Active remote sensing uses artificially generated radiation, emitted by a satellite or ground- based instrument and received by the same instrument or another receiver. This radiation interacts with the earth's surface or atmosphere, and from the measurements meteorological and other information can be derived for eventual use in numerical weather prediction models. Typical wavelengths used include microwaves (wavelengths of a few millimetres) to the optical and ultraviolet regions, depending on the application. The method contrasts with passive remote sensing techniques where an instrument measures the natural radiation signature from the atmosphere or surface.

A scatterometer is a radar system which measures the level of transmitted energy which has been backscattered from the surface at two or more azimuth angles. Over the ocean, backscattering is due to in- phase (Bragg) reflections of the microwave radiation transmitted by the scatterometer. This backscattering is due to the presence of trains of wind-generated waves of the order of 5- 20 cm wavelength. Because these small waves tend to lie perpendicular to the wind direction, there is a larger backscatter in the up or downwind look direction than the crosswind direction. Wave amplitude, and hence backscatter, also increases with increasing wind speed. From an empirical model relating wind vector and backscatter for a given geometry, both wind speed and direction close to the surface can be derived from two or more backscatter measurements of the same area taken from different look directions.

The second generation of scatterometer was included as part of the Advanced Microwave Instrument (AMI) on board the European Space Agency's European Remote Sensing Satellite, ERS-1, launched in 1991. An identical AMI instrument was launched on ERS-2 in 1995; this satellite took over from ERS-1 as the 'operational' satellite in June 1996, and continued to produce data - although a technical failure limits coverage to the North Atlantic. The Satellite Active Sensing Group is currently involved in investigating new scatterometer instruments for use in NWP. Wind retrievals from the QuikScat/SeaWinds instrument, launched in 1999, have been assimilated into the Met Office's operational NWP model since December 2002, and similar data from the ASCAT instrument on MetOp will become operationally available by early 2007.

The GPS radio occultation (RO) technique is based on measuring the path of radio waves propagating between two satellites through the atmosphere. This path is bent by atmospheric refractive index gradients. The variation of ray bending with height above the surface can be inverted to yield the refractive index as a function of height. In regions where the atmosphere is dry, this information can be combined with the ideal gas law and the hydrostatic equation to give a temperature profile. The accuracy of the temperature profiles is ~1.5K with a vertical resolution of ~1 km. The measurements are globally distributed and have an-all weather capability. Recent work within the Satellite Active Sensing Group has focused on how GPSRO data should be assimilated into an NWP system. A 16-day NWP global model assimilation trial performed at the Met Office, using refractivity profiles from the CHAMP instrument and all other available data sources, demonstrated significant improvements in the 250 hPa temperature field RMS fit to radiosondes in the southern hemisphere, with an improvement in fit of ~0.1K at 24 to 96 hour forecast ranges. We have developed the capability to operationally assimilate GPSRO data from missions such as CHAMP, COSMIC and GRACE, and became the first operational NWP centre in the world to begin operational assimilation of GPSRO on 26 September 2006, using data from the CHAMP and GRACE- A instruments.

The Met Office is a partner in the EUMETSAT GRAS Meteorology SAF:

1. to develop data assimilation methods using the GRAS radio occultation instrument, scheduled for launch on Metop;
2. to deliver the Radio Occultation Processing Package (ROPP) to assist other NWP centres to exploit RO data in their models;
3. to analyse the data flow and validate the quality of data delivered to users by near- real time continuous monitoring.

Path delays to GPS satellite signals during propagation through the troposphere to a ground receiver can be modelled with millimetric accuracy. These delays can be used to produce estimates of vertically integrated water vapour(IWV); ground-based GPS data is therefore of potential use for both NWP and nowcasting applications. We worked closely with several other groups from the geodetic and meteorological communities in Europe through the recently completed EU COST Action 716 and TOUGH projects and ongoing E-GVAP project to develop the networks, processing and assimilation techniques necessary to exploit this potential source of data. Assimilation experiments have demonstrated slight positive forecast impacts overall, and significant improvements to short range forecasts in some cases.

The Doppler Wind Lidar technique, as its name suggests, uses a lidar to detect the Doppler shift of the light backscattered from atmospheric molecules or aerosols. The Doppler shift is directly related to the wind speed in the line-of-sight of the lidar beam; since vertical wind speeds are relatively small, the instrument is assumed to measure the horizontal wind speed at the azimuth angle of the beam - the 'horizontal line-of-sight' wind (HLOS). It would need an azimuth-scanning instrument, or two synchronised instruments in different orbits, to measure the true vector wind.

The ESA Atmospheric Dynamics Mission (ADM) or 'Aeolus' is planned for launch in 2008. This mission is dedicated to the measurement of wind profiles using the DWL technique, but being a demonstrator mission, is limited to deriving the HLOS only. Such a measurement can only be sensibly interpreted by assimilation into NWP models. The Satellite Applications section is starting to investigate how HLOS winds can be assimilated so this data source can be used soon after launch of ADM-Aeolus.