This paper presents the development of an ultra-wideband bistatic radar operating over the frequency range of 0.7-2.1 GHz based on the Radio-Frequency System-on-Chip (RF-SoC) platform and its application to snow measurements. The system utilizes Global Positioning System Disciplined Oscillators (GPS-DO) to synchronize the transmitter and receiver mounted on two small unmanned aerial systems (sUAS). We developed an arbitrary waveform generator to synthesize and transmit chirp signals. A data acquisition system was designed to capture signals with up to 2.2 GHz bandwidth and record the radar data. The transmit and receive antennas were developed using a four-element Vivaldi antenna array over the operating frequency range. Bistatic measurements were performed to determine the Brewster angle to estimate snow dielectric properties. The separation, incidence, and reflection angles were adjusted to collect radar data over an angular region extending from 27 to 68 degrees over snow at the Study of Precipitation, the Lower Atmosphere and Surface for Hydrometeorology (SPLASH) field site near Gothic, Colorado.

Snow's vital role in the cryosphere impacts global climate, river flow, and the growth and shrinkage of ice sheets. Small Unmanned Aircraft Systems (sUAS) integrated with low-power ultra-wideband (UWB) radars have great potential to monitor snow to support scientific research and operational applications. We developed both monostatic and bistatic UWB radars operating over frequencies extending from about 0.65 GHz to 2.2 GHz for measurements of snow over land, sea ice, and ice sheets and measurements of soil water content, vegetation, and river bathymetry. The UWB bistatic radar is designed to operate on two coordinated sUAS to perform reflectivity measurements to estimate dielectric properties for determining snow water equivalent (SWE) from radar-generated snow thickness data.

We developed a high-sensitivity airborne multichannel ultrawideband (UWB) frequency-modulated continuous-wave (FMCW) radar for snow depth measurements. This low-transmit power eight-channel radar has a near-ideal point target response and provides a multilook ability. We developed a T-shape Mills-Cross antenna array to obtain a small, overlapped footprint. We integrated it with a 2 degrees of freedom (DOF) gimbal mechanism to correct pointing errors in the roll and pitch movements of the aircraft. We performed airborne UWB radar measurements in conjunction with in situ characterization and dielectric measurements with a monopole dielectric probe over snow in Grand Mesa, CO, USA, from 24th January to 14th February 2023. The radar mapped top and bottom snow interfaces and internal density changes of 1.2-2.6 m of snow with high signal-to-noise of about similar to 45 dB with only 10 mW transmit power. The multichannel system with gimbaled antenna mount allowed us to capture quasi-specular returns even with aircraft roll deviations as large as 15 degrees. The comparison between radar data and in situ measurements shows excellent agreement between the two in terms of snow depth and internal snow layers.

This paper presents results from a field deployment of a small Unmanned Aircraft System (sUAS) radar in Colorado-Grand Mesa during Spring-2022 for measurements over snow. The stand-alone, low-power, compact, and Frequency Modulated Continuous Wave (FMCW) radar on a sUAS is used for these measurements. The radar operates over the frequency range of 2.8-5.8 GHz. The transmit signal is obtained by down-converting 77–81 GHz chirp generated with an automotive radar. The radar operates with a low output power of only 3 dBm (2 mW) to mitigate interference to nearby communication systems with a chirp duration of $250 \mu \text{s}$ . The received signal is up-converted back to 77–81 GHz for digitization and processing using the automotive radar data capture board. The radar data are processed with a fully focused Synthetic Aperture Radar (SAR) algorithm after applying phase and amplitude corrections to the transmitter chirp to obtain nearly ideal point target responses. The radar mapped air-snow and snow-ground surface interfaces as well as the snow internal layers with snow depth exceeding 2 m in areas covered with 15–25 m tall trees. In addition, the radar-generated snow thicknesses are within ±10 cm of in-situ measurements.

This paper details a new approach for enhancing the radiation characteristics of a coplanar Vivaldi antenna (CVA) array. First, the inter-element mutual coupling is reduced by introducing slots in the conventional CVA array. Then, a low-profile dielectric patch antenna (DPA) is situated between the CVA flares when it acts as a traveling wave radiator at the higher frequency band. The DPA is excited by the loop current at the radiation part of the CVA, and a set of TE modes with omnidirectional radiation patterns are excited. This combination enhances the directivity in the broadside direction of the CVA and suppresses the grating lobes by as much as 10 dB. The 4 $\times $ 1 linear array covers the frequency range of 2.77-13.6 GHz, with 132% fractional bandwidth. Then, four of the proposed linear arrays are connected to constitute an 8 $\times $ 2 planar array to achieve a gain of 15.2-24.6 dBi. The proposed arrays are fabricated, tested, and mounted on a small unmanned aerial system (sUAS) for radar measurements. The real-life field radar results with the proposed arrays are presented including an echogram of the scanned area.

