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The COSMOS Airborne Campaigns. Status October’06. N. Skou, S. S. Søbjærg, J. Balling, S. S. Kristensen, and S. Misra Ørsted•DTU Technical University of Denmark ns@oersted.dtu.dk. L-band Radiometer System.
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The COSMOS Airborne Campaigns.Status October’06 N. Skou, S. S. Søbjærg, J. Balling, S. S. Kristensen, and S. Misra Ørsted•DTU Technical University of Denmark ns@oersted.dtu.dk
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
40 degHornPattern HPBW=30.6° i.e.: FPL = 932 m FPX = 714 m from 1000 m altitude
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 msec 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 msec in this mode.
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 msec integration): as above. • Fast data alternatively raw samples plus above integrated to 14.7 msec.
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 msec 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
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
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
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
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!
What are our Priorities? • To detect the situation • Mitigate if possible
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)
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.
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.
