculation Return sentence.Substring(sentence.IndexOf("*") + 1) = _ GetChecksum(sentence) End Function ' Calculates the checksum for a sentence Public Function GetChecksum(ByVal sentence As String) As String ' Loop through all chars to get a checksum Dim Character As Char Dim Checksum As Integer For Each Character In sentence Select Case Character Case "$"c ' Ignore the dollar sign Case "*"c ' Stop processing before the asterisk Exit For Case Else ' Is this the first value for the checksum? If Checksum = 0 Then ' Yes. Set the checksum to the value Checksum = Convert.ToByte(Character) Else ' No. XOR the checksum with this character's value Checksum = Checksum Xor Convert.ToByte(Character) End If End Select Next ' Return the checksum formatted as a two-character hexadecimal Return Checksum.ToString("X2") End Function End Class </pre> |
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12楼#
发布于：2005-03-11 09:13
<h3>Real-Time Satellite Tracking</h3>
Knowing the location of satellites is important when determining howprecise readings are and how stable a GPS fix is. Because GPS precisionwill be covered in detail in Part Two of this series, this section willfocus on interpreting satellite location and signal strength. <p>There are twenty-fouroperational satellites in orbit. Satellites are spaced in orbit so thatat any time a minimum of six satellites will be in view to usersanywhere in the world. Satellites are constantly in motion, which isgood because it prevents the existence of "blind spots" in the worldwith little or no satellite visibility. Just like finding stars in thesky, satellite locations are described as the combination of an azimuthand an elevation. As mentioned above, azimuth measures a directionaround the horizon. Elevation measures a degree value up from thehorizon between 00 and 900, where 00 represents the horizon and 900represents "zenith," directly overhead. So, if the device says asatellite's azimuth is 450 and its elevation is 450, the satellite islocated halfway up from the horizon towards the northeast. In additionto location, devices report each satellite's "Pseudo-Random Code" (orPRC), which is a number used to uniquely identify one satellite fromanother.</p><p>Here's an example of a $GPGSV sentence:</p><pre>$GPGSV,3,1,10,24,82,023,40,05,62,285,32,01,62,123,00,17,59,229,28*70</pre><p>Each sentence contains up to four blocks of satellite information,comprised of four words. For example, the first block is "24,82,023,40"and the second block is "05,62,285,32" and so on. The first word ofeach block gives the satellite's PRC. The second word gives eachsatellite's elevation, followed by azimuth and signal strength. If thissatellite information were to be shown graphically, it would look likeFigure 1-1.</p><p><img src="http://www.codeguru.com/dbfiles/get_image.php?id=8079;lbl=FIGURE11_JPG;ds=20041229"></p><p><b>Figure 1-1:</b> Graphical representation of a $GPGSV sentence,where the center of the circle marks the current position and the edgeof the circle marks the horizon.</p><p>Perhaps the most important number in this sentence is the"signal-to-noise ratio" (or SNR for short). This number indicates howstrongly a satellite's radio signal is being received. Remember,satellites transmit signals at the same strength, but things such astrees and walls can obscure a signal beyond recognition. Typical SNRvalues are between zero and fifty, where fifty means an excellentsignal. (SNR can be as high as ninety-nine, but I've never seenreadings above fifty, even in wide open sky.) In Figure 1-1, the greensatellites indicate a strong signal, whereas the yellow satellitesignifies a moderate signal. (In Part Two, I will provide a way toclassify signal strengths.) Satellite #1's signal is completelyobscured. Listing 1-7 shows the interpreter after it is expanded toread satellite info.</p><p><b>Listing 1-7:</b> The interpreter is improved to interpret the location of GPS satellites currently in view.</p><pre>'******************************************************* '** Listing 1-7. Extracting satellite information '******************************************************* Public Class NmeaInterpreter ' Raised when the current location has changed Public Event PositionReceived(ByVal latitude As String, _ ByVal longitude As String) Public Event DateTimeChanged(ByVal dateTime As DateTime) Public Event BearingReceived(ByVal bearing As Double) Public Event SpeedReceived(ByVal speed As Double) Public Event SpeedLimitReached() Public Event FixObtained() Public Event FixLost() Public Event SatelliteReceived(ByVal pseudoRandomCode As Integer, _ ByVal azimuth As Integer, _ ByVal elevation As Integer, _ ByVal signalToNoiseRatio