The detection of the sky-averaged 21-cm signal from the Cosmic Dawn requires antennas whose beam pattern varies minimally with frequency, since beam chromaticity couples the bright, spatially structured Galactic foreground into spurious spectral structure that can obscure the faint cosmological signal. We present a Particle Swarm Optimization (PSO) framework that optimizes a double-ridged transverse-electromagnetic mode (TEM) horn antenna operating from 60 to 85 MHz against a cost function targeting beam chromaticity directly, and benchmark it against both an unoptimized baseline and an established directivity-based cost function [Restrepo, O. A., Lucero, F. I., Chaparro, G. et al. [2023] J. Astron. Instrum. 12, 2350005]. The designs are evaluated by convolving CST-simulated beam patterns with the Global Sky Model to produce antenna-temperature spectra over Local Sidereal Time, and assessed through a mock signal-injection test and a spectral-index analysis. The proposed cost function reduces the directivity smoothness metric by 97% and the beam-solid-angle variation by 52%, and yields the lowest beam-induced foreground residual of the three designs at every foreground order, falling below the depth of an injected 500[Formula: see text]mK signal already at fourth order. In the cold-sky window used for a global-signal integration it is also the least chromatic ([Formula: see text]). We show that the single-power-law spectral index is an incomplete chromaticity metric — its full-day scatter is dominated by Galactic-transit hours excluded from the science integration — and that the foreground-fit residual is the more direct measure of recoverability. These results demonstrate that targeting beam chromaticity directly produces a TEM horn better suited to foreground subtraction than either the baseline or a directivity-based optimization, and provide an observation-driven framework for antenna design in global 21-cm experiments.
Radio and mm-wavelength astronomical instrumentation systems require anti-aliasing and band-defining filters with sharp band edges, high reproducibility, low thermal variability, and low susceptibility to radiofrequency (RF) interference. As large-scale deployments involving thousands of RF channels become more common, there is an increasing need for filter solutions that balance technical performance, scalability, and cost-efficiency. In this work, we present a practical framework for the design and implementation of high order stripline filters tailored for wideband digital readout systems. Emphasis is placed on achieving low unit-to-unit variation ([Formula: see text] on frequency response metrics), steep roll-off ([Formula: see text] [Formula: see text]dB/GHz), high stopband isolation ([Formula: see text] [Formula: see text]dB), minimal in-band ripple ([Formula: see text] [Formula: see text]dB), and environmental stability (thermal drift [Formula: see text]% across 0–115 ∘ C). These performance targets are realized specifically in stripline filters, which rely on an embedded layout structure, material selection, and electromagnetic shielding. While these filters are complex to design, for low production runs, their precision and process uniformity make them ideal for scalable batch fabrication. Case studies from radio astronomy applications validate the proposed approach against demanding real-world requirements, demonstrating that the combination of careful material stack-up and repeatable design methodology support scalable deployment of high-order filters for next-generation radio telescopes.
Instruments targeting 21 cm emission at high redshifts need a spectral dynamic range of better than ten thousand to distinguish the 21 cm background against bright foregrounds. Systematics arising from the antenna pattern are a leading limitation for current instruments and must be addressed in future experiments. Antenna pattern measurements could help reach this precision. Pattern measurements are complicated by the large scale of the instruments and interaction with the local environment. In-situ beam mapping methods have been investigated but the required accuracy remains ill defined. One consideration is whether the calibration source is in the far field. Near field measurements require more elaborate measurement and such an expense must be well motivated. The far field distance is set by the effective size of the antenna. Reflections and interactions with surroundings extend the effective size of the antenna to scales well beyond the physical aperture. Here we give a new, instrument-agnostic method for calculating beam calibration requirements. Using 21cm models and instrument noise we prescribe bounds on the geometric reflection size scales. These scales must be shown via measurement to be below noise. This prescription depends weakly on instrument-specific noise and for interferometers, on the characteristic baseline length, but is otherwise independent of any detailed simulation of antenna or analysis pipeline. Example calculations for HERA-like and EDGES-like instruments find cosmological structures map to reflection scales of 100 m. This far field distance puts ground-based transmitters close to the horizon and drone sources well above typical or legal operating heights. A near-field measurement approach is necessary. Phase-locked systems have been demonstrated with promising results but more work is necessary to validate an antenna pattern at the necessary dynamic range.
