Vector3 getZUnitVector( )
{
Vector3 zUnitVector( 3 );
// Set components of unit-vector.
zUnitVector[ 0 ] = 0.0;
zUnitVector[ 1 ] = 0.0;
zUnitVector[ 2 ] = 1.0;
return zUnitVector;
}
/*!
* This sub-routine computes the velocity vector, given a magnitude, a unit normal vector from the
* point on the surface where the regolith is lofted from, and the two conic angles which describe
* the velocity vector's direction relative to the normal vector. A backwards approach is used
* to go from the velocity vector in the final rotated intermediate frame back to the body fixed
* frame. More details are given in the thesis report and author's personal notes.
*
*/
void computeRegolithVelocityVector( std::vector< double > regolithPositionVector,
const double velocityMagnitude,
const double coneAngleAzimuth,
const double coneAngleDeclination,
std::vector< double > &unitNormalVector,
std::vector< double > ®olithVelocityVector )
{
// form the velocity vector, assuming that the intermediate frame's z-axis, on the surface of
// the asteroid, is along the final velocity vector
regolithVelocityVector[ 0 ] = 0.0;
regolithVelocityVector[ 1 ] = 0.0;
regolithVelocityVector[ 2 ] = velocityMagnitude;
// get the rotatin matrix to go from the rotated intermediate frame back to the initial
// frame where the z-axis was along the normal axis and the x-axis was pointing towards north
// direction
std::vector< std::vector< double > > zBasicRotationMatrix
{ { std::cos( coneAngleAzimuth ), std::sin( coneAngleAzimuth ), 0.0 },
{ -std::sin( coneAngleAzimuth ), std::cos( coneAngleAzimuth ), 0.0 },
{ 0.0, 0.0, 1.0 } };
std::vector< std::vector< double > > yBasicRotationMatrix
{ { std::cos( coneAngleDeclination ), 0.0, -std::sin( coneAngleDeclination ) },
{ 0.0, 1.0, 0.0 },
{ std::sin( coneAngleDeclination ), 0.0, std::cos( coneAngleDeclination ) } };
std::vector< std::vector< double > > nonTransposedRotationMatrix( 3, std::vector< double > ( 3 ) );
matrixMultiplication( yBasicRotationMatrix,
zBasicRotationMatrix,
nonTransposedRotationMatrix,
3,
3,
3,
3 );
std::vector< std::vector< double > > intermediateFrameRotationMatrix( 3, std::vector< double > ( 3 ) );
matrixTranspose( nonTransposedRotationMatrix, intermediateFrameRotationMatrix );
std::vector< std::vector< double > > rotatedIntermediateFrameVelocityVector
{ { regolithVelocityVector[ 0 ] },
{ regolithVelocityVector[ 1 ] },
{ regolithVelocityVector[ 2 ] } };
std::vector< std::vector< double > > intermediateFrameVelocityVector( 3, std::vector< double > ( 1 ) );
matrixMultiplication( intermediateFrameRotationMatrix,
rotatedIntermediateFrameVelocityVector,
intermediateFrameVelocityVector,
3,
3,
3,
1 );
// now obtain the basis vectors for the intermediate frame expressed in the
// body fixed frame coordinates
std::vector< double > xUnitVector( 3 );
std::vector< double > yUnitVector( 3 );
std::vector< double > zUnitVector( 3 );
zUnitVector = unitNormalVector;
std::vector< double > bodyFrameZUnitVector { 0.0, 0.0, 1.0 };
// get the intermediate RTN frame at the surface point
std::vector< double > unitR = normalize( regolithPositionVector );
std::vector< double > unitT = normalize( crossProduct( unitR, bodyFrameZUnitVector ) );
// get the x basis vector, pointing to north
xUnitVector = normalize( crossProduct( unitT, zUnitVector ) );
// get the y basis vector
yUnitVector = normalize( crossProduct( zUnitVector, xUnitVector ) );
std::vector< double > zPrincipalAxisBodyFrame { 0.0, 0.0, 1.0 };
std::vector< double > zNegativePrincipalAxisBodyFrame { 0.0, 0.0, -1.0 };
// check if the position vector is along the poles
const double positionDotPrincipalZ
= dotProduct( normalize( regolithPositionVector ), zPrincipalAxisBodyFrame );
const double positionDotNegativePrincipalZ
= dotProduct( normalize( regolithPositionVector ), zNegativePrincipalAxisBodyFrame );
if( positionDotPrincipalZ == 1.0 )
{
// the position vector is pointing to the poles, hence x basis vector pointing to the north
// direction wouldn't work
xUnitVector = { 1.0, 0.0, 0.0 };
yUnitVector = { 0.0, 1.0, 0.0 };
}
else if( positionDotNegativePrincipalZ == 1.0 )
{
xUnitVector = { -1.0, 0.0, 0.0 };
yUnitVector = { 0.0, 1.0, 0.0 };
}
// put the basis vectors in a 3x3 matrix
std::vector< std::vector< double > > intermediateFrameBasisMatrix
{ { xUnitVector[ 0 ], yUnitVector[ 0 ], zUnitVector[ 0 ] },
{ xUnitVector[ 1 ], yUnitVector[ 1 ], zUnitVector[ 1 ] },
{ xUnitVector[ 2 ], yUnitVector[ 2 ], zUnitVector[ 2 ] } };
std::vector< std::vector< double > > bodyFrameVelocityVector( 3, std::vector< double >( 1 ) );
matrixMultiplication( intermediateFrameBasisMatrix,
intermediateFrameVelocityVector,
bodyFrameVelocityVector,
3,
3,
3,
1 );
// return the final regolith velocity vector, expressed in body frame coordinates
regolithVelocityVector[ 0 ] = bodyFrameVelocityVector[ 0 ][ 0 ];
regolithVelocityVector[ 1 ] = bodyFrameVelocityVector[ 1 ][ 0 ];
regolithVelocityVector[ 2 ] = bodyFrameVelocityVector[ 2 ][ 0 ];
}