As Integer) ' Processes information from the GPS receiver Public Function Parse(ByVal sentence As String) As Boolean ' Discard the sentence if its checksum does not match our ' calculated checksum If Not IsValid(sentence) Then Return False ' Look at the first word to decide where to go next Select Case GetWords(sentence)(0) Case "$GPRMC" ' A "Recommended Minimum" sentence was found! Return ParseGPRMC(sentence) Case "$GPGSV" Return ParseGPGSV(sentence) Case Else ' Indicate that the sentence was not recognized Return False End Select End Function ' Divides a sentence into individual words Public Function GetWords(ByVal sentence As String) As String() Return sentence.Split(","c) End Function ' Interprets a $GPRMC message Public Function ParseGPRMC(ByVal sentence As String) As Boolean ' Divide the sentence into words Dim Words() As String = GetWords(sentence) ' Do we have enough values to describe our location? If Words(3) <> "" And Words(4) <> "" And Words(5) <> "" _ And Words(6) <> "" Then ' Yes. Extract latitude and longitude Dim Latitude As String = Words(3).Substring(0, 2) ; "0" ' Append hours Latitude = Latitude ; Words(3).Substring(2) ; """" ' Append minutes Latitude = Latitude ; Words(4) ' Append the hemisphere Dim Longitude As String = Words(5).Substring(0, 3) ; "0" ' Append hours Longitude = Longitude ; Words(5).Substring(3) ; """" ' Append minutes Longitude = Longitude ; Words(6) ' Append the hemisphere ' Notify the calling application of the change RaiseEvent PositionReceived(Latitude, Longitude) End If ' Do we have enough values to parse satellite-derived time? If Words(1) <> "" Then ' Yes. Extract hours, minutes, seconds and milliseconds Dim UtcHours As Integer = CType(Words(1).Substring(0, 2), _ Integer) Dim UtcMinutes As Integer = CType(Words(1).Substring(2, 2), _ Integer) Dim UtcSeconds As Integer = CType(Words(1).Substring(4, 2), _ Integer) Dim UtcMilliseconds As Integer ' Extract milliseconds if it is available If Words(1).Length > 7 Then UtcMilliseconds = _ CType(Words(1).Substring(7), Integer) ' Now build a DateTime object with all values Dim Today As DateTime = System.DateTime.Now.ToUniversalTime Dim SatelliteTime As New System.DateTime(Today.Year, _ Today.Month, Today.Day, UtcHours, UtcMinutes, UtcSeconds, _ UtcMilliseconds) ' Notify of the new time, adjusted to the local time zone RaiseEvent DateTimeChanged(SatelliteTime.ToLocalTime) End If ' Do we have enough information to extract the current speed? If Words(7) <> "" Then ' Yes. Convert it into MPH Dim Speed As Double = CType(Words(7), Double) * 1.150779 ' If we're over 55MPH then trigger a speed alarm! If Speed > 55 Then RaiseEvent SpeedLimitReached() ' Notify of the new speed RaiseEvent SpeedReceived(Speed) End If ' Do we have enough information to extract bearing? If Words(8) <> "" Then ' Indicate that the sentence was recognized Dim Bearing As Double = CType(Words(8), Double) RaiseEvent BearingReceived(Bearing) End If ' Does the device currently have a satellite fix? If Words(2) <> "" Then Select Case Words(2) Case "A" RaiseEvent FixObtained() Case "V" RaiseEvent FixLost() End Select End If ' Indicate that the sentence was recognized Return True End Function ' Interprets a "Satellites in View" NMEA sentence Public Function ParseGPGSV(ByVal sentence As String) As Boolean Dim PseudoRandomCode As Integer Dim Azimuth As Integer Dim Elevation As Integer Dim SignalToNoiseRatio As Integer ' Divide the sentence into words Dim Words() As String = GetWords(sentence) ' Each sentence contains four blocks of satellite information. 'Read each block and report each satellite's information Dim Count As Integer For Count = 1 To 4 ' Does the sentence have enough words to analyze? If (Words.Length - 1) >= (Count * 4 + 3) Then ' Yes. Proceed with analyzing the block. Does it contain any ' information? If Words(Count * 4) <> "" And Words(Count * 4 + 1) <> "" _ And Words(Count * 4 + 2) <> "" _ And Words(Count * 4 + 3) <> "" Then ' Yes. Extract satellite information and report it PseudoRandomCode = CType(Words(Count * 4), Integer) Elevation = CType(Words(Count * 4 + 1), Integer) Azimuth = CType(Words(Count * 4 + 2), Integer) SignalToNoiseRatio = CType(Words(Count * 4 + 2), Integer) ' Notify of this satellite's information RaiseEvent SatelliteReceived(PseudoRandomCode, Azimuth, _ Elevation, SignalToNoiseRatio) End If End If Next ' Indicate that the sentence was recognized Return True End Function ' Returns True if a sentence's checksum matches the calculated ' checksum Public Function IsValid(ByVal sentence As String) As Boolean ' Compare the characters after the asterisk to the calculation Return sentence.Substring(sentence.IndexOf("*") + 1) = _ GetChecksum(sentence) End Function ' Calculates the checksum for a sentence Public Function GetChecksum(ByVal sentence As String) As String ' Loop through all chars to get a checksum Dim Character As Char Dim Checksum As Integer For Each Character In sentence Select Case Character Case "$"c ' Ignore the dollar sign Case "*"c ' Stop processing before the asterisk Exit For Case Else ' Is this the first value for the checksum? If Checksum = 0 Then ' Yes. Set the checksum to the value Checksum = Convert.ToByte(Character) Else ' No. XOR the checksum with this character's value Checksum = Checksum Xor Convert.ToByte(Character) End If End Select Next ' Return the checksum formatted as a two-character hexadecimal Return Checksum.ToString("X2") End Function End Class</pre> </p> |
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13楼#
发布于：2005-03-11 09:12
<h3>Are We Fixed Yet?</h3>
The $GPRMC sentence includes a value which indicates whether or nota "fix" has been obtained. A fix is possible when the signal strengthof at least three satellites is strong enough to be involved incalculating your location. If at least four satellites are involved,altitude also becomes known. The third word of the $GPRMC sentence isone of two letters: "A" for "active," where a fix is obtained, or "V"for "invalid" where no fix is present. Listing 1-6 includes code toexamine this character and report on fix status. <p><b>Listing 1-6:</b> The interpreter now knows when the device has obtained a fix.</p><pre>'******************************************************* '** Listing 1-6. Extracting satellite fix status '******************************************************* Public Class NmeaInterpreter ' Raised when the current location has changed Public Event PositionReceived(ByVal latitude As String, _ ByVal longitude As String) Public Event DateTimeChanged(ByVal dateTime As DateTime) Public Event BearingReceived(ByVal bearing As Double) Public Event SpeedReceived(ByVal speed As Double) Public Event SpeedLimitReached() Public Event FixObtained() Public Event FixLost() ' Processes information from the GPS receiver Public Function Parse(ByVal sentence As String) As Boolean ' Discard the sentence if its checksum does not match our ' calculated checksum If Not IsValid(sentence) Then Return False ' Look at the first word to decide where to go next Select Case GetWords(sentence)(0) Case "$GPRMC" ' A "Recommended Minimum" sentence was found! Return ParseGPRMC(sentence) Case Else ' Indicate that the sentence was not recognized Return False End Select End Function ' Divides a sentence into individual words Public Function GetWords(ByVal sentence As String) As String() Return sentence.Split(","c) End Function ' Interprets a $GPRMC message Public Function ParseGPRMC(ByVal sentence As String) As Boolean ' Divide the sentence into words Dim Words() As String = GetWords(sentence) ' Do we have enough values to describe our location? If Words(3) <> "" And Words(4) <> "" And Words(5) <> "" And Words(6) <> "" Then ' Yes. Extract latitude and longitude Dim Latitude As String = Words(3).Substring(0, 2) ; "0" ' Append hours Latitude = Latitude ; Words(3).Substring(2) ; """" ' Append minutes Latitude = Latitude ; Words(4) ' Append the hemisphere Dim Longitude As String = Words(5).Substring(0, 3) ; "0" ' Append hours Longitude = Longitude ; Words(5).Substring(3) ; """" ' Append minutes Longitude = Longitude ; Words(6) ' Append the hemisphere ' Notify the calling application of the change RaiseEvent PositionReceived(Latitude, Longitude) End If ' Do we have enough values to parse satellite-derived time? If Words(1) <> "" Then ' Yes. Extract hours, minutes, seconds and milliseconds Dim UtcHours As Integer = CType(Words(1).Substring(0, 2), _ Integer) Dim UtcMinutes As Integer = CType(Words(1).Substring(2, 2), _ Integer) Dim UtcSeconds As Integer = CType(Words(1).Substring(4, 2), _ Integer) Dim UtcMilliseconds As Integer ' Extract milliseconds if it is available If Words(1).Length > 7 Then UtcMilliseconds = _ CType(Words(1).Substring(7), Integer) ' Now build a DateTime object with all values Dim Today As DateTime = System.DateTime.Now.ToUniversalTime Dim SatelliteTime As New System.DateTime(Today.Year, _ Today.Month, Today.Day, UtcHours, UtcMinutes, UtcSeconds, _ UtcMilliseconds) ' Notify of the new time, adjusted to the local time zone RaiseEvent DateTimeChanged(SatelliteTime.ToLocalTime) End If ' Do we have enough information to extract the current speed? If Words(7) <> "" Then ' Yes. Convert it into MPH Dim Speed As Double = CType(Words(7), Double) * 1.150779 ' If we're over 55MPH then trigger a speed alarm! If Speed > 55 Then RaiseEvent SpeedLimitReached() ' Notify