The Radio wave Observations at the Lunar Surface of the photoElectron Sheath (ROLSES) instrument is a radio telescope system designed to characterize the radio and plasma wave environment of the nearside lunar surface at frequencies between 2 kHz and 30 MHz. The ROLSES sensor consists of a set of four 2.5 meter radio monopole antennas onboard the Intuitive Machines (IM 1) lander, Odysseus. The antennas were stowed during launch and deployed after landing on the lunar surface using a frangibolt mechanism. The frequency range is well suited to observing radio waves at frequencies below 15 MHz that cannot be observed from Earth due to the ionospheric cutoff. Radio waves from the Sun, the Milky Way galaxy, Jupiter, Earth's auroral region, and ground-based radio transmitters were expected to be present on the lunar surface. Radio data from each of the 4 antennas, after passing through an isolating pre-amp and signal conditioning analog electronics, were digitized to 14 bits at 120 mega samples per second and then digitally processed by a Field Programmable Gate Array (FPGA) that performs onboard spectral analysis via a Fast Fourier Transform (FFT). Time averaged spectral values are then stored and returned. Also telemetered to Earth are raw waveforms (unprocessed time sequence data) that are useful in studying dust impact on ROLSES antennas. ROLSES data are sent to the lander and subsequently downlinked for further processing. ROLSES is part of NASA's Commercial Lunar Payload Services (CLPS) program. Odysseus landed close to the south pole at Malapert A (80 S). This paper describes the design and operations of the ROLSES instrument and presents initial observations made during transit and surface operations in February 2024. We also describe next version of this instrument (ROLSES 2) currently under development.
Recent experiments in cosmology, particularly those aimed at detecting the faint, redshifted, global 21 cm hydrogen line (depth < ~200 mK, z > 7.5), have imposed stringent new requirements on radiometer calibration. In this work, we present a framework for circuit modeling and parameter inference to strengthen these calibration pipelines. This new approach enables in situ characterization of otherwise immeasurable systematics using physically motivated models. A combination of frequentist and Bayesian techniques are employed in a pipeline that supports iterative modeling, robust parameter estimation, and detailed uncertainty quantification. The framework is applied to the REACH telescope, where the precise correction of variations in the radio signal paths arising from component aging or environmental effects is critical. Circuit models of REACH's calibration sources are developed, with the goal of predicting source temperature corrections that are conventionally obtained from laboratory measurements. By fitting the models to measured data using a convolutional cost function, a strong agreement with RMS residuals no worse than -37 dB is obtained. However, Bayesian inference reveals that the resulting temperature corrections can have uncertainties on the order of 1 to 2 K, caused by reflection coefficient degeneracies, measurement noise, and errors in the models. To combat this, posteriors obtained from laboratory measurements are employed as updated priors, reducing correction uncertainties down to 75 mK. Ultimately, the framework provides a means of dynamically accounting for drift in system non-idealities over time, addressing the increasing precision demands of global 21 cm radio astronomy.
This paper presents the design and testing of a millimeter-wave frequency comb generator developed for the Black Hole Explorer (BHEX) mission, a space Very-Long-Baseline Interferometry (VLBI) mission concept. The heart of BHEX is a dual-band receiver, centered at 90 and 270 GHz. This novel comb generator is based on a microwave phase modulator producing phase-coherent comb signals with multi-octave bandwidth, which will be used to track instrumental delays when injected into the receiver system. The comb generator was tested with astronomical receivers, which confirms its expected operation.
Direction of arrival (DoA) estimation plays a crucial role in various areas of signal processing. Although existing methods perform satisfactorily at moderately high signal-to-noise ratio (SNR) levels of the order of 0 dB, they often lack accuracy in low SNR scenarios (<-10 dB), which is critical in science applications like radio frequency interference (RFI) mitigation for radio astronomy. In this context, it is essential to estimate the DoA of a source of RFI at low SNR to prevent high-gain radio telescope receivers from saturating. In this paper, a novel DoA estimation method is proposed, which is designed specifically for low SNR scenarios. This method involves multiple subarrays and decomposition techniques to enhance the SNR of the RFI. By comparing the signal subspace with the noise subspace, we can identify the potential incident direction and perform refinement in subsequent steps. Accuracy is further improved by combining all DoA candidates together and utilizing a clustering method to remove outliers. We conducted simulations using Automatic Dependent Surveillance-Broadcast (ADS-B) signals and sine waves in Additive White Gaussian Noise (AWGN) and multipath fading channels for signals that will be detected by a 100m radio telescope. The results demonstrate that our proposed method outperforms prior algorithms in low SNR conditions.
