The potential of space-borne GNSS-Reflectometry (GNSS-R) technique for soil moisture retrieval has been demonstrated in recent studies using observations from the NASA’s Cyclone Global Navigation Satellite System (CYGNSS) and the UK’s Technology Demonstration Satellite, TechDemoSat (TDS-1). Spire Global operates a constellation of CubeSats performing GNSS based science and Earth observation. In December 2019, Spire launched two new satellites with GNSS-R payloads with plans to launch two more follow-on GNSS-R missions in January 2021. In this study, we highlight the capabilities of the Spire’s current and future GNSS-R missions compared to CYGNSS for global soil moisture monitoring and present the results of an inter-comparison between CYGNSS and Spire GNSS-R observables over land with NASA’s Soil Moisture Active Passive (SMAP) observations. The comparison of level-1 data and various statistical parameters was performed after data collocation both trackwise and also within a 6km regular grid. The results of the study were used for intercalibration of CYGNSS and Spire’s GNSS-R measurements for developing a combined GNSS-R soil moisture product.

Spire Global operates the world’s largest and rapidly growing constellation of CubeSats performing GNSS based science and Earth observation. Currently, the Spire constellation, with many satellites in polar orbits, performs a variety of GNSS science, including radio occultation (GNSS-RO), ionosphere and space weather measurements, and precise orbit determination. These satellites have been primarily tasked to perform GNSS-RO to produce accurate profiles of atmospheric temperature, pressure, and water vapor and to collect millions of daily ionospheric total electron content measurements. Previous work showed that grazing angle reflections of GNSS signals off of ocean and sea ice surfaces serendipitously collected during radio occultation measurements had the potential to perform precision altimetry (< 10 cm) over sea ice surfaces. In 2019, Spire reprogrammed its STRATOS GNSS science receiver to collect grazing angle reflection observations on Spire's large constellation of orbiting GNSS-RO satellites. To accomplish this, the open-loop tracking used in GNSS-RO collection was modified to perform open-loop prediction and tracking of grazing angle reflections between 5-30 deg elevation. Initial results confirm coherency of reflections over most sea ice surfaces and some open ocean surfaces. Full altimetric processing has been performed and is being productionized, confirming sub-10 cm precision over sea ice where reflections were coherent, with some initial measurements showing altimetric height precision less than 2 cm RMS relative a mean sea surface (e.g., DTU18). Due to the large number of current and planned GNSS-RO satellites as Spire's constellation scales to over 100 operating GNSS-RO satellites, this technique has excellent potential to complement other sensors such as ICESat-2 and Cryosat-2. A larger production period has now begun on multiple Spire satellites that will result in much larger quantities of diverse cryospheric measurements (sea ice as well as ice sheets will be sampled). We will present further results of this new and potentially revolutionary technique to use existing orbiting GNSS-RO satellite constellations to perform precision sea ice altimetry.