We developed and deployed a high sensitivity and low transmit power airborne ultra wide band (UWB) frequency-modulated continuous-wave (FMCW) radar for snow depth measurements. The radar has a near-ideal point target response so that we can produce near-real-time snow thickness maps after each survey flight. The improved performance is achieved by carefully designing the radar hardware to reduce internal reflections between various components, third-order products generated by mixers, higher order harmonics generated in multipliers and nonlinear devices, and amplitude and phase errors in transmitted chirp signals. In addition, we performed extensive linear and nonlinear system simulations to predict degradations in the radar hardware in advance and applied the remedies to correct them. These improvements allowed for near-real-time data products to be generated by reducing the need for advanced signal processing techniques. We also developed a T-shape Mills-Cross antenna array to obtain a small overlapped footprint of transmit and receive antennas. We performed measurements over snow in Grand Mesa, CO, USA, from March to April 2022, and the radar mapped the top and bottom interfaces and density changes of 1.2–2.1 m of snow. We generated a snow thickness map from the data collected over the grid flown and compared results with in situ measurements. The comparison between radar estimates and in situ measurements shows that the average snow depths obtained from the radar data are within a standard deviation from the mean of in situ measurements.

This paper presents the design and development of a digital two-channel chirp synthesizer using a field-programmable gate array (FPGA) device. To achieve an integrated solution, the design was implemented on radio-frequency system-on-chip (RF-SoC) technology that includes digital-to-analog converters (DACs) and other radio-frequency components on-chip. To overcome the timing errors in high-speed design with DACs operating at 6.144 GHz, a memory-stitching concept was used. A prototype was developed to validate this concept by generating a baseband chirp with a bandwidth of 1.7 GHz and a sweep time of 36 $\mu \text{s}$ . The synthetic chirp was upconverted to 3.572-5.272 GHz for use as the transmit signal for an ultra-wideband radar to characterize the chirp using a 1 km long optical delay line. The transmit signal was analyzed in terms of phase and amplitude errors and corrected for these errors. The root-mean-square (RMS) frequency deviation of the predistorted chirp from linearity over the 1.7 GHz bandwidth is 9.64 kHz, realizing a chirp linearity of 0.00057%. The measurement data show comparable performance of our chirp synthesizer against a commercially available arbitrary-waveform-generator (AWG) operating at a sampling rate of 60 GHz. The reported chirp synthesizer can be used in frequency-modulated continuous-wave (FM-CW) and stretch radars. Such radars are widely used for a variety of remote sensing measurements.

This paper presents the design and development of a high-sensitivity ultra-wideband radar for sounding and imaging 5-km thick polar ice. The design is optimized to detect internal layers in ice from the surface to the bed with fine resolution. The radar consists of 8 transmitters and 16 receivers. Each transmit/receive module is designed to operate either as a standalone system or as a channel of multi-channel radar. Each subsystem is designed and optimized, and then integrated. Each transmitter's output power is 53 dBm and operates over the frequency band of 170 - 470 MHz. A push-pull amplifier configuration is used in the transmitter chain to reduce the second harmonic distortion of the transmitted chirp. A high-power, high-speed PIN diode transmit/receive switch is also designed to operate the radar with a single antenna array. The High-power switch cascaded with a low-power CMOS switch provides 80 dB isolation between the transmitter and receiver. The receiver subsection is designed to provide optimal performance with the digital subsystem. The receiver offers a 20-dB gain over the frequency of operation. Based on laboratory tests, the radar will have a loop sensitivity of about 250 dB for airborne measurements.

This letter presents a novel approach to build a compact lightweight unmanned aerial vehicle (UAV) radar for remote sensing applications. The proposed radar exploits the recent advancement of an automotive radar chip for broadband chirp generation and rapid data processing. To compensate for the path losses and improve penetration at the millimeter-wave (mm-wave) frequency range, up- and down-converters are developed to generate an ultra-wideband (UWB) (3.25–5.15 GHz) chirp signal. The total payload of the radar is 2.5 kg. The proposed radar is installed on an UAV and tested in the field at a 100-m altitude above the ground surface. The results show that the compact low-power UWB radar can be used to map vegetation and soil moisture with fine resolution.

