Patent Application
20130050024
Embodiments of the invention generally relate to a system for the passive monitoring of non-cooperating vehicles and, more specifically, to a system for the passive location of non-cooperating vehicles using satellite-based transmitters with compensation for ionospheric delay.
The position and path of aircraft in the National Airspace System (NAS) has traditionally been determined by primary radars which transmit powerful radio frequency (RF) pulses to locate aircraft by “listening” for echoes. Pragmatic cost concerns have led to the development of secondary radars, which require specialized equipment on the ground and in the aircraft. When interrogated with a coded message, the aircraft system transmits encoded return pulses. Secondary radar improves the primary radar coverage and is used for a variety of other purposes including collision avoidance. The NAS is actively deploying Automatic Dependent Surveillance-Broadcast (ADS-B) surveillance technology for tracking aircraft to address the limitations of existing radar infrastructure. ADS-B is part of the Next Generation Air Transportation System and will be required for the majority of aircraft operating in the United States by the start of the next decade. ADS-B periodically broadcasts its own state vector (i.e., identification, altitude, heading, speed, position) and other information without knowing what other vehicles or entities may be receiving it. No pilot or controller action is required for the information to be issued. Surveillance information is dependent on the navigation and broadcast capability in the source ADS-B equipped aircraft.
Like current secondary radar, ADS-B requires specialized equipment onboard every aircraft in the airspace to be effective. Aircraft without ADS-B equipment must be detected by primary radars. It should be noted that incorporating the ADS-B data into an air traffic management system requires the construction of ground stations to receive the data. However, even with the addition of all planned ADS-B ground stations, there are significant gaps in the coverage of the NAS.
Passive radar is an alternative to conventional primary and secondary radar systems. Passive primary radar eliminates the cost of operating a primary radar transmitter by utilizing existing radio sources as the transmitter in the radar problem. Eliminating the transmitter means that only the relatively low-cost, portable receiver and signal processing circuitry is required to detect and monitor aircraft. Passive Coherent Location (PCL) is a passive radar system in which there is no dedicated transmitter. The receiver uses third party transmitters in the environment and measures the time difference of arrival between the signal arriving directly from the transmitter and the signal arriving via reflection from an object in order to determine the bistatic range of the object.
Passive bistatic detection alone is not a new concept. The fundamental principle of bistatic detection is to take advantage of strong signals already present in the environment, and detect their reflection from a target (i.e., aircraft). Prior research on bistatic radars has resulted in the development of several systems utilizing terrestrial-based transmitters. Existing passive bistatic radar systems utilize terrestrial transmitters and have acquisition times that are slow relative to monopulse radars (e.g., 0.1 to 1 s). One of the best known products is Silent Sentry®, a PCL system available from Lockheed Martin Corporation that uses frequency modulated (FM) radio transmissions. The Silent Sentry system uses indigenous radio and television signals to locate aircraft.
Although the Silent Sentry system does not require cooperating aircraft or illuminators, it operates at a different frequency band than embodiments of the invention and uses terrestrial illuminators. This is fundamentally different from embodiments of the invention which use satellite-based signals to locate aircraft. This difference is especially important when operating in environments where terrestrial signals are absent or compromised. There are many satellite transmitters that provide continuous signals to the entire United States. Unlike terrestrial transmitters, the satellite view of the target is not blocked by mountainous terrain and multipath issues are dramatically reduced. As compared to terrestrial transmitters, the advantages of spaceborne transmitters include a reduction in multipath and shadowing as well as a reduced reliance on vulnerable proximate infrastructure.
The ionosphere, consisting of layers of charged particles in the upper atmosphere, is known to affect radio waves and is a potentially-limiting factor in global navigation satellite systems (GNSS) like the global positioning system (GPS). It is known that it is necessary to compensate for the ever-changing delay of the ionosphere in order to achieve the best-possible accuracy with these systems. Dual-frequency systems as well as geographic augmentation systems are two such approaches to improving GPS accuracy.
