A Feasibility Study for Signal-in-Space Design for LEO-PNT Solutions With Miniaturized Satellites

The global navigation satellite systems (GNSSs) are increasingly suffering from interferences, such as coming from jammers and spoofers, and their performance is still modest in challenging urban and indoor scenarios. Therefore, there are efforts worldwide to develop complementary positioning, navigation, and timing (PNT) solutions. One such complementary method under current research is the so-called LEO-PNT, namely, PNT solutions based on low-Earth orbit (LEO) satellites, and in particular on small-sized or miniaturized satellites. Such satellites have low-to-moderate costs of building, launching, and maintenance. Several challenges are to be overcome when designing a new LEO-PNT solution, concerning all three satellite segments: 1) the signal-in-space (SIS) or space segment; 2) the ground segment; and 3) the user/receiver segment. This article presents a survey of the SIS design challenges under the inherent constraints of wireless-channel propagation impairments as well as some design recommendations for SIS features. We address different constellation types, achievable coverage limits, and geometric dilution of precision (GDOP) bounds, as well as achievable carrier-to-noise ratios (CNRs) under a realistic wireless channel model, based on a MATLAB Quadriga simulator. We also discuss several optimization criteria regarding LEO-PNT SIS design, by taking into account the tradeoff between a low cost/low number of satellites in orbit on the one hand, and a sufficient coverage and good CNR for PNT purposes on the other hand.publishedVersionPeer reviewe

Empowering the Tracking Performance of LEO PNT by Means of Meta-Signals

Global Navigation Satellite Systems (GNSSs) are by far the most widespread technology for Position Navigation and Timing (PNT). They have been traditionally deployed exploiting Medium Earth Orbit (MEO) or Geostationary Earth Orbit (GEO) satellite constellations. To meet future demands and overcome MEO and GEO limitations, GNSSs based on Low Earth Orbit (LEO) constellations have been investigated as a radical system change. Although characterized by a higher Doppler effect, a PNT service supplied by means of LEO satellites can provide received signals that are about 30 dB stronger. Moreover, existing LEO constellations and the forthcoming mega-constellations, which are designed for broadband internet coverage, can be exploited to provide a piggybacked PNT service. With this cost-effective solution, a secondary PNT service might be subject to an economical use of resources, which may result in substantial bandwidth limitations. At the same time, the introduction of meta-signals in the GNSS literature has brought a new receiver signal processing strategy, particularly effective in terms of available bandwidth exploitation. It allows to increase the positioning accuracy exploiting a wideband processing approach, which might be challenging under severe Doppler conditions. A narrowband implementation of the meta-signal concept, namely Virtual Wideband (VWB) can tolerate harsh Doppler conditions while also reducing the processed bandwidth. It is thus more effective when addressing a secondary PNT service, where a limited frequency occupation might be an essential requirement. The aim of this work is to show the applicability of a VWB receiver architecture on signals provided by a piggybacked PNT service, hosted on a broadband LEO constellation. We demonstrate the capability of this implementation to bear high Doppler conditions while empowering the potential of LEO PNT

LEO-PNT With Starlink: Development of a Burst Detection Algorithm Based on Signal Measurements

Due to the strong dependency of our societies onGlobal Navigation Satellite Systems and their vulnerability to outages, there is an urgent need for additional navigation systems. A possible approach for such an additional system uses the communication signals of the emerging LEO satellite mega-constellations as signals of opportunity. The Doppler shift of those signals is leveraged to calculate positioning, navigation and timing information. Therefore the signals have to be detected and the frequency has to be estimated. In this paper, we present the results of Starlink signal measurements. The results are used to develope a novel correlation-based detection algorithm for Starlink burst signals. The carrier frequency of the detected bursts is measured and the attainable positioning accuracy is estimated. It is shown, that the presented algorithms are applicable for a navigation solution in an operationally relevant setup using an omnidirectional antenna

Starlink receiver prototyping for opportunistic positioning

openTraditional GNSS systems for positioning (PNT), such as GPS and Galileo, use Medium Earth Orbits (MEO). Recently, the possibility to use Low Earth Orbit (LEO) orbits for PNT has been investigated, which offer several advantages over the traditional MEO, e.g., higher power and wider band. Among the available signals of opportunity (SOOPs), this thesis project investigates the feasibility of utilizing Starlink signals, primarily designed for global Internet coverage, for positioning purposes. The Starlink signal structure is not publicly available, but the literature suggests the presence of nine equidistant spectral peaks within a band of approximately 1 MHz in the signal spectrum of each satellite, centered at frequency 11,325 GHz. The method proposed in this thesis involves the acquisition and tracking of these peaks, on a signal sampled at a lower frequency than the estimated bandwidth for the entire Starlink channel of 240 MHz, in order to reduce receiver complexity. For the acquisition phase, once the IQ components have been extracted from the signal, the optimal acquisition window length is selected as the trade-off between noise and Fast Fourier Transform (FFT) computational performance. The peak detection threshold is chosen based on the Gaussian distribution of noise and a predefined false alarm probability. This enables the selection of peaks above the noise floor in each acquisition instance, facilitating the detection of potential satellites. Then, similar to standard GNSS receivers, a tracking loop (a third-order PLL assisted by a second-order FLL) is implemented to estimate the Doppler frequency shift of the peaks over the entire captured window. However, as opposed to standard GNSS signals, Starlink does not use a PRN code to identify the individual satellites. To resolve the ambiguity in satellite identification, a method is proposed to compare the Doppler frequency shifts estimated from peak tracking with the Doppler frequency shifts predicted by a visibility prediction tool, which provides the ability to associate each identified peak with a specific Starlink satellite. The tool uses Two-Line Element Sets (TLEs) data and a simplified perturbation model (SGP4) to propagate the satellite orbits. The method is applied to a signal captured using a basic configuration with a Ku-band Low Noise Block (LNB) converter, and the data acquired consist of raw In-phase and Quadrature-phase (IQ) samples with a bandwidth of 4,096 MHz around 11,325 GHz. The results show that the method allows to acquire several satellites in the captured signal, and to track the corresponding peaks for positioning purposes

