MGEX Constellations

Phase center coordinates of the GNSS antenna and the LRA of GIOVE-A/B satellites as recommended for use within the MGEX project are provided in the following table. A machine-readable version of the phase center offset information for each satellite is provided as part of the IGS14 ANTEX product. All values refer to the IGS-specific spacecraft coordinate system illustrated in blue in Fig. 1. The spacecraft coordinate system is aligned with the main body axes and originates near the anti-Earth panel. By convention of the IGS

• the +z_IGS-axis is oriented along the boresight direction of the antenna,
• the y_IGS-axis is parallel to the rotation axis of the solar panels and the positive y_IGS-direction is defined through the adopted +x_IGS-direction,
• the +x_IGS-axis completes a right handed system and is chosen such that the Sun is always located in the +x_IGS hemisphere during nominal yaw-steering.

The detailed orientation of the +x_IGS and +y_IGS-axes for individual satellites is defined as shown in the drawings.

GNSS antenna offsets for GIOVE-A/B are based on calibrated phase centers for each frequency band and center-of-mass information for the begin of life (BoL) as provided by ESA.

For the modeling of satellite laser ranging measurements nominal coordinates of the effective LRA reflection point have been specified by ESA as part of the ILRS mission support request.

Similar to the GPS satellites the attitude of the GIOVE satellites is actively controlled to orient the +z_IGS axis towards the Earth. At the same time the spacecraft is continuously rotated about this axis to maintain the y_IGS-axis perpendicular to the plane made up by the Sun, Earth, and satellite. The +x_IGS-panel is always sunlit, while the -x_IGS-panel is oriented towards “deep space”. Details of the attitude control during noon- and midnight turns in periods when the Sun is close to the orbital plane are described in [2] and [3].

References

[1] Zandbergen R., Navarro D.; “Specification of Galileo and GIOVE Space Segment properties relevant for Satellite Laser Ranging”, ESA-EUING-TN/10206, Issue 3.2, 08/05/2008, Galileo Project Office, ESA, Noordwijk;
[2] Johnston A.G.Y., Holt A.P., Jackson C.D., “GIOVE-A AOCS : An Experience from Verification to Flight”, 7th Int. ESA Conference on Guidance, Navigation & Control Systems, 2-5 June 2008, Tralee, County Kerry, Ireland (2008).
[3] Zentgraf P., Fischer H.-D., Kaffer L., Konrad A., Lehrl E., Müller C., Oesterlin W., Wiegand M.; “AOC Design and Test for GSTB-V2B”, 6th Int. ESA Conference on Guidance, Navigation and Control Systems, 17-20 Oct. 2005 in Loutraki, Greece (2005).

Reproduced from DOI 10.1016/j.asr.2015.06.019 with permission of Elsevier; satellite image courtesy JAXA.

Phase center coordinates of the GNSS antennas and the LRA as recommended for QZS-1 processing within the MGEX project are provided in the following table. All values refer to the spacecraft coordinate system illustrated in Fig. 1. The spacecraft coordinate system is aligned with the main body axes and originates near the center of the launch adapter plane. In accord with IGS conventions, the individual axes are aligned in the following way:

• the +z_IGS-axis is oriented along the boresight direction of the L-ANT antenna;
• the +y_IGS-axis is parallel to the rotation axis of the solar panels and oriented such that the LRA is located in the first quadrant of the x_IGS/y_IGS-plane relative to the L-ANT;
• the +x_IGS-axis completes a right handed system;

While the center of mass (CoM) may shift by roughly 3 cm over the mission life-time, the value for mid 2012 is adopted as a conventional value for a harmonized processing. Begin-of-life (BoL) and End-of-life (EoL) values are given for information, only. CoM coordinates previously reported in [2] for satellite laser ranging support are superseded by more recent values provided in [1].

A machine-readable version of the phase center offset information for QZS-1 is provided as part of the IGS14 ANTEX file.

EVENTS

During nominal operations the +z_IGS-axis is continuously oriented towards a point at +83° East longitude and +5° latitude. In addition, the NAVIC satellites employ a yaw steering to keep the y_IGS-axis perpendicular to the Sun direction. The +x_IGS-panel remains sunlit at all times, while the -x_IGS-panel points to “deep space”. Further information and a mathematical formulation of the NAVIC attitude control law are provided in [3] and [4].

References

[1] A.S. Ganeshan, priv. comm. (9-10 Feb. 2015)
[2] ILRS Retroreflector Information for IRNSS
[3] Montenbruck O., Schmid R., Mercier F., Steigenberger P., Noll C., Fatkulin R., Kogure S., Ganeshan A. S. (2015) GNSS satellite geometry and attitude models. Advances in Space Research 56(6):1015-1029. DOI 10.1016/j.asr.2015.06.019