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The test case below fails because the computed altitude rate is NaN. It is a tricky case where the point is exactly on Earth's pole and traveling perpendicular to Earth's axis. The NaN seems to be created when computing dr = dr2.sqrt() where d(dr)/dt = 1/2 dr2**(-1/2) d(dr2)/dt which when dr2=0 and d(dr2)/dt = 0 reduces to 0/0 causing the NaN.

Alternatively if one projected the point into the local tangent plane to the ellipsoid then the altitude rate is the velocity in the zenith direction, which is clearly zero.

    @Test
    public void testGp() {
        AbsoluteDate epoch = AbsoluteDate.ARBITRARY_EPOCH;
        OneAxisEllipsoid earth = ReferenceEllipsoid.getWgs84(FramesFactory.getGCRF());
        FieldGeodeticPoint<DerivativeStructure> gp = earth.transform(
                new PVCoordinates(new Vector3D(0, 0, earth.getC()), new Vector3D(0, 1, 0)),
                earth.getBodyFrame(), epoch);
        assertThat(gp.getAltitude().getPartialDerivative(1), closeTo(0.0, 1e-12));
    }