Synthetic Aperture Ladar



R.L. Lucke and L.J. Rickard
Remote Sensing Division

M. Bashkansky, J.F. Reintjes, and E.E. Funk
Optical Sciences Division

Introduction: Synthetic aperture radar (SAR) is a long-established imaging technique for air- or space-borne radars: the motion of the platform sweeps out a "synthetic aperture" that can be many times larger than the radar's antenna. By maintaining phase coherence of the radar's RF radiation over this imaging time and using heterodyne detection to measure the phase of radiation reflected from the scene, the imaging resolution can be driven by the size of the synthetic, rather than of the physical, antenna. This provides high resolution in the dimension of the scene that is parallel to the direction of flight (azimuth resolution). High resolution in the other dimension (range resolution) is provided by an FM-chirped (i.e., frequency varying) waveform that supports accurate time-of-flight measurements: light that returns from one range is distinguished by its time delay (which changes its heterodyne beat frequency at the detector) from light that returns from another. If the same techniques can be extended to visible or infrared light, the potential exists for extremely high resolution with a very modest aperture size.1 Using 1.55-μm light, we have produced the first laboratory demonstration of a synthetic aperture ladar (SAL) image,2 using precisely the SAR technique outlined above. Developing SAL into an operational system will be difficult, in large part because the laser wavelength for SAL is 103 to 104 times shorter than the RF wavelength for SAR. This means that phase coherence is harder to maintain. But SAL has the advantage of building on three decades of SAR legacy, and many of the techniques developed for SAR, especially for signal processing, can be used by SAL.

High Resolution at Long Range: The laws of physics dictate that the highest angular resolution that any remote sensing system can achieve is about λ/D, where λ is the wavelength of the radiation and D is the diameter of the system's RF antenna or optical aperture. The resolution with which the system can image the scene is therefore R x λ/D, where R is the range to the scene. For a satellite-based system in orbit around a planet, the range to the scene is at least a few hundred kilometers. If resolution of a few centimeters is desired, the diameter of the aperture must then be several meters (for visible or near infrared light). Only for the very biggest and most expensive satellites is it possible to contemplate flying such a large mirror. But a satel-lite, by its very nature, can sweep out a synthetic aperture many meters across in a few milliseconds.

SAL techniques have great potential for solar-system remote sensing. A candidate landing zone could be examined at the scale of all significant hazards before a vehicle is committed to it. Detailed scientific information could be obtained on small-scale geological features, such as strata on the walls of river valleys or lava channels. Furthermore, SAL is an active sensing method; it can supply images of places where sunlight is absent, such as wintertime polar regions, and increase coverage in single-pass encounters with targets like asteroids. In principle, SAL could offer similar benefits for Earth-surface measurements. But such a system would require further developments in atmospheric compensation techniques because the atmosphere can degrade beam quality substantially at visible wavelengths.

Laboratory Demonstration: NRL has taken the first step in developing a true imaging SAL. Figure 1 shows the experimental apparatus. Ninety percent of the output of a 1.5-μm laser is sent into a fiber-optic circulator that sends light through a lens to the target. The target was an aluminum mounting plate on which the letters "NRL" had been made with retroreflective tape. The 4% of the light that is reflected back from the polished end of the fiber constitutes the local oscillator (LO) for the heterodyne detection. (In heterodyne detection, the signal of interest is mixed with a similar LO signal to enable detection of a more tractable lower frequency signal.) A small fraction of the light returning from the target re-enters the fiber, thereby mixing with the LO light to provide a heterodyne signal, which is sent to the detector. The target is translated across the beam to provide the synthetic aperture. (As Galileo might have said, it doesn't matter whether the aperture or the target moves.) Half of the other 10% of the laser light is sent to a second interferometer, which is used as a reference to monitor nonlinearities in the laser's frequency chirp.

Fig 1





FIGURE 1
Experimental setup (from Ref. 2).

The object is a flat mirror perpendicular to the beam, so there is only one range element. Therefore, variations in the heterodyne beat frequency can be due only to scan nonlinearities. These variations are measured and used to correct data from the primary interferometer. (The second interferometer would not be needed in an operational system using a highly stabilized laser.) The last 5% of the laser light is sent to an HCN cell, which sets an exactly reproducible starting frequency for the laser's chirp.

Figure 2 shows the image provided by this apparatus — the first-ever true SAL image. On the left is the raw image. It is dominated by speckle, as is the case for any coherent-light imaging system. (SAR images also have speckle.) The circle overlaid on this image shows the approximate size of the resolution element that would result from the system's physical aperture alone (i.e., the resolution that could be obtained if there were no system-scene motion, and hence no synthetic aperture generation). The resolution actually obtained is better by a factor of about 50. The image on the right shows the result of filtering the image to reduce the effect of speckle.

Fig 2a
(a) Raw image   (b) Filtered image
FIGURE 2
The image generated (from Ref. 2).

Engineering Problems: Now that the feasibility of SAL has been demonstrated, "only" engineering hurdles stand in the way of developing an operational system. But they are formidable. First of all, one needs high-power lasers (up to kilowatts or even more) that have very long coherence times (up to many milliseconds) and fast pulse-repetition rates (up to 100 kHz) that are tunable over about a 1 GHz bandwidth and can be space-qualified. Maintaining the phase coherence needed for synthetic aperture image processing requires compensating for platform vibrations to an accuracy better than the wavelength of the light used. This problem requires sensitive accelerometers but will be easier to deal with for the smooth motion of a spacecraft than for an airborne system. To some extent, problems with laser coherence times and platform stability can be alleviated in postprocessing by the focusing methods developed for SAR.

[Sponsored by USAF]

References

1R.L. Lucke and L.J. Rickard, "Photon-Limited Synthetic Aperture Imaging for Planet Surface Studies," App. Opt. 41, 5,084-5,095 (2002).
2M. Bashkansky, R.L. Lucke, E. Funk, L.J. Rickard, and J. Reintjes, "Two-Dimensional Synthetic Aperture Imaging in the Optical Domain," Opt. Lett. 27, 1,983-1,985 (2002).