Patent Description:
High precision microwave radiometers sensing natural emission in the passive-protected microwave spectrum of <NUM>-<NUM> were demonstrated by Gamma Remote Sensing AG with the ELBARA-I through ELBARA-III systems.

Low-mass low-power microwave radiometer sensing natural emission in the passive-protected microwave spectrum of <NUM>-<NUM> are known, produced by a few companies/university spinoffs.

These groups have each developed compact L-band radiometers capable of air-borne measurement from UAVs, but their designs incorporate less-stable electronics, questionable calibrations, and larger heavier inferior antennas by comparison with our invention presented in the description part.

The Soil Moisture Company's design uses a fixed-wing unmanned aircraft. The lack of look-angle control in combination with brightness temperature's high angular dependence makes the fixed wing solution quite difficult to implement for accurate retrievals of ground properties. The design also relies on a constant view of sky to measure the different between ground and sky ("differential correlating").

Multiple motors and RF remotes operating on multi-rotor UAVs create a dynamic electromagnetic field environment that can disrupt the sensitive radiometer. The UAV-borne versions of Balamis and Skaha's radiometers are highly effected by this radiofrequency interference (RFI) from the drone itself. Use of a "digital backend", such as those used on all of these systems, allows post-processing and filtering, but also is prone to instability due to local-oscillator drift. Reliance on digital filtering also may cause RFI to saturate amplifiers and compromise the signal before it reaches the backend.

The antennas used in the state-of-the-art systems so far utilize (<NUM>) standard patch antenna, (<NUM>) colinear dipole array, (<NUM>) circular polarized patch array/backfire antenna, which are determined from pictures, videos and documentation available on these products of the groups mentioned above.

Standard patch or patch arrays will have lower gain and higher loss (lower efficiency) than the antenna introduced with this invention. The antennas used in the state-of-the-art normally provide non-symmetric antenna patterns. Non-symmetric antenna patterns result in elongated elliptical ground pixels in the resulting data. The state-of-the-art either uses single polarization (<NUM>), or dual polarization with the same antenna (<NUM>) and (<NUM>).

Measuring two polarizations with the same antenna causes correlation and cross-talk which can make it difficult or impossible to obtain two independent orthogonal linear polarized measurements required by most retrieval algorithms.

With regards to <NPL>; and <NPL>). IEEE, this radiometer uses a single polarization at nadir incidence, providing too little information for reliable soil moisture retrieval.

Other types of antennas that have been considered for radiometry include; horn antennas, collinear arrays, dual feed dual polarization patch arrays, and circular arrays. Horn antennas are much too large and massive to be flown on a UAV or considered portable. The other antenna designs also have disadvantages, mainly concerning their total weight and resolution.

The closest prior art found is <CIT>, which discusses a combined active/passive UAV-borne radar/radiometer with improved resolution. The concept is based on software defined radios (SDRs). The system described in this patent application has a number of disadvantages in comparison to the invention proposed. Concerning the radiometer electronics, no front-end bandpass filter was described, although the ability to filter RFI from the cross-correlation function exists with SDRs, if the RFI saturates the LNA, the data will be unusable. The problem concerning drift associated with the SDR local oscillator (LO) as mentioned previously, is not solved in <CIT>. Additionally, the design of the antenna is not explicitly described, it only mentions an antenna "surpassing patch antennas" and mentions a possible mesh backfire antenna, which has not yet been demonstrated, and has known disadvantages. It is doubtful whether the described apparatus can be moved sufficiently well by a drone.

Different antenna designs for radiometers are known from <NPL>, <NPL> and <CIT>. Some of these antennas are explicitly non-standard patch array antennas, because they are used in extreme environments, like in space near Jupiter. These antenna designs must be adapted to high electromagnetic, and cosmic, radiation and require non-standard materials. As a rule, they do not use printed patches, and even if they do, the feed network is not on the same plane as printed patches, respectively they use machined aluminum patch elements with a coaxial cabling system prone to losses and phase errors. Alternatively, they require a dense dielectric layer in the form of a honeycomb structure. The honeycomb structure results in an effective dielectric of the substrate, made of carbon-loadecl Astroquartz. It is doubtful that these radiometers and respectively, antenna designs, can be operated with a drone in the earth's atmosphere, and if, a sufficiently good measurement accuracy would not be achievable.

