Carols campaigns 2009: First results

CAROLS CAMPAIGNS 2009: FIRST RESULTS

M. Zribi1, M. Pardé2, D. Hauser2, P. Fanise2, J. Boutin3, C. Albergel4, J. C. Calvet4, M. Crapeau7, M. Dechambre2, Y. Kerr1, E. Lopez- Baeza5, A. Mialon1, G. Reverdin3, A. Ruis6, K. Saleh5, J. P. Wigneron7

1- CESBIO (CNRS/IRD/CNES/UPS), 18 avenue Edouard Belin, 31401 Toulouse Cedex 9, France Tél: +00216 71 75 00 09 , Fax: 00216 71 75 02 54, e-mail: [email protected]

2- LATMOS/CNRS, 10-12 av. de l'Europe, 78140 Vélizy, France 3- LOCEAN, UPMC - Tour 45-55-5eme étage, Paris, France

4- Météo France, CNRM/GMGEC/UDC, 42 av. G. Coriolis, 31057 Toulouse, France 5- Facultat de Fisica. Universitat de Valencia C/ Dr Moliner, 50. Burjassot. 46100 Valencia, Spain

6- IEEC/ICE-CSIC, Campus UAB/Fac Ciencies, Torre C-5-parell-2ª planta, 08193 Bellaterra, Spain 7- INRA-Bordeaux, 69, route d'Arcachon, F-33612 Cestas Cedex, France

ABSTRACT

The CAROLS, L band radiometer, is built and designed as a copy of EMIRAD II radiometer of DTU team. It is a Correlation radiometer with direct sampling and fully polarimetric (i.e 4 Stockes). It will be used in conjunction with other airborne instruments (in particular the C-Band scatterometer (STORM) and IEEEC GPS system, Infrared CIMEL radiometer and one visible camera), in coordination with in situ field campaigns for SMOS CAL/VAL. The instruments are implemented on board the French research airplane ATR42. A scientific campaign with thirteen flights is realized over south-western France, Valencia site and Bay of Biscay (Atlantic Ocean) in spring 2009. In order to qualify the radiometer data, different types of aircraft movements were realized: circle flights, wing and nose wags. Simultaneously to flights, different ground measurements were made over continental surfaces and ocean. Results show a good quality of data. For continental surfaces, important Radio-Frequency Interferences (RFI) were observed over a large part of the studied region.

Index Terms: radiometer, CAROLS, radar, SMOS

1. INTRODUCTION

Passive microwave remote sensing of moisture and salinity has been a focus of research for several decades. The Soil Moisture and Ocean Salinity (SMOS) satellite mission is based on a dual polarized L-band radiometer using aperture synthesis [1]. It is proposed by European Space Agency (ESA). In the context of SMOS mission validation, we proposed to design, build and operate an L-Band radiometer from aircraft to be used in conjunction with other airborne instruments (scatterometer – or C-Band radar –, visible wide

angle camera, GPS and infra-red radiometer) in coordination with in situ field campaigns. In spring 2009, a scientific campaign has been realized in order to acquire different types of brightness measurements over ocean and land surfaces, after two campaigns of instruments validation in 2007 and 2008. In section II, we present the instruments used in our campaign. In section III, we describe the proposed flights and studied sites. Data acquisition and first results are presented in section IV. Conclusions are gathered in section V.

2. CAROLS INSTRUMENTS AND INTERFACE WITH ATR-42

2.1 CAROLS Instruments

a) CAROLS radiometer The receiver is a fully polarimetric correlation radiometer with direct sampling [2-3]. Two high speed data converters (max106) converts directly sample the L-band signals. The sampling frequency has been set to 139.4 MHz to prevent the aliasing. The calibration is carried out using internal loads and a noise diode, adding around 120K to the radiometer input. This setup ensures frequent internal calibration capability, and it allows the radiometer to calibrate the possible phase difference between the two inputs channels in front of the digital correlator. The characteristics of receiver are:

- Frequency band : 1400-1427MHz - Digital I/Q demodulation and correlation for accurate estimation of 3rd and 4th Stockes - Data integrated to 8ms recorded on primary storage PC - The raw data pre-integrated to 1.8μS is recorded on dedicated PC On Figure 1, we illustrate the resolution of the radiometer by comparing the theory with the real sensitivity depending on the integration time. We show that for an

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integration time longer than 300 ms, the sensitivity is better than 0.1 K. Concerning stability, on Figure 2 we show measurements when the target is a cryogenic load (with a stable temperature of 79 K). The black points are measurements calibrated using only the first calibration time, the circles indicate measurements calibrated using both calibration times, 20 minutes apart. We show that using both calibration times, variation of temperatures are lower than 0.1 K, if we use only the first calibration point, variation is more than 0.25 K after 30 mn.

