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Space


Space Surveillance - Overview

The United States has deployed a wide range of systems for monitoring the space activities of other countries.(1) For the most part, the primary mission of these sensors has been to provide warning of strategic missile attack. But the growing number of satellites in orbit has increased the requirement to keep track of new launches and impending decays of satellites, in order to avoid confusing these events with hostile missile launches. In addition, the increasing importance of military space operations has made the tracking and characterization of space systems a significant mission in its own right.

Debris orbiting around the earth increasingly is a concern to all space operations due to potential collisions with existing or new space vehicles. Low inclination, low orbiting objects (LILO) are of particular threat because current ground detection systems don't adequately address them, either due to the location of the ground detection system or due to the mode of operation of the ground detection system. Since all assets pass through this `debris field` periodically, attention to these objects is paramount. Because of the orbital speeds and the energy involved, an object only 10 cm long could destroy a satellite, while an object only 1 cm could disable a spacecraft. Smaller objects could also cause significant damage to spacecraft, including disabling vital on-board systems.

NASA has estimated that more than 20,000 objects currently orbiting around Earth are larger than a softball, more than 500,000 objects currently orbiting around Earth are larger than a marble, and that million more objects are currently orbiting around Earth that are so small they cannot be accurately tracked. Of the objects orbiting around Earth, the greatest concentration of orbital debris is thought to be around 800-850 km from Earth, with most objects orbiting within 2000 km from the Earth. This is the Low Earth Orbit (LEO) range.

The number of orbiting objects makes it difficult to catalog and to track the debris. Additionally, the amount of orbital debris is expected to increase over the next few years and decades.

Satellite tracking systems, both optical and radar, are among the most sophisticated and expensive military sensor technologies. Spacetrack radars typically have ranges and sensitivities ten to a hundred times greater than radars for tracking aircraft or surface targets. And optical tracking systems use telescopes that rival all but the largest civilian astronomical observatories. A modest satellite tracking radar or telescope typically costs a few tens of millions of dollars, while the more elaborate radars can cost well in excess of $100 million.

The earliest, and still the least expensive, form of satellite tracking systems rely on sun light reflected off a spacecraft. Visible against the pre-dawn or post-dusk sky, the largest low orbiting spacecraft, such as space stations or imaging intelligence satellites, are of magnitude 0, comparable to the brighter stars in the sky, and many other low-orbiting satellites are visible to the unaided observer.(2) Even satellites at geosynchronous altitudes are visible with relative modest optics, under optimal lighting conditions.(3)

The capabilities of telescopes to observe satellites is primarily a function of the aperture of the primary optical surface of the telescope, as well as the properties of the means used to form the image. Telescopes with mirrors up to four meters in diameter have been used for satellite tracking, while telescopes with meters in excess of eight meters in diameter are used for astronomical applications. Initially, satellite tracking cameras used film systems, but more recently electronic charge-coupled devices (CCDs) have replaced film systems. CCDs provide an instantaneous read-out of the image, avoiding the time-consuming processing required by film systems. These electronic cameras have enabled scientific telescopes of modest apertures of a few meters to obtain recognizable images of large spacecraft in low orbits.(4)

The primary limitation on the resolution of ground-based optical sensors is the turbulence of the Earth's atmosphere. Recently, two new techniques have been introduced to overcome these limitations. Speckle imaging techniques take advantage of the short exposure time of CCDs to produce images of targets with exposure times that are shorter than the time scale of the fluctuations in the Earth's atmosphere, effectively freezing the effects of atmospheric turbulence. Electronically superimposing a number of such images produces a picture of a satellite whose resolution is limited by the capabilities of the telescope itself.(5)

Several other developments in recent years have opened the prospect for greatly improved optical imaging capabilities at significantly reduced costs. New techniques for casting thin mirrors have led to a revolution in optical astronomy, with monolithic mirrors as large as eight meters being produced at significantly lower cost than the four meter mirror that were previously the astronomical standard. Improved construction and control techniques have permitted fabrication of single-aperture telescopes with apertures of up to ten meters. And new aperture synthesis signal processing techniques have permitted the combination of images from multiple apertures to form images that are the equivalent of telescopes with apertures of many dozens of meters.

Although most optical sensors rely on reflected sunlight or emitted infrared energy for satellite tracking, active optical sensors are finding increasingly application. By illuminating a target with coherent laser radiation, these systems can image satellites that are not illuminated by sunlight at night, as well as targets that may be obscured by sky-glow during daylight hours. The use of active illumination also permits direct measurement of the range to the target, as well as facilitating characterization of the satellite's structure.