Spire Global, Inc. operates a large and rapidly growing constellation of CubeSats performing GNSS-based science and Earth observation. In a few short years, Spire has grown from a modest CubeSat kickstarter campaign to a paradigm-shifting provider of satellite data to NOAA, NASA, and other customers of Earth observations. Spire specializes in using science-quality observations of GNSS signals (e.g., GPS, GLONASS, Galileo, QZSS, etc.) to derive valuable information about the Earth environment. Currently, these observations include radio occultations to profile the neutral atmosphere with high accuracy and vertical resolution for applications such as NWP assimilation and climate monitoring, as well as to measure ionosphere slant total electron content and scintillation indices for space weather applications. As of May 2020, and after 21 deployments, Spire now has over 80, 3U Cubesats satellites capable of performing a variety of GNSS science, with plans to grow the constellation to well over 100 operational and continuously replenished satellites. Beginning in 2018, Spire began an effort to design and build the first of many GNSS bistatic radar (or GNSS-R) missions for Earth observations for a variety of applications, including soil moisture measurement, wetlands and flood inundation mapping, sea surface roughness and winds, and sea ice characterization. Following an agile model of rapid, iterative satellite development that has been refined over a few years to produce radio occultation payloads optimized for operation on ultra-small, 3U CubeSats, we adopted a very aggressive schedule to adapt the current Spire 3U bus and STRATOS GNSS science receiver to perform GNSS-R measurements, with a launch of satellites in December of 2019, and plans for two more GNSS-R satellites to be launched in 2020. We will discuss the goals and accomplishments of the Spire GNSS-R mission, the design and operational modes of the first batches of Spire GNSS-R satellites, and plans for a full, operational constellation of GNSS-R satellites. The Spire GNSS-R effort also has a parallel path that is already harnessing existing orbiting Spire satellites used for radio occultation to additionally perform grazing angle GNSS-R measurements for high-precision, phase-delay altimetry. This presentation will additionally discuss the unique experience of adapting the current constellation of radio occultation satellites to perform these new and valuable grazing angle GNSS-R Earth observations. We will introduce the concept of phase-delay altimetry and its potential to estimate surface heights on the order of 10 cm using observations of coherent GNSS signals reflected from various Earth surfaces. We will also show sea ice products derived from these new observations. Finally, we will discuss Spire’s potential to rapidly proceed with these measurements from research to operations and to make them available as a new set of Earth observations.

In a relatively short time, Spire has grown from a small start-up company to the largest commercial producer of satellite-based GNSS Earth observation products. As of May 2020, and after 21 satellite deployments, Spire now has over 80, 3U (10x10x10 cm) Cubesats satellites operating in a variety of orbit planes, third only to Planet and SpaceX in the size of its satellite constellation and growing with each launch. Spire satellites host three primary payloads: a dual-frequency GNSS science receiver, an automatic identification system (AIS) receiver for ship tracking, and an automatic dependent surveillance—broadcast (ADS–B) receiver for aircraft tracking. The Earth observations produced with Spire's GNSS science receiver include atmospheric profiles performed by radio occultation (RO), space weather observations (slant total electron content (TEC) and scintillation indices), and GNSS reflectometry (GNSS-R) using signals from the GPS, GLONASS, QZSS, and Galileo constellations. Spire was the first commercial company to produce RO observations and has participated in a number of commercial data pilot programs with NOAA, NASA, USAF, and ESA, Over the past few years, Spire has expanded and matured the Earth observation products available and has continued to improve and grow the size and capabilities of its constellation. Spire now produces thousands of low-latency RO profiles and millions of TEC observations each day, with plans for over 100 RO-producing satellites in the full constellation. Additionally, Spire recently added GNSS-R capabilities by launching the first two GNSS-R scatterometer configuration satelliites in December of 2019, with plans for two more GNSS-R satellites to be launched in mid-2020. Due to its agility and rapid launch cycle, averaging launches of four to eight satellites every six weeks, Spire has the unique ability to improve performance and add capabilities on-orbit that are impossible with traditional, risk-averse satellite missions. Spire has provided RO and space weather data to the second NOAA Commercial Weather Data Pilot program, the US Air Force Commercial Weather Data Pilot program, ESA, and numerous NWP centers and research institutions. Spire is also pioneering the provision of Earth observation data to NASA and ESA researchers through unique data purchase programs. In our talk we will present an overview of the status and capabilities of the Spire satellites and describe the collection of GNSS-based Earth observations using the GPS, GLONASS, Galileo, and QZSS constellations. We will additionally outline our plans for expanding and adding products to the Spire Earth observation constellation.

