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Ørsted • DTU

The COSMOS Airborne Campaigns.Status October’06

N. Skou, S. S. Søbjærg, J. Balling,S. S. Kristensen, and S. Misra

Ørsted•DTUTechnical University of Denmark

ns@oersted.dtu.dk

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Ørsted • DTU

L-band Radiometer System

• EMIRAD-2 is a fully polarimetric radiometer operating in the 1400 - 1427 MHz protected band

• EMIRAD-2 consists of:– 2 antennas, one pointing 40 deg aft, one pointing

nadir. The antennas are Potter horns with no sidelobes– radiometer unit with dual inputs– EGI (INU + GPS) for attitude and navigation– industrial PC for fast data recording– laptop for instrument control and normal data

recording• Installed on 2 small aircraft

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40 degPotterHorn

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40 degHorn

Pattern

HPBW=30.6°i.e.:FPL = 932 mFPX = 714 mfrom 1000 maltitude

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Radiometer Description

• Digital radiometer with subharmonic sampling. A to D converters directly sample the L-band signals with a clock frequency of 139.4 MHz.

• The data from the converters are fed into an FPGA where correlation, calculation of second and fourth order moments of the PDF, and integration is performed digitally

• Data integrated to 8 msec. is stored on the laptop computer also controlling the system. These data will be available in near real time.

• A second data stream - fast data - is implemented for RFI mitigation, done off-line for optimum performance. In the normal mode of operation, data only pre-integrated to 1.8 sec is recorded on a fast HD in an industrial PC.

• The fast data channel can also be operated in a special mode where raw data from the converters are stored. 2 x 32 K samples are stored with a 25% duty cycle. The normal fast data is pre-integrated to 14.7 sec in this mode.

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Data Output

• 8 msec. integration:– <x2> for H-pol– <x2> for V-pol– <x4> for H-pol– <x4> for V-pol– <xy> 0° for 3’rd Stokes– <xy> 90° for 4’th Stokes

• Fast data (1.8 sec integration): as above.• Fast data alternatively raw samples plus above integrated

to 14.7 sec.

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EMIRAD-2 Specifications

• Correlation radiometer with direct sampling• Fully polarimetric (i.e. 4 Stokes)• Frequency: 1400 - 1427 MHz (-60 dB BW; about 22 MHz -3

dB BW)• Digital radiometer with 139.4 MHz sampling • Advanced analog filter for RFI suppression.• Data integrated to 8 msec recorded on PC• Off-line digital RFI filtering in frequency and time

domains. Fast data pre-integrated to 1.8 sec or raw data is recorded on HD

• Sensitivity: 0.1 K for 1 sec. integration time• Calibration: internal load and noise diode• 2 antennas - one nadir pointing, one pointing 40 deg. aft• Antennas are Potter horns (no sidelobes) with 37.6° and

30.6° HPBW

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Block Diagram

Inte-gration andcom-plexcorrela-tor

filter

(FPGA)

