Little is known on the impact of the increase in anthropogenic aerosols on the global climate. It can potentially have a cooling effect via the second indirect aerosol effect which is poorly understood because of the lack of sound measurements. A radar devoted to the observation of the spatial distribution of drizzle, in combination with other sensors, would certainly give a better insight to this phenomenon. IRCTR is currently developing such radar, which will be place in Cabauw (The Netherlands) a meteoroligical site that will provide synergy with other instruments. This paper describs the requirements, the system design and the product obtained.@en

In 2007, IRCTR (Delft University of Technology) installed a new polarimetric X-band drizzle radar (IDRA) at the meteorological observation site of Cabauw, Netherlands. It provides plan position indicators (PPI) with a high range resolution of either 3 m or 30 m. IDRA aims to monitor any precipitation events for the long-term analysis of the hydrological cycle. Due to the specifications of IDRA the spatial variability of a large range of rainfall intensities (from drizzle to heavy convective rain) can be studied. This also permits the assessment of mesoscale changes in the precipitation pattern due to aerosols (second aerosol indirect effect). As a first step in order to achieve these aims, we present the results of an exponential drop-size distribution retrieval using the conventional algorithm that is based on the radar reflectivity Z and the differential reflectivity Zdr. We will exploit the boundaries of applicability of this drop-size distribution retrieval algorithm for light precipitation. Thereby, we will focus on the limits for the extraction of the median drop diameter using the differential reflectivity. We will discuss potential sensor synergies to extract the median drop diameter even in light precipitation.@en

Linear frequency modulated continuous wave (LFM-CW) radars require high linearity and good phase noise characteristics of its modulated signal to obtain a good resolution and sensitivity. Direct digital Synthesis (DDS) is becoming an increasingly competitive alternative to traditional Voltage Controlled Oscillators (VCO) to perform the modulation. Although ther are plenty of puvlications describing the performance of the DDS and he VCO as applied in FM-CW radars. In this paper the performance of DDS is analyzed both theoretically and with measurements. The performance of DDS is analyzed both theoretically and with measurements. The performance of DDS in a linear frequency modulated continuous wave (LFM-CW) radar system, (the Transportable Atmospheric Radar TARA developed in IRCTR), is compared to a VCO demonstrating its superiority. DDS is , therefore, expected to play an important role in radar sytems in the near future.@en

In some earth observation applications there is a requirement for low cost, high performance imaging radar systems small enough to be operated from small, even unmanned, aircraft. The existing pulse radars are non-cost effective complex systems. Therefore they are not suited for these applications. Frequency Modulated Continuous Wave (FM-CW) radar systems are on the other hand generally very compact and relatively cheap to purchase and to use. IRCTR, together with TNO, is investigating the feasibility of combining FM-CW technology and high resolution Synthetic Aperture Radar (SAR) techniques. This research has led to the development of two different systems. An FM-CW millimetre wave SAR sensor (35 GHz) was built and is being tested. The system is battery powered and uses a PC to control the radar and store the data, including GPS and data from motion sensors. The system is mounted in a pod attached to a light motor glider. Furthermore, a detailed system model has been developed in order to estimate and analyze the performance of the demonstrator system. The measuerementsw made with the demonstrator system are evaluated and the findings are used to improve the system model. This improved model will be a strong aid in the design of future higher performance FM-CW SAR systems. The second development regards a miniature P-band (450 MHz) polarimetric FM-CW SAR. This system is intended for use with an ultra-light aircraft. It is designed for tropical forest monitoring studies but could be used in foliage penetration applications as well. This system will be battery powered and uses a small memory stick to store the raw data. Data processing is now foreseen as an off-line activity. The design of the system is still under study and the realization is expected to start in the second half of 2005. This system will employ a Direct Digital Synthesizer to generate the waveform that is transmitted at P-band. Two separate dual polarized patch antennas will be used for transmit and receive.@en

RCTR developed IDRA (IRCTR drizzle radar), a new weather radar aimed at obtaining high temporal and spatial resolution observations of rain, drizzle, low level clouds and fog. The system is placed on top of a 213 m tall meteorological tower in Cabauw (the Netherlands). The radar has been operating on a continuous basis from April till November 2008 and a consistent data set has been obtained. In order to asses the performance of the system a comparison between rainfall-rate retrieved from radar data and measurements from a network of rain gauges placed in the area surrounding the Cabauw tower and operated by the Wageningen University has been carried out. There are a significant number of rain events where both data from rain gauges and radar data were available. The analysis of these events shows a very good agreement on the trend of the time series of rain gauges and collocated radar measurements but significant differences in the actual values retrieved by the two instruments, particularly during the colder season. Importantly, the radar was able to observe light rain rates (below 0.5 mm/h) that were not properly measured by the rain gauges. In the paper, extended information on the used data sets is going to be provided. A detailed analysis of the rain rate comparison, including weather conditions, for two relevant days, one corresponding to the warm season and the other to the cold season is going to be performed.@en

A new method to estimate the Doppler spectrum parameters of atmospheric signals based on adaptive filtering is presented. The new method is compared with the traditional FFT-based method. In order to do so both methods have been applied to process the same atmospheric data set obtained by the FM-CW polarimetric Doppler radar TARA (Transportable Atmospheric Radar) developed by IRCTR. In section 2 the adaptive filtering processing is described. The details of the FFT-based algorithm will not be described. The reader is referred to Doviak and Zrnic (1993) for an extensive discussion. Section 3 describes the practical implementation of both algoritms in the radar. In section 4 the results of both algorithms are discussed and compared. Section 5 outlines the advantages and disadvantages of both methods.@en