Spire Global operates the world’s largest and rapidly growing constellation of CubeSats performing GNSS based science and Earth observation. The Spire constellation, performs a variety of GNSS science, including radio occultation (GNSS-RO), ionosphere and space weather measurements, and precise orbit determination. In December 2019, Spire launched two new satellites to perform GNSS reflectometry (GNSS-R), with plans for two more GNSS-R satellites to be launched in mid-2020. Due to its agility and rapid launch cycle, averaging launches of four to eight satellites every six weeks, Spire has the unique ability to improve performance and add capabilities on-orbit that are impossible with traditional, risk-averse satellite missions. GNSS-R is a relatively new technique based on a passive bistatic radar system. The potential of space-borne GNSS-R observations for ocean and land applications has been demonstrated by other GNSS-R missions, including the NASA Cyclone Global Navigation Satellite System (CYGNSS) and the UK’s Technology Demonstration Satellite, TechDemoSat (TDS-1). We present initial results from these new Spire GNSS-R satellites that are primarily focused on retrieving soil moisture but also estimate other Earth surface properties such as ocean wind speeds and flood inundation/wetland mapping. Prior to the launch of Spire’s GNSS-R satellites and in preparation for Level-2 data production, we developed algorithms and processing chains for land applications. We will present Spire's soil moisture retrieval method using CYGNSS observations. We evaluated the implemented soil moisture change detection algorithm by comparing the Spire’s daily soil moisture product with NASA’s Soil Moisture Active Passive (SMAP) observations and in-situ soil moisture measurements. The results of study indicate remarkable retrieval skills of the GNSS-R technique for soil moisture monitoring at a medium spatial resolution. Spire’s GNSS-R satellites are tuned for land applications with a series of hardware and software optimizations for better signal calibration and acquiring many more data per satellite compared to CYGNSS. A more robust GNSS-R soil moisture retrieval at finer spatial resolution will be possible in the near future after having more Spire satellites in orbit. Spire’s current and future GNSS-R satellites will provide unprecedented sub-daily global coverage and fine spatial resolution. Such intensive data acquisition is of great importance for many land and ocean applications.

Spire Global operates the world’s largest and rapidly growing constellation of CubeSats performing GNSS based science and Earth observation. The Spire constellation, performs a variety of GNSS science, including radio occultation (GNSS-RO), ionosphere and space weather measurements, and precise orbit determination. In December 2019, Spire launched two new satellites to perform GNSS reflectometry (GNSS-R). GNSS-R is a relatively new technique based on a passive bistatic radar system. The potential of space-borne GNSS-R observations for ocean and land applications has been demonstrated by other GNSS-R missions, including the NASA Cyclone Global Navigation Satellite System (CYGNSS) and the UK’s Technology Demonstration Satellite, TechDemoSat (TDS-1). We present initial results from these new Spire GNSS-R satellites that are primarily focused on retrieving soil moisture but also estimate other Earth surface properties such as ocean wind speeds and flood inundation/wetland mapping. Prior to the launch of Spire’s GNSS-R satellites and in preparation for Level-2 data production, we developed algorithms and processing chains for land applications. We will present Spire's Soil Moisture (SM) retrieval method using CYGNSS observations. We evaluated the implemented SM change detection algorithm by comparing the Spire’s daily SM product with NASA’s Soil Moisture Active Passive (SMAP) observations and in-situ SM measurements. The results of study indicate remarkable retrieval skills of the GNSS-R technique for soil moisture monitoring at a medium spatial resolution. Spire’s GNSS-R satellites are tuned for land applications with a series of hardware and software optimizations for better signal calibration and acquiring many more data per satellite compared to CYGNSS. A more robust GNSS-R SM retrieval at finer spatial resolution will be possible in the near future after having more Spire satellites in orbit. Spire’s current and future GNSS-R satellites will provide unprecedented sub-daily global coverage with sub-kilometer spatial resolution. Such intensive data acquisition is of great importance for many land and ocean applications.