of the new speed RaiseEvent SpeedReceived(Speed) End If ' Do we have enough information to extract bearing? If Words(8) <> "" Then ' Yes. Report the bearing information Dim Bearing As Double = CType(Words(8), Double) RaiseEvent BearingReceived(Bearing) End If ' Does the device currently have a satellite fix? If Words(2) <> "" Then Select Case Words(2) Case "A" RaiseEvent FixObtained() Case "V" RaiseEvent FixLost() End Select End If ' Indicate that the sentence was recognized Return True End Function ' Returns True if a sentence's checksum matches the calculated ' checksum Public Function IsValid(ByVal sentence As String) As Boolean ' Compare the characters after the asterisk to the calculation Return sentence.Substring(sentence.IndexOf("*") + 1) = _ GetChecksum(sentence) End Function ' Calculates the checksum for a sentence Public Function GetChecksum(ByVal sentence As String) As String ' Loop through all chars to get a checksum Dim Character As Char Dim Checksum As Integer For Each Character In sentence Select Case Character Case "$"c ' Ignore the dollar sign Case "*"c ' Stop processing before the asterisk Exit For Case Else ' Is this the first value for the checksum? If Checksum = 0 Then ' Yes. Set the checksum to the value Checksum = Convert.ToByte(Character) Else ' No. XOR the checksum with this character's value Checksum = Checksum Xor Convert.ToByte(Character) End If End Select Next ' Return the checksum formatted as a two-character hexadecimal Return Checksum.ToString("X2") End Function End Class</pre><p>As you can see, a whole lot of information is packed into a singleNMEA sentence. Now that the $GPRMC sentence has been fully interpreted,the interpreter can be expanded to support a second sentence: $GPGSV.This sentence describes the configuration of satellites overhead, inreal-time.</p> </p> |
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14楼#
发布于：2005-03-11 09:10
<h3>Direction ; Speed Alerts</h3>
GPS devices analyze your position over time to calculate speed andbearing. The $GPRMC sentence at the beginning of this article alsoincludes these readings. Speed is always reported in knots and bearingis reported as an "azimuth," a measurement around the horizon measuredclockwise from 00 to 3600 where 00 represents north, 900 means east,and so on. A little math is applied to convert knots into miles perhour. The power of GPS is again demonstrated with one line of code inListing 1-5 that figures out whether a car is traveling over the speedlimit.</p><p><b>Listing 1-5:</b> This class now can tell you which direction you're going and help prevent a speeding ticket.</p><pre>'******************************************************* '** Listing 1-5. Extracting speed and bearing '******************************************************* Public Class NmeaInterpreter ' Raised when the current location has changed Public Event PositionReceived(ByVal latitude As String, _ ByVal longitude As String) Public Event DateTimeChanged(ByVal dateTime As DateTime) Public Event BearingReceived(ByVal bearing As Double) Public Event SpeedReceived(ByVal speed As Double) Public Event SpeedLimitReached() ' Processes information from the GPS receiver Public Function Parse(ByVal sentence As String) As Boolean ' Discard the sentence if its checksum does not match our ' calculated checksum If Not IsValid(sentence) Then Return False ' Look at the first word to decide where to go next Select Case GetWords(sentence)(0) Case "$GPRMC" ' A "Recommended Minimum" sentence was found! Return ParseGPRMC(sentence) Case Else ' Indicate that the sentence was not recognized Return False End Select End Function ' Divides a sentence into individual words Public Function GetWords(ByVal sentence As String) As String() Return sentence.Split(","c) End Function ' Interprets a $GPRMC message Public Function ParseGPRMC(ByVal sentence As String) As Boolean ' Divide the sentence into words Dim Words() As String = GetWords(sentence) ' Do we have enough values to describe our location? If Words(3) <> "" And Words(4) <> "" And Words(5) <> "" _ And Words(6) <> "" Then ' Yes. Extract latitude and longitude Dim Latitude As String = Words(3).Substring(0, 2) ; "0" ' Append hours Latitude = Latitude ; Words(3).Substring(2) ; """" ' Append minutes Latitude = Latitude ; Words(4) ' Append the hemisphere Dim Longitude As String = Words(5).Substring(0, 3) ; "0" ' Append hours Longitude = Longitude ; Words(5).Substring(3) ; """" ' Append minutes Longitude = Longitude ; Words(6) ' Append the hemisphere ' Notify the calling application of the change RaiseEvent PositionReceived(Latitude, Longitude) End If ' Do we have enough values to parse satellite-derived time? If