Fig.1 Resolution of the radiometer CAROLS

Fig. 2 Stability of CAROLS measurements

The antenna system is composed by 2 identical large potter horns and bulky waveguide orthomode transducers (OMT) for almost ideal antenna patterns and low loss. Each horn has an identical pattern and a 37.6° half power beam width (HPBW). The performance of the potter horns is very satisfactory with side and back lobes being suppressed more than 40dB while the cross polar level is even smaller. The two antennas with the OMTs are connected to the radiometer unit with coaxial cables (Figure 3). These 4.5 m cables have been chosen carefully to have a very low loss. Moreover, to obtain a correct correlation between H & V, the components used for V and H have to be phase matched. It means that the two signals corresponding to V and H polarisation will arrive in phase at the receiver, this is important for 3th and 4th Stockes parameters computation.

Because the temperature of the cabin in the plane is not stabilised, these coaxial cables are insulated and five temperature sensors are mounted on them inside the insulation. Each antenna has been insulated to prevent a variation in temperature of its structure. Indeed, we look for keeping an identical temperature of each port. The temperature of each orthomode transducer and horn is continuously recording with the use of thermistors.

Fig. 3 CAROLS radiometer over ATR42

b) STORM radar STORM, developed in LATMOS [4], is a C-Band ranging radar (5.35 GHz) based on an FM/CW (Frequency-Modulated-Continuous Wave) transmitted wave form (Figure 4). It transmits alternatively in H and V polarizations and receives simultaneously in both H and V channels. The geometry of observations of STORM is presented in Figure 5. The antenna is mounted so as to point towards the surface with an incidence angle (with respect to nadir) of 20°, when the airplane is horizontal. The antenna system can be controlled either to scan over 360° around the vertical axis at a rate of 2 rotations per minute or to stop at a fixed azimuth angle.

Fig 4. STORM antenna

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Fig. 5 Geometry of STORM observations

c) GPS GOLD-RTR System The GPS system is delivered by IEEC, [5]. It is composed of 3 antennas and a GPS Open Loop Differential Real Time Receiver (GOLD-RT). This instrument uses 10 channels with 64 complex correlators each, to collect L-band reflected signals. The aim is to retrieving information about sea surface roughness in the form of the effective Probability Density Function (PDF) of the slopes. This information could be very relevant for the sea surface brithness temperature modeling. d) Visible wide angle camera (VWAC) A visible camera is part of the standard equipment of the ATR42. It is located a little ahead of the slant radiometer antenna. It allows the identification of area covered by CAROLS nadir antenna measurements. e) Infra-red radiometer (IRR) An Infra-red radiometer is part of the standard equipment of the ATR42. It is located a little ahead of the C-band radar antenna, or depending on the configuration, ahead of the nadir looking antenna. It performs measurements of the radiance and of the brightness temperature over three channels at 8.7, 10.8 and 12 μm. Its sensor is pointing to the nadir with a field of view of 3°. It has been used to provide surface temperature estimates, simultaneously to CAROLS measurements.

2.2 Description of interfaces with ATR42 SAFIRE AIRCRAFT

The ATR42 is available for research experiment since the beginning of 2006. It is an original ATR42-320 specially modified to scientific use. Two configurations are proposed for CAROLS campaign flights, presented in Figure 6. In the first configuration, the CAROLS radiometer is operating through two antennas, one looking at nadir and the other one looking to the right side of the plane (slant antenna). The second configuration is a combination of the slant antenna of CAROLS, situated in the back of the plane, and the STORM instrument situated in the front of the plane (where the nadir antenna was situated before).

Fig. 6 (a) CAROLS instrument with one side looking

antenna and STORM instrument, (b) CAROLS instrument with two antennas (one site and nadir)

A precise measurement allows determining the misalignment of each antenna with the inertial unit.

