Since many years, global navigation satellite systems (GNSS) are used for satellite orbit and attitude determination at a progressive scale. With the increasing popularity of very small satellites, e.g., following the CubeSat standard, the need for an adapted small payload for orbit determination arises. Therefore, we developed a small-sized versatile GNSS payload board based on commercial-of-the-shelf GNSS receivers with extremely small weight, size and power consumption. The board features two separate antenna connectors and four GNSS receivers—two per antenna. This redundancy lowers the risk of total payload failure in case one receiver should malfunction.

Two prototypes of the GNSS positioning board have been successfully launched to space onboard the Astrocast-01 and -02 3-unit cube satellites. In addition to the GNSS payload, both satellites are equipped with an array of three laser retro-reflectors allowing the validation of the orbit with satellite laser ranging. A special feature of the integrated receivers is their multi-GNSS capability allowing the concurrent tracking of satellites from the four major systems GPS, GLONASS, BeiDou and Galileo.

To date we have GPS-only continuous receiver PVT solutions available for both satellites. First orbit determination results indicate that the receivers perform very well. The orbit fit RMS for daily arcs is on the level of 3 meters. As we are restricted to the single frequency receiver solution, however, the results are biased by a systematic error caused by the ionosphere.

Once raw observation data and long continuous observation arcs are available, we will start an extensive quality assessment and orbit validation phase based on a precise orbit determination in post-processing on ground. The tests will especially include an evaluation of the achievable orbit quality and an overall performance estimation of the payload. The orbit determination may be improved by eliminating the ionosphere effects using a linear combination of phase and code observations. This will help to generate reliable orbit predictions needed for the scheduling of SLR observations campaigns. A subsequent orbit validation based on inter-technique comparisons with SLR observations is foreseen. The performance of various single-system and multi-GNSS solutions will be analyzed putting a special emphasis on solutions including observation data from the Galileo system.

We present details on the payload board, as well as the results from the orbit determination based on the receiver PVT. As new observation data of the two satellite missions will become available during the next months, we will be able to present first results for the aforementioned tests.

Spire Global, Inc., is a leading player in the commercial Earth observation nanosatellite sector, specializing in GNSS-based observations such as radio occultation, ionosphere sounding, and precise orbit determination. Spire was the first commercial company to provide low-cost GNSS radio occultation measurements to support critical weather data for assimilation into numerical weather prediction models, including the the first-ever Galileo radio occultations. Spire has ambitious goals of collecting over 100,000 radio occultation profiles per day from all GNSS constellations, providing robust coverage of Earth observation measurements over the entire planet.

Each Spire satellite is equipped with a Spire-designed GNSS science receiver to collect GNSS signals for science and precise orbit determination, and over 70 satellites have been launched into a continuously growing constellation of CubeSats. As the GNSS signal passes through the atmosphere to the receiver, it is refracted by an amount dependent on the atmospheric characteristics along its path. This technique, referred to as radio occultation (RO), can be used to estimate atmospheric properties such as refractivity and temperature with high precision, accuracy and vertical resolution. We will provide an overview of Spire’s radio occultation measurements and how they are processed to produce accurate profiles of the lower atmosphere. Baseline statistics against numerical reanalysis models show that the overall quality of Spire RO profiles is comparable with those of past missions using much larger and more expensive satellites.

In addition to augmenting the global observing system with a significant amount of high quality vertical atmospheric profiles, collected GNSS signals from Spire’s constellation also carry a wealth of information about the ionosphere. Similar to atmospheric soundings, the large quantity of spatially diverse and low-latency ionospheric soundings are the first of their kind and are becoming increasingly valuable for the improvement of space weather forecasting capabilities. We will highlight Spire’s GNSS-based ionospheric observation capabilities by providing an overview of the types of measurements produced, including total electron content (TEC), scintillation and electron density, and reviewing recent results describing the current coverage and quality of the constellation data. Additionally, we will discuss the precise orbit determination of Spire satellites based on GNSS processing and how these observations are potentially useful for estimating thermospheric density to improve orbit drag and space situational awareness models.

Passive remote sensing from space with signals of opportunity has been studied for a long time beginning in the early 90s. The technique uses already existing signals, transmitted from satellites, which are reflected from ground in order to determine properties of Earth's surface. For GNSS reflectometry, the reflected signal is typically correlated with a clean replica generated on-board of the spacecraft. The ESA PRETTY satellite mission, however, will correlate the received reflected signal with the received direct signal. This technique is known as the interferometric approach. The main advantage for the interferometric approach is, that one is not bound to use known signals but can exploit signals with unknown data modulation, opening up the possibility to use more generic signals for Earth observation. PRETTY will focus on low elevation angles, whereby the direct and reflected signal will be received via the same antenna.
The phase-delay altimetric approach at grazing incidence angles was demonstrated initially using GPS radio occultation data from GPS/MET and CHAMP. The derived surface heights had 0.7 m precision in 0.2 second averaging (1 km resolution). This approach was recently also proposed within new spaceborne GNSS Reflectometry experiments, as, e.g., GEROS-ISS or G-TERN for ocean and ice remote sensing, including surface altimetry for elevation angles at the specular point between 5° and 30°. Phase-delay altimetric simulations have been performed for ocean applications within the GEROS-ISS related scientific ESA study GARCA to optimize the observation and data analysis strategy. The simulation results show that phase-delay altimetric retrievals are sensitive to anomalies of the ocean topography and that an altimetric precision of 10 cm in 1 second observation is possible in this respect. Similar precision was demonstrated with airborne experiments. Recent Observation System Simulation Experiments (OSSEs) indicated the large potential of future phase-delay GNSS-R data to significantly improve ocean modelling and forecast systems. This paper will focus on the scientific observables, and the expected results.
The PRETTY satellite, which is currently developed by RUAG Space, TU Graz and Seibersdorf Laboratories, under a contract with ESA hosts two payloads: The first payload is a passive GNSS based reflectometer. The main scientific goal is the precise altimetric determination of water and ice surfaces using the interferometric phase-delay altimetry approach. This methodology will be applied for the first time from space. The second payload is a dosimeter providing Total Ionizing Dose (TID) data from and also coarsely grained Linear Energy Transfer (LET) spectra based on measurements with a pin diode.