Words(1) <> "" Then ' Yes. Extract hours, minutes, seconds and milliseconds Dim UtcHours As Integer = CType(Words(1).Substring(0, 2), _ Integer) Dim UtcMinutes As Integer = CType(Words(1).Substring(2, 2), _ Integer) Dim UtcSeconds As Integer = CType(Words(1).Substring(4, 2), _ Integer) Dim UtcMilliseconds As Integer ' Extract milliseconds if it is available If Words(1).Length > 7 Then UtcMilliseconds = _ CType(Words(1).Substring(7), Integer) ' Now build a DateTime object with all values Dim Today As DateTime = System.DateTime.Now.ToUniversalTime Dim SatelliteTime As New System.DateTime(Today.Year, _ Today.Month, Today.Day, UtcHours, UtcMinutes, UtcSeconds, _ UtcMilliseconds) ' Notify of the new time, adjusted to the local time zone RaiseEvent DateTimeChanged(SatelliteTime.ToLocalTime) End If ' Do we have enough information to extract the current speed? If Words(7) <> "" Then ' Yes. Convert it into MPH Dim Speed As Double = CType(Words(7), Double) * 1.150779 ' If we're over 55MPH then trigger a speed alarm! If Speed > 55 Then RaiseEvent SpeedLimitReached() ' Notify of the new speed RaiseEvent SpeedReceived(Speed) End If ' Do we have enough information to extract bearing? If Words(8) <> "" Then ' Indicate that the sentence was recognized Dim Bearing As Double = CType(Words(8), Double) RaiseEvent BearingReceived(Bearing) End If ' Indicate that the sentence was recognized Return True End Function ' Returns True if a sentence's checksum matches the calculated " checksum Public Function IsValid(ByVal sentence As String) As Boolean ' Compare the characters after the asterisk to the calculation Return sentence.Substring(sentence.IndexOf("*") + 1) = _ GetChecksum(sentence) End Function ' Calculates the checksum for a sentence Public Function GetChecksum(ByVal sentence As String) As String ' Loop through all chars to get a checksum Dim Character As Char Dim Checksum As Integer For Each Character In sentence Select Case Character Case "$"c ' Ignore the dollar sign Case "*"c ' Stop processing before the asterisk Exit For Case Else ' Is this the first value for the checksum? If Checksum = 0 Then ' Yes. Set the checksum to the value Checksum = Convert.ToByte(Character) Else ' No. XOR the checksum with this character's value Checksum = Checksum Xor Convert.ToByte(Character) End If End Select Next ' Return the checksum formatted as a two-character hexadecimal Return Checksum.ToString("X2") End Function End Class </pre> |
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15楼#
发布于：2005-03-11 09:09
<h3>Wireless Atomic Time</h3>
Time is the cornerstone of GPS technology because distances aremeasured at the speed of light. Each GPS satellite contains four atomicclocks that it uses to time its radio transmissions within a fewnanoseconds. One fascinating feature is that with just a few lines ofcode, these atomic clocks can be used to synchronize a computer's clockwith millisecond accuracy. The second word of the $GPRMC sentence,"040302.663," contains satellite-derived time in a compressed format.The first two characters represent hours, the next two representminutes, the next two represent seconds, and everything after thedecimal place is milliseconds. So, the time is 4:03:02.663 AM. However,satellites report time in universal time (GMT+0), so the time must tobe adjusted to the local time zone. Listing 1-4 adds support forsatellite-derived time and uses the DateTime.ToLocalTime method toconvert satellite time to the local time zone.</p><p><b>Listing 1-4:</b> This class can now use atomic clocks to synchronize your computer's clock wirelessly.</p><pre>'******************************************************** '** Listing 1-4. Add support for satellite-derived time '******************************************************** Public Class NmeaInterpreter ' Raised when the current location has changed Public Event PositionReceived(ByVal latitude As String, _ ByVal longitude As String) Public Event DateTimeChanged(ByVal dateTime As DateTime) ' Processes information from the GPS receiver Public Function Parse(ByVal sentence As String) As Boolean ' Discard the sentence if its checksum does not match our ' calculated checksum If Not IsValid(sentence) Then Return False ' Look at the first word to decide where to go next Select Case GetWords(sentence)(0) Case "$GPRMC" ' A "Recommended Minimum" sentence was found! Return ParseGPRMC(sentence) Case Else ' Indicate that the sentence was not recognized Return False End Select End Function ' Divides a sentence into individual words Public Function GetWords(ByVal sentence As String) As String() Return sentence.Split(","c) End Function ' Interprets a $GPRMC message Public Function ParseGPRMC(ByVal sentence