The object of the present invention is to create an overall lightweight radiometer, with an improved highly-efficient and directional low-mass antenna and/or an optimized radiometer electronics, allowing more stable and more accurate measurements of brightness temperature, especially detectable from an unmanned aerial vehicle (UAV) such as a multi-copter drone. The complete radiometer with low-mass antenna can also be mounted on weather stations, ground vehicles, etc. Advantages can be reached by introducing the radiometer electronics and highly-efficient and directional low-mass antenna, which together form the radiometer. While they could be used with other electronics and antennas, herein low-mass means masses below <NUM> kilograms for the complete radiometer.

The new patch array antenna uses air as a substrate material instead of a printed circuit board of a dielectric material as is typically done with patch antennas. Due to this new antenna design, a <NUM> degree half power full antenna beamwidth provides a far superior resolution and decreased Ohmic losses, critical for radiometry.

The invention comprises a low-mass low-power microwave radiometer electronics optimized for sensing natural emission in the passive-protected microwave spectrum of <NUM>-<NUM>. The dual-polarization brightness temperatures measured by the invention can be used with established retrieval algorithms to extract parameters such as: soil moisture, snow wetness, snow density, sea-surface-salinty, biomass in forests, internal ice temperature of glaciers, moisture content in construction materials (e.g. concrete), etc..

The design of our radiometer electronics uses multiple analog bandpass filter stages on the front-end to eliminate unwanted emissions from the motor, and also utilizes a high sampling rate of the detector to filter time-dependent spurious RFI signals.

Direct-detection is performed with a square-law power detector, as part of the radiometer electronics, which eliminates the need for a local-oscillator and guarantees that the correct frequency is being detected by the measured response of the passive analog filters.

The electronics are further optimized by using two-point internal calibration, eliminating the need for sky viewing, which also provides a better accuracy, than known from prior art.

Commercially, such described radiometers can be used by agriculture companies that develop tools for farmers, natural disaster prediction and modelling professionals, and in general by scientific research and aerospace agencies. Microwave brightness temperature data converted to maps of soil moisture would be used to feed into irrigation schedules to optimize water use efficiency and increase yield by decreasing crop stress. Soil moisture is a main input to land-slide and debris flow models that attempt to predict these natural disasters. The end users could be governments, insurance companies, or researchers in this field.

Further understanding of various aspects of the invention can be obtained by reference to the following detailed description in conjunction with the associated drawings, which are described briefly below.

It should be noted that in the differently described embodiments, the same parts are provided with the same reference symbols or the same component names, the disclosures contained in the entire description being able to be applied analogously to the same parts with the same reference symbols or the same component symbols.

A preferred exemplary embodiment of the subject matter of the invention is described below in conjunction with the attached drawings.

In the following a low-mass low-power microwave radiometer <NUM> and its components are described for sensing natural emission in the passive-protected microwave spectrum of <NUM>-<NUM> in the L-band (IEEE) of the UHF range. The radiometer <NUM> in general comprises a radiometer electronics <NUM> and an antenna <NUM>, which are connected in operation state and could each be operated with different electronics or antenna, whereby their combination gives the best possible results, as measurements showed. Stable and accurate low-mass, low-volume, and low-power microwave radiometer <NUM> in a small package, allowing high gain, directionality of microwave detection and low-loss accurate radiometry is possible due to new radiometer electronics <NUM> and/or new design of the antenna <NUM>. The most important use will be measurements of soil moisture and vegetation optical depth.

Resulting microwave brightness temperature data, converted to maps of soil moisture, can be used to feed into irrigation schedules to optimize water use efficiency and increase yield by decreasing crop stress. L-band radiometers are often used in research for snow-melt detection, soil physics studies, sea salinity, and many other applications. Drone-based mapping of L-band brightness temperatures will also be useful for satellite downscaling studies and many other L-band retrieval parameters under current development.