Ground-based radar systems have been used since the late 1950s to track civilian and military satellites.(6) Radars have several advantages over optical tracking systems, including the ability to observe targets 24 hours a day, and during cloudy or overcast conditions. Today the United States and the Commonwealth of Independent States both deploy extensive networks of radars which perform the satellite tracking function, as well as other duties, such as detection of missile attack. The performance of a radar is a function of the range to the target and the target's size or radar cross-section, as well as the radar's transmitting frequency and power, and the diameter of the transmitting antenna. Radars used for the initial detection of targets typically are able to locate an object with an accuracy of about 1,000 meters, while tracking radars have accuracies of from 10 to 300 meters.(7)

As radar technology has advanced, the problem has taken on a new dimension. Today's modern and sophisticated large phased array radars (LPARs) can serve many functions. They can provide early warning of missile or bomber attack. LPARs can track satellites and other objects in space and observe missile tests to obtain information for monitoring purposes. They are also an essential component of present generation ABM systems, providing initial warning of an attack and battle management support, distinguishing RVs from decoys, and guiding interceptors to their targets. In some cases, distinguishing an LPAR designed for one of these functions from one designed for an ABM role can be rather difficult.

In fiscal year 1976, the Space Infrared Sensor Program and the early phases of the SBSS Program were initiated. During its conceptual phase, SBSS had been referred to as Deep Space Surveillance Satellite or Low Altitude Surveillance Satellite.(28)

The 1977 Hysat Study, a part of the Deep Space Surveillance System program (DSSS), was sponsored by the USAF Space & Missile Systems Organization. Fairchild investigated the applicability of nuclear radioisotope heat sources for this mission. The rather sizable electrical power requirement (1500-3500 watts (e)) is provided by rollup solar arrays, alongside or atop the spacecraft, and attached to the upper body.(29)

The Space Based Surveillance System (SBSS) concept, which called for the deployment of four satellites in equatorial orbits at an altitude of 1100 kilometers, with the possibility of additional satellites in inclined orbits for polar coverage. The satellites were to be launched by the Shuttle using the Inertial Upper Stage, and have a design life of five years.

Although most optical sensors rely on reflected sunlight or emitted infrared energy for satellite tracking, active optical sensors are finding increasingly application. By illuminating a target with coherent laser radiation, these systems can image satellites that are not illuminated by sunlight at night, as well as targets that may be obscured by sky-glow during daylight hours. The use of active illumination also permits direct measurement of the range to the target, as well as facilitating characterization of the satellite's structure.

American systems of this type include the Teal Amber laser radar at the Malabar Optics Laboratory in Florida, which has a total of three optical tracking receiver systems, and eight laser transmitters.(30) Others include the LARIAT (Laser Radar Intelligence Acquisition Technology) system at Cloudcroft, Arizona, and the 60 centimeter aperture Teal Blue laser radar is operational at the AMOS facility on Mt. Haleakala in Hawaii.

The Maui Optical Tracking and Identification Facility (MOTIF) is located at the Air Force Maui Optical Site (AMOS) on Mount Haleakala in Hawaii. MOTIF includes a pair of 1.2 meter surveillance and tracking visible light and infrared telescopes, which operate at ranges of over 35,000 kilometers.(31) AMOS is host to one of the operational GEODSS stations. In addition, a 1.6 meter aperture telescope is used to provide 0.3 meter resolution images of satellites at ranges of over 750 kilometers, with tracking capabilities up to 35,000 kilometers, using reflected visible light and infrared.(32)

AMOS upgrades include installation of a 4 meter telescope, designated Advanced Electro-Optical System (AEOS).(33) Advanced research is currently under way that could eventually lead to an even more capable system, applying synthetic aperture techniques to combine the images from nine 2-meter diameter telescopes to provide images equivalent to those of a 12-meter telescope, at a cost of about $20 million.(34)

Space Surveillance Network Radar Sensors and Field of View at 500 km Altitude
Radar

Space Surveillance Network Optical Sensors and Field of View at 500 km Altitude
Optical

While assured access is a priority, space situational awareness (SSA) underpins all that DOD does in space from launch to disposal and supports the protection of critical space assets upon which national leadership, warfighters and civil and commercial space operators depend. The Air Force developed a foundational SSA architecture that will afford the best mix of near earth and deep space sensors, providing quality information to decision makers. While the Air Force routinely tracked some 23,000 objects at the Joint Space Operations Center (JSpOC), DOD sensors are unable to detect and reliably track what are estimated to be more than 500,000 man-made objects in orbit today.

Currently, SSA sensors are tracking where objects should be. Space domain awareness is the next evolution, facilitated by the JSpOC Mission System (JMS), and will allow knowing where objects are, when they move unexpectedly, and provide the data for the Commander, Joint Functional Component Command for Space (JFCC-Space). This will allow and JFCC-Space forces to respond appropriately.

Numerous types of systems exist or are proposed for the purpose of space object detection and characterization. The proposed systems include ground-based radar, ground-based phased array radar, ground-based optical detection, space-based optical detection, space-based IR detection, and space-based radar. Currently, there are three main Space Situational Awareness (SSA) systems. These include the U.S. Space Surveillance Network (SSN), the European Space Agency's (ESA) Space Situation Awareness Program (Space Surveillance and Tracking--SST), and the International Scientific Optical Network (ISON). Additionally, nations interested or active within space exploration, such as Russia, China, Norway, Australia, India, Japan, South Africa and the UK have or plan to have equipment for surveilling space. The Space Data Association (SDA) includes all the major satellite communications companies, which typically utilize data from SSA systems to protect their assets.