During the ExoMars Schiaparelli separation event on 16 October 2016 and Entry, Descent, and Landing (EDL) events 3days later, the Giant Metrewave Radio Telescope (GMRT) near Pune, India, was used to directly observe UHF transmissions from the Schiaparelli lander as they arrive at Earth. The Doppler shift of the carrier frequency was measured and used as a diagnostic to identify key events during EDL. This signal detection at GMRT was the only real-time aliveness indicator to European Space Agency mission operations during the critical EDL stage of the mission. Plain Language Summary When planetary missions, such as landers on the surface of Mars, undergo critical and risky events, communications to ground controllers is very important as close to real time as possible. The Schiaparelli spacecraft attempted landing in 2016 was supported in an innovative way. A large radio telescope on Earth was able to eavesdrop on information being sent from the lander to other spacecraft in orbit around Mars. This provided real-time (after accounting for one-way light time) basic information to mission controllers.

A low cost, low power, and low mass GNSS receiver (called Cion) has been developed and is currently flying on the CICERO cubesats. The receiver was designed in less than a year by JPL for Tyvak and GeoOptics for use in the GeoOptics CICERO constellation and leverages 25 years of JPL GNSS reciever design experience. Cion uses a commercial off-the-shelf (COTS) computer along with existing space qualified RF down-converters, software, and firmware to produce atmospheric Radio Occultation (RO) data. By combining a FPGA with dual core ARM processor and an embedded system controller, the Xilinx Zynq processor is an enabling technology that provides a customizable digital signal processing platform integrated into the computer (System on a chip) and enables off-the-shelf hardware to become the main engine behind this software defined radio. Using Linux for the on-board computer allows for fast development times and liberal use of existing open source software libraries. The parts of the receiver that require real-time implementation are performed in the Field Programmable Gate Array (FPGA), which can also be reprogrammed in flight. While the Zynq is not rad hard, the silicon on insulator (SOI) technology is rad tolerant 'by accident', allowing for its use in many space-based applications. Early results show that the Cion is working as designed, has demonstrated the first known GLONASS occultations, and obtains high quality atmospheric profiles with excellent lower troposphere penetration (near Earth's surface).

NASA's Soil Moisture Active Passive (SMAP) mission has been tuned to perform a Global Navigation Satellite Systems Reflectometry (GNSS-R) experiment. The motivation of this study is to assess the capabilities of GNSS-R for soil moisture determination, biomass monitoring and cryosphere studies. The use for first time of the Polarimetric Ratio (PR) from a spaceborne platform shows significant sensitivity to soil moisture. Additionally, the investigation for first time of the leading and trailing edges width sensitivity to Above Ground Biomass (AGB) and rough topography shows promising results. Better understanding of these effects in the reflected waveforms will improve the development of retrieval algorithms.

The European Space Agency's ExoMars Trace Gas Orbiter (TGO) arrived at Mars on October 19, 2016, three days after releasing the Schiaparelli Lander on a ballistic trajectory to Meridiani Planum. During the separation event, and subsequently during Schiaparelli's Entry, Descent, and Landing (EDL), the NASA-provided Electra Ultra-High Frequency (UHF) payload onboard TGO was used to record signals from the Schiaparelli Lander for post-processing on the ground to recover both tracking of the lander's carrier signal and reconstruction of the lander's 8 kb/s telemetry. In addition, ESA's Mars Express orbiter recorded the Schiaparelli signal, with ground post-processing providing independent tracking of the lander carrier signal, and the Giant Metrewave Radio Telescope near Pune, India was configured to provide real-time detection of the lander carrier signal. While an anomaly in the latter stages of EDL led to loss of the lander, these critical event data sets, and in particular the telemetry reconstruction enabled by the TGO Electra recording, proved essential in enabling detailed diagnosis of the anomaly. While the loss of the lander during EDL precluded the planned surface relay operations, the preparations for that activity provide important lessons learned for future Mars relay support scenarios.

