Sun sensors are becoming increasingly important in modern satellites due to growing expectations of better maneuverability, agility and precision control of even very small craft.
This demand for improved performance and control follows a trend we’re seeing in the in-space propulsion sector, which you can read more about on our recent post discussing CubeSat thrusters.
In this article, we take a high-level look at how sun sensors work, what to take into consideration when searching for a sun sensor for your satellite, and give an overview of some of the products currently available on the global marketplace for space.
If you would like to skip the introductory material and go straight to view the available products, please click here, otherwise please read on.
What are sun sensors?
Sun sensors determine the direction and position of the sun relative to a satellite.
They enable attitude determination of satellites by providing two-axis information on the orientation of the satellite using the azimuth and elevation of the sun vector.
To be able to fully ascertain the attitude the third-axis information needs to be realized, which can only happen by adding an additional sensor such as a magnetometer or earth sensor.
Obviously, a sun sensor will not work in the eclipse period in orbit and, if only a very crude identification of the direction of the sun is needed, it can be determined quite easily using the voltage variations in solar panels.
In general, there are three main categories of sun sensor:
- Coarse analogue sun sensors: sensors that measure the current output, which is proportional to the cosine of the angle between the sun and the normal to a photocell.
- Fine analogue sun sensors: sensors that use an aperture to create a sunspot on either a four-quadrant photodiode or a position-sensitive device.
- Digital sun sensors: sensors that operate based on integrating a 2-dimensional light sensor and signal processing, so as to discriminate between direct sunlight and reflected sunlight.
The following paragraphs aim to provide some insights into the differences between these categories of sun sensors and their relative merits.
Coarse analogue sun sensors
This category of sun sensor consists of models with a single photodiode for which the generated current is a measure of the incidence angle. This current varies with the cosine of the incidence angle in a first approximation.
As the derivative of the cosine is 0 at nominal incidence (or zenith) there is a very small measurement signal for angles nearing zenith.
As temperature variations of the diode will lead to signal variations, it is difficult to discriminate between angle and temperature variations at angles close to zenith.
Next to this, radiation degradation will directly influence the calculated angle because the angle is determined on the absolute magnitude of the current generated.
As a result, the zenith accuracy is seldom higher than around 5° while the accuracy can increase to around 3° for angles from 10-50°.
For higher angles, the accuracy again decreases due to tunneling effects in the cover glass (required to protect the devices against cosmic radiation), changes in the anti-reflex coatings, and variations in the absorption debt.
Next to a low generic accuracy, it should be noted that this type of sun sensor is very sensitive to albedo signals and reflections from satellite parts.
In order to detect the Sun’s attitude in two directions at least three coarse sun sensors are usually needed unless a pyramid coarse sun sensor is used.
These sensors (examples of which are shown in above Figure 2) are basically a number of photodiodes mounted on a common structure so as to define the nominal incidence angles for the individual photodiodes.
By combining the signals of a number of diodes the zenith accuracy can be improved over a limited field of view (FOV).
This is mainly due to the fact that the incidence angle is not near normal for the individual diodes (leading to a higher response) and that the ratio of the currents can be used to calculate the angle, removing the need for an accurate temperature compensation.
Even though these sensors are more accurate and can even have internal redundancy, they are not commonly used on small satellites due to their large volume and high costs.
As can be seen in the picture, these sensors are generally equipped with mission-specific baffles to avoid reflections from spacecraft parts having a negative effect on the measurement accuracy.
Fine analogue sun sensors
These sensors operate by virtue of the fact that a translucent window provides a light spot which shifts over the sensing element with changing Sun attitude. The most common implementation of which is using a four quadrant photodiode and is shown below.
Figure 3 - fine sun sensor operating principle.
A membrane is suspended at a known distance above a sensing element (typically a four-quadrant photodiode or position sensitive device (PSD)).
Sunlight, shining through the translucent part, creates a light spot on the sensor. This light spot generates currents in the four independent diodes which can be used to calculate the Sun’s attitude in two directions, if the height of the membrane with respect to the diodes is known.
The formulae that can be used to calculate this attitude are quite simple:
Formula 1 - angle calculation formulae.
If α is negative: the sun is to the top of the sensor.
If β is negative: the sun is to the right of the sensor (both when viewed from the top side).