Passive radiometry requires a highly efficient antenna <NUM> in order to accurately measure microwave brightness temperatures of the scene, rather than measuring the physical temperature of the antenna <NUM> itself.

A new antenna <NUM> was created for receiving radiation in microwave range. The antenna <NUM> comprises two patch arrays <NUM>, <NUM>'. Each patch array <NUM>, <NUM>' comprises a sandwich structure of a patch substrate layer <NUM>, <NUM>', an air gap <NUM>, <NUM>' and a ground conductor layer <NUM>, <NUM>', wherein the ground conductor layer <NUM>, <NUM>' is separated by the air gap <NUM>, <NUM>' about a distance d, d'. Such distance d, d', respectively the air gap <NUM>, <NUM>', is reached by a multiplicity of spacers <NUM>, <NUM>', located between patch substrate layer <NUM>, <NUM>' and ground conductor layer <NUM>, <NUM>'. The spacers <NUM>, <NUM>' are single components, individually spaced from each other, as clearly visible in <FIG>. The ground conductor layer <NUM>, <NUM>' could be produced as PCB with a grounding metal layer or could be made entirely of metal.

By introducing the air gap <NUM>, <NUM>' instead of a commonly known PCB dielectric material layer, larger printed patches <NUM>, <NUM>' can be used and signal loss can be avoided, as comparative measurements and simulations showed.

For the air gap <NUM>, <NUM>', best results were achieved with distances d, d' between <NUM> and <NUM>, in particular <NUM>. Such air gaps <NUM>, <NUM>' are optimized for radiometry in L-band as described here. The parameter range was simulated for the used frequency and dimension of patch array antenna <NUM> parts to tune to the highest efficiency, or smallest loss. It is important, that the spacers <NUM>, <NUM>' are electrically non-conducting and the connection means <NUM>, <NUM>' too. The spacer <NUM>, <NUM>' can be formed by polymers, like Teflon or PTFE, polyamides or Nylon or Silicone.

A multiplicity of connection means <NUM>, <NUM>' is used to connect separated ground conductor layer <NUM>, <NUM>' and patch substrate layer <NUM>, <NUM>'. The preferred connection means <NUM>, <NUM>' are plastic screws, for example made of polyamides such as Nylon. For the sake of simplicity, the connection means <NUM>, <NUM>' are operatively connected with the spacers <NUM>, <NUM>' in the same positions. But connection means <NUM>, <NUM>' could also be formed by a glued joint.

On a top side of the patch substrate layer <NUM>, <NUM>', usually a printed circuit board, an even number of printed patches <NUM>, <NUM>' is placed, on the top side opposite the ground conductor layer <NUM>, <NUM>'. The shapes of the printed patches <NUM>, <NUM>' are preferably identical, as well as the number of patches <NUM>, <NUM>' of both patch arrays <NUM>, <NUM>'.

Each printed patch <NUM>, <NUM>' is connected via an insed-fed <NUM>, <NUM>' to striplines <NUM>, <NUM>' running symmetrically along the patch array surface to a centred RF coaxial connector <NUM>, <NUM>', in particular for L-band a SMA connector (SubMiniature version A) or type-N connector. Stripline here refers to an impedance-matched printed conductive transmission line atop a substrate layer. Such striplines <NUM>, <NUM>' can also be called micro-striplines <NUM>, <NUM>'. At high frequencies SMA must be replaced with smaller precision coaxial connectors. The size and form of the patches <NUM>, <NUM>' is adapted or optimized to the microwave frequency to be received. Persons skilled know how to simulate and produce such printed patches <NUM>, <NUM>' and their inset-feeds <NUM>, <NUM>' on patch substrate layer <NUM>, <NUM>' surfaces, to optimize for microwave frequency range of interest. The complex multilayered sandwich with patch substrate layer <NUM>, <NUM>', air gap <NUM>, <NUM>' and ground conductor layer <NUM>, <NUM>' introduced here cannot be described and solved with analytical equations. The design needs to be tuned using electromagnetic finite-element software simulations and build/test iteration to obtain the dimensions necessary for the antenna to operate in the passive-sensing protected band between <NUM> and <NUM>, here the center frequency of the designed antenna <NUM> is <NUM>.

