public double[] Interpolate(double jdtdb, EphemerisComponent target, EphemerisComponent center = EphemerisComponent.SolarSystemBarycenter)
{
if (jdtdb < StartEpoch || jdtdb > FinalEpoch)
{
throw new ArgumentOutOfRangeException(nameof(jdtdb), TimeOutOfRange);
}
var targetCoordinates = new double[6];
var centerCoordinates = new double[6];
// Nutation, Libration, Angular Velocity and TT-TDB.
if (target == EphemerisComponent.EarthNutation || target == EphemerisComponent.MoonLibration ||
target == EphemerisComponent.MoonAngularVelocity || target == EphemerisComponent.TTMinusTDB)
{
return(InterpolateChebyshev(jdtdb, target));
}
// Moon at Earth Geocenter.
if (target == EphemerisComponent.Moon && center == EphemerisComponent.Earth)
{
targetCoordinates = InterpolateChebyshev(jdtdb, target);
}
// Earth at Moon Geocenter.
else if (target == EphemerisComponent.Earth && center == EphemerisComponent.Moon)
{
targetCoordinates = InterpolateChebyshev(jdtdb, center);
for (var axis = 0; axis < targetCoordinates.Length; axis++)
{
targetCoordinates[axis] = -targetCoordinates[axis];
}
}
// Anything that relates to either Earth or Moon.
else if (target == EphemerisComponent.Earth || target == EphemerisComponent.Moon ||
center == EphemerisComponent.Earth || center == EphemerisComponent.Moon)
{
var emrat = 1.0 / (1.0 + constants["EMRAT"]);
if (target == EphemerisComponent.Moon || center == EphemerisComponent.Moon)
{
emrat -= 1.0;
}
var embCoordinates = InterpolateChebyshev(jdtdb, EphemerisComponent.EarthMoonBarycenter);
var moonCoordinates = InterpolateChebyshev(jdtdb, EphemerisComponent.Moon);
// Earth/Moon at Planetocenter/SSB.
if (target == EphemerisComponent.Earth || target == EphemerisComponent.Moon)
{
if (center != EphemerisComponent.SolarSystemBarycenter)
{
targetCoordinates = InterpolateChebyshev(jdtdb, center);
}
for (var axis = 0; axis < targetCoordinates.Length; axis++)
{
targetCoordinates[axis] = (embCoordinates[axis] - emrat * moonCoordinates[axis]) - targetCoordinates[axis];
}
}
// Planet/SSB at Earth/Moon Geocenter.
else
{
if (target != EphemerisComponent.SolarSystemBarycenter)
{
targetCoordinates = InterpolateChebyshev(jdtdb, target);
}
for (var axis = 0; axis < targetCoordinates.Length; axis++)
{
targetCoordinates[axis] = targetCoordinates[axis] - (embCoordinates[axis] - emrat * moonCoordinates[axis]);
}
}
}
// Anything that relates to bodies other than Earth/Moon.
else
{
if (target != EphemerisComponent.SolarSystemBarycenter)
{
targetCoordinates = InterpolateChebyshev(jdtdb, target);
}
if (center != EphemerisComponent.SolarSystemBarycenter)
{
centerCoordinates = InterpolateChebyshev(jdtdb, center);
}
for (var axis = 0; axis < targetCoordinates.Length; axis++)
{
targetCoordinates[axis] -= centerCoordinates[axis];
}
}
for (var axis = 0; axis < targetCoordinates.Length; axis++)
{
targetCoordinates[axis] /= constants["AU"];
}
return(targetCoordinates);
}
private double[] InterpolateChebyshev(double tdb, EphemerisComponent component)
{
if (!pointers.ContainsKey(component) || pointers[component].CoefficientSetCount == 0)
{
throw new ArgumentException(EphemerisComponentNotAvailable, nameof(component));
}
var recordPointer = pointers[component];
var interval = (tdb - StartEpoch) / recordSpan;
var segment = (int)Math.Floor(interval);
// Use previous segment if time is final epoch.
if (Math.Abs(tdb - FinalEpoch) < double.Epsilon)
{
segment--;
}
var subInterval = (interval - segment) * recordPointer.CoefficientSetCount;
var subSegment = (int)Math.Floor(subInterval);
var timeSegment = 2.0 * (subInterval - subSegment) - 1.0;
// Load segment if not already loaded.
if (isFirstReading || tdb < coefficients[0] || tdb >= coefficients[1])
{
reader.BaseStream.Seek(dataOffset + segment * coefficients.Length * sizeof(double), SeekOrigin.Begin);
for (var i = 0; i < coefficients.Length; i++)
{
coefficients[i] = reader.ReadDouble();
}
isFirstReading = false;
}
// Default is set to cartesian axes count.
var coordinateCount = 3;
if (component == EphemerisComponent.EarthNutation)
{
coordinateCount = 2;
}
else if (component == EphemerisComponent.TTMinusTDB)
{
coordinateCount = 1;
}
var coordinates = new double[coordinateCount * 2];
for (var axis = 0; axis < coordinateCount; axis++)
{
var p0 = 1.0;
var p1 = timeSegment;
var p2 = 0.0;
var v0 = 0.0;
var v1 = 1.0;
var v2 = 0.0;
var offset = recordPointer.CoefficientCountPerSet * (coordinateCount * subSegment + axis) + recordPointer.Offset;
coordinates[axis] = coefficients[offset];
for (var i = 1; i < recordPointer.CoefficientCountPerSet; i++)
{
coordinates[axis] += coefficients[offset + i] * p1;
coordinates[axis + coordinateCount] += coefficients[offset + i] * v1;
p2 = 2.0 * timeSegment * p1 - p0;
v2 = 2.0 * timeSegment * v1 - v0 + 2 * p1;
p0 = p1;
p1 = p2;
v0 = v1;
v1 = v2;
}
coordinates[axis + coordinateCount] *= (2 * recordPointer.CoefficientSetCount) / recordSpan;
}
return(coordinates);
}
