Thread: programming deadreckoning gps software?

I saw someone talking about this in another thread. I think it should go one step further. Use a Gyro/Compass to determine whether you are turning, speed sensor to detect change in speed, etc. It could almost be done in Hardware (more difficult, but easier to integrate with speed sensors etc). This is quite a cool idea i think!

Further thinking, this could be done in code, as long as:
a) You could create a virtual Com port which you can point the GPS Software to
b) You have a way of getting Vehicle Speed/Direction INTO the computer

Now the following is just pseudocode dribble, but it might help someone who can work out how to achieve the 2 points above:

Code:

Variable Descriptions:
X=Coordinate (the further north you go = the lower the x coordinate; is this how GPS coordinates work? If not, then some of the calculations below might need changing)
Y=Coordinate
V=Velocity (MS)
D=Direction (0-359 degrees)
Timer=500Ms (User Settable)

Sample Pseudocode:
===============
Main.Initialize
    'Get starting coordinates. If no lock is present, read last known coordinates from file
    If Not ReadGPSNMEAString() Then
	ReadLastCoordinatesFromfile()
    EndIf
End Main.Initialize

Event Timer.Tick
    If ReadGPSNMEAString() Then  'GPS Lock exists
	'Bypass coordinate calculations
	TransmitNewGPSData()
    	WriteCoodinatesToFile() 'Always write last coordinates to file - incase user shuts down at this point
    Else  'No Lock exists
	'Process last known Speed/Direction values
	Select Case D '(Direction)
		Case 0:
			X=X-(V*(1000/Timer))
		Case 90:
			Y=Y+(V*(1000/Timer))
		Case 180:
			X=X+(V*(1000/Timer))
		Case 270:
			Y=Y-(V*(1000/Timer))
		Case Else:
			DeltaX=Sin(D) * (V*(1000/Timer))
			DeltaY=Cos(D) * (V*(1000/Timer))
			X=X-DeltaX
			Y=Y-DeltaY
	End Select

	'Send new coordinates to fake com port
	TransmitNewGPSData()
	WriteCoordinatesToFile() 'Always write last coordinates to file - incase user shuts down at this point

	'Get New Speed/Direction coordinates (this is assumed to be the vehicle's direction for until the next timer tick)
	V=GetVehicleSpeed()
	D=GetDirection
    End If
End Event

Procedure ReadGPSNMEAString()
	'Get X & Y Coordinates
	'Calculate velocity and Direction
End Procedure

The virtual COM port is easy enough.
This one... well, the one at the bottom, can take input from one port and send it to a few others. All you would need to do is read from the GPS, process it, and write it to this one.

I think this dual axis accelerometer phidget would be able to supply the speed and heading.

The TomTom Go doesnt use a standard gyro but instead it does use a dual-axis accelerometer to calculate speed, heading and acceleration...etc.

The accelerometer module only supply the forward-backward and lateral g's (I have the ADXL202 module used for measuring g's in the car). This work fine on flat roads for measuring g's but the problem is when the car is driven hard it will tilt naturally and therefore give a wrong g reading, you will get more g...unless the tilt g is substracted.

Applying this for GPS use, the software need to do some calculation such that it doesnt think that the car is accelerating when the GPS looses its signal on a hill...but how can it know as I dont think TomTom Go need any other connection to the car

If someone can work out how the accelerometer is used in the TomTom Go then a PIC MCU can be used to take over the calculation. The PIC get the NMEA data from the GPS, calculate the values read from the accelerometer and spit out the computed locations whenever the gps looses its signal. This way its going to be an add-on hardware module for the gps making it compatible with any gps software. No need for an extra serial port for the accelerometer or an additional software to compute the NMEA sentence and the accelerometer values and no need for a virtual comport to combine the 2 values into one port.

Maybe someone here can reverse engineer the TomTom to see whats going on.

GPS + Accelerometer + PIC = sounds like an interesting project

From TomTom Go review :

TomTom Go has a two plane acceleration sensor with no moving parts unlike conventional gyro systems. The sensor will be good enough for several hundred meters so TomTom Go can continue to work when GPS signals are not available (for example in tunnels with exits).

It does say no moving parts and only use a dual axis accelerometer, the rest must be softwares...the TomTom guys must have done it somehow.