The printed patches <NUM>, <NUM>' are connected via the striplines <NUM>, <NUM>' as a matched micro-strip feed network, connector lines <NUM>, <NUM>' to the RF coaxial connector <NUM>, <NUM>' and their coaxial center conductor <NUM> by wires at the radiometer electronics <NUM>.

The connector lines <NUM>, <NUM>' are fed through the patch substrate layer <NUM>, <NUM>' to be connected to the striplines <NUM>, <NUM>'. The connection between connector lines <NUM>, <NUM>' and the RF coaxial connector <NUM>, <NUM>' is done by soldering. In the radiometer electronics <NUM> the further signal processing, calibration and analysis are performed.

As measurements showed, the patch array <NUM>, <NUM>' showing improved results, high gain, high efficiency, and is extremely lightweight. Improvements can be achieved with different electronics used.

As an option for further improvement, at least one not shown temperature sensor can be arranged at the patch array <NUM>, <NUM>' for improvement in later temperature calibration in the electronics. The temperature sensor is to be connected accordingly with the electronics <NUM>.

Particularly advantageous is a patch array antenna <NUM>, comprising two patch arrays <NUM>, <NUM>' with identical setup as described above, wherein the patch arrays <NUM>, <NUM>' are rotated by <NUM>° relative to each other. This is also shown in <FIG>, wherein symmetric antenna patterns are optimized for receiving horizonal and vertical linear polarizations. Due to alignment, one patch array <NUM> receives horizontal polarized microwaves H, while the second patch array <NUM>' receives vertical polarized microwaves V and forwarding the corresponding signals H, V to the electronics <NUM>. For all embodiments of the patch array antenna <NUM>, the cabling between patch arrays <NUM>, <NUM>' and the electronics <NUM> is in particular a coaxial cable which can carry high frequency electrical signals with low losses. The impedances of patches <NUM>, <NUM>', inset-feeds <NUM>, <NUM>', striplines <NUM>, <NUM>', connector lines <NUM>, <NUM>', RF coaxial connector <NUM>, <NUM>' and cabling to the electronics must be matched.

Here the patch array antenna <NUM> utilizes at least one 2x2 patch array <NUM>, <NUM>' for each polarization providing fully independent polarization measurements. The advantage of using two separate patch arrays <NUM>, <NUM>' instead of array elements with two feeds is the very low cross-polarization leakage.

Compared with a dual fed patch array this design achieves superior cross-polarization isolation. Compared with a dual circularly polarized array, and mathematically converting into dual linear polarization, the invention does not require phase-coherent sampling, which is prone to IQ offset and phase drift. The horn antenna is the only type of comparable antenna that could provide efficiency comparable to that of the air-gapped patch array antenna <NUM> introduced here. But the horn antenna design is much too heavy to be used as part of a lightweight radiometer on an unmanned aerial vehicle (UAV).

By combining the two 2x2 patch arrays <NUM>, <NUM>', one rotated by <NUM> degrees, the invention receives two fully independent linear polarization views of the same ground footprint. Other antenna geometries don't have this luxury and either have significant polarization coupling and/or view different spots on the ground. The measured horizontal and vertical independent linear polarizations allow use of established soil-moisture retrievals techniques such as the Tau-Omega or Two-stream emission models. Other airborne radiometer systems, with inferior antennas, require sophisticated custom retrieval algorithms and assumptions about the two ground footprints, and without the established heritage and validation of the Tau-Omega soil-moisture retrieval algorithm. Of course more than four patches can be used per patch array pattern.

Our prototype of the two patch array <NUM>, <NUM>' antenna <NUM> had a size of <NUM> x <NUM> x <NUM> with a total mass of about <NUM>, when mounted to an aluminum supporting structure. The patch array antenna <NUM> has a 3dB full beamwidth (full width at half maximum) of <NUM> degrees, and an ohmic efficiency of <NUM>.

