Airborne bathymetric charting using pulsed blue-green lasers

Background noise reduction for airborne bathymetric full waveforms by creating trend models using Optech CZMIL in the Yellow Sea of China

Raw full waveforms of green lasers used in airborne LiDAR bathymetry (ALB) are contaminated by background and random noise related to the environment and ALB devices. Traditional thresholding methods have been widely used to reduce background noise in raw full waveforms on the basis of the assumption of constant background noise. However, background noise that is mainly related to background solar radiation and detector dark current changes over time. Thresholding methods perform poorly on the full waveforms with a wide variation range of background noise. A background noise reduction method considering its wide variation is proposed to decrease the background noise by creating trend models. First, each green full waveform is divided into two parts: pulse- and non-pulse-return waveforms. Second, a linear interpolation is conducted on the non-pulse-return waveform to impute the missing noise. Third, a low-pass filter is used to filter the random noise with high frequency in the imputed non-pulse-return waveform and obtain the trend model of background noise of the full waveform. Finally, the derived background noise model is used to decrease the background noise in the pulse-return waveform. The proposed method is applied to decrease the background noise in raw green full waveforms collected by the Optech coastal zone mapping and imaging LiDAR (CZMIL). The mean and standard deviation of residual noise in the CZMIL waveform reduced by the trend model of background noise are -0.03 and 3.5 digitizer counts, respectively. The proposed background noise reduction method is easy to apply and can reduce the background noise to a significantly low level. This method is recommended for preprocessing the raw full waveforms of green lasers collected by Optech CZMIL for ALB.

Graphene Q-switched Tb:LiYF4 green laser

We report the Q-switched operation of a Tb³⁺-laser for the first time, to the best of our knowledge. The passiveQ-switching was realized by a 15% Tb:LiYF₄ gain medium and a single-layer graphene saturable absorber. An average output power of 744 mW at 544 nm was achieved with slope efficiency of 41%, pulse width of 2.9 µs, and repetition rate of 38.7 kHz. The corresponding pulse energy and peak power were calculated to be 19.2 µJ and 6.6 W, respectively.

Interpretation of airborne oceanic lidar: effects of multiple scattering

The effects of multiple scattering on the interpretation of the time dependence of elastic backscattering of laser pulses from the ocean (lidar) are investigated through solving the radiative transfer equation by Monte Carlo techniques. In particular, after removal of the geometric loss factors, it is found that the backscat-tered power is a decaying exponential function of time, over the time interval required for photons to travel four attenuation lengths through the water. The effective attenuation coefficient of this exponential decay is found to be strongly dependent on the parameters of the lidar system and on the optical properties of the water. The significant parameter is the ratio of the radius of the spot on the sea surface viewed by the lidar receiver optics to the mean free path of photons in the water. For values of this parameter near zero, the decay is determined by the beam attenuation coefficient, while for values greater than ~5-6, the decay is given by the attenuation coefficient for downwelling irradiance, often referred to as the diffuse attenuation coefficient. Between these two extremes the interpretation of the effective attenuation coefficient requires, essentially, complete knowledge of the inherent optical properties of the water: the beam attenuation coefficient and the volume scattering function.

Target signatures for laser altimeters: an analysis

The statistical characteristics of the received signal for short pulse laser altimeters are investigated. Expressions are derived for the mean and temporal covariances of the received pulse for a direct detection receiver. The effects of laser speckle, shot noise, and surface profile of the ground target are considered. The results are used to compute the means and variances of the total received energy, propagation delay, and rms width of the received pulse. These parameters are shown to be directly related to the statistics of the surface profile.

Use of water Raman emission to correct airborne laser fluorosensor data for effects of water optical attenuation

Airborne laser fluorosensor measurements of fluorophore concentrations in surface waters are highly sensitive to interference from changes in optical attenuation. This interference can be eliminated by normalizing the fluorescence signal with the concurrent water Raman signal. In our application to remotely monitor chlorophyll a concentrations in fresh waters, the Raman-corrected chlorophyll a fluorescence measurements were found to be highly correlated with chlorophyll a ground truth data. Also, the reciprocal of the water Raman signal was seen to vary directly as the beam attenuation coefficient. It is suggested that this latter relationship be exploited as a means for remotely sensing changes in the optical attenuation of surface waters.

Water depth measurement using an airborne pulsed neon laser system

Initial base-line field test performance results of the National Aeronautics and Space Administration's airborne oceanographic lidar (AOL) in the bathymetry mode are presented. Flight tests over the Atlantic Ocean yielded water depth measurements to 10 m. Water depths to 4.6 m were measured in the more turbid Chesapeake Bay. Water-truth measurements of depth and beam attenuation coefficients by boat were taken at the same time as the aircraft overflights to aid in determining the system's operational performance. Beam attenuation coefficient a and depth d product alphad was established early in the program as the performance criterion index. A performance product of 6 was determined to be the goal. This performance goal was successfully met or exceeded in the large number of field tests executed. Included are selected data from nadir-angle tests conducted at 0 degrees , 5 degrees , 10 degrees , and 15 degrees . Field-of-view data chosen from the 2-, 5-, 10-, and 20-mrad tests are also presented. Depth measurements obtained to altitudes of 456 m are given for additional comparison. This laser bathymetry system represents a significant improvement over prior models in that (1) the complete surface-to-bottom pulse waveform is digitally recorded on magnetic tape at a rate of 400 pulse waveforms/sec, and (2) wide-swath mapping data may be routinely acquired using the 30 degrees full-angle conical scanner. Space does not allow all the 5,000,000 laser soundings to be included. Qualified interested users may obtain complete data sets for their own in-depth analysis.

Technique for remotely measuring surface pressure from a satellite using a multicolor laser ranging system

The optical path length from a satellite to the earth's surface is strongly dependent on the atmospheric pressure along the propagation path. Surface pressure can be measured by using a multicolor laser ranging system to observe the change with wavelength in the optical path length from the satellite to a ground target. The equations which relate surface pressure to the differential path lengths are derived, and the accuracy of the pressure measurement is evaluated in terms of the ranging system parameters. The results indicate that pressure accuracies of a few millibars appear feasible.