The extent to which the ionosphere affects bistatic radar using satellite-based illuminators is less well-described in the literature. Bistatic radar, especially passive bistatic radar, offers advantages as compared to traditional monostatic radars. Passive bistatic radar utilizing non-cooperative spaceborne transmitters offers the potential to locate a target in three-dimensional (3D) space with greater accuracy than GPS. Given the importance of the ionosphere on GPS measurements, it is necessary to determine whether the ionosphere plays any meaningful role in passive bistatic systems.
Embodiments of the invention provide a system for the passive location of non-cooperating vehicles using satellite-based transmitters with ionospheric compensation. The embodiments include unique aspects as to the system, subsystems, algorithms, and implementation thereof. This system meets the need for passively and inexpensively monitoring non-cooperating aircraft.
In an exemplary embodiment, a method is provided for passive detection and monitoring of target vehicles with non-cooperating satellite-based transmitters. The method includes receiving a reference signal from a satellite-based transmitter at a base (e.g., ground) station along a first path and receiving a target signal at the base station reflected from a target vehicle along a second path following illumination of the target vehicle by an illuminator signal from the satellite-based transmitter. An ionospheric delay of the reference signal and the target signal in traversing the ionosphere from the satellite-based transmitter to the base station is determined. A bistatic range is determined as the time difference of arrival at the ground station between the reference signal and the target signal along the first and second paths, and any errors due to ionospheric delay of the reference and target signals. A position of the target vehicle in three-dimensional space is determined based in part on the bistatic range determination. In some embodiments, frequency difference of arrival (i.e., Doppler shift) can be used to determine bistatic velocity.
In an exemplary embodiment, a bistatic radar system is provided for passive detection and monitoring of target vehicles with non-cooperating satellite-based transmitters. The passive system includes a reference antenna for receiving a reference signal from a satellite-based transmitter along a first path and a reference receiver for amplifying the reference signal, the reference receiver implementing passive coherent location. The system further includes a target antenna for receiving a target signal reflected from a target vehicle along a second path, following illumination of the target vehicle by an illuminator signal from the satellite-based transmitter, and a target receiver for amplifying the target signal, the target receiver implementing passive coherent location for detection of target vehicles traversing an airspace, land, or a water surface. In exemplary embodiments, the bistatic radar system could be used for tracking aerial and non-aerial targets, the latter group including ground-based targets such as cars, and maritime targets such as boats, ships, etc. A plurality of analog-to-digital converters converts the amplified reference and target signals into digital signals. A control computer applies a plurality of digital signal processing algorithms to determine a bistatic range of the target vehicle and to determine a position of the target vehicle in three-dimensional space.
These and other advantages and aspects of the embodiments of the disclosure will become apparent and more readily appreciated from the following detailed description of the embodiments taken in conjunction with the accompanying drawings, as follows.
The following description is provided as an enabling teaching of embodiments of the invention including the best, currently known embodiment. Those skilled in the relevant art will recognize that many changes can be made to the embodiments described, while still obtaining the beneficial results. It will also be apparent that some of the desired benefits of the embodiments described can be obtained by selecting some of the features of the embodiments without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptations to the embodiments described are possible and may even be desirable in certain circumstances. Thus, the following description is provided as illustrative of the principles of the invention and not in limitation thereof, since the scope of the invention is defined by the claims.
The system is conceived of as a light-weight, low-cost, portable, and field-deployable station to supplement deficiencies in the National Airspace System (NAS) and homeland security surveillance networks. Potential applications include providing coverage in remote mountainous regions, low-altitude enroute primary radar coverage throughout the continental United States, and low-altitude interdiction efforts in coastal areas. As a field-deployable system, the disclosed embodiments could also be used to quickly restore primary radar coverage in the event that a disaster disables existing primary radars. Additionally, the portable and non-emitting nature of the passive radar permits a wide range of applications where emitting radars are unacceptable.
The terms “base station” and “ground station” are used throughout this description for convenience, but are not used in a limiting manner. Base station is used generically and can refer to a fixed or moving ground platform, a fixed or moving sea-based platform, or an airborne platform. The tracked targets can be airborne vehicles, land-based vehicles, or water surface-based vehicles.
