HRDI Data are Produced
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.
when combined with data from other UARS instruments, will greatly
enhance our understanding of the processes that occur in the earth's
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.
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
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).
is yawed thru 180 degrees approximately every 36 days, in order to
keep the sun on the same side of the spacecraft.
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.
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.
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.
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.
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.
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:
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)).
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.
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
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.
(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.
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 .
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:
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.
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.
are then written out to the Level 2B file, processing continuing until
the entire Level 2A file has been read and processed.
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).
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.
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).
HRDI products are available (or will soon become available) in the
level 3A format:
and MLT winds (meridional and zonal) in meters/second
MLT 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)
to the measured parameters and their variances, the data collection
conditions are included in the data set (time, location, spacecraft
orbital parameters, etc.).
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.
Update: Thursday, 12-July-2001