NOTE: The illumination given in Figure 3 above is for positive α and positive β.
High quality analogue fine sun sensors, such as the Bradford mini-FSS, and the Lens R&D BiSon64-ET and MAUS, can provide an accuracy of better than 1° at zenith and 3° over the entire FOV on the basis of these formulae only, though higher accuracies will always require the use of a number of calibration tables.
Using calibration tables accuracy below 0.5° can be realized, although it should be noted that this is without measurement errors caused by albedo signals, which can reach up to 10° or even 15° depending on orbit height, pointing and cloud cover.
In order to minimize albedo errors stray light baffles can be added as an alternative to creating larger (and more expensive) sensors, for example:
For sensors with this type of accuracy it is important to note that the re-mounting accuracy is part of the pointing budget.
Because of this, all sensors shown in Figure 4 above have a similar mounting strategies that use a ‘highly-toleranced reference hole’; a slotted hole with a high tolerance in one direction combined with an oversized hole that ensures the sensors can be put back in the same orientation with a high degree of confidence.
Figure 6 - Lens R&D BiSon64-ET mechanical interface.
The reference hole shown on location E5 in combination with the slotted hole detailed in location H7/8 will ensure repeatability of the X- and Y-axis where the flatness of the mounting feed specified in location B9 will guarantee the Z-axis repeatability.
This repeatability is determined by the mechanical tolerances on the mounting holes, the accuracy of the attachment hardware, and the distance between the mounting points.
For very small sensors, such as those shown in Figure 7 below, the re-mounting accuracy can easily exceed the specified accuracy due to the close proximity of the mounting holes in combination with a poor mechanical tolerance on the mounting hardware.
One should be aware of these issues when using very small sensors while desiring a high pointing accuracy. In the end the obtainable re-mounting accuracy could mandate a re-calibration after the sensors are mounted to obtain the desired accuracy.
This will obviously complicate the manufacturing process and might well compensate for a slightly more expensive sensor that doesn’t require such a re-calibration.
It should be noted that the re-mounting accuracy is not only an issue to be considered with the smallest sensors. For the Solar MEMS sensors shown below, the mounting holes are specified to be 3.2 mm wide.
The advised mounting hardware is a stainless steel screw according to ISO 4762-2005. The outer diameter of these screws is only 3 mm so there is 0.2 mm of play between the screw and the side of the hole.
The distance between the two holes of the A60 is 4 cm, which means that it is possible to rotate the sensor by as much as 0.4 mm over 4 cm or 0.57°. This is more than the specified 0.3° accuracy of the sensor.
Although these deviations may not be important for the application, it is good to be aware of them, much like the implementation differences that exist.
Solar MEMS Technologies sun sensors for instance use four quadrant photodiodes, but in an immersed configuration as shown in Figure 9.
Figure 9 - immersed sun sensor configuration A60.
In an immersed configuration, the cover glass that provides a certain degree of radiation shielding is used as a spacer to hold the membrane through which the sun is shining. This allows for the development of a very compact sensor core and potentially a very small sun sensor.
There are however a number of disadvantages to such an approach, the first of which is that it is very difficult to provide additional radiation shielding without causing multiple reflections.
Another more significant disadvantage is the fact that due to the refraction inside the glass, there is a cross-coupling between the X- and Y-axis which causes the formulae given in Formula 1 above to be rendered invalid.
As a consequence, this type of sensor will always need a calibration table or a more complicated formula. In addition, it will require a temperature compensation if full accuracy is to be obtained because of the refractive index sensitivity to the operating temperature.
Other implementations use a position-sensitive device instead of a four quadrant photodiode. This type of sensor typically uses a much smaller pinhole-like aperture, through which a small sun spot is created on the position-sensitive device; a specific type of silicon detector with four electrodes.
By reading out the sensor, the centroid of this light spot can be determined and thus the direction of the incident sunlight. This is an alternate way to produce an analogue sun sensor which is not often chosen because PSD detectors are more prone to radiation degradation, and the smaller pinhole creates less signal thus mandating the availability of proximity electronics.
Despite the smaller pinhole, the sensors have a similar albedo sensitivity to a four quadrant based detector because the relative amount of signal generated is mainly depending on the FOV, accommodation and shielding provisions.
