A method, performed by each of a plurality of satellites of a secondary Global Navigation Satellite System, GNSS, in a Low Earth Orbit, LEO, comprising: receiving multiple GNSS signals, in a first frequency band, from Line-of-sight, LOS, satellites of primary GNSSs in a Medium Earth Orbit, MEO; receiving candidate sets of orbit and clock corrections for the LOS satellites of the primary GNSSs, wherein the candidate sets of orbit and clock corrections represent different sets of orbit and clock corrections for the same satellite of the primary GNSSs and each candidate set of orbit and clock corrections is obtained by a different dissemination source; performing a Position-Velocity-Time, PVT, calculation based on code and/or carrier pseudo-ranges between the LOS satellites of the primary GNSSs and a respective satellite of the plurality of satellites of the secondary GNSS, wherein the code and/or carrier pseudo-ranges are derived from the received multiple GNSS signals, and wherein the code and/or carrier pseudo-ranges are corrected by a single set of the candidate sets of orbit and clock corrections; determining a short term prediction model for an orbit and clock of the respective satellite of the plurality of satellites of the secondary GNSS based on the PVT; transmitting, in a second frequency band, which is different or identical to the first frequency band, a navigation message modulated onto a LEO navigation signal intended for terrestrial user equipments, wherein the navigation message includes the short term prediction model, and wherein the method further comprises: after receiving the candidate sets of orbit and clock corrections, determining an optimal set of orbit and clock corrections from the received candidate sets of orbit and clock corrections, wherein the single set of the candidate sets of orbit and clock corrections is the optimal set of orbit and clock corrections, and wherein the determining the optimal set of orbit and clock corrections from the received candidate sets of orbit and clock corrections is performed based on a prediction error of each of the candidate sets of orbit and clock corrections. receiving multiple GNSS signals, in a first frequency band, from Line-of-sight, LOS, satellites of primary GNSSs in a Medium Earth Orbit, MEO; receiving candidate sets of orbit and clock corrections for the LOS satellites of the primary GNSSs, wherein the candidate sets of orbit and clock corrections represent different sets of orbit and clock corrections for the same satellite of the primary GNSSs and each candidate set of orbit and clock corrections is obtained by a different dissemination source; performing a Position-Velocity-Time, PVT, calculation based on code and/or carrier pseudo-ranges between the LOS satellites of the primary GNSSs and a respective satellite of the plurality of satellites of the secondary GNSS, wherein the code and/or carrier pseudo-ranges are derived from the received multiple GNSS signals, and wherein the code and/or carrier pseudo-ranges are corrected by a single set of the candidate sets of orbit and clock corrections; determining a short term prediction model for an orbit and clock of the respective satellite of the plurality of satellites of the secondary GNSS based on the PVT; transmitting, in a second frequency band, which is different or identical to the first frequency band, a navigation message modulated onto a LEO navigation signal intended for terrestrial user equipments, wherein the navigation message includes the short term prediction model, and wherein the method further comprises: after receiving the candidate sets of orbit and clock corrections, determining an optimal set of orbit and clock corrections from the received candidate sets of orbit and clock corrections, wherein the single set of the candidate sets of orbit and clock corrections is the optimal set of orbit and clock corrections, and wherein the determining the optimal set of orbit and clock corrections from the received candidate sets of orbit and clock corrections is performed based on a prediction error of each of the candidate sets of orbit and clock corrections. after receiving the candidate sets of orbit and clock corrections, determining an optimal set of orbit and clock corrections from the received candidate sets of orbit and clock corrections, wherein the single set of the candidate sets of orbit and clock corrections is the optimal set of orbit and clock corrections, and wherein the determining the optimal set of orbit and clock corrections from the received candidate sets of orbit and clock corrections is performed based on a prediction error of each of the candidate sets of orbit and clock corrections.