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Parallel Design Approach Shortens Concept-to-Product for SmallSats

Parallel Design Approach Shortens Concept-to-Product Timeline for Small Satellites

Introduction

There's a revolution going on in low Earth orbit, as nano- micro-, and minisatellites take over the industry. These smaller less complicated designs are helping to slash the cost of traditional space craft and making new application technologies and research possible. These developments are also changing how the satellites and their payloads are designed and manufactured, with development cycles now measured in months rather than years.

To keep pace with these accelerated schedules, the optical filters that are an essential element of most instruments can no longer be added at the end of the design cycle. This article introduces several practical strategies for designing filter elements in parallel with the rest of the optical chain, resulting in an accelerated design cycle and reduced overall costs.

Background

Nanosatellites (1-10 kg) are normally built with low-cost, commercially available subsystem components, and were initially developed for use by universities and amateur space enthusiasts. Many of these vehicles are classified as "CubeSats," based on standardized form factors consisting of one or more 10 cm cubes. A fully functional CubeSat can be built with off-the-shelf components for as little as $50,000. For larger, more complex payloads, microsatellites (10-100kg), and minisatellites (100-180 kg) are also available for a fraction of the cost of a traditional spacecraft.

There is a growing consensus within the U.S. military that a large group of small satellites will prove more cost-effective and less vulnerable to attack than the small number of large, costly satellites that currently perform most national security missions. In a Report published by the Center for New American Security, the authors envision applications that include swarms of satellites that look for the telltale exhaust plumes of ballistic missile launches and low-earth hyperspectral imagers that can deploy quickly and provide high-frequency imaging of geopolitically sensitive regions. Civilian applications such as atmospheric and ground environmental monitoring already benefit from lower cost, easier to launch designs. For example, Planet (https://www.planet.com), has pioneered the use of microsatellites for commercial earth observation services. It maintains a fleet of over 130 mass-produced 3 liter CubeSats in a 475 km orbit. Each spacecraft is fitted with multi-spectral imagers that deliver high-definition images for a wide range of applications.

Small Satellites, Big Challenges

Advances in microelectronics, detectors, and other key technologies make it possible to produce compact spectrometers, imagers, radiometers, and other types of sensing devices at much lower costs.

On the up side for designers, the growing availability of standardized sensors, logic and control systems make it possible to acquire the necessary components in a matter of months rather than years. On the down side, the dimensions and mass of off-the-shelf components are fixed. Designers must work within the tight weight and space constraints afforded by the smallsat. Finally, engineers are being challenged by ever shrinking budgets and design/build cycle times.

Although no single strategy can solve these problems, carefully coordinated co-development of the optics, and their associated filters, can reduce the number of design iterations and the attendant costs required to produce a fully functional, space-rated product.

Aerospace Optical Filters 101

Optical filters are used in space-borne sensing and imaging instruments to limit the amount of light hitting their detectors outside the experiment's specific bands of interest. This is necessary because most detectors are inherently responsive across a relatively wide frequency range. For example, one of the most common instruments carried by smallsats are uncooled bolometers that use detectors made of polySiGe, TiNi, LSMO/CTO, and other materials to measure energy in the 7.5 to 14.5 mm region1. To sense shorter wavelengths, detectors made from mercury cadmium telluride (HgxCd1-xTe) can be used into the 5 mm region2.  

Since a sensor's resolution and overall performance can be affected by the signal level it receives, modern filters are designed to minimize losses. Modern materials and manufacturing techniques make it possible for many filters to allow 90% or more of the desired spectra to pass while restricting frequencies outside those bands. High out-of-band blocking, referred to as optical density, helps block unwanted spectra, eliminate stray light and limits the potential for bogus data.

In addition, anti-reflective (AR) coatings can be applied to the sensor itself and/or at the front of the optical chain to maximize the amount of light collected. Anti-reflection coatings can reduce reflection to less than 1% per surface in their targeted spectral band. This level of performance may be required for the system to achieve its desired sensitivity3,4,5.

 

Addressing Optical Filter-Related Issues that can Impact Optics Development

At present, many optics chains are developed with little thought to the optical filters that are required until late in the design cycle. Knowing ahead of time the practical limits of optical filter performance and manufacturability can prevent unnecessary design iterations along with hits to the budget and schedule. Filter-related requirements that can impact your design are myriad, but in most cases, simple adjustments to the design flow of the optics chain can mitigate or eliminate potential problems altogether. Some of the most common issues include:

  • Multiband/bandwidth specifications – Requiring a single filter to address multiple spectra or block across too wide of a range can result in longer development times and unnecessarily high manufacturing costs. For example, achieving OD3 and OD4 blocking is reasonable in the MIR and FIR regions, but requiring OD5, or higher can make your filter very difficult to manufacture.  Increased thickness in the coating and other factors can amplify the mechanical and thermal stresses during the cutting process, making them much more susceptible to delamination.