A unique aspect of the disclosed embodiments is that it utilizes satellite-based transmitters as illuminators (e.g., GPS, Iridium®, XM® Satellite Radio, SIRIUS® Satellite Radio). These sources have been traditionally viewed as transmitting signals that are “too weak” for use as illuminators in monitoring systems, but these weak signals have been successfully utilized in the passive monitoring of non-cooperating vehicles as disclosed herein. A further unique attribute of the disclosed system is that it accommodates observation modes having long “integration” times, e.g., potentially greater than one second. Furthermore, another unique feature of the system is that it does not require any a priori databases of transmitters, but rather uses real-time-derived data to characterize the transmitters that are being utilized.
In systems using radio waves traveling through the earth's atmosphere, the ionosphere (i.e., upper layers of charged particles in the atmosphere) will modify the radio waves in a meaningful way, such as by slowing the speed of the radio waves. This problem is well known with GPS systems and can result in very large position errors on the order of tens of meters or more that must be corrected in order to obtain accurate positioning information. In satellite-based radar, the accuracy goal is on the order of several meters; therefore, the ionosphere must be taken into account. Otherwise, it will not be possible to identify the position of a target (e.g., aircraft) with the desired accuracy.
Embodiments for improving the positioning accuracy of a system for locating target vehicles utilizing radio waves that pass through the ionosphere include, but are not limited to, some or all of the following features: (1) incorporation of real-time ionospheric models, and (2) direct measurement of the ionosphere.
Ionospheric models are generated to help correct GPS and this data can be applied to the bistatic radar system. The ionosphere can introduce position errors of tens of meters or more. GPS implements ionospheric corrections in two ways: with dual frequency systems (historically limited to military use) and through a network of augmentation sensors and systems including Wide Area Augmentation System (WAAS), Local Area Augmentation System (LAAS), and related implementations. These systems broadcast near-real-time data on the ionosphere to the user.
A passive system that measures two radio waves (e.g., a direct path and an illumination plus reflection path), and applies a time-difference technique can compensate for the ionosphere since the ionospheric delay is applied to both signals. This also has the advantage of compensating for other uncertainties such as exist in the position of the satellite. The passive system can also measure the bistatic Doppler shift of the target signal and its direction of arrival.
Embodiments of the inventive system include, but are not limited to, some or all of the following features:
1. One or many antennas, radio receivers, and analog-to-digital-converters included as components.
2. Implementation of passive coherent location for the detection of vehicles traversing airspace, land, or water surface.
3. Use of illuminations from satellite-based transmitters in low earth orbit (LEO), medium earth orbit (MEO), and geo-stationary earth orbit (GEO) regimes.
4. Use of illuminator signals having bandwidths primarily, but not exclusively, in the several megahertz regime.
5. Use of illuminators having radio frequencies in the range between 1 GHz and 4 GHz, including, but not limited to, satellites launched for the purposes of telecommunications, satellite-radio, and navigation.
6. Incorporation of bit-minimization schemes to reduce the required digital bit-rates for signal transmission.
7. Incorporation of efficient signal drift and pulse detection to identify very weak targets having changing radar cross-section (RCS) and accelerating relative to the ground station.
8. Integration (look) times between 0.1 s and 30 s for target acquisition.
The primary application of the disclosed technology is the provision of a system that is capable of filling in the radar coverage gaps within the National Airspace System (NAS). The disclosed system is similar to primary radar in that it does not require aircraft to be equipped with specialized equipment like ADS-B, but unlike primary radar, the disclosed system does not require a transmitter at the base or ground station.
Known systems almost exclusively identify terrestrial-based transmitters as the illuminators for this type of work. For example, the Silent Sentry system utilizes VHF television and radio signals. The disclosed system utilizes satellite-based emitters which present two significant advantages. First, satellites cover regions of the globe that terrestrial transmitters do not (e.g., oceans, mountainous regions). Second, satellites are not as easily compromised by disaster or sabotage as are terrestrial-based transmitters.
