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ASCAT – Metop's Advanced Scatterometer ascat ASCAT – Metop’s Advanced Scatterometer R.V. Gelsthorpe Earth Observation Programmes Development Department, ESA Directorate of Application Programmes, ESTEC, Noordwijk, The Netherlands E. Schied Dornier Satelllitensysteme*, Friedrichshafen, Germany J.J.W. Wilson Eumetsat, Darmstadt, Germany Introduction Figure 1 indicates the form of the variation of Wind scatterometers already flown on ESA’s backscattering coefficient with relative wind ERS-1 and ERS-2 satellites have demonstrated direction (at a fixed incidence angle) for a range the value of such instruments for the global of wind speeds. determination of sea-surface wind vectors. These highly successful instruments – part of If an instrument were to be constructed that the Active Microwave Instrument (AMI) – were determined the sea-surface backscattering conceived about twenty years ago. During coefficient from a single look direction, the the intervening period, there has been a overlapping nature of the curves would limit considerable evolution in the capabilities of its capabilities to providing a rather coarse spaceborne hardware. estimate of wind speed and no information on wind direction. The earliest successful space- ASCAT is an advanced scatterometer that will fly as part of the borne wind scatterometer, that of Seasat, used payload of the Metop satellites, which in turn form part of the two dual-polarised antennas pointed at 45 and Eumetsat Polar System. It is developed by Dornier Satellitensysteme 135 deg with respect to the satellite’s direction under the leadership of Matra Marconi Space**, the satellite Prime of flight to determine sea-surface scattering Contractor, for ESA. From its polar orbit, ASCAT will measure sea- coefficients from two directions separated by surface winds in two 500 km wide swaths and will achieve global 90 deg. The scattering characteristics of areas coverage in a period of just five days. in the coverage region were firstly determined via the forward-looking antenna and sub- sequently, by virtue of the along-track motion of The Advanced Scatterometer, known as the satellite, via the rearward-looking antenna. ASCAT, is the successor to these instruments, Pairs of measurements so obtained could then and will be flown as part of the payloads of be fitted to curves like those of Figure 1 to Metop-1, - 2 and -3. It represents advances in derive estimates of both wind speed and wind increased coverage, reduced data rate and direction. power-efficient low-mass technology. A weakness exists in two-beam single- Principles of wind scatterometers polarisation scatterometers which Seasat Wind scatterometers are instruments that are sought to overcome, with only limited success, used to infer data on wind speed and direction by use of dual polarisation. Owing to the shape from radar measurements of the sea surface. of the backscatter functions shown in Figure 1, They rely for their operation on the fact that two-beam scatterometers are capable of winds moving over the sea influence the radar producing a number of alternative solutions for backscattering properties of its surface in a wind speed and direction from the same set of manner that is related to wind speed and wind measurement data. As an illustration, consider direction. Everyday experience of the sea the case of measuring a real 16 m/s upwind – surface suggests a variation of scattering with the two beams of the instrument would wind intensity: we observe a mirror-like surface observe the characteristic at points A and *Now Astrium GmbH. with no wind, small ripples in gentle airs, and a would indicate a scattering coefficient of **Now Astrium SAS. rough surface under high-wind conditions. around –8.7 dB. The yellow curve in Figure 1 19 r bulletin 102 — may 2000 bull 0 inevitably continue to expand the range of emerging scatterometer applications. 24 Key elements in the specification of ASCAT -5 The engineering specifications of both the ERS 16 and Metop scatterometers derive from a geophysical requirement to determine wind B B -1 vectors over a coverage region consisting of 17.6 m/s 50 km-resolution cells. Determination of speeds s° A A -10 in the range 4 – 24 m/s with an accuracy of 8 2 m/s (or 10%) and directions with an accuracy of ± 20 deg is required. Table 1 shows the key Wind Speed (ms ) engineering performances of the ASCAT and Backscatter (dB) the ERS scatterometers, which meet these -15 requirements, and how the performance has 4 evolved in respect of the following: – Coverage: ASCAT offers twice the coverage of the instrument flown on ERS. Its twin swaths are offset to the left and right side of -20 0 90 180 270 360 the satellite’s ground track by about 384 km, and each offers full performance over a width Relative Wind Direction (degrees) of 500 km. This is especially important for the Metop mission, where quasi-global coverage Figure 1. Variation