As String) As Boolean ' Divide the sentence into words Dim Words() As String = GetWords(sentence) ' Do we have enough values to describe our location? If Words(3) <> "" And Words(4) <> "" And Words(5) <> "" _ And Words(6) <> "" Then ' Yes. Extract latitude and longitude Dim Latitude As String = Words(3).Substring(0, 2) ; "0" ' Append hours Latitude = Latitude ; Words(3).Substring(2) ; """" ' Append minutes Latitude = Latitude ; Words(4) ' Append the hemisphere Dim Longitude As String = Words(5).Substring(0, 3) ; "0" ' Append hours Longitude = Longitude ; Words(5).Substring(3) ; """" ' Append minutes Longitude = Longitude ; Words(6) ' Append the hemisphere ' Notify the calling application of the change RaiseEvent PositionReceived(Latitude, Longitude) End If ' Do we have enough values to parse satellite-derived time? If Words(1) <> "" Then ' Yes. Extract hours, minutes, seconds and milliseconds Dim UtcHours As Integer = CType(Words(1).Substring(0, 2), _ Integer) Dim UtcMinutes As Integer = CType(Words(1).Substring(2, 2), _ Integer) Dim UtcSeconds As Integer = CType(Words(1).Substring(4, 2), _ Integer) Dim UtcMilliseconds As Integer ' Extract milliseconds if it is available If Words(1).Length > 7 Then UtcMilliseconds = _ CType(Words(1).Substring(7), Integer) ' Now build a DateTime object with all values Dim Today As DateTime = System.DateTime.Now.ToUniversalTime Dim SatelliteTime As New System.DateTime(Today.Year, _ Today.Month, Today.Day, UtcHours, UtcMinutes, UtcSeconds, _ UtcMilliseconds) ' Notify of the new time, adjusted to the local time zone RaiseEvent DateTimeChanged(SatelliteTime.ToLocalTime) End If ' Indicate that the sentence was recognized Return True End Function ' Returns True if a sentence's checksum matches the calculated ' checksum Public Function IsValid(ByVal sentence As String) As Boolean ' Compare the characters after the asterisk to the calculation Return sentence.Substring(sentence.IndexOf("*") + 1) = _ GetChecksum(sentence) End Function ' Calculates the checksum for a sentence Public Function GetChecksum(ByVal sentence As String) As String ' Loop through all chars to get a checksum Dim Character As Char Dim Checksum As Integer For Each Character In sentence Select Case Character Case "$"c ' Ignore the dollar sign Case "*"c ' Stop processing before the asterisk Exit For Case Else ' Is this the first value for the checksum? If Checksum = 0 Then ' Yes. Set the checksum to the value Checksum = Convert.ToByte(Character) Else ' No. XOR the checksum with this character's value Checksum = Checksum Xor Convert.ToByte(Character) End If End Select Next ' Return the checksum formatted as a two-character hexadecimal Return Checksum.ToString("X2") End Function End Class</pre> |
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16楼#
发布于：2005-03-11 09:08
<h3>Taking Out the Garbage</h3>
Each NMEA sentence ends with an asterisk followed by a two-letterhexadecimal checksum. A checksum is calculated as the XOR of bytesbetween (but not including) the dollar sign and asterisk. This checksumis then compared with the checksum from the sentence. If the checksumsdo not match, the sentence is typically discarded. This is okay to dobecause the GPS devices tend to repeat the same information every fewseconds. With the ability to compare checksums, the interpreter is ableto throw out any sentence with an invalid checksum. Listing 1-3 expandsthe interpreter to do this.</p><p><b>Listing 1-3:</b> The interpreter can now detect errors and parse only error-free NMEA data.</p><pre>'******************************************************* '** Listing 1-3. Detecting and handling NMEA errors '******************************************************* Public Class NmeaInterpreter ' Raised when the current location has changed Public Event PositionReceived(ByVal latitude As String, _ ByVal longitude As String) ' Processes information from the GPS receiver Public Function Parse(ByVal sentence As String) As Boolean ' Discard the sentence if its checksum does not match our ' calculated checksum If Not IsValid(sentence) Then Return False ' Look at the first word to decide where to go next Select Case GetWords(sentence)(0) Case "$GPRMC" ' A "Recommended Minimum" sentence was found! Return ParseGPRMC(sentence) Case Else ' Indicate that the sentence was not recognized Return False End Select End Function ' Divides a sentence into individual words Public Function GetWords(ByVal sentence As String) As String() Return sentence.Split(","c) End Function ' Interprets a $GPRMC message Public Function ParseGPRMC(ByVal sentence As String) As Boolean ' Divide the sentence into words Dim Words() As String = GetWords(sentence) ' Do we have enough