The International GNSS Service (IGS) offers open access and high-quality GNSS data products from over 500 worldwide reference stations. The data available in IGS is known to enable multiple scientific applications, such as the computation of the Terrestrial Reference Frame, water vapor estimation in the troposphere, ionospheric modelling, etc.

The GNSS Science Support Centre (GSSC), led by ESA’s Galileo Science Office (GScO), is contributing to the International GNSS Service (IGS) hosting ESA’s IGS Global Data Centre. This effort is complemented by the IGS Analysis Centre role provided by ESA Navigation Support Office at ESOC, which makes ESA a very active member in IGS.

ESA, through its Galileo Science Office is currently considering to extend the current datasets offered in the GSSC archive with datasets of GNSS observables coming from other sources, notably from locations traditionally not covered by IGS. We are considering, for example the storage of GNSS data, which could be obtained through ships in the oceans, airplanes at 10 Km height, or from space users (e.g. Earth observation satellites).

Currently, the GScO efforts are focused in the integration of GNSS Space Users Data acquired from high-quality GNSS receivers on-board satellites in the GSSC for scientific applications. As an example, the RINEX files of the GPS dual frequency receiver of the Gravity and steady-state Ocean Circulation Explorer (GOCE) mission have been recently added to the GSSC Archive. Unlike traditional reference station data, this GNSS dataset is provided together with information on the spacecraft dynamics, in order to allow the correlation of the GNSS observations with the GNSS receiver location. Several options on the format standardization for this auxiliary data are currently under consideration.

High-quality GNSS observables from satellites, at the top layer of the ionosphere, may provide improvements of the ionosphere modelling; support scintillation studies; support the modelling and understanding of the magnetosphere and plasmasphere; or even support detailed analysis of GNSS systems performances for space users.

In this paper, we will present the status of the integration of GNSS Space Users Data in the GSSC archive, informing on the set of satellites that have been already included. We will discuss some of the potential scientific applications that these datasets may enable and the way the orbital and attitude information of the spacecraft and on-board GNSS antenna are provided.

We will present the current plans to include future Earth Observation satellites datasets and discuss the interest that making this initiative global (i.e. with international cooperation from other agencies) could have for the scientific community.

GNSS Radio Occultation (RO) measurements are important for numerical weather prediction (NWP) and climate reanalysis applications. They complement the information provided by satellite radiances, because they have good vertical resolution, and they can be assimilated without bias correction to the forecast model. Currently, there are around 2800 RO measurements per day available for operational assimilation, including 1900 measurements from the GRAS RO instrument on the three first generation MetOp satellites.

The MetOp-SG mission will provide operational meteorological observations from polar orbit for the next two decades. The GRAS-2 RO instrument is enhanced to receive signals from four GNSS constellations; GPS, Galileo, Beidou, and QZSS, which provides 5000 vertical profiles per day from MetOp-SG.

Simulation studies based on an Ensemble of Data Assimilations (EDA) and Observing System Simulation Experiments (OSSEs) support the case for increasing the number of RO measurements for operational NWP applications. However, these simulations rely on having a good understanding of the both the instrument and forward model error statistics. For GRAS on MetOp-SG, we have perform simulations to determine the size of the instrument errors and forward model errors in the troposphere, using the specifications for the GRAS-2 RO instrument developed for MetOp SG, and a two-dimensional bending angle forward model.

We demonstrate in this paper that the new GRAS-2 RO instrument shows much improved performance in the lower troposphere, as compared to the first generation, thanks to the processing of multiple correlator signals. The simulation results show that performance is clearly improved when signal data from up to 10 correlators are included in the RO bending angle retrieval processing. The multi-correlator technique is especially adapted for processing the modernized GNSS signals of the European Galileo and the Chinese Beidou constellations.

The bending angle error statistics for the multi-correlator and single-correlator RO instrument has been evaluated and compared to the impact of horizontal gradients in the troposphere as computed for a set of 55 globally representative atmospheric profiles, using two independent wave optics simulation tools.

A key result presented here is that the single-correlator error statistics for rising occultations are comparable to the errors caused by horizontal gradients. This means that it is not generally correct to assume that the forward model error dominate the instrument errors in the troposphere, although this may be true for the first generation of instruments and 1D forward operators. It is probably unwise to base specifications for new instruments on the current NWP forward model error statistics.

This paper shows that the new multi-correlator RO instrument together with improved 2D forward models, both contribute to improved NWP performance in the troposphere.