BADC Help File: ISAMS L2 Data

This file contains a summary of the instrument documentation to help you use the ISAMS L2 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

Restricted access ISAMS data held at the BADC is at level 2 version 8 If you would like to use the level 2 data, please apply for access to ISAMS Level 2 data . You will then be given a web account to access the restricted ISAMS data.

2.1. Horizontal Range

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. Vertical Range

The vertical range depends on measured parameters.

2.3 Temporal Range

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 

Each Level 2 file contains data from a single UT day. ISAMS acquired science data over the following periods:

26 Sep 1991 - 14 Oct 1991 (UARS Day  15 -  33)
28 Oct 1991 - 18 Jan 1992 (UARS Day  47 - 129)
27 Mar 1992 -  2 Jun 1992 (UARS Day 198 - 265)
19 Jul 1992 - 29 Jul 1992 (UARS Day 312 - 322)

2.4 Resolution

2.4.1 Vertical Resolution
Generally 0.05° elevation increments (approx. 2.5km). Please refer to the validation document associated with each version of the ISAMS data.

2.4.2 Horizontal Resolution
Horizontal Resolution is 400km in line-of-sight direction, 400km or 200km along tangent track, depending on measured parameter. 400km is approximately 4° lat near the equator.

2.5 Data Quality

Please, refer to the individual validation reports for details (e.g. Rodgers et al, 1994).

2.6 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)