Ground-Based Telescopes for Space Situational Awareness

The Aerotel 1-meter f/1.75 Cassegrain gimbaled telescope provides passive optical tracking of resident space objects. In this image, the PF Cam instrument can be seen at the telescope’s prime focus (top center), where a secondary mirror would normally be.

Ground-Based Telescopes for Space Situational Awareness

By Gabe Spera

Historically, sensitive radar systems have done the work of tracking and characterizing resident space objects. Although these radars are effective, the data comes with inherent uncertainties. Researchers can supplement these systems with ground-based optical telescopes, passively observing and tracking space objects via reflected sunlight. Depending on how the data are processed, physical attributes can be ascertained beyond a simple orbit determination. For the most part, though, space situational awareness via optical telescopes has been a small niche field. The majority of large astronomical observatories lack the capability to track objects in low and medium Earth orbits, and this has limited their participation.

This situation is rapidly changing, due in part to the proliferation of automated systems, sophisticated extraction techniques, and purpose-built telescope drives. Even small-aperture telescopes can now track objects from relatively low elevations. Newer large-aperture survey telescopes, equipped with gigapixel CCD cameras, are capable of monitoring wide swatches of the night sky to search for changes. For example, the Pan-STARRS observatory at the University of Hawaii features a 1.8-meter telescope with a 1.4 billion pixel camera, and the future Large Synoptic Survey Telescope (LSST) will boast an 8.4-meter telescope with a 3.2 billion pixel camera. In nearly all cases though, such observations are limited to visible wavelengths.

Aerotel captured this image of three actively maintained geostationary satellites (center) with another satellite nearby (lower left). For this observation, the telescope was staring at one spot, with no tracking movement. Thus, the stationary satellites appear as dots, while the background stars, which are moving at the natural sidereal rate, appear as streaks.

Aerotel captured this image of three actively maintained geostationary satellites (center) with another satellite nearby (lower left). For this observation, the telescope was staring at one spot, with no tracking movement. Thus, the stationary satellites appear as dots, while the background stars, which are moving at the natural sidereal rate, appear as streaks.

To assess the implications of these developments for government customers, Edward Laag of the Advanced Sensor Engineering Dept. has been working with Aerotel, an observatory and telescope located on the roof of the Aerospace laboratories in El Segundo, California. “Aerotel is a fast-slewing, highly sensitive 1-meter Cassegrain telescope that can track objects in various orbits,” Laag says. “It can accommodate a variety of instruments, operating at both visible and near-infrared wavelengths. The system has been in operation for about two years and is actively enabling a variety of ongoing research projects.”

The main instrument for Aerotel is a multiband imaging system called PF Cam. Located at the Aerotel’s prime focus, it provides a flat, aberration-free image plane for the placement of various large-format square detector arrays and associated filters. The refractive correcting optics transmit wavelengths ranging from near-ultraviolet and visible to shortwave infrared while preserving the high sensitivity of the primary mirror. There are few reflective losses, and PF Cam is well suited for recording calibrated light curves.

PF Cam currently accepts two high-frame-rate cameras with complementary characteristics—the commercial PCO Edge and the Aerospace-made Angie II. Each uses its own five-position filter wheel. Both employ standard data-transfer links and supply images in a format ingestible by common data-processing routines. The two cameras cannot be operated simultaneously, but they can be quickly swapped—an unusual feature for a prime-focus instrument. This means a low-orbiting object making two sunlit passes on the same night can be observed with both cameras.

For visible light observations, the PCO Edge is preferred for its simplicity. It employs a science-grade CMOS detector, with low noise and quantum efficiencies above 50 percent. At the nominal rate of 30 hertz, the unit can be read out in full-frame mode by a standard desktop computer.

For shortwave infrared and certain visible observations, the Angie II camera may be used. This system incorporates a 2048 x 2048 array of 18-micron pixels., Angie II is a powerful infrared detector with a 1.1 degree field of view, but it is not restricted to infrared operation. It has high quantum efficiency that extends well into traditional V-band (550 nanometers), so it can be used for visible observations and produces high-quality wide-field images.

“Aerotel is a very modern Swiss-army-knife telescope,” Laag says. “It can be configured to operate in a manner similar to survey telescopes, but is not purpose-built like Pan-STARRS or LSST.”

“Going beyond simple imaging,” he says, “there are additional foci that allow more complex instruments to be operated. One in particular is the Aerospace-built VNIRIS spectrometer, which provides continuous wavelength coverage from 400 nanometers to 2.5 microns. When properly calibrated, it can detect certain material properties.”

Laag and other Aerospace researchers are using PF Cam to observe resident space objects with increasing efficiency. The observatory has fielded requests from internal customers as well as government agencies and even outside firms. For customer-requested observations, Aerotel accepts standard two- or three-line element sets or NORAD IDs. Aerospace’s Satellite Orbit Analysis Program (SOAP) can be used to check for suitable target passes beforehand. Over the course of a night, Aerotel can easily generate 100 gigabytes of raw data. “Not long ago, this would have been considered a problem, but today, it is no longer significant,” Laag says. Aerotel data is generally stored on a server that has special access privileges. Depending on the customer, it is internally processed and delivered. Laag expects that in the future, data will be pushed automatically to a temporary site so that customers can retrieve it wherever or whenever they need.

“Having Aerotel on site has created countless benefits, but ultimately, it is not a replacement for traditional mountaintop observing,” Laag notes. “Lessons learned from Aerotel will be applied directly to instruments fielded at more pristine sites around the globe. The plan is to continue to streamline operations until the system can operate virtually autonomously.”

Looking forward, Aerotel observations will help improve space situational awareness and overall mission safety, particularly when supported by Aerospace orbital analysis expertise. The utility of Aerotel can be extended further by coordinating with other telescope facilities such as the newer survey telescopes, other Aerospace assets, space systems, or even sensors not yet imagined, Laag says. In fact, the limiting factor is not the technology itself, but the processors and algorithms needed to parse through the enormous volumes of data produced.

Related publication:  Crosslink, Fall 2015, Understanding Space Debris