values to describe our location? If Words(3) <> "" And Words(4) <> "" And Words(5) <> "" _ And Words(6) <> "" Then ' Yes. Extract latitude and longitude Dim Latitude As String = Words(3).Substring(0, 2) ; "0" ' Append hours Latitude = Latitude ; Words(3).Substring(2) ; """" ' Append minutes Latitude = Latitude ; Words(4) ' Append the hemisphere Dim Longitude As String = Words(5).Substring(0, 3) ; "0" ' Append hours Longitude = Longitude ; Words(5).Substring(3) ; """" ' Append minutes Longitude = Longitude ; Words(6) ' Append the hemisphere ' Notify the calling application of the change RaiseEvent PositionReceived(Latitude, Longitude) End If ' Indicate that the sentence was recognized Return True End Function ' Returns True if a sentence's checksum matches the calculated ' checksum Public Function IsValid(ByVal sentence As String) As Boolean ' Compare the characters after the asterisk to the calculation Return sentence.Substring(sentence.IndexOf("*") + 1) = _ GetChecksum(sentence) End Function ' Calculates the checksum for a sentence Public Function GetChecksum(ByVal sentence As String) As String ' Loop through all chars to get a checksum Dim Character As Char Dim Checksum As Integer For Each Character In sentence Select Case Character Case "$"c ' Ignore the dollar sign Case "*"c ' Stop processing before the asterisk Exit For Case Else ' Is this the first value for the checksum? If Checksum = 0 Then ' Yes. Set the checksum to the value Checksum = Convert.ToByte(Character) Else ' No. XOR the checksum with this character's value Checksum = Checksum Xor Convert.ToByte(Character) End If End Select Next ' Return the checksum formatted as a two-character hexadecimal Return Checksum.ToString("X2") End Function End Class</pre> |
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17楼#
发布于：2005-03-11 09:07
<h3>The Core of An Interpreter (Continued)</h3>
The next step is to perform actual extraction of information,starting with latitude and longitude. Latitude and longitude are storedin the form "DD°MM'SS.S" where D represents hours (also called"degrees"), M represents minutes, and S represents seconds. Coordinatescan be displayed in shorthand, such as "DD°MM.M" or even "DD°." Thefourth word in the sentence, "3939.7," shows the current latitude ashours and minutes (39°39.7'), except the numbers are squished together.The first two characters (39) represent hours and the remainder of theword (39.7) represents minutes. Longitude is structured the same way,except that the first three characters represent hours (105006.6').Words five and seven indicate the "hemisphere," where "N" means"North," "W" means "West," and so forth. The hemisphere is appended tothe end of the numeric portion to make a complete measurement.</p><center><p><img src="http://www.codeguru.com/RealMedia/ads/adstream_lx.cgi/ew/ewsoftware/www.codeguru.com/vb/mobile/pocketpc/article/c8079__2/i/345006380/accessunit/default/empty.gif/64613530646434653432323961653830?_RM_EMPTY_"></p></center><p>I'vefound that NMEA interpreters are much easier to work with; they areevent-driven. This is because data arrives in no particular order. Anevent-driven class gives the interpreter the most flexibility andresponsiveness to an application. So, I'll design the interpreter toreport information using events. The first event, PositionReceived,will be raised whenever the current latitude and longitude arereceived. Listing 1-2 expands the interpreter to report the currentposition.</p><p><b>Listing 1-2:</b> The interpreter can now report the current latitude and longitude.</p><pre>'******************************************************** '** Listing 1-2. Extracting information from a sentence '******************************************************** Public Class NmeaInterpreter ' Raised when the current location has changed Public Event PositionReceived(ByVal latitude As String, _ ByVal longitude As String) ' Processes information from the GPS receiver Public Function Parse(ByVal sentence As String) As Boolean ' Look at the first word to decide where to go next Select Case GetWords(sentence)(0) Case "$GPRMC" ' A "Recommended Minimum" sentence was found! Return ParseGPRMC(sentence) Case Else ' Indicate that the sentence was not recognized Return False End Select End Function ' Divides a sentence into individual words Public Function GetWords(ByVal sentence As String) As String() Return sentence.Split(","c) End Function ' Interprets a $GPRMC message Public Function ParseGPRMC(ByVal sentence As String) As Boolean ' Divide the sentence into words Dim Words() As String = GetWords(sentence) ' Do we have enough values to describe our location? If Words(3) <> "" And Words(4) <> "" And