- Section 1: Introduction
- Section 2: Galileo IOV Reference Frame
- Section 3: Attitude Law
- Section 4: Mass and Centre Of Mass
- Section 5: Navigation Antenna Phase Centre Corrections
- Section 6: Geometry
- Section 7: Laser Retro Reflector Location
- Section 8: Satellite Group Delay
- Section 9: Glossary
- Section 10: GNSS Bibliography
The Galileo satellites metadata are information about the satellite properties which need to be known in order to properly implement advanced processing algorithms for precise orbit determination or Precise Point Positioning (PPP). These include physical characteristics such as mass, area or reflectivity, the attitude law, and antenna parameters such as phase center offsets and variations. More information about PPP and GNSS data processing can be found in ,  or . For Galileo SIS details, please refer to .
As depicted in the graph above, the Galileo IOV Reference Frame origin is located at one of the satellite corners. The +Z axis is normal to the separation plane and points to the same direction of the L-Band Navigation Antenna.
The X axis is normal to the clock panel and points towards the clock panel while the Y axis completes the right-handed orthogonal system.
The nominal IOV Galileo spacecraft attitude is as follows: its body is fixed in a way it keeps the Z axis towards the Earth Centre (in order to illuminate the Earth with its Navigation Antenna), the Y axis is perpendicular to the Sun and the X axis points towards deep space. Please take into account that this does not meet the GPS block II/IIA attitude convention. It is important to keep the clock panel toward Deep Space so it is protected from the Sun, avoiding thermal variation.
In order to maintain the nominal attitude it is necessary to turn (“yaw“) about its Z axis while rotating its solar panels around the Y axis.
The required rotation is defined with respect to an orbital RF (Reference Frame). The orbital RF has its +Z-axis pointing towards Earth Centre, the +Y-axis perpendicular to the orbital plane (“across- track”), and the +X-axis completing the right-handed orthogonal system and pointing mainly in the flight direction (“along-track"). The yaw steering angle (Ψr) is defined as follows:
Where is the reference vector of the Sun in the orbital Reference Frame. Please take into account the term “atan2” means arctangent taking into account the sign of the inputs to determine the proper quadrant of the computed angle.
The position of the Sun is denoted by the following unit vector:
Where β = β(t) denotes the elevation of the Sun with respect to the orbital plane at time “t” and η = η(t) is the geocentric angle between the satellite and the orbit noon (consider “noon” as the point in the satellite orbit where the satellite is closer to the sun) measured in the orbital plane and growing with the spacecraft orbital motion.
In order to keep the yaw change rate (and also the rate of change of this derivative) low when the Sun is close to the orbital plane while the satellite approaches the orbit noon or midnight , the vector is substituted by an auxiliary Sun reference vector which allows keeping a minimum angular distance between and the spacecraft Z axis.
The region where the auxiliary Sun reference vector is used is:
with βx = 15.0deg andβy = 2.0deg. The auxiliary reference Sun is given by:
Where means the sign of at the beginning of the auxiliary region.
The Reference Frame used in the ANTenna EXchange (ANTEX) format and in most of the GNSS software packages follows the GPS Block II/IIA attitude conventions. This convention implies that the +X axis points toward the Sun and not towards Deep Space. In order to cope with this 180 degrees reversal in the X axis it is necessary to change the sign when calculating the yaw steering angle in order to meet the standard. Therefore:
In the following tables the mass and Centre Of Mass (COM) of each spacecraft can be found. These measurements were performed on May 2013. Please note that both mass and the Centre Of Mass (COM) may vary depending on the propellant consumption. Updated values are provided by the Laser Ranging Service ILRS website.
|Mass [Kg]||Centre of Mass|
|X [mm]||Y [mm]||Z [mm]|
While the satellite motion is defined with respect to the Centre Of Mass (COM), the mean phase Centre is defined with respect to other point, the Antenna Reference Point (ARP). The difference between both points (mean phase centre and ARP) is known as the Phase Centre Offset (PCO).
As stated above, the antenna PCO for Galileo IOV spacecrafts is different to GPS in the sense that the offsets are given with respect to a well-determined physical reference point located close to the antenna ground plane: The Antenna Reference Point (ARP). The ARP location with respect to Galileo spacecraft mechanical and ANTEX Reference Frames can be found in the table below.
|ARP in mechanical RF [mm]||ARP in ANTEX RF [mm]|
Please note that although the ANTEX format strictly requires satellite antenna PCOs to refer to the COM, the PCV (Phase Center Variation) information presented refers to the ARP. This allows updating a single vector each time the Center Of Mass changes.
