Swath Mapping Lidar with Fiber Lasers and PN Code Modulation

We propose to develop a swath mapping lidar with a fiber laser operated from telecommunication industry and pseudo noise (PN) code modulation and correlation detection technique. We will use a recently developed 2x8 pixel HgCdTe avalanche photodiode (APD) array detector to achieve a near quantum limited receiver sensitivity and, thus the longest possible range. The proposed lidar is especially suitable to map small bodies, such as asteroids or comet cores where there is a greater uncertainty in their orbit and and shape. There can also be potential hazard objects near the asteroid. Investigations of asteroids including near Earth objects (NEOs) will provide us with greater insight into the solar system formation and evolution, the physical forces acting on the objects, and their orbit and potential encounters with Earth. Lidar measurements can determine the body's shape and orbit dynamics of the asteroid with high geodetic accuracy. A lidar investigation at asteroids can benefits from the versatility of the proposed lidar, which can operate in a long-distance survey mode, a precision global mapping mode, a high-resolution 3-D targeted area-mapping mode, and a real-time multi-pixel proximity ranging mode to assist landing and sample collection. Current planetary lidar cannot cover such a wide span of ranges. Additionally, conventional planetary lidar are usually too expensive to be considered for typical asteroid missions. We propose to demonstrate a low-cost multifunction lidar based on our patented return-to-zero pseudo-noise (RZPN) laser modulation and correlation detection technique. The receiver detects the laser pulse pattern instead of individual laser pulses by correlating the received signal with the original RZPN pattern. The technique has already been widely used in radio frequency (RF) ranging and the Global Position System (GPS). This approach enables the use of low peak-power lasers at high pulse rate for very long-distance range measurement without aliasing. The newly available HgCdTe APD array detectors give high quantum efficiency over a broad spectral range in the infrared which allows us to use existing fiber lasers from the terrestrial laser communication industries. The new HgCdTe APD array detector has single photon sensitivity which makes it possible to achieve a multi-pixel flash lidar type measurement from 10's to 100's of kilometer altitude orbits. The proposed lidar can be reconfigured in space. The individual pixel output from the multi-pixel detector can be combined to improve sensitivity. The receiver signal integration time can be continuously adjusted to allow a trade-off between the receiver sensitivity and the measurement rates for intermediate range measurements. The laser power can be continuously adjusted according to target distance. In addition to range measurements, the lidar can also measure the surface reflectance from the number of received photons and effectively image the asteroid with the laser illumination without the need for sunlight. For this MatISSE program, we plan to build a prototype instrument and perform environmental tests of the key subsystems. Our work will mature the instrument from TRL-4 to TRL-6 at the end of the program. The proposed lidar is directly relevant to the MatISSE program by providing a new instrument for near Earth object and asteroid studies called for by the recent Planetary Science Decadal Survey. Our approach provides multiple measurement capabilities with a single cost-effective instrument. It also provides a low-cost alternative to the existing planetary and landing lidar.