In another embodiment the patch array antenna <NUM> can comprise two patch arrays <NUM>, <NUM>' which are sharing one patch substrate layer <NUM> and one ground conductor layer <NUM>, wherein two pattern of patches <NUM>, <NUM>' are printed on the frontside surface of the same patch substrate layer <NUM>, but rotated by <NUM>° to each other. The other components like air gap <NUM>, <NUM>', spacers <NUM>, <NUM>', RF coaxial connectors <NUM>, <NUM>' are used as stated above. Patch, microstrip, and air-gap dimensions must be tuned by means of simulation to reach the desired resonant frequency and matching characteristics.

Received microwave thermal emission signals are very weak and need to be amplified for further signal processing. Frequent internal calibration is required to correct gain and offset drifts in time by the optimized electronics <NUM> as depicted in <FIG> schematically. Gain drifts are correlated with offset drifts, which are both functions of temperature, and can both influence the resulting brightness temperature measurement and retrieval. Therefore also the electronics <NUM> was optimized, resulting in an electronics <NUM> as follows.

The radiometer electronics <NUM> comprises the following components in direction of signal transmission: Antenna inputs <NUM>, n-port switch <NUM>, calibration matched load <NUM>, inverted LNA (low noise amplifier) and active cold load <NUM>, with output terminated by a LNA termination <NUM>, isolator <NUM>, first bandpass filter <NUM> (especially for L-band, <NUM>-<NUM>), first LNA <NUM>, second bandpass filter <NUM>, second LNA <NUM>, third bandpass filter <NUM>, square law power detector <NUM>, lowpass filter <NUM>, ADC (Analog to Digital Converter) <NUM> and computer unit <NUM>. All components are in particular arranged in a housing. The inputs and outputs are indicated in the schematic drawing <FIG>.

The calibration matched load <NUM> and/or inverted LNA and active cold load <NUM> with an LNA termination <NUM> are placed and connected to the n-port switch <NUM>. The calibration matched load <NUM> is not in the main RF chain and acts only as an ambient brightness temperature source. The inverted LNA and active cold load <NUM> and the LNA termination <NUM> are not in the main RF chain and are optional. The LNA termination <NUM> is a passive electronic component, only to terminate the inverted LNA and cold load <NUM>. This prevents reflections and non-linear behavior of the inverted LNA <NUM> that would change its apparent microwave brightness temperature.

In the antenna inputs <NUM>, the signals of at least one antenna <NUM> are fed, most preferred from a two-part patch array antenna <NUM> as disclosed above. All components are adapted to the frequency range of interest, wherein some components are for temperature calibration tasks, as the calibration matched load <NUM> and the inverted LNA and cold load <NUM>, terminated with the LNA termination <NUM>.

The inverted LNA and cold load <NUM> is not actually used as an amplifier and is not in the main RF chain. Both components <NUM>, <NUM> are not in the main RF chain and only act as a calibration cold brightness temperature source. Temperature sensors, for example thermocouples are not depicted here. For a person skilled in the art, it is known how the wiring and/or microwave transmission between the components is done and which technical requirements to be met by the components for the frequency range of interest. The whole radiometer electronics can be manufactured on one PCB using microstrip or CPW to attach the components.

The electronics <NUM> shows a simple direct-detect RF front-end with a square-law power detector <NUM> back-end. In contrast to the recent trend of digital sampling backends, this established and simple method was used in this design for the sake of simplicity, low-power consumption, and stability. Digital backends have been shown to provide mitigation to Radio Frequency Interference (RFI), but with a high sampling rate of the power detector we can also filter RFI in the time domain. The RF front-end is constructed using commercial components.

The RF front-end comprises: switch <NUM>, in particular a four-port switch <NUM>, isolator <NUM>, inverted LNA <NUM>, LNAs <NUM>, <NUM> and L-band filters <NUM>, <NUM>, <NUM>, and an integrating low-pass filter <NUM> as smoothing or integration RC filter on the DC output of the detector.

As bandpass filters <NUM>, <NUM>, <NUM> in particular ceramic cavity bandpass filters are used, for tuning to the passive-protected RF band/ theoretically free from radio-frequency interference.

