Abstract:
Provided is a bistatic and multistatic system for detecting and identifying a target in close proximity to an orbiting satellite. An electromagnetic fence is established to surround the satellite, using a ground-based communication uplink from a gateway antenna. A contact or breach of the electromagnetic fence by the target is detected by the satellite, or at other sensor locations, and an exact position, range and ISAR image of the target is calculated using scattered RF energy from the fence. Identification data is transmitted to satellite system monitors, whereby the data is used to decide on a corrective course of action.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to situational awareness and target or object detection in a space environment. More particularly, this invention relates to a system and method for bi-static and multi-static radar detection of unknown targets/objects. 
   BACKGROUND 
   A long standing challenge for governments and the commercial space industry is developing an effective means for detecting when an unknown object or target is approaching a space-borne platform, such as a satellite. Detection and identification of targets may be necessary to protect satellites from damage and/or functional degradation. The need for “space situational awareness” is accented by an ever increasing effort to use satellites and other space-borne platforms for more than just observation and communication purposes. 
   Satellite operations often include a gateway antenna on Earth which establishes an uplink with one or more satellites orbiting the Earth. The satellite may be in a low Earth orbit (LEO), medium Earth orbit (MEO) of a geo-stationary orbit (GEO). The uplink is via a transmitted radio frequency signal which is typically in one of several frequency bands, to include C, X, Ka, and Ku bands. Once the link is established, complex tracking algorithms and hardware ensure the link is not broken as the satellite orbits. If the satellite becomes damaged, however, the link may be broken despite the best efforts of the antenna and tracking subsystems. 
   A reliable, cost effective means for detecting and identifying an object approaching a satellite, or other space-borne platform, does not exist in the open literature. Detection alone is an inadequate defense mechanism, as an approaching object may or may not be man-made, and it may or may not pose an ultimate threat to the platform. Identification, while certainly preferred, often requires expensive detectors and processors that may occupy more space, and utilize more power, than is available on the platform. 
   Hence, there is a need for a system for space situational awareness that detects and identifies a target as it approaches a space-borne platform, thereby giving operators a chance to take corrective and/or defensive measures. The system must be able to operate within existing satellite system architectures and envelopes, and it must provide reliable data that can be used in a decision making process. 
   SUMMARY 
   The bistatic and multistatic system for space situational awareness herein disclosed advances the art and overcomes problems articulated above by providing a system for detecting and identifying a target as the target approaches a space-borne platform. 
   In particular, and by way of example only, according to an embodiment, a method for establishing relative situational awareness between a space-borne platform and a target is provided, including: establishing an electromagnetic fence around the space-borne platform, the fence extending from a radio frequency signal source, said signal extending outwardly in space beyond the space-borne platform; detecting a target contacting the electromagnetic fence; calculating a distance from the target to the space-borne platform; and generating a radio frequency image of the target. 
   In another embodiment, provided is a system for detecting and identifying a target in proximity to a space-borne platform, including: an electro-magnetic fence positioned around the space-borne platform; a radio frequency signal source for generating the electro-magnetic fence; a detector positioned on the space-borne platform for detecting a contact of the electromagnetic fence by the target; and a processor for processing directed and reflected radio frequency signals to detect and identify the target. 
   In yet another embodiment, provided is a system for detecting and identifying a target in proximity to a space-borne platform, including: a means for establishing an electro-magnetic fence around the space-borne platform; a means for detecting a contact of the electromagnetic fence by the target; and a means for calculating positional information of the target and for identifying the target. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of the operating environment of the present system, according to an embodiment; 
       FIG. 2  is a flow chart of response events, according to an embodiment; 
       FIG. 3  is a schematic of a bistatic radar detection/range determination system, according to an embodiment; 
       FIG. 4  is a schematic of a sensor system, according to an embodiment; and 
       FIG. 5  is a schematic of a Doppler effect for target identification, according to an embodiment. 
   

