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Lidar for Science and Resource Management

Experimental Advanced Airborne Research Lidar (EAARL)

Introduction

The Experimental Advanced Airborne Research Lidar (EAARL) is an airborne lidar system that provides unique capabilities to survey coral reefs, nearshore benthic habitats, coastal vegetation, and sandy beachesThe Experimental Advanced Airborne Research Lidar (EAARL) is an airborne lidar system that provides unique capabilities to survey coral reefs, nearshore benthic habitats, coastal vegetation, and sandy beaches. [larger version]

The Experimental Advanced Airborne Research Lidar (EAARL) is an airborne lidar system that provides unique capabilities to survey coral reefs, nearshore benthic habitats, coastal vegetation, and sandy beaches (Wright and Brock, 2002). Operating in the blue-green portion of the electromagnetic spectrum, the EAARL is specifically designed to measure submerged topography and adjacent coastal land elevations seamlessly in a single scan of transmitted laser pulses.

Four main features separate the EAARL from traditional airborne bathymetric lidar:

  1. a relatively short (1.3 ns) laser pulse
  2. a radically narrowed receiver field-of-view (FOV)(1.5-2 mrad)
  3. digitized signal temporal backscatter amplitude waveforms
  4. software as opposed to hardware implementation of real-time signal-processing elements

The short pulse and narrow FOV are beneficial in coastal sub-aerial and submerged environments to determine bare-Earth topography under short vegetation, as well as in riverine environments where large changes in topography occur over a very small area. The small receiver field-of-view (FOV) rejects ambient light and scattered photons from the water column and bottom-reflected backscatter (Feygels et al. 2003), thereby ensuring relatively high contrast and short duration of the bottom return signal. The EAARL system can accommodate a large signal dynamic range, thereby making it suited to mapping topography over a variety of reflective surfaces in the coastal zone, ranging from bright sand to dark submerged sea-bottom.

Design Considerations

EAARL uses a very low-power, eye-safe laser pulse, in comparison to a traditional bathymetric lidar system that allows for a much higher pulse-repetition frequency (PRF) and significantly less laser energy per pulse (approximately 1/70th) than do most bathymetric lidars. The laser transmitter produces up to 10,000 short-duration (1.3 ns), low-power (70 J), 532-nm-wavelength pulses each second. The energy of each laser pulse is focused in an area roughly 20 cm in diameter when operating at a 300-m altitude. Based upon test flights over typical Caribbean coral reef environments, EAARL has demonstrated penetration to greater than 25 m, and can routinely map coral reefs ranging in depth from 0.5 to 20 m below the water surface.

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%. The fourth channel is available for either water Raman or 1064-nm infrared backscatter depending on the application. All four channels are digitized synchronously with digitization beginning a few nanoseconds before the laser is triggered and ending as long as 16,000 ns later. A small portion of the outgoing laser pulse 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. The backscattered laser energy for each laser pulse is digitized into 65,536 samples, resulting in over 150 million digital measurements being taken every second.

The resulting waveforms are partially analyzed in real time to locate the 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. In addition to the lidar, the EAARL sensor suite includes a digital three-band color infrared camera, a red-green-blue (RGB) digital camera, a dynamically tuned Inertial Measurement Unit (IMU), and precision kinematic Global Positioning System (GPS) receivers that together provide for sub-meter geo-referencing of each laser and photographic pixel.

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: 20 scanlines per second
Laser sample per raster: 120 rasters/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: 0 m
Maximum measurable water depth: 26 m

Post-Processing (ALPS)

Post-processing of EAARL data is accomplished using a custom-built Airborne Lidar Processing System (ALPS) that combines laser return backscatter digitized at 1-ns intervals with aircraft positioning data derived from the IMU and GPS receivers. The ALPS software enables the exploration and processing of lidar waveforms and the creation of Digital Elevation Models (DEMs) for bare-Earth, canopy-top, submerged topography, and vegetation canopy structure. The EAARL system utilizes Earth-centered coordinate and reference systems, thereby eliminating the need for referencing submerged topography data to relative water level or tide gauges.

Applications

The EAARL has been operational since the summer of 2001, when it was used to survey the coral reef tract in the northern Florida Keys (Wright and Brock, 2002; Brock et al., 2004). Subsequent surveys in 2002, 2004, and 2006 along the Florida reef tract have enabled the creation of submerged topography products at Biscayne National Park (Brock et al., 2006a), Florida Keys National Marine Sanctuary (Brock et al., 2007), and Dry Tortugas National Park (Brock et al., 2006b). Several surveys have been conducted using the EAARL system in a variety of coastal communities, including barrier islands along the Atlantic coast (Nayegandhi et al., 2005) and around the margins of an urbanized Gulf of Mexico estuary (Brock et al., 2002, Nayegandhi et al. 2006).

An EAARL survey was also conducted along the Platte River in central Nebraska in March 2002 (Kinzel and Wright, 2002). The Platte River is a braided sand-bedded river that presents technological and logistical challenges with regard to collecting topographic and bathymetric measurements. The vertical accuracy of the system when compared with Real-Time-Kinematic (RTK) GPS surveys was estimated to be 16 22 cm root mean square error (RMSE) in the Platte River (Kinzel and Wright, 2002). Subsequent shallow river surveys conducted in the Lower Boise River, Idaho, yielded vertical accuracies of 14 18 cm RMSE (unpublished data). Further studies have estimated the vertical accuracy of the EAARL system to range from 10 14 cm (RMSE) for submerged topography (0.1 2.5 m water depth) and 16 20 cm for sub-canopy topography under a variety of dense coastal vegetation communities along the margins of Tampa Bay, Florida (Nayegandhi et al., 2007).

Mosaic of 2001-2002 & 2006 Florida Keys EAARL Submerged Topography
Mosaic of 2001-2002 & 2006 Florida Keys EAARL Submerged Topography. [larger version]
Mosaic of 2003 St. John EAARL First-surface & Submerged Topography
Mosaic of 2003 St. John EAARL First-surface & Submerged Topography. [larger version]

Specific design considerations for the EAARL system, which make it an ideal system for mapping the topography and morphologic habitat complexity of shallow reef substrates, include the high spatial (20-cm-surface footprint at nominal flying altitude of 300 m AGL) and sample (1-ns digitizing interval) resolution. The EAARL system is also designed to map substrates in shallow water (< 5 m) where traditional hydroacoustic ship-borne instruments cannot operate efficiently, field surveys are time-consuming and cost-prohibitive, and traditional bathymetric lidars are not applicable. These design considerations are also uniquely well suited for the seamless and simultaneous mapping of shallow, clear channel-bed topography in rivers and streams, and for resolving the bare-Earth topography of the surrounding floodplain.

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