In addition, single filters capable of more than two bandpass regions or of blocking extremely wide spectral bands are difficult to achieve due to the limits of the frequency response and index of refraction currently achievable from available coating materials and the interactions that can result when they are combined. 

Fortunately, these, and other related issues, can be avoided by understanding the true requirements of your system and planning around them. Often several simpler, more easily manufactured filters can be easily designed into the optics chain if planned for early in the process.

  • Manufacturability – Many factors affect a filter's manufacturability. The most common cut substrates are diameters and squares. Designing your system to require specialty shaping can greatly increase your cost or make your filter unmanufacturable. For example, the mechanical characteristics of some substrates may limit the shapes that can be easily cut. Sapphire substrates are extremely hard making shaping after the coating almost impossible. And, as we discussed earlier in this article, special care must be taken when machining coated surfaces to avoid damaging the microns-thin filter layer.

Fortunately, early definition of your filter requirements can give your optical filter manufacturer the time to identify the options from thousands of available substrate/coating combinations that offer the best possible trade-offs between performance, manufacturability, and cost. 

  • Temperature compatibility – It's important to select a substrate/coating combination that exhibits acceptable mechanical behavior (expansion/contraction) across the instrument's full temperature range. Temperature variations can also affect a filter's response, which tends to shift down at lower temperatures.

In both cases, an early review of your application's requirements with an optical design engineer will help you find the best solution to meet them and, in some cases, provide guidance for revising the instrument's thermal design to support a narrower temperature range..

  • Angle (FoV) effects - Variations in an optical chain's frequency response across the span of its aperture tend to grow larger as its FoV grows wider. In designs where a wide FoV is required, angle sensitivity can be mitigated by placing spacers with a high index of refraction in the optical chain. It should be noted that some bandwidths are more difficult to equalize and require special attention. But, regardless of the spectrum(s) of interest, early analysis of the impact of FoV effects makes it easier and more cost-effective to plan where and how the spacers will be used.

Co-development Strategies

As we've seen, co-development of your instrument's optical filters with the rest of the optical chain can help you maintain or accelerate your development schedule, prevent cost overruns, and greatly reduce the potential for project failure. Here are a few strategies to follow when planning a co-development effort for your next project.

  • Practice early engagement - Getting your optics vendor involved in the development as early as possible can eliminate many of the problems that require extensive revisions late in the design cycle, where their impact on the schedule and budget is amplified.
  • Actively manage filter requirements –working closely with your designated filter vendor during the requirements development phase can help you avoid two of the most common pitfalls that occur in developing sensor/imager optical chains.
    • It's important to understand the limits of filter technology and apply them to how you specify the functionality for each element in your system. Although spreading your requirements across several elements may raise the potential increased signal losses, exploring trade-offs with a filter specialist can help minimize or eliminate the impact on your experiment.
    • Be careful if you decide to make the filter elements compensate for shortcomings in other parts of your optical chain or the sensor it feeds. In some cases, filters are an economical way to help compensate for the unwanted characteristics of other elements in your design, But, in other cases, the added requirements may cause unanticipated impacts to performance, cost, or schedule. A clear examination of this strategy's benefits and costs at the top of the design cycle is an easy and highly effective way to identify and manage these risks.
  • Actively manage lead times - Some materials, components, and processes used to produce filters are available from a very limited number of vendors, making them subject to factors that result in unexpectedly long lead times. Specifying your requirements and reviewing them with a trusted vendor as early as possible can help you anticipate these delays and, in some cases, avoid them by using viable alternatives.
  • Benefit from others' experience – It's important to make sure that the vendor you choose for each element in the optical chain (including filters) has the expertise to both respond effectively to your directions, but also provide guidance to the project that can be used to optimize your Instrument's design and its manufacture.

 

Conclusions

As smallsats continue to enjoy increased popularity, extensive use of standardized construction is helping to keep these satellites ready for rapid launch times. This is not necessarily true for their payloads, which will need to be produced more quickly than ever while meeting increasingly demanding performance requirements. Incorporating co-design practices for two or more interdependent system elements can help identify potential problems early in the development cycle and make it easier to find an optimal solution that reduces impact on schedule and budget.

Andover Corporation is an ISO 9001 - AS 9100 – ITAR qualified optics manufacturer specializing in standard and custom high-precision filters and coatings for scientific, military, and aerospace applications, including spacecraft. We take pride in our collaborative approach to optical design, which helps our customers turn their concepts into successful products, on-time and on budget.

If you have questions about a new project or have problems with an existing one, we invite you to contact us for a consultation. You can call us at (888) 893-9992, or internationally at +01 (603) 893-6888, or leave us a message at our contact page https://www.andovercorp.com/contact/

 

References:

https://www.edmundoptics.com/knowledge-center/application-notes/optics/optical-filters/

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