   DETAILED DESCRIPTION 
   Before proceeding with the detailed description, it should be noted that the present teaching is by way of example, not by limitation. The concepts herein are not limited to use or application with one specific type of space situational awareness system. Thus, although the instrumentalities described herein are for the convenience of explanation, shown and described with respect to exemplary embodiments, the principles herein may be equally applied in other types of space situational awareness systems. 
     FIG. 1  shows an operational environment for the bistatic and multistatic system  101  of the present disclosure. A gateway antenna  100 , positioned on Earth  102 , transmits an uplink radio frequency (RF) signal  104  to a satellite  106  orbiting Earth  102 . The satellite  106  may be orbiting in any one of several orbits to include LEO, MEO and GEO. Further, satellite  106  may be a microsatellite, as that term is commonly used in the space satellite industry. While  FIG. 1  depicts one operational environment, it can be appreciated that other environments may be contemplated without exceeding the scope of this application. For example, an alternative source of RF signal  104  may be a second space-borne platform. Likewise, satellite  106  may be any of a number of space-borne platforms, to include manned platforms. 
   Gateway antenna  100  may be any of a type of antenna well known in the art. Typically, the frequency of signal  104  is in the C, X, Ku or Ka band, however, other frequency bands may be used as well. As shown in  FIG. 1 , signal  104  forms a conical beam  108  as it propagates toward satellite  106 . The base radius of beam  108 , and the direction of the beam  108 , can be changed automatically by controlling the parabolic antenna that is part of gateway antenna  100 . Satellite  106  is positioned within the cone of beam  108 , which is to say satellite  106  is surrounded on all or most sides by RF energy transmitted from gateway antenna  100 . In this way, conical beam  108  is an electromagnetic fence surrounding satellite  106 . 
   As satellite  106  continues to orbit Earth  102 , an unknown object or target  110  may enter the path of satellite  106 , or the target  110  may intentionally approach satellite  106  from any angle or orientation. At some point, as target  110  approaches satellite  106 , target  110  may contact conical beam or electromagnetic fence  108 , thereby scattering some of the RF energy of the fence  108 . The degree to which RF energy is scattered is dependent, in part, on the characteristics of target  110 , to include angular rotation, size, and speed. Further, the bandwidth of the spectral signal generated when target  110  contacts or breaches fence  108  is dependent on target characteristics as well. Scattered RF energy  112  is detected by a receiver subsystem (not shown) in satellite  106 , setting off a chain of events in response to the approach of target  110 . 
   The detection and characterization of target  110  may be referred to as relative situational awareness with respect to satellite  106 . System  101 , required to detect and identify the target  110 , may include in at least one embodiment a source (e.g. antenna  100 ) of an uplink RF signal  104 , an electromagnetic fence  108  resulting from a radio frequency uplink signal  104 , and subsystems (not shown) on board the satellite  106  to detect, process and utilize scattered RF energy resulting from the breach of fence  108  by target  110 . 
   Referring for a moment to  FIG. 2 , a diagram of response events, for at least one embodiment, is presented. As shown, an initial warning signal is generated and sent to a satellite  106  system operator or system monitor, notifying the monitor that a signal has been detected, block  200 . Contemporaneously, a processor in satellite  106  uses directed RF energy from antenna  100  and scattered RF energy data to calculate a distance to and a position of target  110 , block  202 . Also, an inverse synthetic aperture radar (ISAR) image of target  110  is generated (block  204 ) using angular rotation data derived from the received RF energy  112 . 
   Processed data, which may include both positional data as well as image data, is communicated to the system monitor (block  206 ) for review. Of note, the system monitor may be a person, or alternatively, the monitor may be an automated system for receiving and analyzing data, and for initiating certain response functions. Once data is transmitted to the system monitor, a series of decision cycles may follow. An initial assessment may be made regarding the status of target  110 , i.e. friend or foe, block  208 . In this context, “friend” typically refers to a known system that may or may not be in an expected location. The term “foe” may be used to identify any unknown target, to include natural objects, or it may identify known but “unfriendly” targets. 
   If target  110  is identified as “friend”, a decision is made regarding whether satellite  106  must be moved out of its current orbit into an alternate orbit, block  210 . If necessary, satellite  106  will be repositioned (block  212 ) in order to avoid physical contact with target  110 . If no movement is required, a notice may be sent to appropriate system monitors and supervisory personnel (block  214 ), informing each of the approach of target  110  and the actions taken in response. 
   If target  110  is identified as a “foe”, in one embodiment satellite  106  is moved to avoid any possibility of physical contact between the satellite  106  and the target  110 , block  216 . An additional response may be required (block  218 ) from satellite  106 , and if so that response may be concurrent with movement or completed after satellite  106  has been repositioned, block  220 . Once a decision is made regarding an additional response, a notification process begins (block  222 ) to notify concerned parties and system monitors of the ongoing or recently completed actions. 
   In at least one embodiment, a sensor subsystem  300  of system  101 , such as that depicted in  FIG. 3 , may be used to transmit and receive RF energy. In  FIG. 3 , a transmitter subsystem  302  and a receiver subsystem  304  constitute the sensor subsystem  300 . The transmitter subsystem  302 , which may include a plurality of components such as an amplifier  306 , pulse generator  308  and frequency synthesizer  310  is used to generate an RF signal. A processor  312  controls the pulse generation and other functions. The signal is passed to a modulator  314  prior to transmitting the signal toward a target  316  and a known reference  318 , which may be satellite  106 . 
   Similarly, in receiver subsystem  304 , two RF signals (generally pulsed signals) reflected from the reference  318  and the target  316  respectively are received by one or more detectors or modulators, e.g. modulators  320  and  322 . One or more processors  324  and  326  control the processing of the received signals by directing the functions of a coherence detector  328 , an oscillator  330  and a timing alignment mechanism  332 . A synchronized, aligned signal is transmitted from a processor  326  as coherent RF data. The processed data is then used to verify detection and to identify a target, e.g. target  316 . 
   Considering now a determination of the distance to, and the position of, the target (block  202   FIG. 2 ), the position of target  110  relative to satellite  106  as depicted in  FIG. 1  may be calculated based on the receipt angle of the incoming scattered RF signal  112 , taking into account the angular rotation of both target  110  and satellite  106 . Satellite  106  may, in at least one embodiment, employ a single receiver subsystem to formulate bistatic measurements. Alternatively, multiple receiver subsystems (multistatic detection) may be used to detect and receive scattered RF energy  112 . More important, perhaps, than the orientation or position of target  110 , is the distance to target  110 , the determination of which will now be discussed. 
   Referring to  FIG. 4 , a spatial relationship between gateway antenna  100 , satellite  106  and target  110  is presented. As shown, antenna  100  has a known gain (G T ) and a known Power (P T ). Gateway antenna  100  is considered the transmitting antenna, hence the identifier (“T”). Similarly, a receiving antenna on satellite  106  has a known gain (G R ) as well. Target  110 , which is at a distance R 1  from antenna  100  and R 2  from satellite  106 , has a radar cross section or RCS (“σ”) which may be calculated based on reflected energy returns as it contacts fence  108 . The distance R 1  may be determined within an acceptable and relatively small margin of error based on the established orbit of satellite  106  (i.e. a LEO, MEO or GEO orbit) because target  110  must be in the same orbit in order to encounter satellite  106 . 
   The ultimate variable of interest is “R 2 ”, which is the distance or range from satellite  106  to target  110  at any given moment in time. The unknown “R 2 ” may be calculated using the equation 
   
