Sun sensor systems 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.
How does a sun sensor work?
Sun sensors in satellite’s navigational systems determine the direction and position of the sun relative to the satellite itself.
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 analog sun sensors: analog sun 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 analog sun sensors: fine sun 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.
Sun sensors are also used for solar array pointing – orienting photovoltaic energy collection systems (e.g. deployable CubeSat solar panels) for the most effective irradiance – and can also play a role in fail-safe recovery processes and gyroscope updating.
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 tunnelling 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 analog 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.
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:
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 analog 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.
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.
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 analog 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
In practice, digital sun sensors are currently a much smaller market segment than analog sensors.
There are various sun sensors on the market which are advertised as digital, but which are actually analog 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.
Understanding your requirements
We recommend a simple four-step approach for the preliminary selection of any new piece of hardware or software for a satellite or other space system.
Note that this is just a basic guide based on what we’ve learned helping hundreds of buyers select products within our marketplace and get rapid responses from suppliers.
It is just meant to help engineers make an initial assessment and shouldn’t replace formalised systems engineering approaches such as the INCOSE Model-Based Systems Engineering (MBSE) CubeSat frameworks.
- Specify your currently known mission parameters,
- Record all currently known overall design specifications of the system,
- Consider the range of technology that will be used in the satellite and in ancillary sub-systems, and
- Take into account the key performance criteria of the kind of product you wish to procure.
These criteria are explained in more detail below.
The first step is to fully understand the currently known mission parameters, including both the critical applications and desirable, but not necessarily essential, objectives.
Typically the more precise mission parameters will only be established later in the process – usually iterated upon in a number of loops by considering the “system of systems.”
But having an idea of what functions your selected technology is likely to need to perform, and on what schedule and duration, will make selecting the most suitable model mch easier.
Also consider the launch stresses, testing processes and regulatory compliance that the product will need to go through in order to make it into orbit, as well as any obsolescence procedures once the mission is complete.
Overall design specifications
Next, keep to hand all currently known design information about the entire unit.
This can include the volume, weight, primary structural material and more basic things such as the location, storage and transport arrangements of the major components.
You will need to make sure that the new piece of technology you choose will be suitable for these parameters.
Full range of technology
Once you are clear on exactly what tasks the new product will need to perform and the design characteristics of the satellite or other unit that it will work within, the next consideration is the full range of technology that will sit alongside the product to make sure that everything is compatible.
You may not yet know the entire range of accompanying technology (and you might need to first choose the product model you are interested in in order to make decisions on other components), but make sure you have access to all available technical specifications of sub-systems and structural components that are most likely to be used, as per the current mission plans.
It is important to understand how different sub-systems and components will interface with each other to create a high-performing satellite.
Balancing the available mass, power and volume budgets is also important, which can only be done with a clear plan of which components will be used.
Also consider how the product will work with the planned or existing ground segment to ensure effective data transfer and communication stability.
Now that you have a clear idea of what sort of product is needed for your mission, system, and existing platform setup, the next step is to compare the commercially-available products that meet these criteria according to the most relevant performance metrics.
Key performance criteria for sun sensors
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.
If you need more information on other smallsat and CubeSat sub-systems we have also put together roundups of several categories such as star trackers, on-board computers, and GPS receivers available on the global market.
Please click on any of the links or images below to find out more about the systems. You can also submit a request for a quote, documentation or further information on each of the products listed or send us a more general query to discuss your specific needs, and we will use our global networks of suppliers to find a system to meet your specifications.
The CubeSense S by CubeSpace
Offering Sun measurements with an accuracy level of <0.2° (3σ) over the whole FOV. The CubeSense S sensors are designed to be completely immune to albedo effects making them highly robust and versatile.
All the sensors are calibrated in CubeSpace’s state-of-the-art dark, clean, optics calibration room, using a sun-simulator and highly accurate rotation stage. The CubeSense product also has flight heritage for a wide variety of missions and orbits.
The SS200 by Hyperion Technologies
The SS200 is a lightweight (weighing just 3g) and low power sun sensor CubeSat system. 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 Fine Sun Sensor by Space Inventor
A small factor, two-axis, standalone fine sun sensor with two different estimate modes; table-based (from ground calibration) or polynomial fit (on-orbit or ground calibration). The sensor provides an estimate of sen sun vector and multiple measurements can be combined to increase the precision of the sun vector estimate.
Everything except the photodiodes are shielded in an aluminum casing and the CAN interface, together with Space Inventor’s parameter system, is designed to provide a simple to setup and intuitive user interface for the sun sensor. The sun vector, along with calibration parameters, raw values and intermediate calculations, are exposed by the parameter system.
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:
- 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.
