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StatusOngoing
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Status date2008-10-20
This activity developed the “Atmospheric Propagation and Profiling System” (ATPROP) advanced ground-based microwave radiometer, for radiowave propagation assessment at Ku, Ka, Q/V and W bands. The design of ATPROP is based on the requirements of SatCom, SatNav systems and Space Science Missions.
ATPROP consists of two independent subsystems, operating at Ka band, near the 60 GHz oxygen absorption band and at 15/90 GHz. ATPROP has a full non-GEO satellite tracking capability and uses switched Dicke reference to improve stability by. His performances allow to derive accurately atmospheric attenuation, sky noise, wet delay, and cloud, vapour and air temperature profiles.
Today satellite-based systems such as telecommunication, navigation and earth observation are fundamental to the course of modern life. In order to guarantee an unrestrained functionality of those systems, the assessment of atmospheric propagation effects and meteorological parameters is a key issue.
Radio propagation in the Earth’s atmosphere is affected by the atmospheric constituents, mainly gases and hydrometeors, which interact with electromagnetic radiation. The goal of this study is to assess radio propagation in the spectral range from 10 to 90 GHz, a spectral region which is becoming more and more important in the applications mentioned above.
For this purpose a highly stable microwave radiometer system (ATPROP) has been designed, constructed and tested which allows the investigation of
- Sky noise due to emission of atmospheric components such as gases and hydrometeors,
- Atmospheric attenuation due to gases and hydrometeors,
- Atmospheric excess path length mainly due to atmospheric gases.
For these purposes the system should allow observations with high spatial and temporal resolution and be capable of operating in a stable way for long periods, in remote locations, under severe environmental conditions and with no or sporadic manned instrument control.
The project aims at the accurate assessment of propagation parameter by a ground-based system. The microwave radiometer system ATPROP was designed to achieve these requirements and to allow automatic stand-alone operation at arbitrary locations over long time periods. Therefore key issues of the study are the high precision and stability of the radiometer, accurate calibration procedures, flexible observation modes and suitable retrieval algorithms for propagation and meteorological parameters and a thorough assessment of the ATPROP specifications.
ATPROP employs a unique set of frequency channels by combining a standard profiling system with additional channels at 15 and 90 GHz. The first channel frequency should allow an enhanced performance for determining attenuation in the transition phase between non-raining clouds to precipitation. On the other hand the 90 GHz channel is most suitable for detecting clouds with low water content. In addition, an infrared radiometer can be used in conjunction with the temperature profiler to derive cloud base height.
All frequency channels point in the same azimuth and elevation direction. The software allows various observation modes including zenith pointing, full sky mapping as well as tracking of GPS/Leo satellites.
All ATPROP channels include a novel calibration concept including a Dicke switch and noise diode injection. This allows a highly stable operation over long time periods. In addition, a high duty cycle can be achieved with no interruptions necessary for relative or tipping curve calibrations.
The ATPROP system consists of four direct detection receiver packages. In the figure above the K-band humidity profiler is shown. The architecture is comparable to a filterbank spectrometer which acquires the 7 water vapour line channels all in parallel. Each channel is equipped with its individual filter and total power detector for a 100% duty cycle operation.
A key feature is the calibration frontend comprising a noise injection section (gain drift calibration) and a magnetically switchable Dicke reference (system noise temperature drift compensation). The calibration cycles are adjusted to yield an optimum radiometric resolution of <0.2 K RMS (one second integration time) and an excellent long term stability. Allan Variance measurements resulted in a noise reduction for integration periods of typically 4000 seconds which makes the system ideal for precision wet delay determination.
The optical resolution of each receiver package has been optimized for radiometer portability and the available observation modes. E.g. the temperature profiler (50-60 GHz band) HPBW is only 2°, so that the instrument is capable of performing boundary layer scans down to 5° elevation with high vertical temperature resolution of 50 m on the ground. The K-band radiometer beams have a beam width close to 4° (HPBW) which is a good compromise for a full sky scanning mode with approx. 400 point for full sky coverage.
The instrument’s temporal resolution is one second. The fast sampling rate is important for full sky scanning, LWP time series cloud variability detection and satellite tracking schemes. The software includes a tracking mode that reads RINEX navigation files in order to scan all visible GPS or Galilieo satellites for wet path delay in the line of sight, LWP and atmospheric attenuation. A single scan of 10 satellites takes about one minute. The ATPROP receivers are thermally stabilized to better than 50 mK over the full operating temperature range (-35°C to +45°C). The instrument can be used under harsh environments and in high altitudes up to 6000 m.
The project started with a critical review of atmospheric microwave radiometry for accurately deriving propagation parameters between 10 and 90 GHz. The baseline design of such a system including hardware and software components was reviewed.
In the second phase of the project the technical design was completed and an analysis of failure mode effects & criticality performed.
The third phase included the manufacturing of the ATPROP system and its operational tests. Retrieval algorithms for various propagation and meteorological parameters were developed.
The final fourth phase, concerning the field testing of ATPROP at the KNMI Remote Sensing Area in Cabauw, The Netherlands, in the framework of the Dutch CESAR project is still ongoing.
A critical review of the system requirements has been performed including a frequency selection study. Based on the requirements the “Atmospheric Propagation and Profiling System” (ATPROP) has been designed and constructed. In order to achieve the longterm stable operation a new calibration approach including a Dicke switch has been designed. After laboratory checks the ATPROP system has been transported to the KNMI measurement site in Cabauw, The Netherlands.
Currently ATPROP is tested in various observation geometries at Cabauw. Retrieval algorithms for the various propagation and meteorological parameters have been developed and are applied to the data of the test campaign. ATPROP performance is currently specified by investigating radiometer stability during quiet atmospheric periods, simulated clear sky emissions calculated from radiosoundings at the close by De Bilt station, and intercomparison with further instrumentation available at Cabauw (rain gauge, cloud radar, rain radar, dual channel mw radiometer) provided by KNMI.