Systems described in prior art patents and technical literature use radio transmissions ranging from VHF (about 100 MHz) to K-Band (about 12,000 MHz). Some satellite-based emitters operate specifically at C (4,000 MHz) and K (12,000 MHz) bands. In exemplary embodiments, the disclosed system can operate in L-band (1000-2000 MHz) and S-band (2000-4000 MHz). In other embodiments, the system could be able to operate in K-band.
Different radio sources use different bandwidths which determine how accurately (or inaccurately) the position of a reflector can be determined. Known systems have bandwidths that range from about 100 KHz (e.g., cell-phone or FM Radio), to a few MHz (e.g., television), to 300 MHz (satellite television). The embodiments disclosed use transmissions having bandwidths of between 1 and (about) 10 MHz. This is a technological sweet spot that enables accurate target location without prohibitive electronics requirements.
Several aspects of data processing are common to receivers implementing passive coherent location (PCL). These include some method of filtering to remove unwanted out-of-band signals and some method of filtering to remove interfering in-band signals, and the application of a range-gated coherent detector (also called a matched filter). The disclosed system also implements such filtering techniques but with the unique aspect of utilizing specific methods to minimize the data rates in the system, thus minimizing the cost.
A second unique aspect of the disclosed system is what happens after application of a matched filter. Known systems use detection algorithms that include Doppler filtering and thresholding, and that exhibit reduced sensitivity to accelerating targets. Consequently, “look times” are limited to less than one second. The embodiments disclosed herein utilize unique algorithms that allow the detection of accelerating targets and consequently increase look-times and sensitivity (e.g., 10 s or more).
The embodiments of the system operate by time-difference-of-arrival principles as illustrated in
A simplified outline of the major system components of the disclosed system in an exemplary embodiment is shown in
An overview of the software processing in an exemplary embodiment is depicted in
The first processing stage suppresses interfering signals, including the illuminator, from the target data, so that the data are noise-dominated. Pre-correction compensates for the relative motion of the receiver and illuminator, and may include anticipated target characteristics. Correlation compares target and illuminator signals to determine the presence of likely echoes. Characteristics of the range gating, integration, and echo identification stages determine the allowable target ranges, speeds, and accelerations. Characteristics of the target vehicle will govern design of these stages and heavily influence computational requirements. The final stage of target localization and classification fuses data from multiple data pipelines, and can include data from multiple illuminators as well as multiple receivers.
Some technical attributes of this exemplary system include:
1. Subtraction can occur by correlation and subtraction, DSP signal processing (Weiner filtering), image “cleaning,” or phased array processing.
2. Correlation is accomplished by means of direction correlation (“X” engine), Fourier transform correlation (“FXF” engine), or overlapping FXF engines, depending on the target domain for the radar.
3. Bit minimization techniques are used to minimize data transport requirements through the stages while maintaining integrity of the signal, and can use as few as one bit to represent each sample.
4. Echo identification stages use specific algorithms to identify stationary, constant-velocity, constant acceleration, and higher-order motion targets using efficient algorithms and specific manipulation of the data sets. Algorithms include but are not limited to de-drift, de-chirp, various chirp transforms, and Doubling Accumulation Drift Detection (DADD).
An implementation of a functional system utilizing non-cooperating spaceborne transmitters and longer acquisition times is the S-band Array for Bistatic Electromagnetic Ranging (SABER), implemented in the Technology Demonstrator Array (SABER-TDA) at Embry-Riddle Aeronautical University (ERAU) in Daytona Beach, Fla. The SABER system utilizes passive radar techniques to covertly and inexpensively locate aircraft proximate to and far from the base station. SABER is unique in that it utilizes non-cooperating spaceborne transmitters as illuminators in the bistatic radar problem. In addition, SABER is implemented using a collection of commercial off the shelf (COTS) components and internal OTS components, which keeps the system costs low.
