The COSMOS Airborne Campaigns. Status October’06
The EMIRAD-2 L-band radiometer, operating within the 1400-1427 MHz range, features a dual-antenna setup designed for high-precision data acquisition from aerial platforms. It combines advanced digital signal processing with real-time data integration for rapid meteorological and environmental observations. Although challenges arose in maintaining calibration stability under elevated temperatures, strategies using internal calibration standards helped mitigate data accuracy issues. This document reviews the system's specifications, campaign statuses, and ongoing RFI mitigation strategies.
The COSMOS Airborne Campaigns. Status October’06
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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.