PSLV Orbital Experimental Module (POEM), also known as PS4 Orbital Platform (PS4-OP), utilizes the spent fourth stage (PS4) of ISRO's PSLV launcher to provide a long-duration in-orbit platform for hosting the payloads. In this regard, the POEM-3 was the third mission of the POEM series. ISTRAC/ISRO ground stations at different locations provide Telemetry, Tracking and Command (TTC) services to POEM missions, which require a precise prior knowledge of their orbit in terms of mission operations, generating antenna pointing angle and schedule planning of ground stations. To perform these essential operations, the flight dynamics packages: Satellite Precise Orbit Determination (SPOD), Satellite Precise Orbit Prediction (SPOP), and Orbit Products generation packages are designed and developed by the Flight Dynamics Team at ISRO Telemetry Tracking and Command Network (ISTRAC) as per requirements given by PSLV/ISTRAC project. The software packages are used to carry out POEM series flight dynamics operations on a daily basis from the launch day onwards. PSLV fourth stage was lowered to 350km orbit at 9.6 degrees inclination after deploying XPOSAT spacecraft at 650km altitude to reach the POEM-3 operational orbit altitude by carrying out PS4 engine restarts twice. The POEM-3 was the first low-altitude mission of the POEM series to ensure its decay within 3 months under the influence of natural forces, mainly the Earth's atmospheric drag. POEM-3 was also the first mission where Navigation with Indian Constellation (NavIC) data was ON till two days before the re-entry date. Based on NavIC short-arc data similar to 5-10min, daily two times Orbit Determination (OD), prediction and product generation were carried out using SPOD and SPOP. The SPOD package iteratively estimates POEM-3 orbit using the least square differential correction method. The orbit propagator used within the SPOD is a high-fidelity orbit propagator, SPOP, which was also being used to generate the ephemeris and re-entry prediction once the orbit determination was carried out.
In this paper, we present initial results from test observations conducted with a compact, broadband (meter-decameter range) mobile antenna array designed for autonomous operation in field conditions. Powered exclusively by solar energy and independent of stationary infrastructure, the system is rapidly deployable and well-suited for remote low-frequency (<100 MHz) radio astronomy. Solar radio bursts were used as illustrative examples to demonstrate system functionality and sensitivity under natural conditions. In addition to solar activity, the antenna array is capable of detecting other astrophysical sources such as Jupiter and Cas A, confirming its broader applicability. Key technical characteristics of the antenna and its integrated solar power station are also described.
The Collaboration for Astronomy Signal Processing and Electronic Research (CASPER) toolflow is a widely used framework for designing and implementing digital signal processing systems, particularly in the field of radio astronomy. It provides a set of tools and libraries that enable researchers to create custom hardware and software solutions for processing astronomical data. The CASPER toolflow has been instrumental in the development of Field-Programmable Gate Array (FPGA)-based digital instruments for various radio telescopes, enabling for real-time data processing and analysis. However, the current frontend tool that CASPER uses for high-level FPGA design is based on Model Composer integrated into MATLAB/Simulink, which is a proprietary software. In this paper, we introduce Scilab as a new frontend tool for the CASPER toolflow. Scilab is an open-source software platform for numerical computation and data visualization, which offers a similar environment to MATLAB/Simulink for designing CASPER blocks, generating FPGA Intellectual Property (IP) cores, and simulating Digital Signal Processing (DSP) systems. We present our implementation of Scilab in the CASPER toolflow and demonstrate its capabilities by developing an FPGA-based spectrometer on a RFSoC4[Formula: see text] × [Formula: see text]2, a commonly used CASPER platform well suited to radio astronomy applications. We have also developed Scilab support for other CASPER compatible platforms. Our results show that Scilab can successfully be used as an alternate frontend for CASPER-based designs.