SABER is a passive bistatic radar that uses emissions from non-cooperating earth-orbiting spacecraft transmitters to detect targets such as aircraft. Key signal processing algorithms implement passive coherent location (PCL) and allow the system to use longer integration times and exhibit improved sensitivity over other systems. The SABER technology demonstrator array (SABER-TDA) was developed to verify the efficacy of this approach. It includes both hardware and software elements.
SABER-TDA utilizes the geostationary XM® Satellite Radio as the illuminator in the radar problem. SIRIUS® Satellite Radio could also be used as the illuminator. In its initial implementation, SABER was limited to a single 2 MHz downlink channel providing a coarse 150 meter resolution. Techniques utilizing wider-bandwidth signals and exploiting peak-up algorithms could provide precisions of up to 5 meters in three-dimensions. The implemented system utilizes the 2.3 GHz downlink of the geostationary XM® Radio satellites as the radar illuminator. The single 2 MHz downlink channel has a coherence length of about 150 meters and exhibits generally well-behaved ambiguity and autocorrelation characteristics. Other choices are possible worldwide. For example, the Solaris Mobile W2A satellite can be used outside of the continental U.S. (CONUS) footprint of the illuminator discussed herein. Systems based on other illuminators such as K-band Direct-Broadcast Satellite (DBS) television or the L-band Iridium® constellation may also be feasible.
The ionosphere consists of layers of charged particles in the uppermost layers of the Earth's atmosphere. As an imperfect, conducting vacuum, the ionosphere affects radio signals that propagate through it. At microwave frequencies, these effects include primarily delay, attenuation, and Faraday rotation. The delay is known to be a potentially dominant source of error in global navigation satellite systems like GPS.
The ionosphere is characterized by the total electron content (TEC), which is a measure of the charge density. The delay of a signal can be expressed in meters as a function of the frequency and the TEC by the expression:
where f is the frequency in hertz (Hz) and the TEC is the electrons in a one square meter column of the atmosphere. This expression is valid for a signal traversing normal to the ionosphere, e.g., at zenith relative to a ground-based observer. In the general case, the prior equation can be modified to use the slant TEC instead. The slant TEC is given by the expression:
where Re is the radius of the Earth, H, is the height of the ionosphere, and θ is the elevation angle through which the ray passes.
Values of the TEC vary significantly from day to night and over periods of solar activity, and can rapidly vary over timescales of minutes or hours. Uncertainty in the delay contributions of the ionosphere led to compensation systems in GPS. For example, dual-frequency positioning systems exploit the frequency-dependence of the signal delay equation to measure the ionospheric delay in real-time for the user. Civilian users utilize augmentation systems, such as the Wide Area Augmentation System (WAAS), to refine their position. These augmentation systems deliver representative models of the ionosphere to the user to refine the positioning to remove the effect of the ionosphere.
GPS utilizes ranging information to determine the distance to each satellite and ionospheric effects directly affect the ranging accuracy. For example, at 1.575 GHz, a TEC of 50 TECU produces a range error of 8.1 meters vertically, or 17.4 meters at 20 degrees elevation. Passive coherent location systems, on the other hand, are less-sensitive because they are difference-based geometries. At the XM Radio frequency (2.32 GHz), the equivalence is about 0.25 nanoseconds (ns) per TECU difference (7.5 cm) vertically. The equivalence is almost doubled to 0.51 ns per TECU difference (15.3 cm) vertically at the lower frequency of 1.62 GHz used by the low earth-orbiting (LEO) Iridium system.
As illustrated in
D
L
=D
G
+ΔD
I
I
L
=I
G
+ΔI
I
E
L
=E
G
The bistatic radar measures only the difference between the direct and illumination plus echo paths, as in the bistatic range given by
R
B=[(IG+EG)−DG]+(ΔII−ΔDI)
Whereas the first set of terms represents the bistatic geometry, the latter term of the measurement represents the error due to the ionosphere. Contributions to the delay that are the same on both the illumination and the direct path will cancel each other. Since each path is lengthened by the same amount, the difference between the path lengths is unchanged, and the geometry is preserved.
