Appendix B: Metop Operational Orbit
The orbit parameters for the operational baseline mean orbit, with respect to the inertial Mean-of-Date System J2000.0, are given below. The orbit propagation is carried out with a geopotential model GEM10B of order 36 and degree 36, with lunisolar perturbations and a medium air drag model MSIS 77.
The repeatability cycle is 29 days/412 orbits and the Mean Local Solar Time at ascending node is 21 h 30 min. As the right column in the table shows, this orbit is almost equivalent to a 5 days/71 orbits repeat cycle.
|Mean Element||Baseline 29 days/412 revs orbit||Value for a 5 days/71 revs orbit|
|Semi-Major Axis||a||7,195,605.347 m||7,197,939.000 m|
|Right Ascension of Ascending Node||Ω||62.4731° + 0.98564735°
N = number of Julian days from 1 Jan 2000
|62.4731° + 0.98564735°
N = number of Julian days from 1 Jan 2000
|Argument of Perigee||ω||90.0°||90.0°|
The Metop nominal attitude is the Local Normal Pointing (LNP) law augmented by the Yaw Steering Mode (YSM).
LNP forces the satellite to point to the local normal direction (which for an oblate planet like the Earth is generally different from the direction towards the Earth centre).
Additionally to LNP, the satellite platform slowly moves about its centre of mass with a slow rocking motion about all three axes (X or pitch, Y or roll, Z or yaw) with the largest oscillation amplitude being on the yaw axis. This is the concept of yaw steering.
Yaw steering angles are modulated on board to compensate for the apparent drift of the sub-satellite point due to Earth rotation. Use of the yaw steering law is beneficial to those instruments with swaths significantly extending sideways (e.g. ASCAT). The amplitude of the rotation about each angle is effected on board (and modelled on-ground), first computing the angle amplitude and then applying the three rotations as follows:
In the reference frame selected for the Metop CFI Software (positive X pointing away from solar panel, positive Y towards the flight direction, positive Z towards outer space) the signs in front of CX and CY in the following equations are negative.
ae = semi-major axis of the Earth reference ellipsoid (km)
a = mean semi-major axis of the Metop reference orbit (km)
ee = eccentricity of the Earth reference ellipsoid
i = mean inclination of the Metop orbit (radians)
k = (ωe /ωn) with ωe Earth angular velocity (radians/s) and ωn Metop angular velocity along its reference orbit (radians/s)
In the spacecraft reference frame (positive X pointing from spacecraft centre of mass to solar panel, positive Y opposite to flight direction, positive Z towards outer space) the signs of CX and CY are positive. In this case the numerical value for CX is positive for Metop and the value for CY is negative:
In the spacecraft reference frame and with the Metop mission reference orbit data, the maximum amplitudes are:
CX Pitch maximum amplitude (radians): +0.002899 CY Roll maximum amplitude (radians): -0.00089 CZ Yaw maximum amplitude (radians): +0.068766
In the Metop CFI Software reference frame the signs of CX and CY are swapped with respect to the signs of the numerical values obtained in the Metop spacecraft reference frame. The CZ sign is unaffected by the choice of attitude reference frame.
η: pitch steering angle ξ: roll steering angle
ς: yaw steering angle
PSO ("Position sur l'Orbite", in radians) is the angle measured in the satellite orbital plane from the latest ascending node to the current satellite position along the orbit.
The main concepts are already explained above in B.1.
The average operational values for maximum propagation error (in metres) after 48 hours are:
in radial direction: 6-7 m in along-track direction: 50-60 m in cross-track direction: 8-10 m
which are well below (i.e. better than) the required values.
The Metop attitude control system is a closed-loop one, therefore no attitude prediction is performed on ground. Our reference attitude is the LNP+YSM law (see above, B.2). Telemetry from sun sensors, earth sensors and gyros is processed on ground to check that attitude deviations from the LNP+YSM reference values stay within certain mission-defined thresholds. Deviations of less than 0.02° in pitch, roll and yaw are routinely observed.