Knowing the location of satellites is important when determining how precise readings are and how stable a GPS fix is. Since GPS precision will be covered in detail in part two of this series, so this section will focus on interpreting satellite location and signal strength.
There are twenty-four operational satellites in orbit. Satellites are spaced in orbit so that at any time a minimum of six satellites will be in view to users anywhere in the world. Satellites are constantly in motion, which is good because it prevents the existence of “blind spots” in the world with little or no satellite visibility. Just like finding stars in the sky, satellite locations are described as the combination of an azimuth and an elevation. As mentioned above, azimuth measures a direction around the horizon. Elevation measures a degree value up from the horizon between 0° and 90°, where 0° represents the horizon and 90° represents “zenith,” directly overhead. So, if the device says a satellite’s azimuth is 45° and its elevation is 45°, the satellite is located halfway up from the horizon towards the northeast. In addition to location, devices report each satellite’s “Pseudo-Random Code” (or PRC) which is a number used to uniquely identify one satellite from another.
Here’s an example of a $GPGSV sentence:
$GPGSV,3,1,10,24,82,023,40,05,62,285,32,01,62,123,00,17,59,229,28*70
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 of each block gives the satellite’s PRC. The second word gives each satellite’s elevation, followed by azimuth and signal strength. If this satellite information were to be shown graphically, it would look like figure 1-1.
(Figure 1-1: Graphical representation of a $GPGSV sentence, where the center of the circle marks the current position and the edge of the circle marks the horizon.)
Perhaps the most important number in this sentence is the “signal-to-noise ratio” (or SNR for short). This number indicates how strongly a satellite’s radio signal is being received. Remember, satellites transmit signals at the same strength, but things like trees and walls can obscure a signal beyond recognition. Typical SNR values are between zero and fifty, where fifty means an excellent signal. (SNR can be as high as ninety-nine, but I’ve never seen readings above fifty even in wide open sky.) In Figure 1-1, the green satellites indicate a strong signal, whereas the yellow satellite signifies a moderate signal (in part two, I will provide a way to classify signal strengths). Satellite #1’s signal is completely obscured. Listing 1-7 shows the interpreter after it is expanded to read satellite info.
Listing 1-7: The interpreter is improved to interpret the location of GPS satellites currently in view.
'** 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" ' A "Satellites in View" message was found
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) & "°" ' 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) & "°" ' 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
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