Horizon Crossing Indicators (HCI) are normally used on spin stabilized spacecraft to provide both attitude control and timing information for spin control or to trigger onboard experiments.
The Model RH 310 HCI is a radiation hardened device designed to accurately determine the transition from space to the Earth’s horizon and back to space by using a small field of view (FOV) optical system. The sensor operates by detecting the Earth atmosphere’s carbon dioxide emissions in the 14 to 16-micron band. The standard system consists of a separate sensor head and an electronics package. The electronics package can accommodate both single and dual sensor heads and is fully redundant. The standard system is designed for spin rates between 3 and 10 rpm, however other spin rates can be accommodated with minor modifications to the design. The sensor produces two separate output pulses, each 80ms long with amplitude of +5V. There are two separate outputs, one from an Earth-space crossing and one from a space -Earth crossing. The leading edge of these pulses indicates a horizon crossing. The system is powered from 28V input with 1W maximum power consumption. The unit is designed to withstand radiation dose of 100krads (total dose) within the sensor head and 20krads at the electronics box.
An optional sun gate and moon pulse width discriminator circuit can be used to inhibit crossing pulses from these sources. The picture below shows the two RHCIs mounted on the STRV1d satellite prior to environmental testing.