     
       
         
           
             
               
                 
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   The variable “R 2 ” is analogous to the bistatic link budget, and the bistatic radar equations may therefore be used to calculate a signal-to-noise ratio (S/N). In general, S/N is a function of the distances, R 1  and R 2  ( FIG. 3 ). In a bistatic radar system, S/N is minimized when R 1 =R 2 . It can be appreciated, therefore, that S/N increases as target  110  comes closer to either satellite  110  or antenna  100 . By knowing the S/N, the range R 2  may be calculated as given in (1) above, wherein
         R 1 =distance from transmitter to target   P T =power of the transmitting antenna   G T =gain of the transmitting antenna   G R =gain of the receiving antenna   λ=wavelength of the transmitted signal   σ=RCS of the target   k=Boltzman&#39;s Constant   T=system temperature   B=receiver bandwidth   and,   S/N=signal-to-noise ratio       

   The processor in satellite  106  may be used to calculate R 2 , or alternatively data may be sent to a ground station processor for use in calculating the desired target-to-satellite range. Subsequent range calculations can be used to determine if target  110  is moving closer to or further from satellite  106 . Further, in at least one embodiment, relative changes in R 2  can be used to calculate a closing velocity for target  110 . The combination of range and velocity data may also be used to calculate an estimated path of flight for target  110  once target  110  fully breaches fence  108 , and is no longer in contact with fence  108 . 
   For a target in motion, i.e. one that is rotating or moving with respect to pitch, yaw and/or roll, different components or facets of the target may have different velocities relative to a stationary antenna that is illuminating the target. For example, three different velocity vectors V 1 , V 2  and V 3  are shown in  FIG. 5  which represent the relative velocities, with respect to antenna  100 , of different facets on target  110 , as the target is illuminated by antenna  100 . Of note, illumination in this context includes a contact or breach of fence  108  as antenna  100  projects a RF signal toward satellite  106 . As a function of the rotation depicted in  FIG. 5 , slight variations in Doppler frequency are produced, and these frequency variations may be detected by a receiver sensor either on the ground or mounted on satellite  106 . 
   Stated differently, when a rotating target, e.g. target  110 , is illuminated, there will be a shift in the Doppler frequencies of reflected energy between any two adjacent point scatterers. These differential Doppler frequencies may be received and processed by a receiver subsystem on satellite  106 , or by another sensor suite, and may be used to characterize the reflectivity of target  110  in the Doppler domain. Inverse Synthetic Aperture Radar (ISAR) techniques may use the differential Doppler information to generate a cross-range or azimuth resolution of target  110 . In particular, a Fourier transform or other transformation algorithm may be used to process the Doppler data. 
   Down-range or line-of-sight radar measurements are simply a measure of the distance between an illuminating antenna, e.g. antenna  100 , and a target  110 , and these measurements may be referred to as conventional radar measurements. Inverse Synthetic Aperture Radar (ISAR) techniques combine the azimuth data derived from differential Doppler frequencies, and the line-of-sight range data derived from a more traditional radar measurement, to generate a two-dimensional ISAR image of target  110 . 
   In an embodiment of the system disclosed herein, one or more receiver subsystems positioned on satellite  106  or elsewhere collect reflectivity data as a rotating target  110  contacts and breaches fence  108 . The reflectivity data, indicative of both the Doppler domain and the line-of-sight range distance to target  110 , is transmitted to a processor wherein an ISAR image of target  110  is produced. In this way, satellite  106  may detect (at a known range) and identify target  110  as the target contacts the electro-magnetic fence  108  surrounding satellite  106 . 
   Changes may be made in the above methods, devices and structures without departing from the scope hereof. It should thus be noted that the matter contained in the above description and/or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method, device and structure, which, as a matter of language, might be said to fall therebetween.