The square-law power detector <NUM>, whose output voltage is proportional to the square of the amplitude-modulated input voltage, is then low-pass filtered by lowpass filter <NUM> and sampled by the A/D converter <NUM> at ~<NUM>. The lowpass filter <NUM> cutoff frequency or time constant is adjusted according to the A/D sample rate. The small computer unit <NUM> collects and saves the detector data and the optional temperature sensor data as well as controlling the switch.

The radiometer electronics <NUM> is nominally calibrated at ~<NUM> intervals using the two internal calibration loads <NUM>, <NUM>/<NUM>, an ambient temperature monitored matched load <NUM> and an active cold load <NUM>/<NUM>. The physical temperature of the active cold load is also monitored, and its brightness temperature is characterized as a function of physical temperature using measurements of well characterized cold sky brightness temperature.

The components are chosen to deliver <NUM>-Ω input impedance. For improved calibration, temperature sensors should be placed at the used antenna. In case of an above disclosed two-part patch array antenna <NUM>, a temperature sensor should be located on each patch array <NUM>, <NUM>'.

The radiometer electronics <NUM> offers adjustable integration time and fast time domain sampling. For the UAV-borne radiometer, adjustable integration time allows for better radiometric accuracy for high flight altitude missions when spatial resolution is inherently lower, and vice-versa by sacrificing some radiometric accuracy. Extremely high spatial resolution is possible at low flight altitudes. For use as a ground based radiometer, integration time can be long resulting in low noise effective delta temperature (NEDT), or enhanced radiometric resolution.

Fast time domain sampling allows filtering of radio-frequency interference (RFI). As opposed to other designs, that use software defined radios (SDR) for filtering RFI in the frequency domain, the invention utilizes the theory of Gaussianity of noise in the time-domain to filter samples containing interference. Receivers using heterodyne or super-heterodyne detection, such as SDRs, are subject to uncertainty in the received frequency, and thus brightness temperature, from drifts in the temperature and power dependent local oscillator. The direct detection, with a front-end bandpass filter, used here provides a balance between simplicity and stability. The other designs discussed in the prior art above do not incorporate a front-end filter, which would allow RFI from a broad range of frequencies to saturate the first amplifier stage and compromise the measurements.

The resulting radiometer <NUM> is a direct detection, total-power, internally calibrated radiometer operating at the frequency band <NUM>-<NUM> and has the following advantages due to the special antenna <NUM> design and/or the electronics <NUM> over the state-of-the-art are:.

Claim 1:
Patch array antenna (<NUM>) for a portable stable low-mass microwave radiometer (<NUM>) for measuring microwaves in the spectral range between <NUM> and <NUM>, connectable to a radiometer electronics (<NUM>),
characterized in that
the patch array antenna (<NUM>) comprises at least one patch array (<NUM>, <NUM>') with a patch substrate layer (<NUM>, <NUM>') in form of a printed circuit board of a dielectric material on which a pattern of printed patches (<NUM>, <NUM>') is printed on a front side of the patch substrate layer (<NUM>, <NUM>'),
wherein the printed patches (<NUM>, <NUM>') are interconnected via inset-feeds (<NUM>, <NUM>') to strip lines (<NUM>, <NUM>') on the same plane as the printed circuit board printed patches (<NUM>, <NUM>'), wherein connector lines (<NUM>) are connected to the strip lines (<NUM>, <NUM>') and fed through the patch substrate layer (<NUM>, <NUM>') through a ground conductor layer (<NUM>, <NUM>'), which is spaced apart fixed to the backside of the patch substrate layer (<NUM>, <NUM>') at a defined distance (d, d') forming an air gap (<NUM>, <NUM>') only between both layers (<NUM>, <NUM>', <NUM>, <NUM>'), wherein the air gap (<NUM>, <NUM>') is formed by a multiplicity of individual single spacers (<NUM>, <NUM>'), and such that the connector lines (<NUM>, <NUM>') are led through the ground conductor layer (<NUM>, <NUM>') into at least one RF coaxial connector (<NUM>, <NUM>') fixed at a backside of the ground conductor layer (<NUM>, <NUM>'), which is or are directly without detour connectable via cables with the radiometer electronics (<NUM>).