At Solar MEMS we are fascinated by sun sensors: they are our core products and we know them inside out. Today, we would like to explain how the most common types of sun sensors available in the aerospace market work.
Sun sensors are navigation instruments that are used in space to establish the direction and position of the sun in relation with the satellite or spacecraft. In the aerospace sector they are mainly used to monitor attitude, to guide the orientation of solar panels and other functions.
These devices are used to determine attitude in space, providing data about the orientation of the satellite in relation with the solar vector, which is the angle at which the sun rays reach the satellite or spacecraft.
Sun sensors work by allowing light to enter through a small window (which may be of varying shapes and sizes) in the top of the sensor device, which consists of several or many photosensitive units.
These photosensitive units work by using the photoelectric effect: they convert the photons that reach them into electrons, and thereby into electric currents that can be made into a signal that sends information.
Light enters the window and projects an image onto the base of the device. At this point, the photosensitive cells measure the amount of light and it can be used to calculate the angle of incidence.
There are several special types of sun sensors, and each project should use the one which is best suited to the characteristics and requirements of the satellite. At Solar MEMS we offer personalized engineering services which can determine the needs of each mission and adapt to them accordingly, creating customized space software and hardware systems.
The main categories of sun sensors that are used in the aerospace market are Fine analogue sun sensors, Coarse analogue sun sensors and Digital sun sensors. Let’s take a closer look at how each of these works.
How fine analogue sun sensors work
This type of sun sensor uses the window to project the incident solar light onto a photodiode with various quadrants, or a position sensitive device (PSD).
As shown in the image above, the device works when the light enters a window in the sensor, creating a point or area of light that moves in accordance with the attitude of the satellite in relation to the sun.
This point or area of light creates currents in the independent photodiodes (or PSD) which tells us the position of the sun from two directions or axes.
These sensors are especially sensitive to albedo effects, which are reflections caused by other objects of the satellite or other celestial bodies (the Earth), whose light can also enter the window and prevent us from correctly distinguishing the sun rays, leading to imprecision.
How coarse analogue sun sensors work
These are analogue sun sensors that measure the amount of incident light without using windows nor projections. Incident light is proportional to the cosine of the angle between the sun and the normal vector of the light sensitive of the light sensitive cell, which allows us to calculate the angle of incidence of the solar vector. Solar sensors of this type only use one photodiode and the electric current they create is used to deduce the angle of incidence.
These sensors are very sensitive to variations in temperature, which can cause differences in the current generated and the estimated angle is directly proportional to this current, leading to imprecise readings of the angle of the solar vector. They are not very precise when the incident angle is close to perpendicular. For this and other reasons, coarse sensors are not as accurate as fine sensors.
How digital sun sensors work
These are sensors that use a photo sensitive unit made up of thousands or millions of light-sensitive pixels (CMOS or CCD cameras, linear or two axes) integrated below the window. This requires greater processing power but offers greater sensitivity to distinguish between the sun and confusing reflections (albedo, straylight). These sensors can be more precise than the fine analogue models (fine sun sensor).
The increased number of photosensitive units make them more sensitive and make them capable of other functions. However, these photosensitive units are more sensitive to radiation, which makes them very expensive or of limited use in long-term missions.
At Solar MEMS we specialize in fine analogue sun sensors, and we are developing an advanced digital sun sensor which will be tested on an IOD/IOV mission of the ESA. This sun sensor will be more precise, have a wider field of vision and immunity from the albedo effect and straylight.