It is now recognized that the enormous challenge of scientifically understanding the Earth system requires careful strategic decisions on what missions are deployed. In a recent report, the National Research Council developed a "value framework" for Earth observing systems with a focus on prioritizing NASA observations that merit long-term continuity. In this paper, we refer to this framework to discuss how high value observations arise from opportunistic use of signals generated by Global Navigation Satellite Systems (GNSS) such as GPS. The increasing number of GNSS constellations internationally, likely to be permanently deployed, suggests that the geosciences community will benefit by adopting these signals for a variety of remote sensing needs. We describe recent progress in using these observations scientifically and developing technology to exploit them. We conclude that dedicated constellations of GNSS science instruments in low Earth orbit capable of receiving both direct and reflected GNSS signals will provide excellent science return in a broad range of areas, and constitute a high value Earth observing system.

Quantifying freeze/thaw status of the landscape (soils and vegetation) is important for identifying the availability of liquid water in the high latitudes to support ecosystem processes controlling carbon sequestration and release. A better understanding of the seasonal transition of high latitude landscapes between frozen and thawed states is therefore critical for predicting carbon cycle feedbacks in a warming climate. However, observations of freeze/thaw state are not at sufficient temporal and spatial resolution to observe fast (~days–weeks) transitions in highly heterogeneous regions. Here, we present new evidence that reflected Global Navigation Satellite System (GNSS) signals captured by the SMAP radar receiver have the potential to quantify changes in high-latitude soil freeze/thaw state at kilometer-scale resolution. Boreal wetland observations indicate a ~10dB seasonal difference in the signal-to-noise ratio (SNR), in agreement with simulations from a simple reflectivity model. Our analysis indicates that ground-reflected GNSS signals as a form of L-band bistatic radar could be a robust observable to quantify soil freeze/thaw state at process-relevant scales. We describe the caveats and potential limitations of this approach, and highlight areas for continuing research.

Global Navigation Satellite Systems Reflectometry (GNSS-R) ocean applications includes scatterometry and altimetry. In this work, an investigation is performed on polarimetric scatterometry over ocean surface using data from a GNSS-R experiment on-board the Soil Moisture Active Passive (SMAP) mission, and on ocean surface topography from the Cyclone Global Navigation Satellite System (CyGNSS) mission using new retrieval algorithms. The former one provides global coverage because of the Sun Synchronous Orbit (SSO), while the latter one focuses on tropical latitudes providing a spatial sampling of 32 swaths. First results from SMAP over the Artic Sea show clearly sea ice effects on the reflected waveforms.

The Gravity Recovery and Interior Laboratory (GRAIL), a NASA Discovery mission, twin spacecraft were launched on 10 September 2012 and were inserted into lunar orbit on 31 December 2011 and 01 January 2012. The objective of the mission was to measure a high-resolution lunar gravity field using inter-spacecraft range measurements in order to investigate the interior structure of the Moon from crust to core. The first step in the lunar gravity field determination process involved correcting for general relativity, measurement noise, biases and relative & absolute timing. Three independent clocks participated in the process and needed to be correlated after the fact. Measuring the absolute time tags for the GRAIL mission data turned out to be a challenging task primarily because of limited periods when such measurements could be conducted. Unlike the Gravity Recovery and Climate Experiment (GRACE), where absolute timing measurements are available using the GPS system, no absolute timing measurements were available on the far side of the Moon or when there were no DSN coverage periods. During the early cruise phase, it was determined that a direct absolute timing measurement of each spacecraft Lunar Gravity Ranging System (LGRS) clock could be directly observed by using a DSN station to eavesdrop on the Time Transfer System (TTS) S-band inter-satellite ranging signal. By detecting the TTS system directly on earth, the LGRS clock can be correlated directly to Universal Time Coordinated (UTC) because the TTS and LGRS use the same clock to time-tag their measurements. This paper describes the end-to-end preparation process by building and installing a dedicated hardware at Goldstone station DSS-24, selecting favorable lunar orbit geometries, real time signal detection and post processing, and finally how the absolute timing is used in the overall construction of lunar gravity fields.

This paper will describe the TriG architecture, and how the new features will benefit the next-generation of global network instruments, as well as current test results.