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| How HRDI Data are Produced | |
HRDI and UARSThe High Resolution Doppler Imager (HRDI) is the primary instrument onboard the Upper Atmospheric Reseasch Satellite (UARS) for measuring the dynamics of the stratosphere and mesosphere. The goal of HRDI is to measure wind velocities in the stratosphere, mesosphere, and lower thermosphere during the day with an accuracy of 5 m/s, and also to measure the winds in the mesosphere and lower thermosphere at night. HRDI determines winds by measuring the Doppler shifts of atmospheric absorption and emission features. Line of sight wind measurements are taken in two directions, thus allowing the wind vector to be formed. This data, when combined with data from other UARS instruments, will greatly enhance our understanding of the processes that occur in the earth's middle atmosphere. The fall 1991 UARS launch provided coverage of at least two northern hemisphere winters, a key requirement, but interruption of data taking by HRDI from 2-June-1992 thru 22-July-1992, caused by a failure of the UARS solar array, created a significant data loss. The primary products of HRDI measurements are winds in the stratosphere (10 to 40 km.), and mesosphere/lower thermosphere (50 to 125 km). Secondarily, it is possible to recover temperature, O2 atmospheric band volume emission rate, and O3 and O(1D) mixing ratios in the mesosphere and lower thermosphere, and aerosol and molecular extinction coef. in the stratosphere. The UARS spacecraft has a nearly circular, 585 kilometer apogee orbit about the earth, with an inclination of roughly 57 degrees to the equatorial plane. The orbit precesses about 5 degrees per day relative to the sun, thus over a period of about 72 days the entire diurnal cycle can be sampled (for each day's worth of data, the local solar time will be nearly a constant function of latitude). The satellite is yawed thru 180 degrees approximately every 36 days, in order to keep the sun on the same side of the spacecraft.Instrument SamplingWith this orbit, as HRDI makes its measurements above the limb of the earth , the instrument should provide coverage of the earth from -72 to 72 degrees latitude in total. Since HRDI is typically viewing on only one side of the spacecraft (northward or southward) during a single orbit, the coverage it can provide is about 35 degrees smaller (typically -72 to 40 if viewing south, or -40 to 72 if viewing north). This coverage is also mitigated by the available sunlight - HRDI can provide some measurements at night, but only provides full sets of measurements during the daytime. The HRDI instrument provides a wind profile measurement on the order of once per minute. During periods when the spacecraft is in darkness, wind measurements are confined to a narrow altitude band (near 95 km) in the mesosphere/lower thermosphere, where an emission layer provides the necessary signal level to measure winds accurately. The operating mode of the instrument determines which of the other quantities are measured, and how often a measurement is made. HRDI measures the Doppler shift of absorption and emission lines of molecular oxygen in the atmosphere, mainly looking above the limb of the earth. Its main product, winds, are measured by finding the Doppler shift of the atmospheric lines, then adjusting this value to compensate for the velocity of the spacecraft. In a typical science operation, the telescope is pointed at a 45 (or 135) degree angle to the spacecraft (s/c) velocity vector and 'scan' up or down in tangent height altitude. The telescope pauses in its up/down motion to collect several integration periods (each is 0.125 seconds long) of data at each of several tangent points (selected heights above the limb of the earth). Different wavelength bands are used for different altitude ranges, to maximize information content in the collected signals for the altitude in question. HRDI currently samples the stratosphere using the 'b' and 'gamma' bands, and the mesosphere/lower thermosphere('MLT') using the 'a' band. A typical operational cycle will be to scan up and down at a fixed telescope azimuth (ex. 45 deg. with velocity vector), then slew the telescope to a new azimuth (ex. 135 deg. with velocity vector), repeat the scan cycle, then slew back to the original azimuth to start a new cycle. This is done so that the same volume in space can be seen from two directions (looking forwards and, later, looking backwards), to allow us to form a true wind vector from the two LOS (line of sight) components. Since the HRDI telescope is capable of viewing either side of the spacecraft (north or south) or either region of the atmosphere, sampling can be changed from day to day to study different aspects of the atmosphere.Data Processing OverviewRaw instrument and spacecraft telemetry are transmitted from the UARS spacecraft, and eventually received at the NASA Goddard Space Flight Center (GSFC) Central Data Handling Facility (CDHF). These data are processed at the CDHF to produce daily files of results for each instrument at several levels of complexity/reduction. The files are archived of the CDHF system, and later the highest level products are copied to the GSFC Distributed Active Archive Center (DAAC) for long-term storage. Data processing at CDHF for HRDI data can be outlined as follows: The raw telemetry is used to generate Level 0 files (containing raw instrument values, ex. counts). These files are further processed to produce Level 1 files (in which measured levels have been converted to meaningful scientific values (ex. volts)). The Level 2A HRDI data processing software reads these Level 1 files, gathers the data into significant bundles ('tangent heights' and 'scans'), and processes the bundles to produce Level 2A files (atmospheric measurements - spectra, etc. - valid along the line of sight of the instrument at each tangent height). A 'tangent height' is defined as the data taken during the time in which the instrument is staring at a certain location in space (usually refered to as being at a given distance above the earth's surface). A 'scan' is a collection of consecutive tangent heights, between which the instrument's movement direction (up or down) is constant. A scan is intended to contain a column of tangent heights above the same point on the earth's surface. When a scan of data has been accumulated, the data are analyzed by first finding the average signal in each channel at each tangent height (HRDI produces 32 channels of data, 31 from the interferometer and one from the photometer). The signal is corrected for instrument dead time, the dark counts (background) values are subtracted from the data, the signal level for each channel is converted to Rayleighs per wavenumber, and the line of sight velocity of the atmosphere is calculated using the Doppler shift of an atmospheric absorption or emission line. The line of sight (l.o.s.) velocity is then corrected for the spacecraft velocity, and a weighted average of the data (which may include data with different s/c velocity components) is formed. The scan (which now consists of averaged measurements, calculated l.o.s. velocities, and the associated collection conditions for each of several tangent heights) is then written out as a Level 2A data record. Level 2A files are read, in turn, by the HRDI Level 2B processing software, gathered into meaningful bundles (sets of scans which are consecutive in time and are of the same 'type' (the instrument was operating in the same mode)), and processed to produce Level 2B files . The Level 2B processing software reads the 2A file, then analyzes the data to produce 'profiles' (measurements of various quantities (ex. wind) at each of several tangent heights in a column in a radial direction above the earth), as follows: The 'average' values for each scan are determined (latitude, longitude, time, etc.), along with the atmospheric state for the scan's location. The atmospheric state, including the temperature, pressure, O2 density, and aerosol and molecular extinction coefficients, is determined from the results of prior processing if such results are available. If prior results are not available model values are used. 'Kernels', which show how much of a single LOS measurement comes from a certain altitude along the line of sight, are calculated for each species to be recovered, and stored in a scratch file. The recovery at an individual location is inverted, and these are averaged with nearby recoveries using a sequential estimation technique to reduce noise. The profiles are then written out to the Level 2B file, processing continuing until the entire Level 2A file has been read and processed. Level 2B files are read by the HRDI Level 3 processing software, which interpolates the profile data to standardized grid points (locations and times), and writes this data as Level 3 files (one file for each type of profile in the Level 2B files, and for each gridding method). In the Level 3 data processing software, each profile is read sequentially from the Level 2B source file, and the location, time and type (which region of the atmosphere does the profiles contain data for, and what type of data (velocity, temperature, etc.) does it contain) are stored. The sequence of profiles of each type is divided into subsequences called 'semi-orbits'. The HRDI Level 3 processing analyzes all data (for each atmospheric region and data type) for one semi-orbit at the same time, pairing profiles for different atmospheric regions (so a single Level 3A profile can represent all results in a profile above a single point (i.e. both mesospheric/lower thermospheric and stratospheric results), filling points on the standard latitude and/or time grid by interpolation, and transforming data onto a pressure grid using NMC pressure/altitude data. Each Level 2B file should produce several Level 3 files, depending on the operating mode of the instrument at the time the data was produced (a different Level 3 file is produced for each species/measurment type and grid).Measured ParametersThe following HRDI products are available (or will soon become available) in the level 3A format: stratospheric and MLT winds (meridional and zonal) in meters/secondMLT O2 band volume emission rates in photons/cm**3/sec MLT temperatures in degrees Kelvin mesospheric O3 mixing ratios * (unitless) mesospheric O(1D) mixing ratios * (unitless) srtatospheric molecular and aerosol extinction coefficients * in km**-1 (* = data product not yet available) In addition to the measured parameters and their variances, the data collection conditions are included in the data set (time, location, spacecraft orbital parameters, etc.). Measurements are made approximately every minute, and have 2.5 km vertical and 250 km horizontal resolutions. The measurement interval and resolution can vary according to the operational mode of the instrument. HRDI Home Page Last Update: Thursday, 12-July-2001 |
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