The SSN and SST both comprise two main parts: surveillance and tracking. The surveillance segment monitors a large area of the sky (creates a "fence") and passively waits for objects to pass through that area of the sky (i.e. cross the fence). The tracking segment has a very small field of view and is an active system. Data (orbit parameters) from the surveillance segment are refined by the tracking segment if the rough estimate is accurate enough for the tracker to find it in its small beam. In June of 2014, the U.S. Air Force awarded Lockheed Martin (L M) a $914M contract for a ground-based radar system called "Space Fence". The Space Fence is an upgrade to the existing SSN and consists of two S-band phased arrays.

Each of the existing or proposed SSA systems has drawbacks. Most drawbacks are related to the limited data acquisition abilities of the systems, the inability of surveillance systems to attain precision orbit parameters of newly detected objects, the lack of 24/7 availability (e.g. ground optical systems), and the system costs and complexity. The principal limitation of all present methods is the perceived need to comprehensively search all space, since an arbitrary object at an arbitrary time could be almost anywhere in the sky. Larger aperture sensors, while providing greater sensitivity, narrow the field of view and enlarge the space to be searched. It would be useful to develop a system and method for detecting and determining the orbit for earth orbiting objects that is relatively simple, that is highly accurate, that limits a search to a portion of available space, and that is able to quickly detect and catalog earth orbiting objects.

The Space Fence ground sensor replaces the already retired Air Force Space Surveillance System and was expected to greatly increase the ability to understand the battlespace and inform warfighter decisions. The increased Space Fence sensitivity, coupled with the increased computing capabilities of JMS, will yield a greater understanding of the space operating environment and associated threats, while increasing our knowledge on over one-hundred thousand objects including debris, active and inactive satellites, and the international space station. The uncued nature of the Space Fence greatly increase the opportunity to discover satellite breakups, collisions, or unexpected satellite maneuvers. The Air Force awarded the Space Fence contract to Lockheed Martin in June 2014, with a current projected initial operating capability in the second quarter of FY19.

The Space Fence will be the most significant improvement in near Earth SSA capability in nearly 50 years. It will work in conjunction with the JSpOC and the rest of the Space Surveillance Network to provide an integrated picture of the space operating environment for the warfighter. The delivery of the Kwajalein radar in 2019 will give JFCC-Space nearly complete coverage for detection of near Earth objects as well as improved ability to detect unforeseen or unannounced space events. The Space Fence will not solve all the near Earth needs alone, but will operate in conjunction with the legacy missile warning radars and other space surveillance network sensors.

References

1. Jasani, Buphendra, "Military Space Activities," Stockholm International Peace Research Institute Yearbook - 1978, (Taylor and Francis, London, 1978).

DeVere, G.T., and Johnson, N.L., "The NORAD Space Network," Spaceflight, July 1985, vol. 27, pages 306-309.

North American Aerospace Defense Command, "The NORAD Space Detection and Tracking System," factsheet, 20 August 1982.

2. King-Hele, Desmond, Observing Earth Satellites, (Macmillan, London, 1983).

3. Manly, Peter, "Television in Amateur Astronomy," Astronomy, December 1984, page 35-37.

4. The 2.3 meter telescope at Kitt Peak, Arizona has been used to produce images of the Hubble Space Telescope (McCaughrean, Mark, "Infrared Astronomy: Pixels to Spare," Sky & Telescope, July 1991, pages 31-35) and the Mir space station ("Satellite Trackers Bag Soviet Space Station, Sky & Telescope, December 1987, page 580).

5. George, E.V., "Diffraction-Limited Imaging of Earth Satellites," Energy & Technology Review, August 1991, page 29.

6. Jackson, P., "Space Surveillance Satellite Catalog Maintenance," AIAA Paper 90-1339, 16 April 1990.

7. Thomas, Paul, "Space Traffic Surveillance," Space/Aeronautics, November 1967, pages 75-86.

28. U.S. General Accounting Office. DOD Acquisition: Case Study of the Air Force Space Based Space Surveillance System. (Report to Congressional Requesters), July 31, 1986, page 2.

29. Bernard Raab, "Nuclear-Powered Infrared Surveillance Satellite Study," Inter-society Energy Conversion Engineering Conference, 1977, Fairchild Space & Electronics Company, Germantown, Maryland.

30. "Requests for Proposals - Air Force Space Technology Center," SDI Monitor, 25 May 1990, page 125.

31. Kiernan, Vincent, "Air Force Begins Upgrades to Satellite Scanning Telescope," Space News, 23 July 1990, page 8.

32. Covault, Craig, "Maui Optical Station Photographs External Tank Reentry, Breakup," Aviation Week & Space Technology, 11 June 1990, page 52-53.

33. Congressional Record, 3 August 1990, page S 12318.

34. "LLNL Space Imaging Tests Slated for Maui Telescope," Space News, 19 February 1990, page 12.

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