Temperature and humidity profiles may vary significantly with time and location. The major advantage of satellites is their capacity for regular, long-term observation on a global scale.
Atmospheric sounding using spaceborne instruments basically works like this: the brightness temperatures measured in the spectrum lines of CO2 absorption bands is strongly correlated to the vertical profile of the atmospheric temperature. The radiant energy measured by IASI is the sum of thermal emissions from the different absorbing layers of the atmosphere.
By selecting wavelengths so that emissivity (absorption) is maximum in a given layer and weak in the other layers, and assuming that emittance is known at this level (this is possible for gases such as CO2), we can use measurements at each wavelength to derive the temperature of the corresponding layer. The set of measurements provides the whole temperature profile.
Similarly, if the temperature profile is known, measurements in water vapour (or ozone) absorption bands allow us to derive the humidity profile (or ozone vertical distribution).
The NOAA/HIRS operational sounder (which offers a spectral resolution of about 10 cm-1) currently provides an accuracy of around 2.5 Kelvin for temperature and 40% for humidity.
Better spectral resolution (0.3 to 0.5 cm-1) is required to improve accuracy. The targets for IASI are one Kelvin and 10%. That kind of resolution would enable us to derive contents of chemical components present in small concentrations in the atmosphere but which play a major role in the greenhouse effect, i.e., ozone, methane, nitrous oxide and carbon monoxide.
In addition, IASI has the ability to measure the total column content of the main greenhouse gases. It will supply valuable data for scientific studies to achieve a closer understanding of climate processes and to represent them better in global models.
The methods to derive these products with the required accuracy are studied by the ISSWG in the framework of the Preparatory Programme. The studies have shown that very accurate Radiative transfer models (Line by Line models), like the French model 4A/OP, include all the spectral features needed for IASI simulation. The accuracy of existing models has been compared to existing in situ (airborne) measurements allowing to quantify forward model errors needed in inversion techniques. It was also shown that IASI will enable some studies in the atmospheric chemistry domain.
(See Documentation on Science results)
Overall, the scientific studies have shown that mission requirements will be met and that for some parameters the accuracy, which could be achieved, is better than specified. Moreover, additional products of interest for atmospheric and climate studies can also be derived.
Science Plan Objectives: The science plan IASI_Science_Plan_Issue1_released_version.pdf (pdf - 665.76 KB) (PDF) describes the scientific work needed to meet the IASI mission objectives and it provides a framework for required scientific research and development activities.
The IASI instrument observes and measures twice a day the spectrum of infrared radiation emitted by the Earth from a low altitude sun-synchronous orbit, over a swath width of 2000 km. These measurements are compatible in terms of sampling, resolution, accuracy and overall performances with the mission objectives of providing information on:
- Profiles of temperature in the troposphere and lower stratosphere with an accuracy of 1 Kelvin, a vertical resolution of 1 km in the low troposphere and a horizontal sampling of 25 km.
- Profiles of water vapour in the troposphere with an accuracy of 10% on relative humidity, a vertical resolution of 1 km in the lower troposphere and a horizontal sampling of 25 km.
- Total amount of ozone with an accuracy of 5% and a horizontal sampling of typically 25 km, possibly also ozone vertical distribution with an accuracy of 10% and a vertical resolution providing two or three pieces of independent information.
- Fractional cloud cover and cloud top temperature/pressure.
- Sea and land surface temperatures.
IASI is a key element of the meteorological sounding package of EUMETSAT Polar System (EPS).
Some of the IASI features have been adjusted to be compatible to the other meteorological instruments. This refers to the simultaneous operation and identical geographical coverage for all instruments and co-registration between the infrared (IASI, HIRS) and microwave (AMSU-A, MHS) instruments.
Furthermore, co-registration of 1 km is required between the IASI sounder and the AVHRR imager. The IASI scan pattern have been synchronized with those of the microwave instruments MHS and AMSU-A.
The defined target life time for IASI is five years. The probability of IASI to being fully functional and operating without significant loss in performance after five years in orbit operation must be at least 0.8. For METOP-A, to this day, the five years ended. No redundancy or performance decrease was reported during the successive REVEX (Exploitation Review) for the instrument.
The operation of IASI mission plan includes two distinct phases after the launch. The first one is the functional in-flight commissioning (duration for METOP-A: 10 weeks), and the second one is the CAL/VAL (duration for METOP-A: 25 weeks). The CAL/VAL enabled to disseminate the products to a restricted scientific community (early dissemination) since May 24th, 2007. The operational phase began on July 19th, 2007 and will last until the rest of the life-time of METOP-A satellite.
Very quickly, the IASI system, composed of the IASI instrument and its ground segment, enabled a daily permanent coverage of at least 95% of the Earth without any significant systematic gap in coverage was performed.
Over the Atlantic Ocean, Europe and the Mediterranean Sea the objective was to reach 99 % coverage per day. This objective has been attained.
The operational processing of IASI data on-ground provides the users community with sounding data not later than 135 minutes after sensing. The software for this processing has been developed by CNES (L1/PPF as known as OPS) and provided to EUMETSAT that integrated it to the METOP satellite data processing operational centre.