3. DESCRIPTION OF SITES AND FLIGHTS

3.1 Sites and ground measurements

During CAROLS campaigns, three sites were selected, in order to estimate soil moisture or ocean salinity. SMOSMANIA siteSMOSMANIA (Soil Moisture Observing System – Meteorological Automatic Network Integrated Application) implements soil moisture measurements in a portion of the automatic ground station network of Météo-France (the RADOME network), [6]. The SMOSMANIA network permits to monitor soil moisture in south-western France thanks to automatic, real-time in-situ measurements of soil moisture and soil temperature profiles (-5, -10, -20, -30 cm). The soil moisture measurements are obtained with horizontally installed ThetaProbes. Twelve ground stations were installed in 2006 allowing observing the hydrologic and climate gradient between the Mediterranean and the Atlantic. The soil moisture data acquisition has been fully operational since January 2007. With continuous measurements, punctual estimations were performed over different test fields, with gravimetric and thetaprobe surface moisture estimations and vegetation description. Valencia site

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The Valencia Site is located in south-eastern Spain, about 80 km inland to the west of Valencia. Within the Valencia validation site, an area of 30 km x 50 km was selected for the experiment. The land use in this area is dominated by vineyards and bare soil (>70%), and orchards (18 %). 20 sampling teams were established, during airborne measurements, with different sampling strategies. In general, each sampling plot was about 20x20 m2 and volumetric SM measurements were performed by means of soil cylinders of known volume and of ThetaProbe Delta-T capacity probes. Bay of BiscayConcerning the ocean measurements, the experiment took place in the Bay of Biscay. In order to support Airborne CAROLS experiment, a ship campaign was organized under the airplane track. Measurements of sea surface salinity (SSS), sea surface temperature (SST), wind speed, foam, wave spectrum, heat and momentum flux were collected on the ship. Three drifters measuring SSS, twelve drifters measuring SST, one drifter performing acoustic wind measurements were deployed.

3.2 CAROLS flights

13 flights were realized during CAROLS campaign 2009 over studied sites. Departure was from Francazal military base in Toulouse. Table 1 illustrate details of all flights.

Date Site of measurements

Instruments Departure UTC

04/27/2009 Valencia Carols, GPS, IRR

12h00

28/04/2009 SMOSMANIA Carols, GPS, IRR

10h00

05/04/2009 Gulf of Biscay Carols, STORM, GPS, IRR

17h00


17h00


17h00


10h00

05/18/2009 Gulf of Biscay Carols, GPS, IRR

17h00


12h00


17h00


17h00


17h00


10h00


12h00

Table1. Description of CAROLS flights

For ocean, seven flights were realized in night, in order to avoid sun glint. Figure 7 illustrates three types of flights. All measurements are realized in an altitude between 2000 and 3000m. During ocean measurements, different options were tested: wing wags with ± 25° of roll; nose wags with ± 5° of pitch; and circular flights with 15° positive and negative roll angle. These movements are useful for qualification of instrument behaviour and validation of emissivity models. The flights and ship trajectories are coordinated in order to have simultaneous ground measurements with carols data acquisitions. The first two types of flights have as objective to identify salinity variations from costal area up to an approximate longitude of 5°W. The third type has as objective to analyse brightness temperatures in costal area.

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Fig. 7 CAROLS ocean flights types

For SMOSMANIA site, three flights were realized at 2000m altitude. Acquisitions were realized between Atlantic Ocean and Mediterranean Sea with a double estimation in the two directions of flights (Figure8).

Fig. 8 SMOSMANIA flight description

For Valencia site, we proposed three flights. The flight plan includes ten single altitude lines performed at approximately 5000 m above Sea level. Measurements allow to cover completely the studied site (30km X 50km) with CAROLS estimations (Figure 9).

Fig. 9 Valencia flight description

3.3 CAROLS acquisition and calibration

CAROLS radiometer was present in all CAROLS flights, with one or two antennas. Flights were made with different altitudes. As a function of altitude, and type of surfaces (land or ocean), we proposed different acquisition programs. The acquisition program concerns three parameters: - The calibration cycle duration - The duration of internal calibration periods: the radiometer input can be switched to an internal load targets. These load targets provide a warm calibration point typically around 315 K (exactly equal to their physical temperature which is monitored by the radiometer system). A noise diode, injecting around 120 K of correlated noise into the 2 channels, provides a second calibration point as it is added to the internal load target. With the internal calibration constants determined continuously from the calibration sequences, and with knowledge of external parameters, such as losses and reflections, based on measurements in the laboratory, a set of gain and noise temperature can be found for each measurement point and applied to the data.- The timing to switch between the two antennas (for flights with two antennas). All these parameters are programmed during the flight. Generally, before important events like noise and wing wags, circle flights, we realized a large period of internal calibration to have a high precision of measurements after. In order to calibrate brightness temperatures measured by CAROLS, we need to correct cables and OMT losses. For the first one, we consider brightness measurements of cryogenic charge with and without cables before and after airborne campaigns. Knowing with a high precision their physical temperature, we can estimate cables losses. For the four cables, we retrieve a value between 0.19 and 0.2 dB. For OMT losses, laboratory measurements are very complicated. We need to do measurements over natural targets like lakes for example. In our case, lakes are contaminated with RFI. For this reason, we choose to estimate OMT losses over ocean, by considering low incidence angle measurements and emissivity model estimations based on TRAP software developed by IFREMER.