DESIGN AND ASSESSMENT OF A LEO GNSS MINICONSTELLATION FOR POSITIONING, NAVIGATION, AND TIMING (PNT)

Recently, there has been a resurgent demand in the United Arab Emirates for more accurate positioning, navigation, and timing signals, especially for some targeted applications such as autonomous vehicles and flying taxis. The existing Global Navigation Satellite Systems (GNSS) provide real-time positioning accuracy for up to several meters, while the targeted applications require fast convergence of centimeterlevel positioning accuracy. Recent studies have shown that transmitting GNSS signals from a Low Earth Orbit (LEO) instead of a Medium Earth Orbit (MEO) would enhance positioning accuracy. The main objective of this thesis is to design and simulate an optimum scenario of a mini-LEO constellation transmitting GNSS signals in LEO and assess its performance using a GNSS simulator tool. The second objective is to evaluate the performance of a ground-based GNSS receiver receiving GNSS signals from LEO regarding the receiver’s time to lock, locking period, continuity, Position Dilution of Precision (PDOP) and 3D positioning accuracy. The final objective is to compare the performance of the simulated mini-LEO GNSS constellation with the existing MEO GPS and Galileo. Skydel GNSS simulator tool, single frequency L1/E1 ublox receiver, Systems Tool Kit (STK), and u-center software were used to conduct this research. The best simulated LEO scenario had a design consisting of 35 satellites at 800 km altitude, distributed into 5 planes, with 7 satellites in each plane, the planes were 45° apart and the satellites were 30° in each plane. The results showed a range of PDOP values from 2.1 to 3.3, 3D positioning accuracy of 5.86 m, and the time the receiver took to lock was about 1 minute with a maximum locking period of 3 minutes and with no continuity. The results obtained from the simulated LEO constellation assessed using the ublox receiver were no better than those of the simulated MEO GPS and Galileo. The main reason behind the obtained results is that the current GNSS receivers are not designed to cope with the higher dynamics of the satellites in LEO

In-Orbit Demonstration of Precise Point Positioning for Real-Time On-Board High-Accuracy Orbit Estimation of LEO Satellites

Satellites in Low Earth Orbit (LEO) are typically equipped with GNSS (Global Navigation Satellite Systems) receivers to obtain real-time positioning, velocity, and timing. Until now, onboard GNSS positioning accuracy in LEO has been limited. Where more accurate state vector estimates are required the raw GNSS data must be post-processed on the ground. Precise Point Positioning (PPP) is a GNSS positioning technique which allows users to obtain absolute high accuracy positioning. While PPP has been extensively used for precise Earth-based navigation, it has not been used for enhancing the positioning accuracy of satellites in LEO until now. This paper describes how Fugro has utilised the PPP technique to estimate real-time nominal positioning at sub-decimetre levels of accuracy onboard satellites in LEO. This technology was demonstrated on Loft Orbital’s YAM-3 satellite. To the knowledge of the authors, this is the first time that such a level of positioning accuracy has been achieved on-board a LEO satellite in real-time. The system architecture used to deliver PPP-enabling corrections to LEO and the architecture onboard is described. The results from the demonstration are presented and some use cases that benefit from this enhanced onboard position, velocity, and time solution are highlighted

The Joint ESA/NASA Galileo/GPS Receiver Onboard the ISS the GARISS Project

ESA and NASA conducted a joint Galileo/GPS space receiver experiment on-board the International Space Station (ISS). The objectives (Enderle 2017) of the joint project were to demonstrate the robustness of a combined Galileo/GPS waveform uploaded to NASA hardware already operating in the challenging space environment - the SCaN (Space Communications and Navigation) software defined radio (SDR) testbed (FPGA) - on-board the ISS. These activities data included the analysis of the Galileo/GPS signal and on-board Position/Velocity/Time (PVT) performance, processing of the Galileo/GPS raw data (code- and carrier phase) for Precise Orbit Determination (POD), and validate the added value of a space-borne dual GNSS receiver compared to a single-system GNSS receiver operating under the same conditions. This paper will provide a general overview of the Galileo/GPS experiment called GARISS - on-board the ISS, describe design, test and validation and also the operations of the experiment. Further, the various analysis conducted in the con is joint project and also the results obtained will be presented with a focus on the (Precise) Orbit Determination results

