Lidar Simulation Model: Instruments

Lasers

The LSM currently simulates the performance of coherent Doppler lidars as laser-based remote wind sensors with an emphasis upon realistic representations of the atmosphere along individual line of sights. The existing atmospheric data bases support 2.0158, 9.11, and 10.59 m wavelength lidars.

In late 1997, upgrades to the atmospheric databases to include additional wavelengths, ranging from 0.3-1.6 m range was started in order to fully support other laser types in the LSM such as direct detection lidars and DIAL.

Scanner Model

The scanner model computes the latitude and longitude of each lidar shot as a function of the laser pulse or atmospheric level, nadir scan angle and azimuth scan angle. The model uses an oblique spherical triangle algorithm (Kells, L. M. et. al, 1940) that solves for a spherical triangle defined by the north pole, the position of the satellite and the position of the shot. The Shot Coverage Model (SCV) supports conical, fixed-beam and step-stare beam scanners. The timing of the shots are determined by the shot management algorithms.

Scanner System Geometry

The scan geometry angles and swath width are computed as shown below

where

a - satellite to shot to center of the earth angle (rad),

p - the constant, 3.14159,

Z_s - satellite altitude (km),

R_e - radius of earth (km),

f - nadir scan angle (rad),

g - satellite to earth's center to shot angle (rad),

Q - slant path elevation angle (rad),

SW - swath distance (km).

Conical Scanner

The conical scanner gives the latitude and longitude of the lidar shot and the azimuth scan angle for a counter-clockwise scanning lidar. The user can apply a cosine modification to the conical scan.

Fixed Beam Pointing

The fixed-beam pointing gives the latitude and longitude of the lidar shot for up to twenty telescope azimuth dwell angles. Each telescope is configured with a firing time and a prf associated at a fixed azimuth angle, a cycle wait period between firing times and a cycle wait period between a set of telescope angles.

Step-Stare

The Step-Stare scanner gives the latitude and longitude of the lidar shot for up to twenty telescope azimuth dwell angles. Each telescope is configured with a firing time and a prf associated at a fixed azimuth, a firing time and prf associated with a clockwise or counter-clockwise conical scan and a cycle wait period between a set of telescope angles.

Laser Shot Management

Unlike most passive sensors in space, active laser based systems have limited lifetimes (pulses) and are ultimately constrained by available platform power. Such conditions call for some form(s) of resource management that will optimize the number of useful observations and the potential impact on the primary mission objective - e.g., improved understanding of the global circulations and transports.

Management of the lidar pulses is one of the primary foci of the research under this contract. The objectives of shot management include:

1) to extend mission lifetime;

2) to optimize, within a scan, the distribution of shots to obtain best wind measurements; and

3) to optimize the global distribution of shots within an orbit to favor regions of high ageostrophy (i.e., tropics, jet streams, major mountain ranges, etc.).

To meet these objectives, seven modes of shot management have been defined in the table below.

Note 1: Ratio of shots taken per orbit for each mode compared to Mode 1.

While the detailed options of scheduling lidar pulses are unlimited, the general sense of the management is to use a finite number of shots to achieve the best set of data for a given mission objective. For example, if the mission objective is to provide full global coverage every 12 hours, then a combination of modes 2 and 3 is in order. If the mission objective is to provide direct measurements of winds in regions of ageostrophic flows, then a combination of modes 4 and 5 may be proper. If the mission objective is to provide data preferentially in regions where a forecasting model is having difficulty, the mode 7 would be employed.

The SCV model is designed to invoke modes 1 through 5. The most common mode combination is 2 plus 3. Mode 2 applies only to the conical scanner.