Although the Intelligence Advanced Research Projects Activity (IARPA; Riverdale Park, MD) agency is aware of the technical ability of ground-based interferometers such as the existing Navy Prototype Optical Interferometer (NPOI) and the under-construction (first fringes in 2019) Magdalena Ridge Observatory Interferometer (MROI) to observe geostationary earth-orbiting (GEO) satellites, it has launched the Amon-Hen program to solicit designs for a less-expensive interferometer system to improve space situational awareness and accountability as the deployment of these GEO satellites increases worldwide (see figure).1
Operating at altitudes of more than 35,700 km, GEO objects are too small to be imaged in detail by ground-based telescopes. And while ground-based interferometers can easily resolve objects to an accuracy of a few thousandths of a second of arc, the dimness (GEO objects are 100X fainter than what can be resolved by existing systems) reduces the signal-to-noise ratio of the observed interference fringes, increasing the required mirror sizes and arm separations and, consequently, driving up overall system size and cost up to $1 billion.
NPOI and MROI framework
Ground-based interferometers direct the light from a faint object in the sky to several mirrors spatially separated by tens- to hundreds of meter-long arm lengths. These images interfere at a central detector, producing fringes that dictate the physical shape of an object and its location in the sky.
The existing NPOI site uses an array of 12 cm apertures located at six different imaging stations that can be moved along the three 250 m array arms of the system, with the current 98 m baseline enabling around 1 milliarcsecond (mas) resolution (a 432 m baseline capability will be commissioned next year). Alternatively, the MROI will be a 10-mirror imaging site operating from 0.6 to 2.4 μm with a 347 m baseline, enabling 0.3 mas resolution—more than 100X that of the Hubble Space Telescope.
While NPOI and MRO (the telescope only) have performed limited research on GEO satellite imaging that is published, the results are only the very first steps on the long road of science and technology development required to measure enough fringes to generate actual images. Amon-Hen seeks to develop new approaches to make interferometers that can make more fringe measurements at lower overall system cost.
"DARPA sent out a similar proposal for GEO satellite interferometry called Galileo in 2012 and selected Lockheed Martin for phase I of the project," says Michelle Creech-Eakman, project scientist at MROI and physics professor at New Mexico Institute of Mining and Technology (Socorro, NM). "While some of the goals were achieved, DARPA canceled phase II in recognition of the tremendous challenges this project presents."
More compact, less expensive
Using longer baselines for higher-resolution imaging, or possibly more and smaller apertures, cost reduction is critical for the Amon-Hen program, which targets an interferometer system cost of less than $25 million in a footprint smaller than existing ground-based interferometry sites.
The proposed interferometer must achieve a 12.5 nanoradian angular resolution with image interpretability for GEO objects of magnitude 11 or even dimmer. The mirror aperture size is flexible, but desired to be no more than 2 m for any one aperture, and the design should have a larger number of apertures to reduce or eliminate the need for guide stars/adaptive optics. Data must be gathered in a one-hour timeframe and processed within 24 hours.
The 33-month program began in August 2017 and has received proposals from AMP Research (Naples, FL), AOSense (Sunnyvale, CA), Composite Mirror Applications and GEOST (both in Tucson, AZ), Integrity Applications (aia; Kihei, HI), Lowell Observatory, and others.
1. See https://goo.gl/hTgHth.