The Asteroid Tracker Challenge is part of NASA’s Asteroid Grand Challenge Series, and in this particular challenge, we are tasking competitors to optimize the use of an array of radar dishes when tracking Near Earth Objects from the time they become visible over the horizon till the point at which they cease to be visible. This tracking allows scientists to gather information from each object such as composition, spin rate, among other properties.
Near Earth Object(NEO) detection and characterization is a critical need for NASA and the United States. NASA has been directed to develop capabilities to observe, track and characterized NEOs and other deep space objects that could pose a threat to the Earth. As a result, NASA is developing concepts for a highly capable deep space radar array consisting of sets of commercially available monolithic antennas.
One of the challenges faced by NASA is determining the optimum selection of individual antennas within the array for a given track observation. This is a complex analysis and goes directly to development of the concept of operations and cost of operations (in terms of maintenance and total capacity required).
High Level Requirements
This challenge, in Phase 1 will devise the optimum subarray selection per set of object tracks provided in a predefined configuration using a fixed number of dishes . Phase 2 will build on Phase 1 results but will focus optimal optimizing configuration and looking at placement of additional dishes.
The following requirements must be met:
- Be able to model phased array antenna beams using a predetermined set of dish and beam properties.
- Take, as input, trajectories of a number of NEO and for each, be able to provide the optimal selection of subarrays to track the object for its entire visible path (or, if defined, a minimum time period of observation that gives sufficient scientific observation value).
- Be able to read properties from configuration files – i.e. dish properties, array configuration, trajectory data, etc.
Atomized Project Plan
So what’s coming up, and how do I register? Simply select any of the contest names below to find out more about each specific contest, and register to participate.
|Asteroid Tracker - Phase 1 – Marathon prep – API reference documentation||04/30/2014||05/05/2014||First2Finish|
|Asteroid Tracker - Phase 1 - Visualization Assembly||05/08/2014||05/19/2014||Frist2Finish|
|Asteroid Tracker - Phase 1 – Private Testing||06/30/2014||07/11/2014||Testing|
|Asteroid Tracker - Phase 1 – Marathon Match||07/25/2014||08/10/2014||Marathon Match|
|Asteroid Tracker - Phase 1 Project Delivery||08/16/2014||08/16/2014||-|
|Asteroid Tracker - Phase 2 – Architecture for Code bundling||TBD||Architecture|
|Asteroid Tracker - Phase 2 – Code bundling into PC Executables||TBD||Assembly|
|Asteroid Tracker - Phase 2 – Scoring Function Update||TBD||Assembly|
|Asteroid Tracker - Phase 2 - Invitational Testing||TBD||Testing|
|Asteroid Tracker - Phase 2 – Marathon Match||TBD||Marathon Match|
|Asteroid Tracker – Phase 2 Project Delivery||TBD||-|
*PLEASE NOTE: The contest start and end dates are estimates only, and may shift forward or backwards depending on contest progress. To verify a contest start date, please select the contest name to view the registration page.
This challenge requires an understanding of antenna theory, radar and electromagnetics. The level of understanding is NOT greater than that which could be learned through web resources and/or undergraduate physics / engineering references. Some general references are provided, as well as specific examples and references applicable to NASA’s use cases:
Amateur Radio / Layman Resource
MIT Open Course on Electromagnetics
MIT Open Course on Receivers, Antennas and Signals
Institution of Engineering and Technology
MIT / Lincoln Laboratory
Fenn, et. al., The Development of Phased Array Radar Technology
MIT / Lincoln Laboratory
Herd, Phased Array Radar Basics
NASA Application (Deep Space Radar Measurement and Uplink Array)
Vilnrotter et. al., Planetary Radar Imaging With the Deep Space Network’s 34 meter Uplink Array
Davarian, Uplink Arrays for the Deep Space Network
D’Addario, Uplink Array Demonstration with Ground-Based Calibration
Monolithic Antenna (or just Antenna)
A single, standalone antenna element.