Two cases of the geometry are considered herein. In the first case, it is assumed that the illuminator is very far away from the ionosphere and that the direct path and illumination rays are parallel. This case is applicable to geostationary illuminators. In the second case, it is assumed that the spacecraft is close to the ground station and that the rays are no longer parallel. This case is applicable to a low earth-orbiting (LEO) spacecraft.
The second case is illustrated in
As illustrated in
The anticipated effect of the ionosphere was determined by analysis of historical US-TEC data accessed from the National Geophysical Data Center archives. The US-TEC data has a geographic resolution of one degree in latitude and longitude and a temporal resolution of 15 minutes and a root mean square (RMS) accuracy of 2.4 Total Electron Content Units (TECU). Along a great circle, one degree of arc is about 111 km. This is near the anticipated limits of the SABER instrument to detect target aircraft.
The first analysis considers the difference in TEC between adjacent cells within the map. Each cell represents a one degree change in latitude or longitude. The derivative of the TEC is taken per cell to achieve a per cell differential. For derivatives along constant parallels, this figure is scaled with latitude so that the figure represents “delta TEC per 111 km” movement on the ground.
The second analysis considers the case when the illuminator is a low-earth orbiting satellite. This model used an ionospheric height of 400 km and a hypothetical satellite at an altitude of 500 km above the surface of the earth. The ground station receiver and target are separated by one degree of arc (111 km along the surface of the earth).
TEC values of 25-50 TECU are expected, although notable space weather events can lead to a much higher TEC. Analysis of
The analysis of the expected TEC distribution of the ionosphere has been combined with the geometry of a passive coherent location system utilizing satellite illuminators to predict the effect of the ionosphere on the accuracy of such a system. This analysis is motivated because ionospheric delay is an important factor in GPS accuracy, up to 20 meters or more. The ionosphere has large features and is slowly changing geographically compared to the expected distances between the ground station and the target aircraft in this bistatic radar and, as a result, the errors are generally small.
The expected worst-cases errors for the analyzed data set are on the order of a few meters or less (0.52 meters for XM® radio; 1.5 meters for Iridium® satellites). Reductions as compared to GPS are achieved from two factors. First, the bistatic geometry renders any consistent delay between the two paths meaningless, because the bistatic system is concerned only with the differences in the path lengths. This can be considered as the bistatic system constantly probing the ionosphere with the direct path signal. The second effect is gained in the XM® radio case from the increase in frequency. Because of the frequency dependence of the ionospheric delay, values are only 46% at XM of what they would be for the same TEC at GPS.
This analysis has determined that the contribution of the ionosphere is a meaningful, but secondary, effect to the accuracy of a passive bistatic radar system utilizing satellite-based illuminators. For coarse measurements utilizing large range cells (e.g. 150 meter granularity), the effect of the ionosphere is negligible. For more precise measurements approaching the 5 meter level, the ionosphere can contribute a large portion of the total error.
It should be noted that the error is lessened for closer range targets. For the application as a terminal area radar (e.g., 10 km), the errors will be reduced accordingly and negligible compared to the expected accuracies. Additional analysis of ionospheric data should verify the conclusions and statistically expected accuracy degradation over different periods of solar activity.
The corresponding structures, materials, acts, and equivalents of all means plus function elements in any claims below are intended to include any structure, material, or acts for performing the function in combination with other claim elements as specifically claimed.
Those skilled in the art will appreciate that many modifications to the exemplary embodiments are possible without departing from the scope of the present invention. In addition, it is possible to use some of the features of the embodiments disclosed without the corresponding use of the other features. Accordingly, the foregoing description of the exemplary embodiments is provided for the purpose of illustrating the principles of the invention, and not in limitation thereof, since the scope of the invention is defined solely by the appended claims.
This application claims the benefit of provisional patent application Ser. No. 61/527,405, filed on Aug. 25, 2011, and provisional patent application 61/593,630 filed on Feb. 1, 2012. The specification and drawings of the provisional patent applications are specifically incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 61527405 | Aug 2011 | US | |
| 61593630 | Feb 2012 | US |
