Josu Uusitalo

SMOS Calibration Subsystem

Interferometric radiometry is a novel concept in remote sensing that is also presenting particular challenges for calibration methods. In this paper, we describe the calibration subsystem (CAS) developed for the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS) interferometer of the Soil Moisture and Ocean Salinity (SMOS) satellite. CAS is important for the overall performance of the payload as it calibrates out the differences between the multiple receivers of MIRAS. SMOS is in the final phase of development and is due to launch in 2008.

The Helsinki University of Technology/Ylinen Electronics airborne L-band interferometric radiometer

An airborne L-band two-dimensional interferometric radiometer by aperture synthesis, HUT-2D, is under development and construction at LST/HUT (Laboratory of Space Technology/Helsinki University of Technology) in co-operation with Ylinen Electronics Ltd. The instrument consists of 36 antenna/receiver elements and an FPGA-based digital correlator and it will be accommodated onboard the LST/HUT remote sensing aircraft, Short SC-7 Skyvan.

Ground calibration of SMOS: NIR and CAS

Ground calibration of the calibration subsystems of MIRAS (Microwave Imaging Radiometer using Aperture Synthesis) has been performed. The MIRAS instrument is the payload of European Space Agencys (ESA) Soil Moisture and Ocean Salinity (SMOS) mission. The calibration subsystems are Calibration Subsystem (CAS), which is a noise distribution network, and Noise Injection Radiometer (NIR), which measures the noise levels of CAS and the average incident brightness temperature. This paper presents the used measurement approaches, related uncertainties, and the calibration results for the NIR and CAS. The results show that in spite of uncertainties, the characterization methods allow accurate ground calibration of the two subsystems. The performance of both subsystems meet the requirements.

Calibration subsystem for spaceborne interferometric radiometer

The Calibration Subsystem (CAS) for SMOS mission is described. The Engineering Model (EM) of CAS was developed, qualified, and characterized; results are presented in this paper. Furthermore, the method to model the subsystem parameters in-orbit is presented; based on EM measurements, the feasibility of this method is successfully demonstrated.

Evaluation and comparison of RFI detection algorithms

Anthropogenic Radio Frequency Interference (RFI) is an increasing problem in microwave remote sensing radiometry. Therefore, there is a growing interest on methods to detect and filter RFI. In this study, the performance of several different detection algorithms has been studied and compared to detect Continuous Wave (CW), QPSK modulated, and pulse modulated (with 0.1%, 1%, and 10% duty cycles) RFI. The mission scenario corresponds a spaceborne, polar-orbiting, conically scanning microwave radiometer. However, the qualitative results (e.g., the relative performances of algorithms) are applicable to other scenarios as well. It has been shown that RFI detection thresholds in 1 K range can be achieved in this scenario if complementary RFI detection algorithms can be incorporated in the system, e.g., in a digital RFI processor. Even 0.1 K level can be achieved for pulsed RFI with low duty cycles. Kurtosis and spectral kurtosis are effective in detecting pulsed RFI but not optimal in detecting constant envelope signals (such as CW or QPSK). Spectral Density Estimation and polarimetry, on the other hand, have a performance that is independent on the modulation or duty cycle of the RFI - they are sensitive to the average power.

Development and characterization of fully polarimetric noise injection radiometer for MIRAS

An L-band noise injection radiometer (NIR) has been designed and implemented by Helsinki University ofTech- nology Laboratory ofSpace Technology f or the SMOS (Soil Moisture and Ocean Salinity) mission ofESA (1). The work is performed as a part of ESAs MIRAS Demonstrator Pilot Project-2 (MDPP-2) under a subcontract for EADS-CASA. Other partners in the MDPP-2 NIR project are Toikka Engineering Ltd. and Ylinen Electronics Ltd. NIR will work as a part ofthe MIRAS (Microwave Imaging Radiometer Using Aperture Synthesis) instrument. Its main purpose is (1) to provide precise measurement ofthe average brightness temperature scene for absolute calibration of the MIRAS image map and (2) to measure the noise temperature level ofthe internal active calibration source f or individual receiver calibration. The performance of NIR is a decisive factor ofthe MIRAS perf ormance. The challenge in the implemented, so-called blind correlation, method is the fact that there is additional noise in the correlated signal due to using the noise injection method. The main objective ofthis paper is to demonstrate the f ofthis technique. I. INTRODUCTION The precision of a noise injection radiometer is based on comparing the measured signal to two reference sources, the noise temperatures of which are known. This will remove the effect of the receiver gain and offset variations. The length of the noise pulse is then proportional to the antenna temperature (2). NIR will also be used in the MIRAS array as a regular receiver unit for interferometric image creation. In addition to measuring the horizontally and vertically polarized antenna noise temperature and the calibration net- work noise temperature, the MDPP-2 NIR was designed to provide fully polarimetric measurement capability. The Stokes parameters are retrieved using the same correlator, which the MIRAS uses for solving the correlation for the interferometric image creation. The so-called modified Stokes parameters are defined under the Rayleigh-Jeans approximation as (3)

Measurement of the HUT-2D aperture synthesis radiometer four-element subassembly

An airborne L-band aperture synthesis radiometer, HUT-2D, is under development in Helsinki University of Technology, Laboratory of Space Technology (HUT/LST) in co-operation with Ylinen Electronics Ltd. The airborne platform for the instrument will be the HUT/LST remote sensing aircraft, Short SC-7 Skyvan. A four-element subassembly has been manufactured and test measurements are in progress to verify the chosen technology.

Comparison of ERS-2 SAR, HUTSCAT and CARABAS results for boreal forest

A combined airborne and spaceborne radar remote sensing campaign on boreal forest was conducted in Tuusula, near Helsinki, in June and September 1997 in the framework of the EU EUFORA project. Airborne radar data was collected by the HUTSCAT ranging scatterometer on 6 and 12 June, and by the CARABAS SAR on 30 September. Spaceborne radar data include ERS-2 SAR images acquired on 1, 12, and 17 June. Ground truth data includes detailed information on weather, canopy, and soil parameters. The feasibility of various radar frequencies and polarizations for retrieval of forest parameters was investigated.