In the case of the NEO Array each monolithic antenna consists of a 12-meter Cassegrain parabolic reflector antenna, antenna pedestal (structure) and the electronics and motors necessary to point, steer and transmit/receive signals. For the NEO Array, each monolithic antenna is essentially a standalone “ground station”. Each physical (real world) antenna has a specific antenna pattern “fingerprint”. These patterns must be measured to determine the actual pattern and how it differs from theoretical.
Antenna Array (or just Array)
A combination of multiple monolithic antennas into an integrated system. The NEO array will use phased array signal processing techniques to actively combine the transmit and receive signals to achieve much greater performance (power, range, sensitivity, etc) than could be achieved by monolithic antennas individually. The “Array” in the context of the NEO array refers to the complete set of monolithic antennas and the infrastructure required to support and operate them.
Antenna Subarray (or just Subarray)
This refers to a specific combination of monolithic antennas used to support a given tracking event. A “subarray” can consist of any number of monolithic antennas from one to the total number of antennas in the entire array system. A subarray is used to synthesize a RF beam for a specific tracking event.
This refers to the path along which most of the desired energy (and sensitivity) of an antenna is focused. This is also referred to as the “main lobe” of the antenna pattern.
Array (or Subarray) Beam
This refers to the synthesized antenna beam resulting from the combination (through constructive interference and superposition of RF energy) resulting from the simultaneous operation of properly phased monolithic antennas that make up the subarray.
REF (Antenna Array Analysis in MATLAB): http://www.mathworks.com/help/phased/examples/antenna-array-analysis-with-custom-radiation-pattern.html
REF (Subarray Analysis in MATLAB): http://www.mathworks.com/help/phased/examples/subarrays-in-phased-array-antennas.html
This refers to directivity that is not in the direction of the main antenna beam. These are typically undesirable. Side lobes are a result of the interference (constructive and destructive) and superposition of RF energy as it is influenced by the design of the antenna. All antennas (other than the theoretical isotropic) have sidelobes which can be predicted mathematically. Real-world antennas generally follow the theoretical predictions, but will have additional effects introduced by imperfections in the physical antenna.
The “gain” is the amount of “increase” or “amplification” that a signal receives as a result of the physics of the antenna. An antenna with a gain of 1 (or 0 dB) is referred to as “isotropic” or “omnidirectional” and is used as a reference level. All real antennas have some gain which is reflected by the directivity of the RF energy transmitted in the beam or sensitivity of the received signal. This is a zero-sum process as power “added” to one direction must be made up by power being “removed” from another.
The parabolic reflectors used in the NEO array are highly directional and designed to produce very focused beams in the direction of the dish axis, with very little power being radiated (or sensitivity provided) off-axis. All real-world antennas have imperfections, however, and so some power and sensitivity is available “off axis”. These are referred to as the “side lobes” of the antenna pattern. It is a goal to minimize the effects of the side lobes.
The process by which the amplitude and phase of signals transmitted / received by individual monolithic antennas comprising the subarray are combined (through signal processing) to yield a synthesized beam resulting from the subarray.
The maximum directivity (or sensitivity in the receive path) of the synthesized beam resulting from the combination of antennas in the subarray. In an ideally combined system, the resulting transmit power in the synthesized beam is increased by a factor of N^2 where N is the number of antennas in the subarray.
Azimuth and Elevation
Measures used in a spherical coordinate system to identify a direction in celestial space.
The object of interest for the Radar observation. This could be a satellite / spacecraft or a natural object in space such as a planet or asteroid.
The path traced by a target object across the sky.
Used to specify the orbital characteristics of a spacecraft / space target. Along with
Two Line Element set
“Shorthand” notation specifying a space object’s characteristics and orbit. Used to plan for radar observations, compute target tracks, and plan / observe object trajectories.
The purpose of the array radar is to detect, track, measure and characterize targets such as spacecraft, asteroids and planets from LEO to deep space. Radar works by measuring the characteristics of the RF energy reflected back off of a distant target. An RF signal is transmitted, it travels to the target (losing power and experiencing distortions). The signal is reflected by the target (with some changes to the signal resulting from material characteristics of the target). The signal then travels back to the receiver where is it captured and analyzed. Round trip time-of-flight tells you the range. Doppler shifts inform you of motion and rotations. Polarization and other electrodynamic changes to the RF energy tell you something about the material properties. Finally, precise measurement of the RF signal can provide images of the target object (eg. pictures of an asteroid).