- MASS-X – Integrated Digital Sensor – an integrated sensor series that includes one accelerometer, one magnetometer, and one sun sensor to measure the angle of sun-ray, DNI solar radiation and azimuth and elevation position of the device.
- ACSS – Advanced Coarse Sun Sensor on satsearch – offering 200 kRad (gamma) radiation tolerance and 18.3 g @ 20-2000 Hz random vibration. The ACSS includes an Analog, 15-pin micro connector electrical interface and is made from Aluminum 6082, alodine + black anodizing housing.
Sun sensors by Redwire Space
Redwire Space provides a variety of mission critical space solutions and components for space applications. This portfolio includes a range of sun sensors to suit different satellite form factors and use cases:
Coarse Analog Sun Sensor (CASS) – a 45g system that contains a 2-segment solar cell for attitude determination. Each cell segment generates a short circuit current (ISC) proportional to illumination intensity.
Digital Sun Sensor (±32°) – typically used for medium accuracy attitude determination, the DSS32 provides a large field of view with moderate resolution.
Digital Sun Sensor (±64°) – featuring 2 measurement axes and 5 sensors (up to 8 sensors available). The unit is radiation hard to 100 krad (Si), spaceflight proven, and AS9100D/ISO9001 certified.
Coarse Sun Sensor Pyramid – a 2-axis sensor containing 4 detectors, used for applications including solar array pointing, sun acquisition, and failsafe recovery.
Fine Sun Sensor (±50°) – used in attitude control applications where sub-arcminute accuracy and millidegree angular resolution are required. The system is designed in single or two-axis configurations and each sensor can be custom-designed to meet specific performance requirements for Field of View (FOV), accuracy, resolution, and other key parameters.
Micro Sun Sensor – the smallest sun sensor in Redwire’s portfolio. It provides a cosine output of the sun angle for small satellite applications and can be used for a variety of spacecraft missions requiring sun detection.
Miniature Spinning Sun Sensor (±87.5°) – designed to provide sun aspect angle and sun crossing pulses for spinning spacecraft. The information provided by the system is used to determine spin rate and spin axis orientation relative to the sun.
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 analog 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.
Cosine Sun Sensor – providing coarse information about the polar angle of the sun, derived from the fact that the sensor output varies approximately proportional to the Cosine function of the angle of incidence of sunlight. The sensor has a pointing accuracy typically in the order of ±3 degrees of arc and a minimum operational FOV of 160 degrees of arc full cone angle (±80°).
Redundant Cosine Sun Sensor (CoSS-R) – the design of the CoSS-R exploits the heritage of Bradford’s dual chip detectors while optimising the physical footprint and mass. The sensor has a pointing accuracy typically in the order of ±3 degrees of arc and a minimum operational FOV of 180 degrees of arc full cone angle (±90°).
Fine Sun Sensor (FSS) – an analog 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.
This nano Fine Sun Sensor (FSS) is small in size and requires a current of less than 5 mA. It senses the direction of sunlight in two vectors and provides a pointing knowledge up to 0.1 deg (3-sigma, no albedo). The sensor also features a high updating rate and can be operated as a course sun sensor if required.
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 sun sensor CubeSat and small satellites market. Building on Lens R&D’s BiSon64-ET development experience, the sensor is designed to combine a high radiation tolerance, as needed for interplanetary missions (tested to be > 9Mrad), with the low profile required for efficient mounting on CubeSats.
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 analog 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.
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.
A cost-efficient dual-axis instrument designed for precise sun pointing and tracking, alongside accurate attitude measurement, particularly for miniature satellites. The 5V (< 0.3W) device has a less than 0.5 degree field of view (FOV) accuracy over an FOV of -60 — +60 deg. It detects the incidence angle of solar rays on two orthogonal axes and features low mass (0.03 — 0.05 kg) and volume (48 x 21 x 36 mm), along with a long lifecycle (5 year operational lifetime).
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.
The Sputnix SXC-SD-01 is designed for use in nanosatellite missions. It features a CAN interface that provides single-cable connection and chains of sensors. The system weighs 10g and is a digital sun sensor with two orthogonal axes. It generates the directional cosines of the angles of the direction to the sun, so doesn’t require any calibration. The product is low-power, requiring just 15 mW, and is compatible with Sputnix solar panels.
The SX-SSM-01 is designed for use in orientation system circuits to determine the directional unit vector to the Sun. It has dimensions of 45 mm x 40 mm x 15 mm and a mass of 50 g. The system features a half-angled cone of 45° field of view (FOV) and an accuracy of 0.15°.
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.
Noticed that your product isn’t included in this article? Send us an email today today we’d be happy to work with you to showcase it to the satsearch community!