Finding offset angle for GPS

Bison52

Bison52

Member
Playing around with my Naza GPS trying out RTH failsafe. Flew it out estimated 100 yards and turned off TX. It flew to altitude, flew back to a point about 30 feet to my right, then directly overhead and then landed

Hmmm. My GPS was aligned as closely as possible to centerline of copter. So I got a wheel and measured exactly 100 yards out. Got my nephew to stand at that point and signal when the copter was directly overhead. Went into failsafe and when the copter flew back to the offset point, marked it and measured distance back to actual takeoff/final landing point.

Repeated and both times measured offset of just about 35 feet.

With one side of 300 feet and another of 35 feet, a little trig gives an angle of 6.65 degrees. It just so happens the magnetic declination in Lubbock is ~ 6.5 degrees east.

So I moved my GPS about that much to the right, calibrated, and repeated the test one more time and the return was right overhead.

Thought this was interesting and apparently a good way to determine exactly how much you need to offset your GPS.
 


A

azure

Member
Bison52 said:
Playing around with my Naza GPS trying out RTH failsafe. Flew it out estimated 100 yards and turned off TX. It flew to altitude, flew back to a point about 30 feet to my right, then directly overhead and then landed

Hmmm. My GPS was aligned as closely as possible to centerline of copter. So I got a wheel and measured exactly 100 yards out. Got my nephew to stand at that point and signal when the copter was directly overhead. Went into failsafe and when the copter flew back to the offset point, marked it and measured distance back to actual takeoff/final landing point.

Repeated and both times measured offset of just about 35 feet.

With one side of 300 feet and another of 35 feet, a little trig gives an angle of 6.65 degrees. It just so happens the magnetic declination in Lubbock is ~ 6.5 degrees east.

So I moved my GPS about that much to the right, calibrated, and repeated the test one more time and the return was right overhead.

Thought this was interesting and apparently a good way to determine exactly how much you need to offset your GPS.
Click to expand...

I found my 12º antenna offset by trial and error - but yup, my declination is roughly 12 degrees east for Los Angeles (or clockwise when looking down at the antenna... 12 o'clock being north), which is where I wound up. Really good tip. Thanks.
 

Bison52

Bison52

Member
I knew I was supposed to offset the angle of the GPS by the amount of magnetic declination, I just hadn't done it before this test. I just like it when field results so closely relate to expectations.
 

R

rdalemercer

Member
This is interesting.
My magnetic declination is 5.25 degrees West.

With that information and what I see that you have done (@ 6.5 East, you moved the pointer on your GPS disc to the 'right'), then, using that logic, should move the pointer on my GPS disc to the 'LEFT' by 5.25 degrees?
 

Bison52

Bison52

Member
That's the way I moved mine and it worked. That's also the direction the diagram in the Naza manual shows to move the GPS.

Another way I think of it is that if you had your aircraft facing true north, you'd turn your GPS to point at magnetic north.
 

A

azure

Member
Bison52 said:
That's the way I moved mine and it worked. That's also the direction the diagram in the Naza manual shows to move the GPS.

Another way I think of it is that if you had your aircraft facing true north, you'd turn your GPS to point at magnetic north.
Click to expand...

+1 Same.
 

R

rdalemercer

Member
Bison52 said:
That's the way I moved mine and it worked. That's also the direction the diagram in the Naza manual shows to move the GPS.

Another way I think of it is that if you had your aircraft facing true north, you'd turn your GPS to point at magnetic north.
Click to expand...

Right. I read and undertood the manual -- and when flying, I have not noticed a deviation in forward flight so I have not made a correction on the angle that the pointer is fixed relative to the centerline of the MR.

That being said, I have not tested the failsafe mode (RTH), so I was unaware (hadn't made the connection) that the angle of declination would impact the return point being closer to the initial point by establishing this parameter and setting the GPS disc's angle. Unless I missed it, the method to calculate and set the RTH point to be close to the initial point is not referenced in the manual.

Cheers!
 

Bison52

Bison52

Member
I didn't notice a deviation in straight flight either but on thinking about it I don't think you would with just a basic Naza/GPS setup without waypoints It probably wouldn't show up in straight flight until you were letting the GPS autopilot fly to a waypoint, which is really what RTH is.

I just thought it was interesting. At first when it didn't fly straight back I wondered why and then realized pretty quick it was probably that I needed to adjust the direction the GPS was pointing. If my nephew hadn't been with me and been willing to stand out in a cotton field and wave his arms, I probably wouldn't have bothered with all the measurements. But like I said, it's kind of fun when things actually work out in practice the way they do on paper.

I wouldn't be surprised if at some point the GPS just gets a table of mag decl built in. Then you would just point it straight forward and it would figure out the adjustment based its location. I'm sure that's the way full blown navigation GPS works. Would make it a lot easier to travel with your rig.
 






Bison52

Bison52

Member
I look forward to meeting him. I just recently got into rc and multirotors for the AP aspect and don't really know many in the local rc community yet.

I wonder if he might be the guy flying a LED lit airplane near my neighborhood. I live near the outskirts of town and somebody was flying at night not far from where i live. That led to a report on one of the news stations of UFOs in the area

I knew it was an rc plane because I had seen it from a distance and so had some of my neighbors I have been pretty low key as I have been learning but I have LEDs on my rig and do testing in my front yard.

When that story came out I had a hard time convincing some of the people who had seen me flying that it wasn't me. I told them it couldn't be me because whoever it was managed to fly fairly high for fairly long lengths of time at night without crashing and that left me out. I have since improved but not to that level.

I think after the news report, whoever it was may have gone to ground for a while.

Mike
 


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