The optical interferometer array configuration that attains highest angular resolution for a given number of array elements is a linear array with equal spacing between array elements, in which the array is phased by tracking fringes on the neighbor-to-neighbor baselines. We demonstrate this technique with observations taken with a nearly-linear configuration of the Navy Precision Optical Interferometer (NPOI). This chain bootstrapping technique required an upgrade to the NPOI beam combiner, which we dub New Classic. We describe the control scheme for New Classic for phasing the array and present observations of two stars to demonstrate the technique. To analyze the data, we use the post-processing technique of coherent averaging, in which the fringe phases on the neighbor-to-neighbor baselines are used to correct the phases on the longer baselines before averaging the complex visibilities.
Modern radio interferometers are designed with increasingly sprawling geographical footprints, offering enhanced sensitivity and resolution. However, managing such extensive facilities presents operational challenges that can potentially impede or delay scientific progress. One solution to such obstacles is the `swarm telescope' concept which enables collaborative use of individual telescope systems, overseen by separate institutions, to create a more powerful and manageable facility. We present the design, construction, and commissioning of the Long Wavelength Array – North Arm (LWA-NA) station, a prototype 64-element LWA Swarm telescope. LWA-NA is a cost-efficient, rapidly deployable platform for radio astronomy, and serves as a pathfinder for the larger LWA Swarm project.
Deflection of light along the optical path is a major source of image degradation for ground-based telescopes. Methods have been developed to measure upper atmospheric seeing based on models of the turbulence in the atmosphere, but due to boundary conditions, transmission within telescope enclosures is more complex. The Multi-beam Optical Seeing Sensor (MOSS) directly measures the component of the image quality degradation from inhomogeneity of the index of refraction within the telescope dome. MOSS outputs four near-parallel beams of light that travel along the optical path and are imaged by the telescope's detector, landing like starlight on the telescope's focal plane. By using a strobed light source, we can 'freeze' the instantaneous index variations transverse to the optical path. This system captures both 'dome' and 'mirror' seeing. Through plotting the standard deviation of differential motion between pairs of beams, MOSS enables characterization of the length scale of turbulence within the dome. The temporal coherence of temperature gradients can be probed with different pulse lengths, and the spatial coherence by comparing pairs at different separations across the aperture of the telescope. Optical path turbulence measurements, alongside other telemetry metrics, will guide thermal and airflow management to optimize image quality. A MOSS prototype was installed in the 1.2m Auxiliary Telescope (AuxTel) at the Vera C. Rubin Observatory in Chile, and preliminary data constrain the optical path turbulence with a lower bound of 1.4 arcsec. The optical path turbulence varied throughout the night of observing.
In this paper, we present improvements to the pointing accuracy of the South Pole Telescope (SPT) using machine learning. The ability of the SPT to point accurately at the sky is limited by its structural imperfections, which are impacted by the extreme weather at the South Pole. Pointing accuracy is particularly important during SPT participation in observing campaigns with the Event Horizon Telescope (EHT), which requires stricter accuracy than typical observations with the SPT. We compile a training dataset of historical observations of astronomical sources made with the SPT-3G and EHT receivers on the SPT. We train two XGBoost models to learn a mapping from current weather conditions to two telescope drive control arguments — one which corrects for errors in azimuth and the other for errors in elevation. Our trained models achieve root mean squared errors on withheld test data of 2[Formula: see text]14 in cross-elevation and 3[Formula: see text]57 in elevation, well below our goal of 5[Formula: see text] along each axis. We deploy our models on the telescope control system and perform further in situ test observations during the EHT observing campaign in April 2024. Our models result in significantly improved pointing accuracy: for sources within the range of input variables where the models are best trained, average combined pointing error improved 33%, from 15[Formula: see text]9 to 10[Formula: see text]6. These improvements, while significant, fall shy of our ultimate goal, but they serve as a proof of concept for the development of future models. Planned upgrades to the EHT receiver on the SPT will necessitate even stricter pointing accuracy which will be achievable with our methods.
