BADC Help File: ISAMS L3A Data

This file contains a summary of the instrument documentation to help you use the ISAMS L3A data held at the BADC


1. The Instrument

This is a brief, simplified outline of the ISAMS instrument and the measurement techniques which are employed. Users requiring a more detailed description of the design and operation of the instrument should consult the paper by Taylor et al. (1993).

1.1 Instrument Description

ISAMS is an infrared radiometer which measures thermal emission from the Earth's limb. Measurements are made in eight channels in the 4-17 microns range using pressure modulation and wideband radiometric techniques to select regions of the spectrum appropriate to the species to be measured.

The instrument observes the earth's limb in a direction normal to the orbital track (with a small offset to correct for the Doppler shift due to the Earth's rotation). The tangent point is a great circle distance of approximately 23° (approx. 2500km) away from the sub-satellite track.

A switching mirror is included in the optical path to allow the instrument to view the limb on either side of the spacecraft velocity vector, but in practice viewing on the "Sun-side" of the spacecraft is limited to periods when the Sun is behind the Earth so that solar radiation cannont enter the instrument.

The primary optics of the instrument focus the radiation onto a chopper which modulates between the Earth's limb and cold space at a frequency of 1kHz, giving a zero-radiance reference signal. The gain is measured relative to an internal black body calibration source at 290K.

The incoming beam is divided into 8 separate pressure modulator channels, each of which contains a sample of the gas to be measured. ISAMS uses pressure-modulator cells for each of the following gases : CO2 (times 2), CO, CH4, N2O, NO2 and H2O. The two CO2 channels can be operated at different pressures which ensures that temperature measurements may be made over a extended range.

Each of the two CO2 channels also contains a filter wheel to allow wideband measurements of three key chemical species - ozone (O3), nitric acid (HNO3) and dinitrogen pentoxide (N2O5) - none of which can be isolated in a gas cell. There is also a `window' channel at 12.1 microns which is relatively free of molecular absorption and can be used in the retrieval of aerosol opacities.

1.2 Measurement Techniques

The measurement technique used by ISAMS is known as Pressure Modulation Radiometry. The incoming radiation in each channel passes through a cell containing a sample of the gas to be measured, in which the pressure is modulated at a frequency of about 30Hz.

A multi-layer interference filter in front of the detector restricts the radiation reaching the detector to a narrow part of the vibration-rotation band of the relevant species.

The signal processing electronics demodulates the signal from the detector at the chopper frequency to give a wideband signal for the spectral band defined by the interference filter. This "wideband" signal is then further demodulated at the pressure modulation frequency. This pressure modulated signal originates in or near the spectral lines of the species in the gas cell and is therefore very selective for radiation originating from that species in the atmosphere.

1.3 Data Processing Techniques

The UARS data processing is carried out at the Central Data Handling Facility at the Goddard Space Flight Center using software supplied by the Instrument's Principal Investigator group.

The data processing for UARS instruments consists of a progression through a sequence of `levels' from the raw telemetry at level 0 to geophysical quantities interpolated onto standard grids at level 3. The processing steps for ISAMS are outlined below :

Level 1 processing
At the level 0-1 processing step, instrument-specific effects are removed and a set of calibrated data are derived in physical units (eg. voltages and radiances) tagged with their locations.

Level 2 processing
The level 1 data are then processed further to produce the level 2 product which contains vertical profiles of temperature, pressure and mixing ratios of chemical constituents at the measurement positions. This step involves a complex inversion algorithm which consists of two principal stages :

Firstly the temperature and pressure are retrieved using the radiances from the CO2 channels, using a sequential estimation scheme, or Kalman filter, similar to that described by Rodgers et al (1984) for SAMS data. This method combines the radiance data with retrievals from neighbouring profiles and with climatology to produce a statistically optimal result.

The temperatures and pressures are then combined with the data from the constituent channels to retrieve vertical profiles of the constituents using a maximum likelihood estimator.

Level 3A processing
The level 2 data are profiles located at the measurement positions which are determined by the scan pattern and by the track of the tangent point. The level 2-3A processing step takes these data and interpolates them onto a standard set of vertical levels -- evenly spaced in log pressure, and onto standard times (level 3AT) and standard latitudes (level 3AL).

2. The Data

Public ISAMS data held at the BADC is at level 3A version 10 except for H2O where the recommendation is to use version 9. At version 10 there are no O3 files for October 15 (Day 34) or Aerosol6 files for April 5 (Day 207). The BADC also holds version 8 data. All of these data are available through the WWW or by user-identified FTP (i.e. FTP service is available to registered users only).