Words(5) <> "" _ And Words(6) <> "" Then ' Yes. Extract latitude and longitude Dim Latitude As String = Words(3).Substring(0, 2) ; "0" ' Append hours Latitude = Latitude ; Words(3).Substring(2) ; """" ' Append minutes Latitude = Latitude ; Words(4) ' Append the hemisphere Dim Longitude As String = Words(5).Substring(0, 3) ; "0" ' Append hours Longitude = Longitude ; Words(5).Substring(3) ; """" ' Append minutes Longitude = Longitude ; Words(6) ' Append the hemisphere ' Notify the calling application of the change RaiseEvent PositionReceived(Latitude, Longitude) End If ' Indicate that the sentence was recognized Return True End Function End Class</pre><p>One thing to watch out for here is that some GPS devices will reportblank values when no information is known. Therefore, it's a good ideato test each word for a value before parsing. If you need to type thedegree symbol (°), hold down the Alt key and type "0176" on the numerickeypad.</p> |
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18楼#
发布于：2005-03-11 09:05
<center>
<a href="http://www.codeguru.com/RealMedia/ads/click_lx.cgi/ew/ewsoftware/www.codeguru.com/vb/mobile/pocketpc/article/c8079__1/i/971194714/accessunit/default/empty.gif/64613530646434653432323961653830" target="_blank" ><img src="http://www.codeguru.com/RealMedia/ads/Creatives/default/empty.gif"></a><img src="http://www.codeguru.com/RealMedia/ads/adstream_lx.cgi/ew/ewsoftware/www.codeguru.com/vb/mobile/pocketpc/article/c8079__1/i/971194714/accessunit/default/empty.gif/64613530646434653432323961653830?_RM_EMPTY_"></p></center><p><img src="http://www.codeguru.com/dbfiles/get_image.php?id=8079;lbl=GPSPOCKETPC3_JPG;ds=20041229"></p><p>"I am continually amazed by how little code is required to use atomic clocks in satellites 11,000 miles above my head."</p><p>What is it that GPS applications need to be good enough to handlein-car navigation? Also, how does the process of interpreting GPS dataactually work? In this two-part series, I will cover both topics andgive you the skills you need to write a commercial-grade GPSapplication that works with a majority of GPS devices in the industrytoday.</p><h3>One Powerful Sentence</h3><p>This first part in the series will explore the task of interpretingraw GPS data. Fortunately, the task is simplified thanks to theNational Marine Electronics Association (www.nmea.org), whichintroduced a standard for the industry now in use by a vast majority ofGPS devices. To give developers a head start, I chose to use someVisual Studio.NET source code from my "GPS.NET Global Positioning SDK"component. (The code is stripped of features like multithreading anderror handling for brevity.)</p><p>NMEA data is sent as comma-delimited "sentences" that containinformation based on the first word of the sentence. There are overfifty kinds of sentences, yet an interpreter really only needs tohandle a few to get the job done. The most common NMEA sentence of allis the "Recommended Minimum" sentence, which begins with "$GPRMC." Hereis an example:</p><pre>$GPRMC,040302.663,A,3939.7,N,10506.6,W,0.27,358.86,200804,,*1A</pre><p>This one sentence contains nearly everything a GPS applicationneeds: latitude, longitude, speed, bearing, satellite-derived time, fixstatus, and magnetic variation.</p><h3>The Core of An Interpreter</h3><p>The first step in making an NMEA interpreter is writing a methodthat does two things: separating each sentence into its individualwords and then examining the first word to figure out what informationis available to extract. Listing 1-1 shows the start of the interpreterclass.</p><p><b>Listing 1-1:</b> The core of an NMEA interpreter is a function which divides NMEA sentences into individual words.</p><pre>'******************************************************* '** Listing 1-1. The core of an NMEA interpreter '******************************************************* Public Class NmeaInterpreter ' Processes information from the GPS receiver Public Function Parse(ByVal sentence As String) As Boolean ' Divide the sentence into words Dim Words() As String = GetWords(sentence) ' Look at the first word to decide where to go next Select Case Words(0) Case "$GPRMC" ' A "Recommended Minimum" sentence was found! ' Indicate that the sentence was recognized Return True Case Else ' Indicate that the sentence was not recognized Return False End Select End Function ' Divides a sentence into individual words Public Function GetWords(ByVal sentence As String) As String() Return sentence.Split(","c) End Function End Class</pre> |
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19楼#
发布于：2005-03-10 20:43
<P>进不去!</P>
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