Actual Track

[Rocky 128]

What is actual track?If you read the definition of the variable GPS_GROUND_TRACK, it says that it corresponds to Heading + Magnetic Variation.If you calculate the actual track by taking the current heading (variable PLANE_HEADING_DEGREES_MAGNETIC) and adding the Magnetic Variation (variable MAGNETIC_VAR), the result is different.So what is the correct definition of the actual track?Is it related to the wind?Does it correspond the the current heading corrected with the wind deviation?Please let me know.Eric

[Guest]

Ground Track should be true bearing (vmg) in degrees relative to true north. In other words, this is the direction, relative to true north, that the plane is actually moving.You can, theoretically, calculate it as a vector sum of the aircraft's airspeed and heading and the wind velocity, corrected for magnetic variation. In other words, yes, it is heading, in degrees relative to true north, corrected for wind deviation.Matt

[Guest]

On the default B747, there is a gauge that shows and rotates a "track line" accordingly, using the variables you need to know and how to use them. It's a good place to start at least.

[Rocky 128]

Thanks for your clear explanations.But how do you calculate the true bearing?I mean, do you add or substract the magnetic variation?In other words, is:True Bearing = Magnetic Heading + Magnetic variationorTrue Bearing = Magnetic Heading - Magnetic variation ?Or do you think I can use the FS variable PLANE_HEADING_DEGREES_TRUE ?Eric

[Guest]

I'm not sure exactly what format FS uses to store magnetic variation information. In the Eastern US, though, true headings are 10-15 degrees smaller than magnetic headings. For example, a heading of 150 degrees true is equivalent to 164 degrees magnetic in Albany. You can do it one way, and test it in the sim...and if it's backwards, switch it.On the other hand, I'm not sure that you need to...it seems like everything you could want is there for you in the variables: PLANE_HEADING_DEGREES_TRUE should be the direction relative to true north the plane is actually pointing. PLANE_HEADING_DEGREES_MAGNETIC is the direction relative to magnetic north. GPS_GROUND_TRACK is the direction the plane is actually moving, relative to true north, which can be different from PLANE_HEADING_DEGREES_TRUE if there is wind, if the aircraft is sideslipping, etc.Matt

[Rocky 128]

You're completely right !!I think I can use the variable PLANE_HEADING_DEGREES_TRUE, but I'll check this in Albany to make sure that the difference between magnetic and true heading is correct.On the second point, you're also right: I can read the GPS_GROUND_TRACK variable, but I don't like it much because it is refreshed only every second, which is too slow for my real-time gauge... This is why I'd like to calculate the Actual Ground by myself, to have a normal refresh rate.Thanks again for your help !!Eric

[Guest]

Ahh, I see...calculating actual ground track is going to be extremely difficult, though; that's why FS only calculates it once every second. I'd assume what they do is simply measure the position of the plane every second, and "connect the dots" to get the ground track.You could approximate the ground track by only accounting for airspeed and wind: it would be something likePLANE_HEADING_DEGREES_TRUE + ((wind direction degrees true - PLANE_HEADING_DEGREES_TRUE) * (wind speed / true airspeed))However, this would be inaccurate (1) if the plane is sideslipping: for example, if someone is making a crosswind approach by sideslipping (crabbing will not cause a problem); (2) if the plane is on the ground: this will report that the plane is drifting with the wind even if it is not; and (3) if true airspeed is 0: this will cause a divide-by-zero error and possibly cause FS to crash.Not sure what kind of advice to give you here...what kind of gauge is this, anyway?Matt

[Rocky 128]

I was thinking about calculating this with the formula you wrote. That's exactly what I was doing when I was flying my aircraft... (I am a private pilot, but I don't fly any more).Anyway, the problems you're pointing are right. Problems (2) and (3) can be avoided by switching off the track indicator when the aircraft is on the ground, and by testing the airspeed to make sure it is non zero.The easiest solution is to read what FS calculates, and I'll forget the bad refresh rate...FYI, I am working on an Airbus PFD, which includes the actual track indicator in the heading scrolling tape. This is why I'd like to have a high refresh rate... The new gauge will soon be available for download on my website.Eric

[Guest bartels]

For XML gauges see the 747 gauges for an example, for C gauges check NORTH_VELOCITY and EAST_VELOCITY for your ground track, same as the XML gauges use the atan2() function for it.Arne Bartels

[Guest Eugen]

Hi everybody,I have followed the discussion with great interest so I have put togheter some code. But I am not sure that it shows the actual track. Is it correct to us (A:Magvar, radians) or should it be degreesBrgdsEugen((A:Velocity world X, m/s) (A:Velocity world Z, m/s) atg2 (A:Magvar, radians) -) (A:Wiskey compass indication degrees,degrees) - dnor s1 180 <= if{ l1 } els{ l1 360 - s1 } + l1 -25 > l1 25 < &((A:Velocity world X, m/s) (A:Velocity world Z, m/s) atg2 (A:Magvar, radians) -) (A:Wiskey compass indication degrees,degrees) - dnor s1 180 <= if{ l1 } els{ l1 360 - s1 }

[Guest bartels]

(A:Velocity world X, m/s) (A:Velocity world Z, m/s) atg2 is the track angle relative to true north in radians. If you need it relative to your aircraft you have to rotate it by PLANE HEADING DEGREES TRUE. To get a ground track realtive to magnetic north add or substract MAGVAR (don't test in Europe because MAGVAR is about 1