Evaluation of Galileo FOC Reference Antenna Patterns

Knowledge of the effective power of radio signals transmitted by navigation satellites is essential for link budget calculations for, e.g., GNSS reflectometry experiments or space-borne GNSS tracking. In particular, this is important for receivers in medium or higher Earth orbit as well as lunar orbit. Here, the signal strengths suffers from the large distances between transmitter and receiver as well as reduced power from the sidelobes of the transmit antennas with significantly smaller gain. An example for such a satellite is ESA’s Genesis mission, the first co-location in space of the four space-geodetic techniques GNSS, SLR, VLBI, and DORIS.

In spring 2024, the Joint Research Center (JRC) of the European Commission published the "Galileo Reference Antenna Pattern" (GRAP) of the Galileo Full Operational Capability (FOC) satellites. GRAP describes the equivalent isotropically radiated power (EIRP) of the open signals in the E1, E5a, E5b, and E6 frequency bands. It has a 2-degree resolution in azimuth and nadir direction for nadir angles up to 90° and provides the block-mean of all FOC satellites along with the corresponding standard deviations.

However, GRAP only specifies a minimum EIRP level in accord with the corresponding signal specifications. This might be undesirable for practical applications as the actual power level transmitted by the satellites could be significantly larger. In order to evaluate this potential discrepancy as well as the general agreement of GRAP with measured EIRPs, data from a dedicated measurement campaign are analyzed for the individual Galileo FOC satellites. The EIRP is measured with the 30 m dish antenna of the German Aerospace Center located in Weilheim, Germany, which is regularly used for GNSS signal monitoring. The satellite- and frequency-specific EIRP values are compared to GRAP as well as to the measured mean values. In this comparison, special attention has to be paid to the fact that the EIRP measurements cover all signals within one frequency band, whereas GRAP only refers to the Open Service data+pilot components and does not include the Public Regulated Service signals and the intermodulation product in the E1 and E6 band.