TV

TH

U

V

A/D~~~

~~~ A/D

V

H

V

H

Noisediode

Cal.

Cal.

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Temperature Stabilized Enclosure• 2 digital PI-regulators• stability of microwave section better than 0.02 °C for 15

°C change in ambient temperature• DFE stability better than 0.1 °C for same change

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Problems under Warm Conditions

• Internal temperature (normally around 40 °C) cannot be kept stable

• Calibration severely affected• Eventually the radiometer overheats• This situation prevailed in Australia due to failing

aircraft air-condition• Solution:

– base calibration on internal load and noise diode– make model for noise diode output as function of

temperature by operating radiometer in lab under elevated temperatures

– re-process all data• Result:

– calibrated data but with less accuracy (under normal conditions calibration depends directly on primary LN2 cal. - here only indirectly)

– OK for soil moisture where requirements are modest

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Radiometer Control - screen dump

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Large Antenna on C-130

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EMIRAD-2 on Aero Commander

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EMIRAD-2 on Aero Commander

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EMIRAD-2 on Aero Commander

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CoSMOS “Down Under” Campaign

22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Campaign operations and FlightsAssimilation of root zone soil moisture ASSI

Scaling issues, 2000 metres alt.

SCALwith NAFE on first day

Sun Glint and Topography GLINT

Effect of Water and dew on L-band measurements

WATER 1 flight at start and 1 at end

1 flight over Roscommon

October 2005 November 2005

second flight during last NAFE vegetation sampling

December 2005

campaign extension

flights delayed due to delays in certification

<== end of Campaign

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EMIRAD on HUT Skyvan

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Two Flight Patterns off Norway

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

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CoSMOS-OS Campaign

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5 6 7

Setting up of Operations CenterCampaign operations and FlightsASAR over measurement target Morning passASAR over measurement target Evening pass

Departure of Ferry from Stavanger

(arrival at destination next day)

Departure of Ferry from Newcastle

(arrival at destination next day)

Circle Flightno SAR no SAR

Sun Glint FlightNo SAR

April 2006 May 2006

Orbits far to the east

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Re-processing Status

• Radiometer has been characterized in the lab with internal temperatures in the range 37 - 48 °C

• OMTs have been measured (NWA):– loss in side port: 0.08 dB, in end port: 0.05 dB– S11 around -15 dB– X-pol below 30 dB

• Cable losses are 0.3 dB• Processing algorithms established this week• Bulk processing starts next week• Quality checks• Data delivery before Christmas

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RFI - 8 msec Data and 15 sec Data

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RFI - 15 sec Data: TB and Moment Ratio

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Example from Australia (zoom in on 15 sec data)

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Example from Australia (raw data)

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What are the Dangers of RFI?

• Strong RFI of long duration may elevate brightness temperature by unreasonable amount or even blank radiometer

• Result: loss of data, but you know!

• More likely, but far more dangerous situation: RFI (low level or short pulses) may contribute to your signal with a power corresponding to a Kelvin for example.

• Very difficult to know!

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What are our Priorities?

1. To detect the situation2. Mitigate if possible

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Can RFI be Detected?

• Huge RFI no problem - TB is clearly too large• More normal RFI very difficult in normal radiometer

systems having integration from milliseconds to seconds• Need to have very fast sampling rate - preferably digital

radiometer (EMIRAD-2 has 140 MHz sampling)• Radar signals may be detected as unusually large signals

with suitable but short pre-integration• But most signals can be detected by investigating

statistical properties:– TB is Gaussian which has a fixed ratio of 3 between

4’th and 2’nd order central moments (kurtosis)– Other signals (especially pulsed and continuous)

typically have different value (beware, however: sine with 50% duty cycle also have ratio of 3!!)

• All this has to be done using “raw” data (before integration)

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Can RFI be Mitigated?

• Time domain:– Continuous signals not– Pulse type signals with low duty cycle can: following

the detection in the raw data, inflicted samples are discarded before integration. For typical radar signals the loss of radiometer signal will thus be very moderate.

• Frequency domain:– Even our narrow band (27MHz) may be split into sub-

bands.– Each sub-band is analyzed– Inflicted sub-bands are discarded– Work on both continuous and pulsed signals

• In both cases: consequence is increased T - depending on how much has to be discarded.

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What is Being Done at DTU?

• EMIRAD-2 has collected data in Australia and in the North Sea:– Data pre-integrated to 1.8 microsec. recorded

continuously.– Bursts of raw data (140 MHz sampling) recorded on

special occasions.• For sure, examples of RFI have been captured• Analysis and theoretical considerations are ongoing.

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CoSMOS-OS Flight Line

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Power, Aft Horn, H-pol, 8 msec. Sampling

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Kurtosis, Aft Horn, H-pol, 8 msec. Sampling

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Power, Region of Interest

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Kurtosis, Region of Interest

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Power, Zoom in on Region of Interest

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Kurtosis, Zoom in on Region of Interest

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Power, one 8 msec Window with 1.8 s Sampling

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Kurtosis, one 8 msec Window with 1.8 s Sampling

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Power, Nadir Horn, 8 msec. Sampling

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Kurtosis, Nadir Horn, 8 msec. Sampling

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Power, Zoom in on Region of Interest

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Kurtosis, Zoom in on Region of Interest

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Power, one 8 msec Window with 1.8 s Sampling

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Kurtosis, one 8 msec Window with 1.8 s Sampling

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Kurtosis on Flight Line

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Power

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Power - Land Zoom

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Power - Sea Zoom

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Kurtosis

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Kurtosis - Land Zoom

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Kurtosis - Sea Zoom

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Conclusions and Plans

• Many potentially harmful RFI pulses present• Some can be seen in 8 msec TB data - some cannot!!• Can be seen in kurtosis data• Some not seen in 8 msec TB data can be seen in 1.8 s TB

data.• Example 1: seen both in kurtosis and 8 msec TB data. RFI

adds 3.5 K!!• Example 2: only seen in kurtosis. RFI adds 0.9 K to 8

msec TB data!!!!!!• Kurtosis powerful tool.• Also fast sampling - 1.8 s (TBC) - is powerful• Analysis being carried out this fall and winter• Algorithms for mitigation to be developed• Potential for FPGA implementation - avoids fast data rate

(link to ground / recording capacity)

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What About Future Campaigns?

• Do not trust any L-band radiometer unless it has at least fast sampling (microsecond range) - and preferably kurtosis check

• This goes especially for airborne campaigns• Available radiometers:

– EMIRAD-2– CAROLS (EMIRAD-2 copy, ready next year)– EMIRAD-1 after minor modification.– ????