This paper reports on a new satellite sensor, the Geostationary Earth Radiation Budget (GERB) experiment. GERB is designed to make the first measurements of the Earth's radiation budget from geostationary orbit. Measurements at high absolute accuracy of the reflected sunlight from the Earth, and the thermal radiation emitted by the Earth are made every 15 min, with a spatial resolution at the subsatellite point of 44.6 km (north-south) by 39.3 km (east-west). With knowledge of the incoming solar constant, this gives the primary forcing and response components of the top-of-atmosphere radiation. The first GERB instrument is an instrument of opportunity on Meteosat-8, a new spin-stabilized spacecraft platform also carrying the Spinning Enhanced Visible and Infrared (SEVIRI) sensor, which is currently positioned over the equator at 3.5 degrees W. This overview of the project includes a description of the instrument design and its preflight and in-flight calibration. An evaluation of the instrument performance after its first year in orbit, including comparisons with data from the Clouds and the Earth's Radiant Energy System (CERES) satellite sensors and with Output from numerical models, are also presented. After a brief summary of the data processing system and data products, some of the scientific studies that are being undertaken using these early data are described. This marks the beginning of a decade or more of observations from GERB, as subsequent models will fly on each of the four Meteosat Second Generation satellites.

The Differential Absolute Radiometer is making measurements of the total solar irradiance as part of the Variability of Irradiance and Gravity Oscillations experiment on the Solar and Heliospheric Observatory. We present the measurements made during its 7.8 years of operation, from 1996 until the present (2003). The aging due to UV exposure of the continuously measuring left channel is determined by comparison with the backup right channel; the loss in sensitivity of the left channel is 0.5 W/m2 or 364 ppm over 7.8 years. A raise of the total solar irradiance from a level of 1365.5 W/m2 at the end of the solar minimum in 1996 towards a maximum level of 1368 W/m2 at the beginning of 2002 has been measured by DIARAD.

Life on Earth is possible because the climate conditions on Earth are relatively mild. One element of the climate on Earth, the temperature, is determined by the heat exchanges between the Earth and its surroundings, outer space. The heat exchanges take place in the form of electromagnetic radiation. The Earth gains energy because it absorbs solar radiation, and it loses energy because it emits thermal infrared radiation to cold space. The heat exchanges are in balance: the heat gained by the Earth through solar radiation equals the heat lost through thermal radiation. When the balance is perturbed, a temperature change and hence a climate change of the Earth will occur. One possible perturbation of the balance is the CO2 greenhouse effect: when the amount of CO2 in the atmosphere increases, this will reduce the loss of thermal infrared radiation to cold space. Earth will gain more heat and hence the temperature will rise. Another perturbation of the balance can occur through variation of the amount of energy emitted by the sun. When the sun emits more energy, this will directly cause a rise of temperature on Earth. For a long time scientists believed that the energy emitted by the sun was constant. The 'solar constant' is defined as the amount of solar energy received per unit surface at a distance of one astronomical unit (the average distance of Earth's orbit) from the sun. Accurate measurements of the variations of the solar constant have been made since 1978. From these we know that the solar constant varies approximately with the 11-year solar cycle observed in other solar phenomena, such as the occurrence of sunspots, dark spots that are sometimes visible on the solar surface. When a sunspot occurs on the sun, since the spot is dark, the radiation (light) emitted by the sun drops instantaneously. Oddly, periods of high solar activity, when a lot of sunspot numbers increase, correspond to periods when the average solar constant is high. This indicates that the background on which the sunspots occur becomes brighter during high solar activity.

The Geostationary Earth Radiation Budget (GERB) instrument, to be launched on the MSG-1 satellite in January 2002, will be the first broad band radiometer in a geostationary orbit. GERB data will be processed in near real time in the GERB ground segment, distributed over the Rutherford Appleton Laboratories (RAL) and the Royal Meteorological Institute of Belgium (RMIB). RAL is responsable the calibration of the measured filtered broad band radiances and for their geolocation. RMIB is responsable for the determination of unfiltered radiances and fluxes.Unfiltered radiances and fluxes will be derived at GERB resolution - corresponding to 50 km sampling distance at nadir - and at 3x3 SEVIRI pixel resolution corresponding to 9 km sampling distance at nadir, The time resolution is 15 minutes.The unfiltered radiance and flux products will be available in near real time at RMIB for weather applications. A long term archival and user service will be provided by RAL for GERB resolution radiances and fluxes.

