Space Situational Awareness

ssa_thumbThe overall objective of space situational awareness (SSA) is to know the location of every object orbiting the Earth, to know why it is there, what it is doing now, and predict what it will be doing in the future.It is the ability to track and understand what exactly is in orbit from either space or from the ground. This capability is needed to protect the extensive U.S. investment in space assets for weather, reconnaissance, navigation, and communications. These systems represent hundreds of billions of dollars worth of public and private investment and play a key role in the national economy, U.S. prosperity, and wealth creation.

Satellites from every nation naturally cluster in preferred orbits: Low Earth Orbit (LEO) for weather and reconnaissance, Medium Earth Orbit (MEO) for cellular telephone communication and navigation, Geostationary Orbit (GEO) for Global Positioning Systems (GPS) and communications, as well as Highly Elliptical Orbits (HEO) or Molniya Orbits for communications services and other uses at high northern latitudes. These preferred orbits are littered with spent rockets, dead satellites, and thousands of other bits of debris that are hazards to space operations. By charting and tracking, SSA helps protect space assets and ensure safe operations by providing warnings of potential hazards (natural or manmade, intentional or unintentional) in a manner timely enough to allow preventive actions to be taken.

The greatest challenge to SSA is the existence of totally unknown RSOs in space. These are natural objects like meteorites, debris from launch vehicles, or debris broken off from already orbiting assets. The proliferation of debris in space constitutes one of the primary threats to safe operation of spacecraft. This debris can range in size from the smallest particles to large objects such as rocket bodies. Given the high relative velocities (up to 16 km/sec) in potential collisions in space, even small debris (e.g., < 1cm diameter) can cause significant damage. Most of the 17,000 or so objects greater than 10 cm in diameter are regularly tracked and cataloged by the United States Air Force (USAF). However, there are more than 200,000 objects between 1 cm and 10 cm in diameter that remain largely untracked because of the difficulty in observing them. These objects propose a potential threat to U.S. space assets and the number continues to grow. Recent large jumps in debris have been caused by the destruction of the Chinese satellite in 2007, and the collision between the spent Russian Cosmos 2251 satellite and the Iridium 33 satellite in 2009. For safety and security, these objects must be detected, identified, and assessed without the benefit of any pre-conditional information such as cues or guidance.

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Figure 1. Categories of satellite Earth orbits.

The USAF currently tracks objects in space using a collection of ground-based telescopes and radars, one space-based sensor, and a control center in the Space Surveillance Network (SSN) that is maintained by U.S. Space Command. In general, ground-based radar is used to search and detect space objects because of their ability to perform a wide-area search over a large field of view with a single beam. Ground-based telescopes are used for space object characterization because objects beyond LEO are almost always illuminated by the sun and can be observed as long as the telescope is in the dark. The existing GEO search capability is provided by three Ground-Based-Optical Deep Space Surveillance (GEODSS) Systems. One drawback of telescopes is they are limited by the weather. Another drawback for both ground-based radar and telescopes is they lose track of objects that move temporarily out view, resulting in information voids that must be filled.

These limitations motivate the development of space-based observing systems for the more timely detection and characterization of smaller space objects. Until recently, the Space-Based Visible (SBV) sensor on the Midcourse Space Experiment (MSX) was a space-based asset that was used to detect RSOs. The success of the SBV sensor led to the development and planned deployment in 2010 of the Space Based Space Surveillance (SBSS) Pathfinder satellite, which also employs a visible band sensor for passive observation of RSOs. Success of such systems relies on their ability to detect, resolve, identify, and monitor dim RSOs for all possible threat scenarios. Space sensing, detection, and characterization of small space objects from space-based platforms, such as the SBSS satellite, may be enhanced by exploiting the natural illumination of space objects.

Solar illumination is a major source of natural illumination that contributes to the RSO signature, as it affects both the thermal signature through the heating of the object and the reflected signature. Other sources of natural light include thermal emissions from the Earth and clouds, emissions from the OH airglow layer, direct illumination from the sun, moon, planets, and stars, reflections of sun and moonlight from the Earth and clouds (Earthshine), and scattered atmospheric light. To a lesser extent, skyshine, the electromagnetic (EM) radiation emanating from celestial point sources (e.g., moon, planets, stars) other than the sun and diffuse sources (e.g., zodiacal light, mean stellar radiance, galactic radiance, extra-galactic radiance), also plays a role in the object signature.

To support the space-based space surveillance component of the SSA mission, CPI has developed a natural illumination model of the Earthshine and skyshine incident on RSOs to determine the extent to which natural sources illuminate the RSOs in the ultraviolet (UV), visible, and infrared (IR) regions of the EM spectrum. This provides the capability to predict the illumination of dim RSOs in non-sunlit scenarios such as when a satellite is in the Earth's shadow. This capability allows for the design of planned future sensor systems through modeling and simulation, enabling the more accurate assessment of sensor requirements for space-to-space and Earth-to-space sensing of dimly lit RSOs, as well as anticipated sensor detection thresholds.ssa2 figure


CPI has developed first-principles physics models to extend the applicability of the data to different wavebands, viewing geometries, and solar illumination conditions, where needed. Readily available real-world data from operational space-based sensors are used to generate realistic illumination conditions for specific viewing geometries. This illumination model is embedded into CPI's Shine architecture to facilitate the incorporation of fast running algorithms developed within the computer graphics community, and to take advantage of massively parallel computing hardware. The result is a high performance computational tool that can be incorporated into SSA simulations.