On Sat, 2009-01-10 at 17:13 +0100, Csaba Halász wrote:
> #2 0x000000000080188d in FGNavRadio::update (this=0x98df490,
> dt=0.058333333333333334) at src/Instrumentation/navradio.cxx:613
> 613 double angle = asin( y / x ) * SGD_RADIANS_TO_DEGREES;
> (gdb) p x
> $1 = 3.2580450943147174
> (gdb) p y
> $2 = 3.258443598627875
> (gdb) p y / x
> $3 = 1.0001223139341602
>
> No idea what it is calculating and how to fix. (apart from forcibly
> clamping y/x to [-1, 1] ...)
This bug bit me today. I'm starting this thread from Csaba's first
e-mail since the last thread missed the problem.
More context code:
double x = gs_dist_node->getDoubleValue();
double y = (fgGetDouble("/position/altitude-ft") - nav_elev)
* SG_FEET_TO_METER;
// cout << "dist = " << x << " height = " << y << endl;
double angle = asin( y / x ) * SGD_RADIANS_TO_DEGREES;
y is vertical distance, x is distance to the navaid. It appears the
original author intended the "x" distance to be slant range. Hence we
have (vertical rise)/(slant range) or rise/hypotenuse. As long as that
is true, y/x will never exceed +/- 1 since the vertical rise must always
be less than or equal to the slant range. However, I enabled the cout
line and discovered x is indeed dropping below y. From observation "x"
is horizontal distance not slant range.
I think we need
double angle = atan2( y, x ) * SGD_RADIANS_TO_DEGREES;
Thanks,
Ron
Index: src/Instrumentation/navradio.cxx
===================================================================
RCS file: /var/cvs/FlightGear-0.9/source/src/Instrumentation/navradio.cxx,v
retrieving revision 1.31
diff -u -b -r1.31 navradio.cxx
--- src/Instrumentation/navradio.cxx 25 Dec 2008 23:11:44 -0000 1.31
+++ src/Instrumentation/navradio.cxx 2 Apr 2009 14:49:57 -0000
@@ -610,7 +610,7 @@
double y = (fgGetDouble("/position/altitude-ft") - nav_elev)
* SG_FEET_TO_METER;
// cout << "dist = " << x << " height = " << y << endl;
- double angle = asin( y / x ) * SGD_RADIANS_TO_DEGREES;
+ double angle = atan2( y, x ) * SGD_RADIANS_TO_DEGREES;
r = (target_gs - angle) * 5.0;
r *= signal_quality_norm;
}
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