Martti Kemppinen

Fully polarimetric microwave radiometer for remote sensing

The design, characteristics, and operation of the Helsinki University of Technology Fully Polarimetric Radiometer (FPoR) are described. The developed 36.5-GHz radiometer can be used for airborne remote sensing; however, ground-based and laboratory measurements are also possible. A direct cross-correlation technique with analog correlators, which measures all four Stokes parameters simultaneously, is applied. This paper is the first successful demonstration of an analog direct cross-correlation technique for polarimetric remote sensing radiometry. The radiometer was subjected to a variety of laboratory tests, and considerable attention is given to analysis of the characteristics of the instrument. Owing to the effective active temperature control system of the receiver, the radiometric stability of the instrument was found to be very high; test results showing stabilities below 10 mK and of 4-40 mK on time scales of 800 and 8000 s, respectively, are presented. Furthermore, the absolute accuracy of the system is analyzed to be at a sub-Kelvin level for most measurement conditions. A maritime wind vector experiment was carried out over the Gulf of Finland. The feasibility and performance of the applied correlation technique and the whole radiometer system were verified for fully polarimetric airborne measurements. The obtained brightness temperatures of the first three Stokes parameters show typical harmonic behavior with respect to the surface wind; the results suggest, however, that the model coefficients presented earlier for oceans may not be directly applicable for different conditions.

The theory and mechanical realization of an ideal scanning method for a single-channel imaging microwave radiometer

The scan path of an imaging microwave radiometer is determined so that the desired area is totally covered, while the overlap between successive sweeps is minimized. This minimizes the number of redundant data points, which, in turn, leads to faster and more economical postprocessing. Different scanning methods and sampling techniques are discussed in brief. Conical scan is chosen and three different approaches are presented. A theory that relates the flight velocity to the angular velocity of the antenna beam to provide an ideal scan pattern is formulated for pendulum scan. Results of each scan method are presented and compared, and the best one is chosen. Some considerations about the mechanical realization and an outline of the instrument are presented. >

Airborne imaging radiometer scan simulation

An imaging radiometer scan simulation program is developed for comparing the performance of different scan patterns and process behavior under varying circumstances. The program gives the radiometers antenna beam the desired scan motion over an artificial target scene, computes the antenna radiometric temperature as the convolution of the antenna pattern with the scene brightness temperature deviation, computes a moving average of the antenna output, and records the data values as well as the corresponding coordinates at the sampling moments. The simulation indicated that a helicopter-borne imager needs at least passive attitude stabilization. In addition, the state-of-the-art sampling rate was found to be too slow, if the sampling period is set equal to the integration time. A detailed study revealed the achievable spatial resolution (line pairs/length unit-definition) to be 1.0-1.2/spl times/footprint dimensions, but the integration and sampling periods should be as short as 0.2-0.4/spl times/footprint dimensions. >

93 GHz airborne imaging radiometer

A 93 GHz conically scanning imaging radiometer is under construction at HUT. The system consists of a sensor unit to be mounted under a Bell Jetranger helicopter and a control electronics unit to be strapped on the helicopters back seat. The new features of the compact instrument are the ability to follow any conical scan path, the airborne liquid nitrogen cooled cold calibration as well as the flat panel color display featuring a real-time false colour image and video image window.<<ETX>>

Advanced airplane version of the HUT 93 GHz imaging radiometer

The HUT 93 GHz Airborne Imaging Radiometer (AIR-93) is being modified in an advanced airplane version. The new features include: a) the use of an attitude-dGPS instrument to produce gyro data for radiometer beam active stabilization, b) an improved version of the airborne, liquid nitrogen cooled cold calibration target, and c) the digital, computationally stabilized, position oriented optical comparison image system.

The first results of a novel 93 GHz airborne imaging radiometer

A 93 GHz conically scanning dual-channel (H&V) imaging radiometer has been built at the Helsinki university of Technology (HUT). The system has been designed for the Bell Jetranger helicopter, but can also be installed aboard the HUT remote sensing aircraft. The new features of the compact instrument are its ability to follow any feasible conical scan path, the airborne liquid nitrogen cooled cold calibration load as well as a flat panel color display providing a real-time false colour image and video image window. The very first experiences of the instrument are promising, and some of the data measured so far are presented.

Imaging radiometer scan simulation results

Imaging radiometer operation is simulated by a computer program, which computes the antenna radiometric temperature as the convolution of the antenna pattern with an artificial test target brightness temperature deviation and records the data values at the sampling moments as well as the corresponding pixel coordinates. Run results proved that a well-aligned scan pattern reduces data without loss in image quality. In addition, a helicopter-borne imager was found to need at least passive attitude stabilization and the state-of-the-art sampling rate was found too slow.<<ETX>>

Overview of EMAC-95 snow and ice airborne campaign in Finland

The 1995 European Multisensor Airborne Campaign (EMAC-95) on snow and ice was funded by European Space Agency (ESA) and national authorities. The main test sites were in Finland (sea ice, snow in boreal areas) and Norway (land ice, snow in mountainous areas). The airborne instruments included two microwave radiometer systems (Helsinki University of Technology HUTRAD and UK Meteorological Office system) and two SAR sensors (ESAR from DLR (Germany) and EMISAR from Technical University of Denmark). This paper summarizes the activities in the Finnish test sites. Results from the HUT microwave radiometer measurements in the Campaign are given.

A cost-effective, airborne video system for producing rectified, geo-referenced digital images

Optical images provide a useful reference for passive microwave radiometer data. The images need not have state-of-the-art spatial resolution, but they should include positioning data and be produced and manipulated with ease. The paper introduces the HUT pushbroom video imager consisting of a video camera, frame grabber, controlling PC and optional attitude-DGPS. System calibration and pixel geopositioning are discussed, and an example image is presented.

A contribution to imaging radiometer sampling and integration period determination

Imaging radiometer sampling and integration period determination studies are contributed to by: a) performing an empirical study of the sampling rates effect on the spatial resolution of the resulting image, b) analyzing the effect of supplementing thermal noise on a numerical model expressing the influence of sampling and integration periods on the achievable spatial resolution, and c) discussing the optimal values for the sampling and integration periods on the basis of a scan simulation study.