ESSAR enabling technologies:
ESSAR differs from traditional radar arrays in that coherent transmit from widely-spaced sensors is an inherent feature versus one location transmitting and multiple receiving. The array creates a smaller beam on the object and thus higher resolution AOA as well as enabling higher system sensitivity and operational flexibility. Note that the sensors could be phased arrays themselves; this technology is not limited to reflector antennas.
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An uncued, directed search of the sky for earth-orbiting objects is typically a long and difficult process whether undertaken from ground or space based locations. Obviously if the object is completely unknown, it could appear anywhere in the sky at a given time, or for that matter not at all if it is in the hemisphere of space immediately hidden from view.
Our approach rapidly and cost effectively discovers and catalogs unknown objects in earth orbit. The approach consists of an Equatorial system optimized by horizon pointing with specialized hardware and advanced algorithms.
While the method applies in principle to all orbits, it is of particular interest for LEO/MEO objects usually referred to as “space junk”.
Our system offers advantages over current radar, optical, and laser ranging systems. Existing systems stare straight up into space missing low altitude, low inclination objects. A radar system on the equator pointing towards the horizon (west or east) will eventually (~ every 50 days) ‘see’ propagating objects passing through the equatorial plane.
Other advantages come with a particular instantiation of our system: a ground-based radar array. Unlike optical systems, it can operate 24/7 rain or shine, day or night. An array of widely-space antennas coherently combined to produce a phased array can overcome the other limitation of a single antenna; the amount of power it can safely transmit. This is because the power is distributed among the elements. This mitigates the 1/R4 signal power loss that radar suffers from between signal transmission, reflection, and reception.
Primary benefits: Centimeter size objects detected and characterized at altitudes dependent on the number of dishes in the array
Performance of a system at 7 degrees latitude, looking toward the west
Performance of a non-equatorial system located at 7° latitude is given below. Except for system location, parameters are the same as used for the above calculations. The antenna system was looking due west in the analysis above. LILO targets cannot be seen at this facility with this pointing, although higher altitude targets begin to be visible. As altitude increases, even targets with very low inclination become visible.
The performance advantages are illustrated in the following plots. Analytical Graphics’ Systems Toolkit (STK) was run in the Analyzer mode for a random sampling of satellite orbits ranging in altitude from 160 km to 20,000 km with inclinations ranging from zero to 20° and including both prograde and retrograde motion. The total time for each object within the radar beam was determined for a time span of one year as well as other parameters such as slant range, range rate, and line-of-sight angles. These calculations were repeated for different ground station locations and antenna pointing variations, particularly towards the Equatorial plane and looking vertically into Space. The results from Analyzer were processed and plotted using Mathworks’ Matlab.
Performance of a system at 0 degrees latitude, looking toward the west
Results for our preferred location on the equator and preferred staring antenna direction, toward the western horizon, are given in below. A 12m reflector antenna was assumed, as was a frequency of 2 GHz. The color scale at the right of the figure is in minutes, with the darkest red being about 80 minutes of visibility over a year’s time. Although data for all plots was clipped to 80 minutes for consistency of the colors across coverage maps and ease of comparison (ample time to detect and characterize an object), the actual maximum time for the year is shown in the title bar. Blue colors depict visibility time up to 10 minutes and Black depicts zero coverage.
Object altitude is graphed along the y-axis, ordinate, in this case ranging from160 km to 20,000 km while the object’s inclination is graphed on the abscissa, x-axis, ranging from zero to 20°.
These results show that the best performance (Red) is obtained for low inclination objects, since an object’s transverse velocity, a function of the inclination, carries the object out of the beam faster than if the orbit were not inclined.
Performance of a system at 7 degrees latitude, looking toward the Equator
System performance was also evaluated for this 7 degree latitude system but this time pointing back towards the equator. The lack of LILO visibility with a non-equatorial location can be mitigated somewhat by pointing the antenna back toward the equatorial plane (again, at low elevation) rather than due westward. This will of course, lead to blindness for high orbit, low inclination objects, but it may be an acceptable compromise for some applications.
Example: Our basic approach is a monostatic radar array of large reflector antennas located on the equator and staring toward the horizon. Optional multistatic remote receiving facilities can enhance performance. The monostatic design consists of at least three widely spaced large reflector antennas (e.g., 12m). Despite the very narrow beam, low elevation staring intercepts objects at greater range, resulting in a much bigger spot size than overhead, in contrast to Space Fence.
Performance results: Left: Antenna system is at 7 degrees latitude looking west with elevation 5 degrees from Earth surface. Colors represent amount of time (minutes in one year of data) the space object is visible from Equatorial System. Y-axis is object altitude (160-20,000km). X-axis is object inclination (0-20). Right: Focus is on object altitudes of 160 – 1000 km.
Performance results: Left: Antenna system is on the equator pointing West with elevation 5 degrees from Earth surface. Colors represent amount of time (minutes in one year of data) the space object is visible from Equatorial System. Y-axis is object altitude (160 - 20,000 km). X-axis is object inclination (0-20).
Right: Focus is on object altitudes of 160 – 1000 km.
Definition: A radar array of ground reflectors located near the Equator and pointing towards the horizon for the detection and one-pass orbit characterization of all  Earth-propagating objects.
 Marble-sized LEO objects with a minimal three antenna system; expandable for extended range or smaller object cross section
Performance results: Left: Same as above except antenna system at 7 degrees latitude is looking back towards the Equator. Right: Focus is on object altitudes of 160 – 1000 km.
(1) Example array configuration; (2) Array orientation; (3) System simulation; (4) Specialized hardware and signal processing for detection and one-pass orbit determination
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Multiple independent observations (about 30/sec) throughout the pass resolve cross-velocity and thus an orbit can be obtained.
Highly accurate AOA with ambiguity resolution, aided by the wide array spacing mitigates the ‘too short arc problem’ (object’s time in beam).
Innovative, very wide bandwidth waveform and signal processing approach for accurate range/range rate
Results: AGI’s Orbit Determining Toolkit was applied to convert simulated measurements to orbits, which were then compared to the actual orbit parameters modeled by STK, obtaining position errors of a few meters and velocity errors of a few cm/sec.