Digital sun sensors
Truly digital sun sensors are a much smaller market segment than analogue sensors.
There are various sun sensors on the market which are advertised as digital, but which are actually analogue sun sensors with a digital interface.
These sensors will exhibit the same albedo sensitivity and will not allow the ability to exclude certain parts of the FOV from evaluation through digital programming.
Some models feature novel sensing principles with innovative arrangements of linear arrays and a double slits.
Digital sun sensors can also be developed with low mass and small physical footprints, though there can be a potential trade-off with radiation tolerance.
Please note that this is just a brief introduction to sun sensors of course; in a future article as part of our Let’s talk about series on the blog we’ll go deeper into the topic of attitude control - if you’d like to be informed when this is published, please sign up to our mailing list at the link below.
How to select the best sun sensors for your needs?
In our recent product overview article on CubeSat thrusters, we suggested a basic four-step approach to selecting the right in-space propulsion system for your needs.
This framework can also be used to select the optimum sun sensor, and an overview is given below:
- Specify your exact mission parameters - ensure you are clear on the full range of activities your spacecraft needs to perform in terms of the attitude determination and control.
- Record all known design specifications of the satellite - keep to hand the current specifications of the craft (understanding these may change as the design evolves) - for example, it is vital that sun sensor is correctly accommodated within the spacecraft due to the impact on the system’s ability to use the correct frame of reference.
- Consider the range of technology that will be used in the system - take into account the results of all decisions on what other components and sub-systems have been made. It is important that the sun sensors you choose will work effectively with other components, such as magnetorquers or star trackers in order to establish full attitude knowledge.
- Take into account the key performance criteria - understand how to evaluate available sun sensor products according to the criteria most relevant for your applications. More on this below.
There are a number of design and performance criteria which dictate the selection of a sun sensor model:
- Field of view (FOV)
- Environmental characteristics including operating temperature, radiation tolerance limits, and vibration limits
- Available mass and volume budgets
- Update rate (for units with integrated electronics)
- Power consumption
- Angular resolution
Sun sensors available on the global market
In the article above we have mentioned a number of sun sensor products in order to demonstrate various core features and differences. Below we have included a more comprehensive list of products available on the global marketplace for space.
Please note that this list will be updated when new products are added to the global marketplace - so please check back for more.
The CubeSense S and Cubesense Fine Sun/Earth sensor by CubeSpace
CubeSpace manufactures sun sensor products that are calibrated in a state-of-the-art dark, clean, optics calibration room, using a sun-simulator and highly accurate rotation stage. The sun sensors are immune to albedo effects, making them highly robust and versatile. Two core products are available:
The CubeSense S (pictured) provides Sun measurements with <0.2° (3σ) accuracy over the entire FOV. It weighs 30g, has peak power requirements of 200mW and an FOV of approximately 180°.
The Cubesense Fine Sun/Earth sensor is a CMOS-imager based Sun and Earth sensor with a wide FOV, low power usage and high accuracy. Available in two different formats; a PC104-sized board with two sensor heads featuring full redundancy on memory, and a highly compact single sensor head with integrated electronics.
The SS200 by Hyperion Technologies
The SS200 is a lightweight (weighing just 3g) and low power CubeSat sun sensor. It features an adjustable sampling rate for a high level of operational control. This enables higher sampling rates when required, at a temporary increase in power consumption, and low sampling rates in order to optimise power consumption when suitable. The SS200 delivers below 1° accuracy in the +/-45°, and has a total FOV of approximately 110°.
The BiSon64-ET, BiSon64-ET-B and MAUS by Lens R&D
Lens R&D manufactures a range of high reliability sun sensors suitable for a wide variety of satellite types and mission requirements:
The BiSon64-ET model is a high reliability sun sensor with a nominal field of view of 64° in diagonal. It is a fine sun sensor that is qualified for European Space Agency (ESA) development activities. The sun sensor is designed for demanding missions that require highly reliable sun sensing attitude control and is suitable for use in many orbits.
The BiSon64-ET-B features the BiSon64-ET along with a baffle mounted to the housing in order to reduce albedo stray light.
The MAUS is a fine sun sensor specially developed for the CubeSat and small satellites market. Building on Lens R&D’s BiSon64-ET development experience, the MAUS product focuses on the unique requirements of small satellites and is designed to be used primarily for Low Earth Orbit (LEO) missions.