4. FIRST RESULTS

4.1 RFI contamination

Over the studied sites, we observe two types of RFI: a first type with continuous noise in time. This noise is particularly present in two areas: around Auch city and close to CEL area in south-western France. A second type with periodic RFI pulses: this type of RFI is due to radars and particularly to air control traffic radars or military ones. Ocean measurements show only a limited presence of the second type of RFI.

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In order to correct these data, we propose to use the kurtosis parameter, defined as the ratio:

22

44

μμβ =

where 2 is the second central moment and 4 is fourth central moment Figure 10 illustrates results of application of kurtosis parameter filtering over south-western France. For continuous RFI, correction is more complicated. Other approaches are developed for a better mitigation of this type of RFI [7].

Fig. 10 Correction of CAROLS data, mitigation of RFI over southern west France

4.2 CAROLS data

After calibration, we retrieve the four Stocks parameters, the two brightness temperatures in horizontal and vertical polarisations and the third and fourth parameters. For the two last parameters, because of the important length of cables, we observe that the cables are not completely phase matched, needing therefore a correction to eliminate this effect on correlation between horizontal and vertical measurements. Computation of phase difference angle is based on wing wags movements. Figure 11 illustrates good agreement between measurements and simulated brightness temperatures during wing wags movements.

Fig. 11 TX and TY variations (Gulf of Biscay), with ±25° wing wags (slant antenna), compared with TRAP output

simulations (in red and green)

5. CONCLUSION

In the context of SMOS mission, CAROLS radiometer, in synergy with STORM radar and other instruments will be an essential tool for calibration phase and also improvement of inversion modelling. The CAROLS campaign 2009 has as a first objective acquisition of scientific data for inversion algorithm for soil moisture or ocean salinity estimations. Measurements are acquired over the three studied sites (SMOSMANIA, Valencia site and Gulf of Biscay). In spite some difficulties, particularly linked to RFI presence in continental surfaces, CAROLS data show an excellent quality, able to be used for SMOS mission validation. The next step will be an important experimental campaign in 2010 spring in order to validate SMOS measurements.

Acknowledgements

This work was funded by TOSCA/CNES program. The authors would like to thank CNES for its important support to CAROLS project and all CAROLS campaigns. The authors would like to thank also CESBIO, CNRM, IFREMER, LATMOS, LOCEAN, SAFIRE, DT-INSU, DTU, INRA technical teams, SAFIRE pilots for their support to have successful airborne and ground campaigns.

REFERENCES

[1] Y. Kerr, P. Waldteufel, J.-P. Wigneron, Martinuzzi, J. Font, and M. Berger, 2001, “Soil moisture retrieval from space: The Soil Moisture and Ocean Salinity (SMOS) mission,” IEEE Trans. Geosci. Remote Sensing, vol. 39, pp. 1729–1735,. [2 ] SS. Sobjoerg, Rotboll, J., Skou, N., 2002, The L band ocean salinity airborne campaign, First Results Workshop EuroSATARRS, WISE, LOSAC, 27- 37p. [3] J. Rotbøll, S. S. Søbjærg, and N. Skou, 2003, “A Novel L-band Polarimetric Radiometer Featuring Subharmonic Sampling,” Radio Science, vol. 38, no. 3, pp. 11-1 – 11-7. [4] Mouche, A.; D. Hauser; J.-F. Daloze, 2005, Dual-polarisation measurements at C-Band over the ocean: results from airborne radar observations and comparison with ENVISAT ASAR data, IEEE Trans. On Geoscience and Remote Sensing, 43, 753-769 [5] Cadarech E, A. Rius, A new technique to sense non-Gaussian features of the sea surface from L-band bi-static GNSS reflections, Remote Sensing of Environment, Volume 112, Issue 6, 16 June 2008, Pages 2927-2937 [6] J.C. Calvet, C. Rudiger, N. Fritz, J. F. Mahfouf, 2008, Soil moisture products: Satellite/model/in situ cross-validation, microrad’08, Florence, Italy, 11-14/03/08. [7] M. Pardé, M. Zribi, P. Fanise, M. Dechambre, CAROLS L-BAND RADIOMETER AIRBORNE CAMPAIGN: ANALYSIS OF THE RFI ISSUE, microrad’2010, Washington, DC, USA, 01-04/03/2010.

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Carols campaigns 2009: First results

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