Visibility of LEO satellites under different ground network distributions

The Low Earth Orbit (LEO) satellites have shown various benefits in augmenting the Positioning, Navigation and Timing (PNT) service based on Global Navigation Satellite Systems (GNSSs). The higher number of LEO satellites and their much smaller footprints than those of the GNSS satellites motivate studies of the ground tracking network design to pursue higher visibilities to LEO satellites. This contribution proposes an algorithm, called here ‘MaxVis’ to select network stations for LEO satellites of different inclinations and altitudes. The goal is to increase the general visibility and shorten the visibility gaps of LEO satellite that can be observed from the entire ground network, i.e., when at least one of the network stations are visible to the satellite. A parameter can be set to balance the priority of the two objectives. It was found that LEO satellites with high altitudes and low inclinations tend to deliver high visibility. With only the polar regions excluded from the design area for demonstration purposes, the general visibility could reach above 98% with less than 30 stations when the LEO satellite has an altitude of 1200 km and an inclination of 50 degrees. The visibility could be significantly reduced when island areas are excluded from the design area

Integer ambiguity Resolution in Multi-constellation GNSS for LEO Satellites POD

Precise Orbit Determination (POD) of Low Earth Orbit (LEO) satellites is essential for future LEO-augmented Positioning, Navigation and Timing (PNT) service based on the use of Global Navigation Satellite Systems (GNSS) measurements. Compared with the ambiguity-float LEO satellite POD, Integer Ambiguity Resolution (IAR) reduces number of parameters, eliminates the high correlations between the ambiguities and other estimable parameters, and strengthens model strength. In this study, using real data from Sentinel-6A tracking dual-frequency GPS and Galileo observations, the wide-lane (WL) and narrow-lane (NL) ambiguity fixing rates and the effects of the IAR on orbital accuracy are assessed in the single- and dual-constellation scenarios. Post-processed high-accuracy GNSS satellite clocks, orbits and Observable-specific Signal Biases (OSBs) from the final products of the Center for Orbit Determination in Europe (CODE) and the rapid products of the GeoForschungsZentrum (GFZ) are used for the analysis. Results showed that both the WL and NL fixing rates in the Galileo-only scenario are higher than those in the GPS-only scenario, reaching more than 98%. This implies a better signal quality of the Galileo observations. Applying IAR has improved the orbital accuracy for all single- and dual-constellation scenarios, and was shown to be especially helpful in reducing the once-per-revolution systematic effects in the along-track orbital errors, with over 50% improvement when using the COM products. With the IAR enabled, when using the COM final products, the 3D RMS of the orbital errors amounts to 1.2, 1.2 and 1.1 cm in the GPS-only, Galileo-only and GPS+Galileo combined scenarios, and the RMS of the Orbital User Range Errors (OUREs) amounts to 0.7, 0.7 and 0.6 cm, respectively. When using the GFZ rapid products, the IAR-enabled 3D RMS were 1.8, 2.1 and 1.4 cm in the GPS-only, Galileo-only and GPS+Galileo combined scenarios, with OURE RMS of about 1 cm

Bridging clock gaps in Mega-Constellation LEO satellites

In recent years, mega-constellation Low Earth Orbit (LEO) satellites have been proposed as an augmentation to the Global Navigation Satellite System (GNSS) for positioning on the ground, especially for those in measurement environments with limited satellite visibility. The fast geometry change of these LEO satellites also reduces the convergence time of Precise Point Positioning (PPP) techniques. To realize the benefits brought by these LEO satellites, their precise orbits and clocks need to be delivered to users, which would typically be based on processing the GNSS signals collected onboard LEO satellites. Assuming that this will be possible in the future, during data reception, storage and transmission, however, data gaps could exist in the collected GNSS measurements, which would result in gaps in the LEO clock estimates. The transmission of the LEO satellite clock corrections to users could also experience outages. In this study, taking the Ultra-Stable Oscillator (USO) onboard GRACE FO-1 as an example of LEO satellites that has similar operational conditions to the expected LEO mega-constellations, three different models are proposed for bridging clock gaps varying from 1 to 60 minutes. Model A considers its mid- to long-term systematic effects, Model B bridges the gaps using low-order polynomials employing the data near the gap, and Model C exploits the benefits of both Models A and B. Results show that Model A results in larger errors than the other two models for short clock gaps, while Model B could lead to a dramatic increase in the bridging errors for long gaps, e.g., 1h. Applying Model C for the USO on GRACE FO-1, the mean absolute bridging errors (in range) are within 1cm for gaps shorter than 10min, and within 0.2m for gaps not exceeding 1h. Increasing the polynomial degree of Model C from quadratic to cubic can lead to a reduction in the mean absolute bridging errors to mm- to cm-level