An introduction to fine sun sensors
by Luke Hou
What is Sun Sensor?
A sun sensor is a device that senses the direction of the incident angle of lights. The source of lights is usually the sun. Therefore, we called it the “sun sensor.”
There are various types of sun sensors that differ in their characteristics. However, two classifications are the most commonly used, coarse sun sensor and fine sun sensor. Fine sun sensors can also be classified into two types, analog output fine sun sensor and digital output fine sun sensor. Here are some example products:
- Coarse sun sensor: https://tensortech.com.tw/products/
- Analog-type fine sun sensor: https://www.cubesatshop.com/product/nano-ssoc-a60-analog-sun-sensor/
- Digital-type fine sun sensor: https://tensortech.com.tw/products/
Coarse Sun Sensor (CSS)
The output signal from the coarse sun sensor varied if the sun is entering/leaving its field of view (FOV) or changing its position in the FOV. It often contains a single photoresistor or photodiode. This simple architecture has less power consumption and accuracy compared to a fine sun sensor.
Figure 1. Definition of the fine sun sensor reference frame (photo credit: Tensor Tech CO., LTD.)
Figure 2. Definition of the fine sun sensor reference frame in spherical coordinate style (following ISO 31-1) ; The “x” spot in the diagram represents the sun.
Let’s specified the fine sun sensor reference frame in Figure 1 and define the spherical coordinate of it in figure 2. The coarse sun sensor outputs only the information of the theta angle but the fine sun sensor output both theta and phi angle.
Fine Sun Sensor (FSS)
The analog-type fine sun sensor outputs angle information with voltage levels. Analog-to-digital converters (ADC) are required on the customer’s side.
For digital-type fine sun sensors, analog-to-digital converters or micro-control unit (MCU) is already embedded in the sensor. Therefore, the ADCS/OBC can access the angle information via communication interfaces such as I2C or UART. This type of fine sun sensor often consumes more power than the analog type since part of the data preprocessing is handled by FSS. Analog-type FSS often suffer from the EMI on the transmission line, while digital-type FSS can avoid it. Furthermore, an error correction table is embedded in digital FSS for achieving better accuracy. In short, digital FSS is more modulize and self-contained than analog FSS.
Table 1. Three common types of sun sensors
Figure 3. Example current consumption profile of digital-type fine sun sensor and analog-type fine sun sensor (for explanation only, not measured by instruments)
As figure 3 shows, the consumed current rises while samplings occurred on the digital-type fine sun sensors. That’s why the peak power consumption specs on digital fine sun sensors often seemed much higher than the analog-type. However, the average power consumption of both should not be differed by much.
How does a Fine Sun Sensor Work?
Closely above the quadrant detector (photodiode), a square aperture mask is accurately positioned on the diaphragm. Sunlight reaches the detector through the mask. In these cells, the position of the light spot can be sensed by the photodiode, as figure 4 shows.
Figure 4. In this picture, a quad-segmented photodiode is used. It is divided into four cells, Q1 to Q4. The current induced on each cell represents the area that the whole light spot takes on that cell. We can then calculate the direction of the sun. 
FSS100 from TensorTech is digital-type. Compared with the analog fine sun sensor, it can achieve higher precision because a pre-calibrated error table has embedded. In addition, the I2C address of our FSS is programmable. Therefore, address conflict with some other I2C-based components can be avoided.
Sun sensors are widely used in spacecraft attitude determination systems for measuring the sun vector in spacecraft coordinates. Besides, it can help create meteorological systems, tracking systems, and navigation systems. A sun sensor is also used in weather platforms, solar trackers, and unmanned vehicles.
Q1: How does the error of the fine sun sensor induce?
A1: In sensor technology, there are two types of error, “system error” and “random error.” Let’s firstly discuss the possible cause of system error:
- Differences in each cell of the quad segmented photodiode
- Resistance and capacitance error on passive components
- Misalignment occurred during sensor installations (mounting screws has a smaller diameter than the holes, which incurs misalignments)
- Refraction on the surface of the fine sun sensor and poor modeling of this effect
The misalignment often contributes the most, especially when customers mount these sensors by themselves without post-calibrations. In this case, the fine sun sensor is only trustable on its relative accuracy. However, use Tensor Tech’s product as a case, these system errors could be minimized within 0.1 deg via proper calibrations. That’s why we recommend customers purchase the integrated ADCS solution because every sensor is mounted onto it and calibrated altogether.
Secondly, the random error is caused by the following factors:
- Noises on the output current of the photodiodes
- ADC sampling random error, usually uniformly distributed, and its range depends on the resolution of the ADC
However, all these random errors should not interfere much with the performance of the fine sun sensor if a well designed was made. In practice, the albedo effect may be another cause of flawed attitude determination that cannot be well-calibrated or modeled in a fine sun sensor. 
Q2: How do we calibrate the fine sun sensor?
A2: Let me introduce you to how Tensor Tech conducts this procedure in-house. A dual-axis adjustable platform is set up and mounted with the FSS100 or ADCS100 (with fine sun sensors installed). We assume the readings on two of the manual rotation platforms as the true pointing. Another AM0-grade solar simulator is placed to provide input sun lights. A dark room is suggested as the calibration environment to prevent other sources of disturbances. By recording the pointing determination result of the fine sun sensor and comparing it with the readings on the dual-axis platform, we obtain the correction coefficients for the error table.
Q3: Where can I install the fine sun sensor on my CubeSat? Is there a recommended location?
A3: There are three kinds of common installation styles.
- Two sun sensors style: Usually applied to the satellites where the solar panel will be deployed. One is installed on the side that has deployed solar panels. Another installed on top or bottom of the satellite.
- Six sun sensors style: Install on every side of the satellite. This style is the most reliable architect. It can make sure every side of a satellite can cover with FSS’s FOV. So no matter the satellite is in tumbling or any arbitrary attitude, at least one FSS could be referenced.
- Five sun sensors style: Install on five sides of a satellite. A solar panel or optical payloads might cover the one last side of the satellite. Therefore, the six-side style can not be made. In Tensor Tech’s ADCS100, we only install five FSS. This design is because of our mechanical limitation, which only allows us with the tuna-can volume.
 Spherical coordinate system. (2021).
 E. Boslooper, N. Heiden, D. Naron, R. Schmits, J. Velde, and J. Wakeren. (2012). BepiColombo fine sun sensor. International Conference on Space Optics.
 D. Brasoveanu and J. Sedlak. (1998). ANALYSIS OF EARTH ALBEDO EFFECT ON SUN SENSOR MEASUREMENTS BASED ON THEORETICAL MODEL AND MISSION EXPERIENCE.