The Navigation Antenna on-board each IOV satellites has been chamber-calibrated prior to launch for all five carrier signals. The measured PCOs with respect to the ARP are given in the table below.
|PCO in Mechanical RF [mm]||PCO in ANTEX RF [mm]|
The variation of the electrical phase centre of the antenna with respect to the mean phase centre, for a given direction, is called “Phase Centre Variation” (PCV). Direction-Dependant PCVs can be found in the GALILEO IOV ANTEX file. In order to obtain the ANTEX file please click on the following link. The PCVs are given for a 181 x 15 grid of azimuth and nadir angle-pairs format. The grid step size is 2° in azimuth and 1° in nadir. Due to the mutual geometric arrangement of the single Navigation Antenna antenna elements, the PCVs exhibit threefold patterns on each carrier frequency with azimuth- and nadir-dependent variations in the range of several millimeters.
The Galileo IOV spacecraft is a typical “box-wing” type satellite, consisting of a central cube (the “box”) and two rectangular solar panels (the “wings”) attached to it. Due to the way the attitude of the spacecraft is controlled, only three of the six satellite panels are actually exposed to solar radiation: the –X panel, the –Z panel and the +Z panel. (Note EOL means “End Of Life” and BOL means “Beginning Of Life”).
The optical properties coefficients’ are: α ≡ absorption coefficient, ρ ≡ specular reflection coefficient, δ ≡ diffuse reflection coefficient.
|Dimension of the Box with respect to the mechanical RF||Surface areas of the Box|
|ΔX = 2.611m||±X - panel = 1.32m2|
|ΔY=1.149m||±Y - panel = 3.00m2|
|ΔZ=1.149m||±Z - panel = 3.00m2|
The surface area of each solar array amounts to 5.41 m2 (= 5.000 m x 1.082 m).
|Surface||Material 1||Area [m2]||BOL & EOL|
|Box||-X||Carbon filled Kapton (external MLI)||1.32||0.94||0.00||0.06|
|Surface||Material 2||Area [m2]||BOL||EOL|
|+X||Optical surface radiator||0.78||0.10||0.72||0.18||0.25||0.60||0.15|
|+Z||Germanium coated black Kapton foil||1.28||0.57||0.22||0.21||0.57||0.22||0.21|
|Wing||+Y||Kapton HN (Insulation layer)||1.53||0.90||0.10||0.00||0.90||0.10||0.00|
The center of phase of the LRR (Laser Retro Reflector) is provided in the table below.
|LRR in Mechanical RF [mm]||LRR in ANTEX RF [mm]|
The satellite group delay is the total amount of time it takes to a signal of a particular bandwidth to start out from the on-board frequency oscillator, travel through each code generator, modulator, transmitter, multiplexer, and finally emerge from the satellite’s antenna. Since the group delays among the various signal paths within the satellite are not the same, signals do not exactly emerge from the transmitting antenna at the same time. The difference between the group delay of the radiated signals of two carrier frequencies f1 and f2 is what is commonly referred to as satellite group delay differential or Differential Code Bias (DCB).
The Galileo IOV satellite group delays have been measured on-ground by the spacecraft manufacturer for all three signal bands (E1, E5, and E6) and both on-board subsystems (“primary” and “redundant”). The results are listed in the table below.
|Primary [ns]||Redundant [ns]|
Median values and standard deviations of these (hourly) DCBs estimates are given in the table below. The standard deviations in the table below point to a DCB stability of about σ = ±0.1m (0.3 ns). Evidence for the existence of thermal-dependent fluctuations, as detected in tri-carrier combinations computed for the GPS Block IIF spacecraft series while passing through eclipse season, was not found.
|Antenna Reference Point (ARP)||Physical reference point located close to the antenna ground plane, used to refer the PCO in the Galileo IOV data|
|Antenna Phase Centre Offset (PCO)||Difference between the position of the mean phase centre and the COM|
|Differential Code Bias (DCB)||Difference between the group delay of the radiated signals of two carrier frequencies f1 and f2.|
|Mean Phase Centre||Point for which the phase of the signal shows the smallest (in the sense of “least- squares”) Phase Centre Variation (PCV) for a given nadir angle interval|
|Phase Centre Variation (PCV)||Variation of the Phase Centre location as a function of the direction of the outgoing signal for a specific frequency|
 European GNSS (Galileo) Open Service Signal In Space Interface Control Document. European Union, 2015.
 B. Hofmann-Wellenhof. Global Positioning System Theory and Practice. Springer, 2001.
 J.M. Juan Zornoza J. Sanz Subirana, M. Hernández-Pajares. GNSS DATA PROCESSING. European Space Agency, 2013.
 A. Leick. GPS Satellite Surveying. Wiley-Interscience, 1994.