The present invention relates to the field of topographic radar data synthesis. More specifically, the present invention relates to the field of synthesizing topographic data from synthetic aperture radar data.
There exists a need for topographic (i.e., three-dimensional) data using conventional synthetic aperture radar (SAR) data-gathering techniques. The need for accurate topographic data is demonstrated within the mapping, charting, geodesy, and intelligence communities, among others. Techniques have been developed to obtain topographic information from SAR data. However, such techniques suffer from one or more problems.
One approach to obtaining topographic information from SAR data is the single-pass, two-antenna technique. In this technique, a single SAR system uses two antennas to obtain two images in a single pass. The two antennas point in substantially the same direction but with differing vectors to a target in a vertical plane. The two antennas are therefore able to obtain two complex images that are substantially identical except for phase. For a given pixel, therefore, phase difference between the two complex images is a function of the height of the target above a reference plane at the location of that pixel. Thus, each phase difference can be converted into a target height. This is as taught by Graham, U.S. Pat. No. 3,727,219.
A significant problem exists with the single-pass, two-antenna technique. Generally, the greater the angular separation between the two vectors, the better the resolution of the resultant topographic data. It is therefore desirable that the two antennas be separated by as great an angular distance as possible. In an aircraft, for example, this may be accomplished by the two antennas being either vertically separated (e.g., mounted above and below the fuselage) or horizontally separated (e.g., mounted proximate the wing tips). Since both antennas are mounted to the same aircraft, the angular displacement between the antennas is constrained by the physical limitations of the aircraft.
Additionally, the single-pass, two-antenna technique requires an SAR system with two antennas and attendant electronics. Even when a majority of components are shared, this represents a significant increase in complexity over a single-antenna (i.e., conventional) SAR system. This increase in complexity incurs a corresponding increase in procurement and operational costs.
Another approach to obtaining topographic information from SAR data is the dual-pass, single-antenna technique. In this technique, a conventional single-antenna SAR system obtains one image in each of two passes. Each pass is made so the single antenna points in substantially the same direction but with differing vectors to the target in the vertical plane. The single antenna is therefore able to obtain two complex images that are substantially identical except for phase. As above, the pixel-by-pixel phase differences between the two complex images can be converted into target heights.
A significant problem exists with the dual-pass, single-antenna technique. Accuracy is dependent upon the two passes being substantially identical except for angular separation between the two vectors. That is, the two paths should ideally be parallel in both space and time. This would require a pilot and aircraft, both of which are capable of flying truly parallel courses at exactly identical velocities. Such pilots and aircraft do not exist in the real world. The accuracy obtainable with the dual-pass, single-antenna technique is therefore limited by the accuracy of the two paths.
Additionally, because the real world presents an ever-changing scenario, both passes of the dual-pass, single-antenna technique should ideally be simultaneous. To effect simultaneous passes, two aircraft, each encompassing a conventional single-antenna SAR system, are used. This significantly increases the overall complexity, and incurs a corresponding increase in costs.
To minimize costs, a single aircraft with a single SAR system is normally used to effect both passes. The accuracy of the resultant data is therefore additionally affected by the time between the two passes.
A third approach to obtaining topographical information from SAR data is the single-pass, autofocus-tracking technique. In this technique, a conventional single-antenna SAR system collects SAR phase history data while negotiating an arc in three-dimensional space. This approach results in radar scatters not in the SAR around plane being defocused. A spatially variant autofocus may be used to extract the parabolic phase needed to properly focus the SAR imagery. This parabolic phase can be scaled into a height above and below the ground focus plane.
A problem exists with the single-pass, autofocus-tracking technique in that other sources of parabolic chase shift may produce significant errors in the determined target heights. These parabolic phase shifts must therefore be eliminated and/or compensated for. This typically requires an increase in circuit complexity, with an attendant increase in costs.
Also, the single-pass, autofocus-tracking technique requires a large number of pixels of SAR data to extract a single parabolic phase measurement. To achieve meaningful topographic data, a large number of parabolic phase measurements must be made and the results averaged to reduce the variance of the measurement error. The result is that, for a given amount of SAR data, only a relatively few topographic data may be produced. This results in a coarse topographic image for a fine SAR image.
What is needed, therefore, is a method of producing topographic data from SAR data while maintaining the data resolution and using only conventional (i.e., single-antenna) SAR systems in a single pass.
Accordingly, it is an advantage of the present invention that a method of synthesizing topographic data is provided.
It is another advantage of the present invention that a method of synthesizing topographic data is provided that collects synthetic aperture radar data in a single pass over an arcuate data collection path transiting a single radar aperture having a construct baseline to a target.
It is another advantage of the present invention that a method of synthesizing topographic data is provided that partitions a radar aperture into a plurality of partial radar apertures each having a construct baseline substantially coincident with a construct baseline of the radar aperture.
It is another advantage of the present invention that a method of synthesizing topographic data is provided that processes synthetic aperture radar data collected over each of a plurality of partial radar apertures into complex image data containing Interferogrametric properties.
It is another advantage of the present Invention that a method of synthesizing topographc data is provided that integrates irterferogrametric complex image data into topographic data.
The above and other advantages of the present invention are carried out in one form by a method for synthesizing topographic data of a target. The method incorporates collecting synthetic aperture radar (SAR) data of the target, generating full-aperture complex image data of the target from the SAR data, generating first partial-aperture complex image data of the target from a first portion of the SAR data, and generating second partial-aperture complex image data of the target from a second portion of the SAR data. The method also incorporates deriving first-angle complex image data of the target in response to the first partial-aperture complex image data and the full-aperture complex image data, deriving second-angle complex image data of the target in response to the second partial-aperture complex image data and the full-aperture complex image data, and interferograrmmetrically integrating the first-angle complex image data and the second-angle complex image data to produce the topographic data of the target.
The above and other advantages of the present invention are carried cut in another form by a method for synthesizing topographic data of a target. The method includes transiting a radar aperture partitioned into first and second partial apertures, accumulating first and second portions of synthetic aperture radar (SAR) data over the first and second partial apertures, respectively, producing first-angle and second-angle complex image data of the target in response to the first and second portions of the SAR data, and interferogrammetrically integrating the first-angle and second-angle complex image data to produce the topographic data.