We apply the Spectral Kurtosis (SK) estimator as a radio frequency interference (RFI) mitigation technique. The technique is applied to MeerKAT pulsar data whilst operated in beamformer mode. According to the central limit theorem, the unique probability distributions of all radio sources in the sky would add to a Gaussian distribution. The SK estimate of a Gaussian signal is 1. Therefore, any signal that deviates from 1, above or below a specific threshold, can be accounted for as RFI and eliminated. This forms the basis of the SK RFI mitigation technique. However, care should be taken when the astronomical signal contains non-Gaussian components, which is the case for pulsar astronomy. Initially, the statistics and methodology of the estimator are investigated. Afterward, we look at the effects of data clipping, RFI duty cycle, and SNR on the SK estimator. We provide details on the implementation and how to obtain the thresholds used for RFI mitigation. Signal-to-noise ratio (SNR) optimization studies are conducted for two pulsars, J0835-4510 and J0437-4715, where a 116% and 125% improvement in SNR is obtained, respectively.
The Square Kilometer Array Reconfigurable Application Board (SKARAB) is a Xilinx Virtex-7 FPGA-based platform designed for the MeerKAT array, to be used for both F- and X/B-engines. The MeerKAT F-engine receives digitized data, thus no samplers are required on the SKARAB-based platform. Among the alternative options available, a high-performing ADC mezzanine board can be used in conjunction with the SKARAB, which allows four 1.4-GHz-wide input channels to be digitized and processed. In this paper, we outline how the SKARAB has been successfully used for single-dish applications including imaging, spectroscopy, and spectro-polarimetry.
Suborbital rockets that fly focal-plane cameras that need to be cooled to optimize their operation face a series of challenges around their operation. These include maintaining a high-quality vacuum and the cooling of the detectors in a controlled way. These challenges are further heightened by the requirement that no current flows through the payload systems while the rocket motors are being armed. This paper discusses the novel pumping and cooling system implemented for the 2022 launch of the Rockets for Extended-source X-ray Spectroscopy (tREXS), including the use of a magnetic umbilical to connect the vacuum foreline to the rocket skin.
Moving objects have characteristic signatures in multi-spectral images made by Earth observation satellites that use push broom scanning. While the general concept is applicable to all satellites of this type, each satellite design has its own unique imaging system and requires unique methods to analyze the characteristic signatures. We assess the feasibility of detecting moving objects and measuring their velocities in one particular archive of satellite images made by Planet Labs Corporation with their constellation of SuperDove satellites. Planet Labs data presents a particular challenge in that the images in the archive are mosaics of individual exposures and therefore do not have unique time stamps. We explain how the timing information can be restored indirectly. Our results indicate that the movement of common transportation vehicles, airplanes, cars, and boats, can be detected and measured.
This paper describes the design of a 5.5:1 bandwidth feed antenna and reflector system, intended for use in hydrogen intensity mapping experiments. The system is optimized to reduce systematic effects that can arise in these experiments from scattering within the feed/reflector and cross-coupling between antennas. The proposed feed is an ultra wideband Vivaldi style design and was optimized to have a smooth frequency response, high gain, and minimal shadowing of the reflector dish. This feed can optionally include absorptive elements which reduce systematics but degrade sensitivity. The proposed reflector is a deep parabolic dish with $f/d = 0.216$ along with an elliptical collar to provide additional shielding. The procedure for optimizing these design choices is described.
With the ongoing growth in radio communications, there is an increased contamination of radio astronomical source data, which hinders the study of celestial radio sources. In many cases, fast mitigation of strong radio frequency interference (RFI) is valuable for studying short lived radio transients so that the astronomers can perform detailed observations of celestial radio sources. The standard method to manually excise contaminated blocks in time and frequency makes the removed data useless for radio astronomy analyses. This motivates the need for better RFI mitigation techniques for array of size M antennas. Although many solutions for mitigating strong RFI improves the quality of the final celestial source signal, many standard approaches require all the eigenvalues of the spatial covariance matrix ([Formula: see text]) of the received signal, which has [Formula: see text] computation complexity for removing RFI of size d where [Formula: see text]. In this work, we investigate two approaches for RFI mitigation, (1) the computationally efficient Lanczos method based on the Quadratic Mean to Arithmetic Mean (QMAM) approach using information from previously-collected data under similar radio-sky-conditions, and (2) an approach using a celestial source as a reference for RFI mitigation. QMAM uses the Lanczos method for finding the Rayleigh–Ritz values of the covariance matrix [Formula: see text], thus, reducing the computational complexity of the overall approach to [Formula: see text]. Our numerical results, using data from the radio observatory Long Wavelength Array (LWA-1), demonstrate the effectiveness of both proposed approaches to remove strong RFI, with the QMAM-based approach still being computationally efficient.