2.1 Spatial Coverage

2.1.1 Vertical Coverage

The table below gives an estimate of the vertical coverage of each parameter in the ISAMS Level 3A files. Limitations and further information are contained in the Level 2 dataset document.

Parameter             Pressure Range    Altitude Range  
                             mb              km        
Temperature             100  - 0.01        16 - 80         
Pressure                100  - 0.01        16 - 80         
Carbon Monoxide         10   - 0.03        31 - 75         
Water Vapour            10   - 0.1         31 - 66         
Methane                 10   - 0.3         31 - 57         
Ozone                   100  - 0.2         16 - 60         
Nitric Acid                  *                *            
Dinitrogen Pentoxide    100  - 1           16 - 48         
Nitrogen Oxide          100  - 0.2         16 - 60         
Nitrogen Dioxide        100  - 0.3         16 - 57         
Nitrous Oxide                *                *            
aerosol                  68  - 15          19 - 29         

 * Indicates that this species was not retrieved
   for version 8, 9 or 10.

The vertical coverage of the ISAMS instrument is shown schematically in Figure 1. which compares the vertical coverage of the UARS instruments.

2.1.2 Horizontal Coverage

Views of the limb from the UARS orbit extend to approximately 10° of the pole. To achieve maximum latitude coverage, the instrument views at right angles to the orbit track.

A switching mirror allows the instrument to observe on either side of the orbital track, so that in principle the coverage could extend from 80°N to 80°S, every day. In practice, viewing is only possible from the Sun-viewing side of the spacecraft when the Sun is behind the Earth, so that solar radiation cannot enter the instrument. Generally, therefore the coverage is restricted to the anti-Sun side of the spacecraft between 34° in one hemisphere and 80° in the other.

The spacecraft performs a 180° yaw approximately every 36 days so that the spacecraft orientation relative to the velocity vector is reversed. (eg. the spacecraft flies `backwards' rather than `forwards').

Each spacecraft yaw manoeuvre changes the coverage from high latitudes in one hemisphere to high latitudes in the other. Thus, when the spacecraft files `forwards', the coverage is 80° S to 34°N, and when it flies `backwards' the coverage is 80°N to 34°S.

The latitudinal coverage of the ISAMS instrument is compared with the other UARS instruments in Figure 2.

2.2 Temporal Coverage

ISAMS operated from shortly after the UARS launch in October 1991 until the end of July 1992 when failure of the instrument's chopper halted operations. However there were several periods within these dates when the collection of data ceased because of operational difficulties. These are summarised below :

     Date              UARS day         Comments
12th Sept. 1991          1         Launch 
28th Sept  1991         17         Beginning of operations 
15th Oct.  1991         34         Scan Mirror Problem 
                                   Data collection ceased 
28th Oct.  1991         47         Problem fixed 
                                   Data collection resumed 
18th Jan.  1992        129         Chopper failed 
                                   Data collection ceased 
17th Mar.  1992        198         Chopper recovered 
                                   Data collection resumed 
2nd  June  1992        265         Solar Array Problem 
                                   Data collection ceased 
19th July  1992        312         Problem fixed 
                                   Data collection resumed
29th July  1992        322         Chopper failed 
                                   Data collection ceased 


2.3 Resolution

2.3.1 Vertical Resolution

The UARS level 3A data are interpolated onto a standard set of logarithmically spaced pressure levels. There are 6 levels per decade in pressure, given by the following relation :

p = 1000 * 10 ^ (i/6), where i= 0, 1, 2... 

2.3.2 Horizontal Resolution

The level 3AT and level 3AL products differ in their horizontal gridding.

The 3AT products are interpolated along the tangent-point track at standard output times corresponding to intervals of one UARS Engineering MAjor Frame (1 EMAF = 65.536 s). This period represents an along- track displacement of approximately 400 km between measurements. The profiles are constructed by linear interpolation in observation time from the level 2 profiles. Note that the level 2 files have a higher resolution along track, each level 3 profile being constructed from two level 2 profiles.

The 3AL products are interpolated onto positions defined by the intersection of the tangent track with a latitude circle at standard 4° latitude intervals

Associated with each level 3A data file there is a parameter file (3TP with 3AT and 3LP with 3AL), which contains information on the ISAMS operating conditions which cannot be accommodated in the standard UARS level 3A data format.

2.4 Data Quality

The data quality is summarised in the product quality summary which is available online from the BADC ISAMS documentation directories.

2.5 Units

The units for the parameters present in the ISAMS data files are tabulated below :

Parameter               Units   
Temperatures            K
Pressures               mb
Mixing Ratios           by volume
Aerosol extinction      km(-1)