On long timescales the variation of the total solar irradiance (TSI) received by the Earth is believed to be one of the climate change drivers. Therefore accurate and time‐stable measurements of the total solar irradiance are necessary. The Solar Constant (SOLCON) instrument made TSI measurements in April 1992 and during the International Extreme Ultraviolet Hitchhiker (IEH) 3 STS 95 shuttle flight in autumn 1998. We assume that the SOLCON instrument remained stable within 0.01 % in between those measurements, and we verify this assumption as well as possible. From the SOLCON measurements we conclude the following: (1) The 1998 Space Absolute Radiometric Reference (SARR) adjustment coefficient applicable to the Active Cavity Radiometer Irradiance Monitor II (ACRIM II) during the IEH 3 period is 1.000438 with a one sigma uncertainty of 18 ppm, compared to the 1993 SARR adjustment coefficient of 1.000258. (2) The solar monitor on the Earth Radiation Budget Satellite (ERBS), with a 1993 SARR adjustment coefficient of 1.000453, has not aged within a one sigma uncertainty level of 130 ppm; and (3) the 1998 SARR adjustment coefficients for the Variability of Solar Irradiance and Gravity Oscillations (VIRGO) radiometers have been determined with a one sigma uncertainty of 10 ppm: They are 1.000025 for the Differential Absolute Radiometer left channel (DIARAD‐L) and 1.000279 for the version 1.2 data from the PMO6‐VA radiometer.

The PICARD microsatellite mission will provide 3 to 4 years simultaneous measurements of the solar diameter, differential rotation and solar constant to investigate the nature of their relations and variabilities. The 110 kg satellite has a 42 kg payload consisting of 3 instruments: SODISM, which will deliver an absolute measure (better than 4 milliarcsec) of the solar diameter and solar shape, SOVAP, measuring the total solar irradiance, and PREMOS, dedicated to the UV and visible flux in selected wavelength bands. Now in Phase B, PICARD is expected to be launched by 2005. We review the scientific goals linked to the diameter measurement with interest for Earth Climate, Space Weather and Helioseismology, present the payload and instruments' concepts and design, and give a brief overview of the program aspects.

PICARD is a CNES micro-satellite mission due for flight by the end of 2002, named after the name of a French astronomer who first observed with consistency the solar diameter changes during the Maunder minimum in the 16th century. It consists of two instruments measuring (i) the solar diameter and differential rotation, and (ii) the total solar irradiance. These quantities are fundamental for the understanding of the solar-Terrestrial relations, e.g. the influence of the Sun on the Earth's climate, and of the internal structure of the Sun. The continuous — or nearly continuous — viewing of the Sun from an appropriate orbit, the 5 minutes sampling rate and the very low noise measurements, will allow g-modes detection and precise diameter measurements besides accurately establishing the relationship between irradiance and diameter changes. Providing an absolute measure of the solar diameter to 1 milliarcsecond, PICARD is the first step towards instruments capable of accurate and perennial measurements, for the centuries to come, of the solar-terrestrial influence. The objectives of the mission, instrument capabilities, observing modes and performances are described.

The dispersion of the measurements that contributed to the previously defined space absolute radiometric reference (SARR) is investigated by objective statistical analysis. The estimated standard deviation with which the reference is known is 0.22 W/sq m, corresponding to 0.016 percent of its mean value. Several updates are made in the SARR referenced total solar irradiance data series, which was previously obtained from November 1978 until December 1993. The shift in 1898-1990 of the NIMBUS 7 instrument identified by Lee in 1995 is investigated and taken into account, resulting in new values for the NIMBUS 7 measurements before 1990 and in a new SARR adjustment coefficient for the active cavity radiometer irradiance monitoring (ACRIM) 1 instrument. The data series is extended to the present by adding level 1 data of DIARAD/VIRGO on board SOHO. A preliminary SARR coefficient for level 1 DIARAD/VIRGO was obtained by comparison with SARR referenced ACRIM 2 data.

A simple algorithm is presented for the evaluation of the spectral distribution of the solar radiation on a horizontal surface as a function of the measured total solar irradiance (global radiation). The method, valid for industrial applications (agriculture, aging of materials, etc.), is based on a triangular representation of the spectrum. Straight line algorithms between 300 and 465 nm and 465 and 900 nm are used, which are functions of the global radiation and mean adjustment coefficients. Our method has been tested on a complete year of spectral observations at eight wavelengths, as well as on typical spectral distributions for clear and overcast skies.

Experimental forecasts performed at the Royal Meteorological Institute of Belgium during the winters 1979–1980 and 1980–1981 are described and analysed. These forecasts are based on the evaluation of an air pollution potential index which uses three important meteorological parameters : wind speed, vertical stability and temperature.