What is a horizon sensor? The SOLAR MEMS HSNS

Space sensors are our speciality and our vocation. We have great experience in working with different types, such as solar sensors, star trackers and horizon sensors. In this article we will be looking at the last of these, the horizon sensor, which can determine with precision and reliability the attitude of a vehicle or satellite by taking the Earth’s horizon as its reference. We explain what they are and what they do, a little about their history and our HSNS for micro and nanosatellites.

How can a satellite or spacecraft orient itself in the immensity of space? To prevent them becoming disoriented and to maintain their trajectories they need to use attitude control, which are the actions taken to control the orientation of an object through its relation with a specific reference, which could be the heavens, the Sun, etc. Sensors are used to determine the position of these, and we at Solar MEMS are specialists and have hundreds of them in orbit. One type of sensor that is used to control attitude is the horizon sensor, whose main feature is that it uses the light from the edge of Earth’s atmosphere, the horizon, as its reference point.

What is a horizon sensor?

As we have already said, a horizon sensor is an optical instrument that detects the light at the edge of the Earth’s atmosphere, the horizon. One of the most common ways of making this measurement is to use a thermal infra-red sensor that contrasts the temperature of the atmosphere against that of the cosmic background. Horizon sensors enable satellites and spacecraft to know how they are oriented in relation with the orthogonal axes of the Earth.

A history of horizon sensors

There are few sources better than NASA for learning about the history of any space technology. In this document the American agency explains that “Horizon-sensor development began in 1958 on sensors used for Jupiter rocket reentry experiments and for the Air Force Discoverer program. The Jupiter sensor evolved into sensors used on the Mercury, Nimbus, Biosatellite, ESRO, Vela, and Agena programs. The Discoverer sensor evolved into sensors used on the OGO and Gemini programs”. Would you like to know how horizon sensors have evolved from their origins to the present day? This image from the National Air and Space Museum shows the difference from today’s high-tech devices, like our Solar MEMS’ HSNS that you can see below in this article. As the museum website explains, this sensor “was mounted on the left-hand side of the exterior of the Gemini space capsule. It was used, in conjunction with an electronics package, to sense the position of the Earth’s horizon relative to the capsule’s position in orbit. It contained a pivoting mirror, lens, and infra-red sensor to locate the horizon. The location of the horizon relative to the spacecraft was used to align the spacecraft or its inertial guidance system.”

Solar Mems’ HSNS is a horizon sensor for nano and microsatellites

Horizon Sensor for Nano and Micro Satellites (HSNS) of Solar MEMS is a Quad Thermopile sensor for Earth detection and Nadir vector determination. This device measures the infrared radiation from Space and from Earth with 4 IR-eyes, providing accurate and reliable detection and attitude determination.
It is a very small device, measuring 90 x 92 x 50 mm with a mass of 120 g. These characteristics make it ideal for use in nano and micro space satellites. It also includes a microcontroller for fast assembly and integration with different options like UART or I2C protocols. Following this link, you can download the complete technical file of this Solar MEMS product.