The techniques involved in the design and application of laser altimeter instruments are reviewed, including a description of the instrument subsystems required for the range and waveform measurements. Laser pulse transmitters based on the relatively novel technology of diode-pumped solid-state lasers are considered. Various factors affecting laser altimeter instrument performance are discussed. These include the receiver signal-to-noise ratio, atmospheric propagation, and altimeter platform effects. Some examples of laser altimeter data are presented to illustrate the variety of possible instrument applications. >

Laser altimetry measurements from aircraft and spacecraft

Determining the $\hbox{CO}_{2}$ column abundance from an integrated path differential absorption (IPDA) lidar requires accurate knowledge of the range to the scattering surface, i.e., the column height. We have adapted and tested a ranging algorithm for the airborne IPDA $\hbox{CO}_{2}$ lidar designed at the NASA Goddard Space Flight Center, and have evaluated its accuracy and precision. We applied a quasi-maximum-likelihood method, using cross correlation, to estimate the range from the lidar's 1- $\mu\hbox{s}$ -wide echo pulses. The objective was to show that the use of these temporally long laser pulses allows the determination of the optical path length with required precision. We analyzed airborne measurements made in August 2009 over the Chesapeake Bay near the Eastern Shore of Virginia. The results indicate a ranging precision of better than 3 m, which is sufficient for airborne and space-based retrievals of $ \hbox{CO}_{2}$ column concentration.

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Analysis of Range Measurements From a Pulsed Airborne $ \hbox{CO}_{2}$ Integrated Path Differential Absorption Lidar

Corrections for the errors that occur in pulsed optical ranging such as satellite ranging are generally analyzed. It is shown that corrections for range errors can be described as a linear combination of the atmospheric correction, the spatial correction, and the corrections for the errors in electrical circuits. A method for the calibration and the correction of observed distances by using a standard target is presented. Particularly in local ranging, corrected distances are easily calculated by two standard targets, even if the atmospheric refractive index and the accuracy of the time base of the counter are unknown. Ranging experiments on earth from 100 m to 10 km were made with the laser system. The corrected distances, using two standard targets, agree well with distances measured accurately by a Geodimeter within the pulse width of the laser, 50 nsec (7.5 m), and the resolution of the time interval counter, 10 nsec (1.5 m).

Correction for errors in optical pulsed range measurements.

For high-precision measurements of the CO2 column concentration in the atmosphere with airborne integrated path differential absorption (IPDA) Lidar, the exact distance of the Lidar beam to the scattering surface, that is, the length of the column, must be measured accurately. For the high-precision inversion of the column length, we propose a set of methods on the basis of the actual conditions, including autocorrelation detection, adaptive filtering, Gaussian decomposition, and optimized Levenberg–Marquardt fitting based on the generalized Gaussian distribution. Then, based on the information of a pair of laser pulses, we use the direct adjustment method of unequal precision to eliminate the error in the distance measurement. Further, the effect of atmospheric delay on distance measurements is considered, leading to further correction of the inversion results. At last, an airborne experiment was carried out in a sea area near Qinhuangdao, China on 14 March 2019. The results showed that the ranging accuracy can reach 0.9066 m, which achieved an excellent ranging accuracy on 1.57-μm IPDA Lidar and met the requirement for high-precision CO2 column length inversion.

https://www.mdpi.com/1424-8220/20/20/5887/pdf

High-Precision CO2 Column Length Analysis on the Basis of a 1.57-μm Dual-Wavelength IPDA Lidar

The effect of atmospheric turbulence on the accuracy of optical pulsed range measurements is investigated assuming a Poisson detection process and log normal scintillation. Expressions for the detection cell and the r.m.s. error of detection in the presence of atmospheric scintillation are derived assuming that the optical pulse duration is long compared to the correlation time of the atmospheric scintillation, and evaluated numerically for various optical Gaussian pulse shapes. Numerical results show that in the presence of weak scintillation, the r.m.s. detection error depends primarily on the number of photoelectrons in the pulse, while in the presence of strong scintillation, it depends largely on the strength of scintillation as well as the number of photoelectrons in the pulse. It is also found that the detection error depends largely on the pulse width and the resolution time of the photodetector in the presence of atmospheric scintillation.

Optical pulsed ranging: effect of atmospheric scintillation on measurement accuracy

The ranging error of short pulse optical tracking systems is investigated. The relationships between the tracker characteristics, the characteristics of the received signal and background radiation, and the measurement accuracy are determined. It is found that the effects of characteristics of the tracker, the propagating medium, and/or the target to be tracked will predominate in the derived accuracy expressions. This result is based on the assumption that the transmitted pulse duration is short compared to the delay fluctuations induced by at least one of these three effects.

H.C. Salwen

Papers

Error analysis of optical range measurement systems