Vector<Acceleration, Frame> Ephemeris<Frame>::
ComputeGravitationalAccelerationOnMasslessBody(
not_null<DiscreteTrajectory<Frame>*> const trajectory,
Instant const& t) const {
auto const it = trajectory->Find(t);
DegreesOfFreedom<Frame> const& degrees_of_freedom = it.degrees_of_freedom();
return ComputeGravitationalAccelerationOnMasslessBody(
degrees_of_freedom.position(), t);
}
// A linear acceleration identical for both bodies. The test point doesn't
// move. The resulting acceleration combines centrifugal and linear.
TEST_F(BodySurfaceDynamicFrameTest, LinearAcceleration) {
Instant const t = t0_ + 0 * Second;
DegreesOfFreedom<MockFrame> const point_dof =
{Displacement<MockFrame>({10 * Metre, 20 * Metre, 30 * Metre}) +
MockFrame::origin,
Velocity<MockFrame>({0 * Metre / Second,
0 * Metre / Second,
0 * Metre / Second})};
DegreesOfFreedom<ICRFJ2000Equator> const centre_dof =
{Displacement<ICRFJ2000Equator>({0 * Metre, 0 * Metre, 0 * Metre}) +
ICRFJ2000Equator::origin,
Velocity<ICRFJ2000Equator>({0 * Metre / Second,
0 * Metre / Second,
0 * Metre / Second})};
EXPECT_CALL(mock_centre_trajectory_, EvaluateDegreesOfFreedom(t, _))
.Times(2)
.WillRepeatedly(Return(centre_dof));
{
// The acceleration is linear + centripetal.
InSequence s;
EXPECT_CALL(*mock_ephemeris_,
ComputeGravitationalAccelerationOnMassiveBody(
check_not_null(massive_centre_), t))
.WillOnce(Return(Vector<Acceleration, ICRFJ2000Equator>({
-160 * Metre / Pow<2>(Second),
120 * Metre / Pow<2>(Second),
300 * Metre / Pow<2>(Second)})));
EXPECT_CALL(*mock_ephemeris_,
ComputeGravitationalAccelerationOnMasslessBody(_, t))
.WillOnce(Return(Vector<Acceleration, ICRFJ2000Equator>()));
}
// The acceleration is linear + centrifugal.
EXPECT_THAT(mock_frame_->GeometricAcceleration(t, point_dof),
AlmostEquals(Vector<Acceleration, MockFrame>({
(-120 + 1e3) * Metre / Pow<2>(Second),
(-160 + 2e3) * Metre / Pow<2>(Second),
-300 * Metre / Pow<2>(Second)}), 2));
}
// The test point is at the origin and in motion. The acceleration is purely
// due to Coriolis.
TEST_F(BodySurfaceDynamicFrameTest, CoriolisAcceleration) {
Instant const t = t0_ + 0 * Second;
// The velocity is opposed to the motion and away from the centre.
DegreesOfFreedom<MockFrame> const point_dof =
{Displacement<MockFrame>({0 * Metre, 0 * Metre, 0 * Metre}) +
MockFrame::origin,
Velocity<MockFrame>({10 * Metre / Second,
20 * Metre / Second,
30 * Metre / Second})};
DegreesOfFreedom<ICRFJ2000Equator> const centre_dof =
{Displacement<ICRFJ2000Equator>({0 * Metre, 0 * Metre, 0 * Metre}) +
ICRFJ2000Equator::origin,
Velocity<ICRFJ2000Equator>()};
EXPECT_CALL(mock_centre_trajectory_, EvaluateDegreesOfFreedom(t, _))
.Times(2)
.WillRepeatedly(Return(centre_dof));
{
InSequence s;
EXPECT_CALL(*mock_ephemeris_,
ComputeGravitationalAccelerationOnMassiveBody(
check_not_null(massive_centre_), t))
.WillOnce(Return(Vector<Acceleration, ICRFJ2000Equator>({
0 * Metre / Pow<2>(Second),
0 * Metre / Pow<2>(Second),
0 * Metre / Pow<2>(Second)})));
EXPECT_CALL(*mock_ephemeris_,
ComputeGravitationalAccelerationOnMasslessBody(_, t))
.WillOnce(Return(Vector<Acceleration, ICRFJ2000Equator>()));
}
// The Coriolis acceleration is towards the centre and opposed to the motion.
EXPECT_THAT(mock_frame_->GeometricAcceleration(t, point_dof).coordinates(),
Componentwise(AlmostEquals(400 * Metre / Pow<2>(Second), 1),
AlmostEquals(-200 * Metre / Pow<2>(Second), 0),
VanishesBefore(1 * Metre / Pow<2>(Second), 110)));
}