The NCSS-SA05 and NFSS-411 by NewSpace Systems
NewSpace Systems manufacturers two models of lightweight and small-sized sun sensor that feature a simple interface and wide FOV. The devices are low power for minimal demands on satellite energy systems and over a hundred deliveries of each product have been made to a variety of international satellite missions and constellations in development:
The NCSS-SA05 is a <5g sensor with a 114° FOV and a simple analogue interface. It has PSD architecture and can enable full sky coverage with the use of 6 sensors.
The NFSS-411 is a fine sun sensor with a 140° FOV weighing less than 35g. It features embedded calibration and digital architecture, and can enable full sky coverage with the use of 4 sensors.
Sun sensors by Solar MEMS Technologies
Solar MEMS Technologies is a Spain-based manufacturer specialising in Micro Electro Mechanical Systems (MEMS) technology enabling the fabrication of highly integrated sensing structures. Solar MEMS Technologies develops an array of sun sensors for small and medium satellites that are designed to offer either a wide or narrow FOV and are lightweight and low power. The portfolio also includes a number of Sun Sensor on a Chip (SSOC) models:
- ISS-A5, ISS-A15, ISS-A25 and ISS-A60 - analogue sun sensors.
- ISS-D5, ISS-D15, ISS-D25 and ISS-D60 - digital sensors, MODBUS RTU communication.
- MASS-5, MASS-15, MASS-25 and MASS-60 - magnetometer, accelerometer and sun sensor systems.
- ISS-T5, ISS-T15, ISS-T25 and ISS-T60 - digital sensors for tracking systems, MODBUS RTU communication.
- SSOC-A60 - includes a metal shield and cover glass in the optical eye to minimise the ageing of the device under high radiation levels.
- nanoSSOC-A60 - a two-axis low-cost sun sensor for highly accurate sun-tracking, pointing and attitude determination.
- SSOC-D60 - includes a microprocessor that directly provides the sunlight incident angles and their derivatives without external calculations, via digital interface.
- nanoSSOC-D60 - suitable for nanosatellites and CubeSats.
Sun sensors by Bradford Space
Bradford Space is a multidisciplinary space technology manufacturer operating in the U.S. (California), the Netherlands, Sweden and Luxembourg. The company manufacturers various analogue sun sensor products to suit different mission requirements:
Coarse Sun Sensor (CSS) - a rugged, highly reliable, self-redundant device designed to detect sunlight and provide coarse information for a potentially hemispherical FOV. Proven performance with the product continuously in production since 1975 and used in an array of missions.
Fine Sun Sensor (FSS) - an analogue sensor based on a quadrant detector that measures the solar aspect angle in two axes. Processing the four quadrant outputs results in the two components of the solar aspect angle. Qualified for LEO with many temperature excursions and for very severe radiation regions (e.g. HEO in Van Allen belts).
Mini Fine Sun Sensor (Mini-FSS) - similar to the FSS but with a smaller mass and physical footprint. Designed for long-duration LEO missions, a Mini-FSS variant is also available without read-out electronics that is particularly suited for small satellites.
Antrix’s Four-Pi Steradian Sun Sensor consists of four optical heads. It can be used for attitude control, safe mode operation, solar array pointing, and station-keeping. The system weighs 50g and has a physical footprint of 55 mm x 40 mm x 30mm.
Through careful design and testing, O.C.E. analogue and digital sun sensors have achieved small sizes and weights along with low power consumption. Available with simple, reliable design characteristics with excellent measurement accuracy and FOV, the sensors provide a high quality option for the next generation of micro-satellites.
The S3 (Smart Sun Sensor) is a two-axis solar sensor based on an Active Pixel Sensor (APS) detector. The S3 has been optimized for the commercial market of EO satellites and for GEO Telecommunication spacecraft due to the intrinsic radiation hardness of the detector.
If you would like our assistance in finding a suitable sun sensor for your needs from our extensive network, please fill out a request for information or proposal, and we’ll get back to you as soon as we can.
Sun sensing technology is a large and active area of the market and the listings above are not exhaustive - we are actively working on updating this information and will share any updates via our email list below and on social media.
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