of also has a pair of points at –8.7 dB seperated on a daily basis is needed. scattering coefficient with by 90 deg and represents an ambiguous – Spatial resolution: Although driven by the wind direction (35 deg solution to the wind-measurement problem in requirement for 50 km spatial resolution, incidence) the form of a 17.6 m/s downwind. In order to ASCAT also offers, on an experimental basis, overcome this ambiguity problem, a third the possibility of increased spatial resolution: beam oriented at 90 deg to the flight direction a data product with 25 km spatial resolution was introduced into the design of the will be produced in addition to the 50 km scatterometers flown on ERS-1 and -2. This product. configuration has proven very effective, leading – Radiometric accuracy and inter-beam stability: to a typical ambiguity rejection skill of 97%. In order to meet the stringent requirements on wind speed and direction, it was necessary Figure 2. A typical In addition to their intended use for wind on ERS to apply strict control on overall gain scatterometer data product determination, the ERS scatterometers have variations in the instrument and on gain over land: the image shows been applied in both sea and land applications variations between its beams. Although the the multi-year mean with objectives as diverse as monitoring of sea terminology differs somewhat between the scattering coefficient at 40 deg incidence angle ice, snow, geophysics, soil moisture and ERS and Metop scatterometers, the same (courtesy of by V. Wismann vegetation. Figure 2 is a typical product over fundamental requirements apply: radiometric and K. Boehnke) land. The advanced capabilities of ASCAT will accuracy controls the extent to which the 20 ascat ensemble of beams may be allowed to Table 1. Key engineering performance parameters for ASCAT and the ERS vary, and inter-beam stability controls the scatterometers maximum extent by which the gain of any pair of beams may vary. The use of an on- Performance Parameter Unit ASCAT ERS board calibration network is of great value in this context, but satisfying these specifications Coverage: still represents one of the most challenging Number of swaths 2 1 problems of the ASCAT design. Full performance width km 500 400 – Radiometric resolution and ambiguities: In Reduced performance width km 550 500 order to perform wind extraction on a reliable Length km continuous continuous basis, it is important to limit the level of Mid-swath inclination deg 37.6 29.3 random errors in the measured data. The specification on radiometric resolution limits Spatial Resolution: the level of such errors in the measurement Nominal km 50 45 due to the effects arising from speckle (the Experimental km 25 none statistical behaviour of the radar target) and finite signal-to-noise ratio. The need to provide Radiometric accuracy dB 0.57 - adequate radiometric resolution is a factor in the dimensioning of antenna gain, transmitter Common mode stability - 0.57 power and receiver noise figure. A second factor leading to random errors in the Interbeam stability dB 0.46 0.46 measured data is ambiguity energy. This is spurious energy arising from outside the Radiometric resolution at % 3.0 – 9.9 8.5 – 9.7 measurement cell. The need to control minimum cross wind ambiguities imposes constraints on the antenna side-lobe performances. Radiometric resolution at % 3.0 6.5 – 7.0 ASCAT’s operating principle Ambiguity contribution % 1 (near swath) included in radiometric 3 (far swath) resolution specification In contrast to the scatterometers used on ERS, which relied on the transmission of continuous- wave pulses with durations of around 100 µsec and peak powers of several kilowatts, ASCAT times t1 and t2; because of the linear frequency transmits linear frequency-modulated pulses with modulation, the pulses are offset from the local a markedly longer duration of around 10 msec, oscillator pulse for the duration of the echo at a relatively low peak power of 120 W. by frequencies f1 and f2, respectively. These frequency differences allow the discrimination Echo signals are received by the instrument of signals originating at different ranges. For the and may be thought of as a large number of real received echo, the mixer output is a superimposed echo pulses arriving over a spectrum of signals, which is present for a time range of times corresponding to the width of corresponding to the time that the transmit the instrument swath; signals outside the area pulse is present within the swath. By sampling of interest are largely suppressed by the the mixer output in time and Fourier-transforming antenna side-lobe pattern. The received echo is the time series into a frequency series, the mixed with a suitably delayed pulse, which is a frequency spectrum may be extracted from the frequency-modulated replica of the transmitted data and the height and frequency of its signal.
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