The Trigger Broadcasting Unit (TBU)

In case of trigger event it is necessary to provide the trigger data to ground in a short time. It is expected triggered events occurred at the rate of one event per orbit and the data to be sent to ground is very low: the amount is <1 kb/trigger event. To support the prompt transmission of such event data to ground segment it is necessary a link with Earth independent from the satellite TT&C, which can have a link with ground station only once per orbit. The solutions evaluated for an independent and promt burst position broadcast to ground are by the means of:

  • VHF equatorial network (SVOM and HETE-2);

  • Orbcom satellite network (implemented in Agile satellite);

  • Iridium satellite network (tested by INAF/IASF on baloon and ready to be tested in FEES/IOD n-sat);

  • TDRSS link.


  • 26b

Figure 17. General block diagram of the Trigger Broadcast Unit


The preferred solution is the well proven VHF broacasting based in the utilization of the SVOM equatorial network. Link with SVOM ground segment will be made of 40 ground stations located located around the Earth inside a ± 30°strip.

The satellite to SVOM network link shall be carry out by the Trigger Broadcasting Unit (TBU - see Figure 17) proposed in the baseline as an unit of the Payload Module. The antennas are miniaturised for a better accommodation on PLM and SVM. The VHF ground network is the same of SVOM mission. For the definition of the VHF frequency range a possible choice, according to ITU Article 5, could be 137 – 137.175 MHz, reserved to space research . This band sub-system for trigger transmission is consequently proposed for compatibility with SVOM ground segment. The resources to be allocated for TBU are summarized in Table 7 while Table 8 reports the temperature ranges.


Table 7. TBU resources allocation summary

Mass [kg]


Volume [mm3]


Power [Watt]



Table 8. TBU Temperature Range

Temperature Range

Min. [°C]


Operative Temperature



No-Operative Temperature