Beating the Atmosphere: High-speed Wavefront Sensing for Resolved Space Situational Awareness

The orbital environment has reached a critical density. To manage the exponentially growing threat of space debris, operators rely on Space Situational Awareness (SSA) for collision avoidance. However, current orbital propagation models face a fundamental data deficit. They often rely on Two-Line Element (TLE) sets.

A TLE is a standardised data format that provides a satellite’s orbital position and velocity at a specific point in time. While TLEs are sufficient for general tracking, they lack precision regarding an object’s physical characteristics.

Accurate orbit prediction requires precise modelling of non-gravitational forces, specifically atmospheric drag and solar radiation pressure [1]. These forces are non-linear and heavily dependent on the object’s size, shape, and orientation – parameters that are often unknown for debris or defunct satellite [2]. Without this “characterisation” data, the position of a satellite can drift significantly from its predicted path.

The Physics Problem: Why Adaptive Optics is Mandatory

Ground-based optical telescopes offer a solution for characterising these objects, but they are limited by the medium they look through: the Earth’s atmosphere. Even if these telescopes are usually located in altitude to avoid the most of atmospheric turbulences, they are still concerned by higher atmospheric layers.

The atmosphere is a dynamic, turbulent system. Variations in temperature and wind shear create constantly changing pockets of air with different refractive indices. As light from a satellite passes through these pockets, the wavefront is distorted.

For a large aperture telescope (e.g., 1 meter), this turbulence effectively destroys the instrument’s theoretical resolution. While the telescope should be “diffraction-limited” (resolution defined by λλ/DD ), the atmosphere limits it to the “seeing limit” (defined by the Fried parameter, rr₀ ) [1]. In practice, this reduces a complex, multi-panel satellite to a blurry, fluctuating blob, making identification impossible.

The Solution: Recovering the Diffraction Limit

Adaptive Optics (AO) systems solve this by measuring the wavefront distortion and correcting it thanks to deformable mirrors in real-time [2]. However, because the atmosphere changes milliseconds by millisecond, the AO system must operate at extreme speeds. This requires a closed loop running at kilohertz (kHz) rates to effectively “freeze” and correct the turbulence.

The Enabling Technology: OCAM2K as the WFS Engine

The performance of an adaptive optics system is strictly limited by the speed and sensitivity of its Wavefront Sensor (WFS) camera. In SSA applications, where targets are often fast-moving and faint (magnitude 10 or dimmer), the WFS camera must operate in a photon-starved regime without introducing latency or noise.

Leading research institutions, including the Australian National University (ANU) and the Korea Astronomy and Space Science Institute (KASI), have standardised on the Andor OCAM2K to solve this specific engineering challenge.

They leverage three specific OCAM2K capabilities to turn their telescopes into precision instruments:

• Speed for “freezing” turbulence: With a frame rate of 2,067 Hz, the OCAM2K exceeds the Greenwood frequency of the atmosphere, allowing the control loop to measure distortion before it changes [1] [2]. Greenwood frequency is defining the optimal correction for an AO system and takes into account wind speed and atmospheric
• Sensitivity for faint targets: Utilising the EMCCD technology, the camera achieves sub-electron readout noise (<0.3 e-). This allows for the tracking of faint debris that would be lost in the noise floor of standard CCD or CMOS sensors [2].
• Zero-latency feedback: With a latency of just 43 µm , the OCAM2K maximises the error rejection bandwidth, ensuring the Deformable Mirror correction is applied instantly [2].

Use case 1: KASI Geochang observatory

Benefit: Precision orbit prediction via sharper imaging

The Korea Astronomy and Space science Institute (KASI) developed an AO system for their 100cm telescope to image space objects at altitudes up to 1,000 km. Their primary goal was to determine the “size, shape, and orientation” of debris to improve ballistic coefficient estimation [2].

Using the OCAM2K to guide on the object itself (Natural Guide Star mode), KASI achieved measurable operational improvements.

Note on Natural Guide Star mode: Traditional AO often relies on a bright star nearby or a laser beacon to measure atmospheric distortion. However, in “Natural Guide Star” mode, the system uses the sunlight reflecting off the target satellite itself as the reference point. This allows the system to track objects even where no suitable background stars exist.

• 5x resolution improvement: The system improved the stellar profile sharpness (Full Width at Half Maximum) by a factor of 5 compared to uncorrected images [2].
• Diffraction-limited performance: The system achieved images near the diffraction limit of the telescope, turning a blur into a resolved point source [2].
• Operational impact: By resolving the geometry of debris, KASI can feed precise physical parameters into their dynamic models, significantly reducing the uncertainty in collision warnings [2].

Figure 1: Comparison of stellar object imaging [2]. (a) Without AO correction. (b) With AO correction enabled. The stellar object appears to be better resolved on the image thanks to the AO system.

Use case 2: ANU “AOI” system

Benefit: Object identification and characterisation

The Australian National University (ANU) deployed their “Adaptive Optics Imaging” (AOI) system on a 1.8m telescope to characterise LEO and GEO satellites. The ANU research team identified the OCAM2K as the “ideal choice” for the WFS, noting that “no other camera is capable of the same performance” [1].

The system proved its value by imaging the Cosmos 1656, a defunct Tselina-2 satellite:

• From “blob” to “structure”: In open loop, the satellite was unrecognisable. With the OCAM2K closing the loop, the system resolved distinct features, including the satellite’s body, gravity boom, and panel array [1].
• Precision tracking: The system reduced image jitter by a factor of 5 (from 43 to 0.08 arcseconds), enabling precise astrometric tracking of the object’s position [1].
• Enhanced post-processing: The high frame rate of the OCAM2K allowed the team to capture thousands of frames per second, enabling Multi-Frame Blind Deconvolution (MFBD). This computational technique further sharpened the image, allowing the team to measure the satellite’s panels to within metres [1].

Figure 2: Adaptive Optics Correction of the Cosmos 1656 satellite [1]. Left: Uncorrected open-loop image. Right: Closed-loop image using the OCAM2K-driven WFS, resolving the solar panel array and body structure.

Figure 3: Cosmos 1656 image: shape and angular/physical size analysis.

Conclusion

For Space Situational Awareness, the ability to resolve a satellite’s geometry is the difference between a generic alert and a precise collision avoidance manoeuvre. As demonstrated by the KASI and ANU deployments, the Andor OCAM2K is the enabling technology for high-bandwidth Adaptive Optics. Its unique combination of 2 kHz speed, negligible latency, and sub-electron noise allows SSA systems to beat the atmosphere, protecting critical space assets.

References

1. Copeland, “Satellite and Debris Characterisation with Adaptive Optics Imaging,” Ph.D thesis, Australian National University, Canberra, Australia, 2020.
2. -C. Lim et al., “Development of Adaptive Optics System for the Geochang 100 cm Telescope,” Journal of Space Technology and Applications, vol. 4, no. 3, pp. 185-198, 2024.