Radar is an N^4 problem. This means that the power received by the radar receiver will be approximately (but not greater than) the 4th root of the power transmitted. This is because one-way RF transmission is an R^2 problem. As a RF beam propagates through space it spreads over an increasingly large surface (think of the surface of an expanding balloon… as the radius increases so does the surface covered by the same beam angle, and so less power per unit area is available). Once the signal reaches the target and is reflected back the process must begin again, only now the “transmitting” power is only as much as was available at the target… And so the total power loss for the round trip goes as 1/N^4… 1/N^2 out and 1/N^2 back. This makes sensing targets very far away extremely difficult.
Using a phased array for radar (especially a phased array of individually very powerful transmitters) significantly improves this. As the main beam of the phased array combines the power from each element antenna the power is added together at the target. Since its actually the electromagnetic field strength (measured in volts/m^2) that is being combined, and power goes as the square of voltage, we achieve an increase of N^2 in the beam.
To achieve this it is absolutely vital that the signals reach the target precisely in phase. As such, we need to know very accurately the physical placement of each transmitting / receiving antenna as well as the pointing of the individual element main lobes. To achieve this each site is surveyed precisely and mapped. Each antenna reflector is very carefully measured to determine the individual radiation pattern (to measure the non-ideal shape) and those “real world” parameters are used to compute the actual beam phases in the signal processing system.
Selection and positioning of each monolithic antenna is vital. Individual antennas must be able to see and track the target across the sky and must not be blocked by neighboring antennas (shadowed). Additionally, the RF beam side lobes from each antenna can and will interfere and so their placement is critical as is the selection of which antenna to use in each subarray.
For a an array consisting of a large number of antennas, operations will be something like this…
A given target will be selected (say an asteroid). The basic two line elements will be known so that a target track can be predicted. This tells us where we need to point the main beam of the array and what the expected path will be across the sky. It also tells us when we can expect the target to “rise” and “set” at the horizon.
Given the target (size, approximate distance, desired measurement) a decision will be made as to how much radar power will be used for the observation. This will determine the number of monolithic antennas required to be combined as a subarray to support the track. As power is a factor of N^2 where N is the number of antennas used, this can easily be determined. To this minimum number some margin will be added in case of failure of an individual antenna during the observation.
Having determined the number of antennas required, we need to decide specifically WHICH antennas to use for the track, and when (or if) to add in or remove antennas during the track. This selected set will comprise the SUBARRAY for the track, and can be as few as one antenna (only a single dish tracking) or as many dishes as are in the entire array (full array support).
The selection of antennas needs to take into account several things:
- Shadowing. There is a geometric limit below which a given antenna cannot usefully be used because its beam would be pointing directly at another dish. This not only is limiting to the measurement, it can be dangerous to the equipment (and possibly personnel) that would be in its beam. So, the geometries of each selected antenna (including their predicted view paths throughout the track) must be identified and deconflicted.
- RF interference. The constructive and destructive interference produced by arraying antennas is what yields the benefit of the main beam. It also produces a set of periodic peaks and nulls that are not desired. These peaks and nulls are much more pronounced as the periodicity of the antenna spacing increases – this means that the more regular the spacing of the antennas (with respect to wavelength of the transmitted signal), the worse the interference pattern will be. As such, we want to select antennas that are placed as “aperiodically” as possible. A uniform grid is a very bad idea here.
To add to the complexity, all antennas produce side lobes. Real antennas produce more “jagged” patterns than ideal models. While a very good approximation can be made by using ideal models, we need to compute the interference patterns using actual measured patterns from the antennas.
Once the determination of specific antennas is finalized, the track is planned and executed. The dishes of the subarray track together (steering the beam as the target crosses the sky) and the radar observations are made. Once the track completes, the subarray antennas are “released” back to the pool to be assigned to a new track event.
There will be multiple, simultaneous tracks by the array and so several sets of subarrays must be identified and allocated in real-time.
Karim R. Lakhani