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The alert network based on a French design is one of the key features of the SVOM mission enabling the near real time dissemination of information between the satellite and the ground. It plays a crucial role in the optimization of the synergy for the GRB multiwavelength follow-up. This effort is needed to ensure that the distance to the GRB can be measured.

Given the rather short duration of GRBs, the alert network is designed to dispatch, as soon as possible, all science data needed for ground telescopes to rapidly follow-up of the GRBs detected by ECLAIRs. Once a GRB is detected onboard, the alert message is transmitted to the ground in about 10 seconds at very high frequency (VHF) in a frequency band between 137 and 138 MHz by an onboard antenna. The message is then downlinked through a network of radio stations spread around the Earth’s equator.

At least 43 VHF antennae will be homogeneously deployed in the inter-tropical zone around the Earth, between latitudes -30° and +30°. They will relay every alert message from the satellite to the French Science Center (FSC) located at Saclay in France.

The VHF messages will summarize the main GRB properties that are needed for the follow-up. After a first analysis at the FSC, the messages will be distributed to the whole scientific community, in particular to the GFTs robotic telescopes that will refine the GRB position and give an initial indication of distance. A summary of the GFT results will then be sent back to the FSC in order to share their findings with large observing facilities.
The performance objective of this network is to enable the alert message to be transmitted to the robotic telescopes in less than 30 seconds after detection on board the satellite. The results of the observations from the GFT telescopes are sent back to the FSC and finally disseminated to the large telescopes. These large telescopes, with a smaller field of view, will allow the acquisition of the spectrum of the burst and thus the estimation, by measuring the redshift, of its distance. If everything goes smoothly, it should take less than 4 min to start acquiring GRB optical spectra with large telescopes.

As of August 23, 2023, the deployed network is composed of 41 stations.

Chronology of station installation

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