Authored by Luke Hou

 
 

What is Sun Sensor?

Sun sensors are critical components in satellite attitude determination and control systems (ADCS). These devices precisely measure the incident angle of solar radiation, providing essential data for satellite orientation and positioning in space.  
 

Classification and Types

Sun sensors are broadly categorized into two main classes based on their precision and functionality:
      1. Coarse Sun Sensors (CSS)
     https://satsearch.co/products/bradford-coarse-sun-sensor

      2. Fine Sun Sensors (FSS)
        a. Analog Output Fine Sun Sensors
            https://www.cubesatshop.com/product/nano-ssoc-a60-analog-sun-sensor/
        b. Digital Output Fine Sun Sensors
            https://tensortech.co/product/category/sun-sensor-series

The output signal from the coarse sun sensor fluctuates as the sun enters or exits its field of view (FOV) or shifts position within it. Typically consisting of a single photoresistor or photodiode, this simple design results in lower power consumption but also reduced accuracy.

Specify the fine sun sensor reference frame in Figure. 1 and define the fine sun sensor reference frame in spherical coordinate style in figure 2. The coarse sun sensor outputs only the information of the alpha angle but the fine sun sensor output both alpha and beta angle.
 
The analog-type fine sun sensor outputs angle information with voltage levels. An analog-to-digital-converter (ADC) is often required on the customer's side.

For the digital-type fine sun sensor, an ADC or MCU is already embedded into the sensor. Therefore, the attitude control computer 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. However, noises that occur on the transmission line of analog-type fine sun sensors can be avoided. Furthermore, an embedded error correction table is allowed on these digital-type fine sun sensors for achieving better accuracy.
 
  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 function?

A rectangular window allows a light spot to fall on an array of photodiode cells at the bottom of the chamber. In these cells, the position of the light spot can be sensed electronically, as Figure 4 shows. As for the digital fine sun sensor, the internal MCU will convert analog signal to digital signal before it outputs.  
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. [2]

Tensor Tech’s fine sun sensor is digital-type. Compared with the analog fine sun sensor, it can achieve higher precision, reduce noise, and the output digital signal can directly communicate with the attitude control computer. In addition, the angle information could be accessed via the I2C interface.  

Other applications

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.

 

Common Questions and Answers

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:

1. Differences in each cell of the quad segmented photodiode
2. Resistance and capacitance error on passive components
3. Misalignment occurred during sensor installations (mounting screws has a smaller diameter than the holes, which incurs misalignments)
4. Refraction on the surface of the fine 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, using 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:

1. Noises on the output current of the photodiodes
2. ADCS 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 one 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. [3]

Q2: How do we calibrate the fine sun sensor?

A2: A dual-axis adjustable platform is set up 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.

1. Two sun sensors style: Usually applied to the satellites where the solar panel will be deployed. One installed on the same side with solar panel deployed, another installed on top or bottom of the satellite.
2. Six sun sensors style: Install on each side of the satellite. To make sure every side of a satellite can cover with FSS
3. Five sun sensors style: Install on the five sides of the satellite. One side of the satellite is covered by a solar panel or a mechanical design, so it can’t be installed on this side. In our ADCS, we only install five FSS. Because of our mechanical design, we occupy the tuna can of the 3U satellite.

References

[1] Spherical coordinate system. (2021). Wikipedia. https://en.wikipedia.org/wiki/Spherical_coordinate_system

[2] E. Boslooper, N. Heiden, D. Naron, R. Schmits, J. Velde, and J. Wakeren. (2012). BepiColombo fine sun sensor. International Conference on Space Optics.
https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10564/105641P/BepiColombo-fine-sun-sensor/10.1117/12.2309172.full?SSO=1

[3] D. Brasoveanu and J. Sedlak. (1998). ANALYSIS OF EARTH ALBEDO EFFECT ON SUN SENSOR MEASUREMENTS BASED ON THEORETICAL MODEL AND MISSION EXPERIENCE. https://ntrs.nasa.gov/api/citations/19980203812/downloads/19980203812.pdf