This paper reports the design and application of a ceramic-based ultra-wideband (UWB) dielectric resonator antenna (DRA) mounted on an unmanned aerial vehicle (UAV). The proposed 8×1 DRA array covers the frequency band of 2.5-6 GHz with an 83% impedance bandwidth. The DRAs are excited with a trapezoidal patch at the front face to achieve wider bandwidth. The peak realized gain varies from 12 to 14 dBi with 95% efficiency over the operating band. A pair of transmit and receive antenna arrays is utilized in a UWB frequency modulated continuous wave (FMCW) radar for remote sensing applications. The isolation between the two antenna arrays is more than 45 dB while weighing only 255 gm with a footprint of 250×100 mm 2 . The antennas arrays are used to collect data to generate synthetic aperture radar (SAR) images for developing algorithms to estimate soil moisture and snow thickness.

This paper the design of a lightweight and compact patch antenna array with an inclined proximity coupled feed. The antenna array is designed for operation on a small unmanned aircraft system (sUAS) with an ultra-wideband (UWB) radar for remote sensing of soil moisture and snow. To achieve UWB impedance matching, the design utilizes an inclined wide feed and inter-element coupling. A 3-D printed base (polylactic acid) is used to support the feed structure. The antenna array operates over a 2.5 GHz – 4.5 GHz frequency (57.2% bandwidth) range, and its weight is 112 grams. A prototype $2\times4$ element array is fabricated and tested. The antenna array has an 88.2% average radiation efficiency and a 13.2 dBi average realized gain over the desired frequency band with broadside and stable radiation pattern.

Soil moisture plays a key role in land-atmosphere interactions and is an important variable in hydrological modeling. Satellite-based microwave sensors have been demonstrated for estimating surface soil moisture, but the spatial resolution is poor. Retrieval of soil moisture over inhomogeneous terrain is difficult. There is a need for fine-resolution soil moisture data to support both scientific modeling research and operational applications.The Remote Sensing Center at the University of Alabama has developed a compact, ultra-wideband microwave radar operating over the frequency range of 2-6 GHz for airborne soil moisture measurements. A highly linear, highly configurable chirp is generated by a direct digital synthesizer (DDS) driven voltage-controlled oscillator (VCO) with phase-locked loop (PLL). This system enables us to generate an ultra-linear chirp of up to 10 GHz bandwidth. We have also developed a custom ultrawideband antenna to collect data over incidence angles between 0 and 50 degrees with an adequate signal-to-noise ratio and meet the bandwidth requirements. The radar performance is verified using simulated targets in the laboratory, and the measured impulse response is close to the ideal response.We have integrated the radar with a hybrid multirotor unmanned aircraft system (UAS) that can be operated within FAA limits. The long endurance and low-speed capability of the hybrid multirotor aircraft minimizes motion-related errors. We have operated the radar and collected data at test sites near Tuscaloosa, AL. Radar measurements are being calibrated and validated against detailed in-situ measurements collected using soil moisture probes. In this paper, we will discuss the design and development of the radar, measurements, and field results.

An ultrawide-band radar system was developed to measure snow thickness from an airborne platform. It is a second-generation device, improving upon a system that was deployed to Antarctica in 2018-2019. The radar operates from 2.7-10.7 GHz and from 10-18 GHz for a theoretical resolution of less than 2 cm. It was deployed to the Grand Mesa National Forest in Colorado on a Twin Otter DHC-6 aircraft in March of 2019. The system successfully mapped the air-snow and snow-ground interfaces as well as some internal layering over both vegetative and non-vegetative areas. In this paper, we discuss the design, development and deployment of this radar and show some preliminary results.

A surface-based L-band radar sounding system was developed to measure ice sheet thickness, basal conditions, and ice-shelf melt rate. The radar system operates in a pulse-chirped mode with a peak transmit power of 4 kW through eight channels of 500 W each. The system is equipped with an array of high gain Yagi antennas to achieve the required sensitivity. We have deployed the radar to the East Greenland Ice-Core Project (EGRIP) site in Greenland for field tests during the 2019 Summer field season. We have successfully demonstrated, for the first time, radar sounding of kilometers-thick polar ice sheet at L-band. In this paper, we will present the radar system design, field operation, as well as field results.

An ultra-wideband frequency-modulated-continuous-wave radar was designed and developed at the Remote Sensing Center at the University of Alabama. The radar was deployed to Dome Fuji in East Antarctica for the measurement of near-surface snow accumulation as a part of the ice core drilling site selection effort by the National Institute of Polar Research (Japan) and the Norwegian Polar Institute. The oldest ice is expected to be located in low-accumulation areas of the East Antarctica. The UWB radar is to map near-surface internal layers for estimating snow accumulation over areas surveyed to find an optimum site. The radar operates from 2 to 8 GHz with theoretical range resolution of 2.5 cm. In this paper, the design and development of this radar, as well as some preliminary results, are presented.