EAARL - Airborne lidar system for high-resolution submerged and sub-aerial topography
NASA's Experimental Advanced Airborne Research Lidar (EAARL) is a raster-scanning, waveform-resolving, green-wavelength (532 nm) lidar designed to map near-shore bathymetry, topography, and vegetation structure simultaneously. The EAARL system has been developed by C. Wayne Wright at the NASA Wallops Flight Facility, Wallops Island, Virginia. The EAARL has been operational since the summer of 2001, when it surveyed the coral reef tract in the northern Florida Keys. Subsequently, several surveys have been carried out by the EAARL system in a variety of coastal communities, including barrier islands along the Atlantic coast, and around the margins of an urbanized Gulf of Mexico estuary. The EAARL green-wavelength laser can penetrate water slightly more than one Secchi disk depth, allowing the sensor to map submerged and sub-aerial topography simultaneously.
The EAARL has the unique real-time capability to detect, capture, and automatically adapt to each laser return backscatter without any of the sensor "dead zones" found in discrete return lidars. It can accommodate a large signal dynamic range and is keyed to considerable variations in the vertical complexity of the surface target. These features enable automatic adaptive acquisition of dramatically different surface types, thereby making EAARL uniquely well suited for mapping emergent coastal vegetation, submerged coral reefs, and bright sandy beaches in a single flight. These targets have vastly different physical and optical characteristics which cause the extreme variations in the laser backscatter complexity and signal strength.
The EAARL system uses a "digitizer only" design which eliminates all hardware-based high-speed front-end electronics, start/stop detectors, time-interval-units, range gates, etc. typically found in lidar systems. The EAARL system instead uses an array of four high speed waveform digitizers connected to an array of four sub-nanosecond photo-detectors. Real-time software is used to implement the system functions normally done in hardware. Each photo-detector receives a fraction of the returning laser backscattered photons. The most sensitive channel receives 90% of the photons, the least sensitive receives 0.9%, and the middle channel receives 9%. All four channels are digitized synchronously with digitization beginning a few nanoseconds before the laser is triggered, and ending over 16,000 nanoseconds later. A small portion of the laser is sampled by fiber optic and injected in front of one of the photo-detectors to capture the actual shape, timing, and amplitude of the laser pulse shortly after it is generated. A total of 65,536 total samples are digitized for every laser pulse, resulting in over 150 million digital measurements being taken every second.
The resulting waveforms are partially analyzed in real time to locate key features such as the digitized transmit pulse, the first return, and the last return. The real-time waveform processor automatically adapts to each laser return waveform and retains only the relevant portions of the waveform for recording. Thus, the storage space required for returns from tall trees or deep water is more than the storage requirement for beach or shallow water backscatter.
The EAARL sensor suite includes the raster-scanning, water-penetrating full-waveform adaptive lidar, a down-looking RGB digital camera, a high-resolution multi-spectral color Infrared (CIR) camera, two precision dual-frequency kinematic carrier-phase GPS receivers and an integrated miniature digital inertial measurement unit which provide for sub meter geo-referencing of each laser sample. The nominal EAARL platform is a twin-engine Cessna 310 aircraft, but the instrument may be deployed on a range of light aircraft. A single pilot, a lidar operator and a data analyst constitute the crew for most survey operations.
| NASA EAARL System Specifications |
| Total system weight |
250 lbs. |
| Maximum power requirement |
28 VDC at 24 amps |
| Nominal surveying altitude |
300 m AGL |
| Raster scan rate |
97 knots (50 m/s) |
| Laser sample per raster |
25 rasters per second |
| Swath width at 300 m altitude |
240 m |
| Sample spacing |
Swath center - 2 x 2 m
Swath edges - 2 x 4 m |
Area surveyed per hour
(300 m altitude, 50 m/s) |
43 km2 per hour |
| Nominal power required |
400 Watts |
| Illuminated laser spot diameter on the surface |
20 cm |
| Nominal ranging accuracy |
3 - 5 cm |
| Nominal horizontal positioning accuracy |
< 1 m |
| Digitizer temporal resolution |
1 nanosecond
(13.9 cm in air, 11.3 cm in water) |
| Minimum water depth |
30 cm |
| Maximum measurable water depth |
26 m |
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The EAARL
receiver optics consists of (1) a 15 cm diameter dielectric coated Newtonian telescope, (2) a computer driven raster scanning mirror oscillating at 12.5 Hz producing 25 raster scans each second, and (3) an array of sub-nanosecond photo-detectors each sampling a specific dynamic range fraction of the backscattered laser energy. The computer driven scan mirror position is measured by a precision high speed shaft angle encoder.
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| Sample EAARL waveforms. The left images shows 2 channels saturated from a water surface Fresnel reflection and a submerged topographic return seen in all 3 channels. The right image shows a vegetated region describing the vertical structure of the canopy within a 20 cm footprint. Black waveform indicates the most sensitive channel (90%), red indicates the middle channel (9%), and blue is the least sensitive channel (0.9%). The digital counts are inverted in the EAARL receiver hardware and hence decrease with an increase in backscatter return signal strength. |
The EAARL laser provides for a high sample rate of up to 10,000 samples/second from a very short (1.2 ns) green pulse. The 1.2 ns pulse is approximately 18 cm long in air and 13 cm long under water. The system can make a range measurement with accuracy on the order of 2 - 5 cm depending on variations in the target reflectivity from pulse to pulse. The energy is focused in an area roughly 20 cm in diameter when operating at a 300 m altitude. The green wavelength is critical for mapping submerged topography since green light can pass through the water with minimal loss.
The digitally recorded return signal for each transmitted laser pulse is a time history of the return backscatter photons within the laser footprint. The EAARL waveform represents the amount of photon energy received by each photo-detector on the sensor as a function of a series of equally spaced time intervals. A waveform with a simple (single-mode) shape represents a reflection from a single reflecting surface such as a sandy beach. A water-surface Fresnel reflection is represented by a very strong amplitude backscatter signal that is usually saturated in the sensitive channels, with the range being resolved in the least sensitive channel (Figure 2a). The submerged bottom reflection is weaker and is resolved in the most sensitive channel. A waveform having a complex (multi-mode) shape (Figure 2b) represents multiple reflections from apparently distinct canopy surfaces within the laser footprint.
EAARL surveys have been conducted over portions of the reef tracts fringing the Florida Keys, Puerto Rico, and the U.S. Virgin Islands. The northern Florida reef tract from the northern tip of Elliott Key to south of Carysfort Reef and portions of the Dry Tortugas were swath-mapped during the summers of 2001 and 2002. Reefs along the southwest coast of Puerto Rico near La Parguera, the island of St. John, and the shallow shelf along the northeast coast of St. Croix were surveyed during the spring of 2003. The shallow and emergent portions of the Dry Tortugas were surveyed by the EAARL in August 2004.
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