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More accurate, more symmetrical computation of tangent vector.

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John Denker 2010-02-25 17:35:23 -07:00 • committed by ThorstenB
parent 02f42b8816
commit 214919da20
1 changed files with 27 additions and 18 deletions
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45
src/Instrumentation/navradio.cxx
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@ -61,28 +61,37 @@ static double sawtooth(double xx)
return 4.0 * fabs(xx/4.0 + 0.25 - floor(xx/4.0 + 0.75)) - 1.0;
}
// Calculate a unit vector in the horizontal tangent plane
// starting at the given "tail" of the vector and going off
// with the given heading.
static SGVec3d tangentVector(const SGGeod& tail, const SGVec3d& tail_xyz,
const double heading)
// Calculate a Cartesian unit vector in the
// local horizontal plane, i.e. tangent to the
// surface of the earth at the local ground zero.
// The tangent vector passes through the given <midpoint>
// and points forward along the given <heading>.
// The <heading> is given in degrees.
static SGVec3d tangentVector(const SGGeod& midpoint, const double heading)
{
// The fudge factor here is presumably intended to improve
// numerical stability. I don't know if it is necessary.
// It gets divided out later.
double fudge(100.0);
SGGeod head;
double az2; // ignored
SGGeodesy::direct(tail, heading, fudge, head, az2);
head.setElevationM(tail.getElevationM());
// The size of the delta is presumably chosen to give
// numerical stability. I don't know how the value was chosen.
// It probably doesn't matter much. It gets divided out.
double delta(100.0); // in meters
SGGeod head, tail;
double az2; // ignored
SGGeodesy::direct(midpoint, heading, delta, head, az2);
SGGeodesy::direct(midpoint, 180+heading, delta, tail, az2);
head.setElevationM(midpoint.getElevationM());
tail.setElevationM(midpoint.getElevationM());
SGVec3d head_xyz = SGVec3d::fromGeod(head);
return (head_xyz - tail_xyz) * (1.0/fudge);
SGVec3d tail_xyz = SGVec3d::fromGeod(tail);
// Awkward formula here, needed because vector-by-scalar
// multiplication is defined, but not vector-by-scalar division.
return (head_xyz - tail_xyz) * (0.5/delta);
}
// Create a "serviceable" node with a default value of "true"
SGPropertyNode_ptr createServiceableProp(SGPropertyNode* aParent, const char* aName)
SGPropertyNode_ptr createServiceableProp(SGPropertyNode* aParent,
const char* aName)
{
SGPropertyNode_ptr n = (aParent->getChild(aName, 0, true)->getChild("serviceable", 0, true));
SGPropertyNode_ptr n =
aParent->getChild(aName, 0, true)->getChild("serviceable", 0, true);
simgear::props::Type typ = n->getType();
if ((typ == simgear::props::NONE) || (typ == simgear::props::UNSPECIFIED)) {
n->setBoolValue(true);
@ -945,11 +954,11 @@ void FGNavRadio::search()
// GS axis unit tangent vector
// (along the runway):
_gsAxis = tangentVector(_gs->geod(), _gsCart, gs_radial);
_gsAxis = tangentVector(_gs->geod(), gs_radial);
// GS baseline unit tangent vector
// (perpendicular to the runay along the ground)
SGVec3d baseline = tangentVector(_gs->geod(), _gsCart, gs_radial + 90.0);
SGVec3d baseline = tangentVector(_gs->geod(), gs_radial + 90.0);
_gsVertical = cross(baseline, _gsAxis);
} // of have glideslope
} // of found LOC or ILS