Synchronisation and Signal Delay Calibration in Passive Satellite Tracking

TRACXING SMALL SATELLITES USING TRANSLATED GPS

Doppler-Based Localization Under Hardware, Atmospheric, and Orbital Impairments for Single LEO Satellite Systems

In order to enable a wide range of applications anywhere and anytime, future communication systems are expected to employ low Earth orbit satellites to perform user verification. Single-satellite systems offer a cost-effective verification alternative, reducing implementation complexity and dependence on satellite constellations. This article develops a single-pass, single-satellite localization algorithm independent from global navigation satellite systems, supporting user verification and requiring only coarse coverage-region side information. The algorithm is based on the tracking of phase changes from received pilot signals originated from Doppler shifts, inherently related to the user's position. Our work addresses realistic channel and receiver conditions—encompassing carrier frequency offset, phase noise, and atmospheric propagation effects—while evaluating robustness against orbital perturbations, a combination that has not been jointly addressed in prior studies on Doppler-based localization. The proposed two-stage approach employs an extended Kalman filter for estimation of the referred phase shifts, followed by a weighted least squares solution. Algorithm performance is evaluated through simulations in terms of mean and $90{\text{th}}$ percentile distance error, together with the time to reach a 10-km error level, an accuracy benchmark discussed in verification studies by 3rd Generation Partnership Project (3GPP). Results demonstrate improved accuracy with respect to compatible Doppler-based baselines, with $90{\text{th}}$ percentile errors falling below the 10-km mark under the considered narrowband and line-of-sight conditions, suggesting that the method may support user verification. The time required to achieve such accuracy, particularly, may require a significant portion of the satellite's visibility window in strong phase noise conditions.

GPS/BDS Continuous Network Time Transfer With Estimated Satellite Clocks

The strong coupling between the receiver and satellite clocks implies that traditional precise point positioning (PPP) inevitably inherits the uncertainties of external satellite clock products—such as limited precision and day boundary discontinuities—thereby degrading time transfer (TT) performance. Moreover, most existing network-based solutions are formulated using ionosphere-free (IF) combinations, which not only amplify measurement noise but can also introduce inconsistencies due to misaligned satellite-end products. In this work, we extend the small-scale baseline approach to a large-scale network solution using undifferenced and uncombined observations. Two models are developed: an introduced satellite clock (ISC) model as a control group, and a simultaneously estimated satellite clock (ESC) model, which is particularly robust to day boundary discontinuities in satellite clock products. We evaluate both models using 11 days of GPS and BeiDou Navigation Satellite System (BDS) data from a European network of five stations spanning several thousand kilometers. The results show that, at the 95 % confidence level, the ESC model improves daily frequency stability by 1 %–23 % for GPS and 8 %–25 % for BDS relative to the ISC model when Centre for Orbit Determination in Europe (CODE) products are used. Comparisons across products from different analysis centers (ACs) further indicate that satellite clock products exhibit more pronounced day boundary discontinuities than orbit products. For the longest SPT0-IENG baseline (1455 km), the BDS ISC solution based on GRG products shows more frequent and larger jumps, whereas the ESC model delivers an additional 8.30 % improvement beyond the result obtained with CODE products. In addition, verification using different level products shows that the ESC model can effectively reduce the adverse impact of (near) real-time products. In general, the network-based ESC approach actually mitigates uncertainties in satellite clock products and is well suited for both postprocessed and (near) real-time applications.

A Multi-Constellation Approach to Space-Based RF Emissions Monitoring Through Opportunistic Satellite Cluster Collisions in LEO

Abstract. The deployment of small Low Earth Orbit (LEO) satellites equipped with radio-frequency (RF) tracking sensors marks a transformative shift in global, space-based RF emissions monitoring. Leveraging cost-effective software-defined radio (SDR) technology and advanced micro-antenna arrays, these satellites – organized into tandems or clusters – can efficiently scan the Earth's surface. Their payload can generate Time Difference of Arrival (TDOA) or Angle of Arrival (AOA) observables that can be used to accurately detect and locate RF emitters. Applications range from identifying known distress signals for search and rescue operations to detecting unknown Radio Frequency Interference (RFI) sources that may disrupt protected bands, such as GNSS. Currently operational LEO satellite constellations already offer such data acquisition, with commercial providers offering varying performances based on the number of deployed satellites and revisit times. To achieve unparalleled monitoring coverage and responsiveness, this paper introduces the innovative concept of a multi-constellation approach, where a centralized RF monitoring service provider accesses data from multiple constellations. The cost-benefit of this more expensive solution hinges not on the sum of individual constellation capabilities but on the potential to capitalize on additional multi-constellation opportunistic events, termed ‘collisions.’ These occur when satellites from different constellations form new clusters, allowing simultaneous observation of the same area and enabling additional TDOA or AOA measurements not associated to a single constellation. The effectiveness of these collisions depends on relative geometry and other critical performance indicators, and not all events can be considered advantageous for a target RF detection and localization scenario. This paper develops a methodology to establish metrics that quantify the occurrence and utility of such interactions, providing collision frequency data. The methodology involves an analytical description of satellite constellations with varying orbit sizes, considering their relative kinematics to a reference signal emitter on Earth's surface, the Cramér-Rao Lower Bound (CRLB), Dilution of Precision (DOP), and incorporating constraints on the Line of Sight (LOS). Case studies in LEO mission applications, exemplary of search and rescue operations and spectrum monitoring in the E1/L1 GNSS frequency bands, are discussed. The proposed solution offers a crucial tool for monitoring service providers to evaluate the advantages of multi-constellation data access, assess the expected number of beneficial collisions, and optimize sensor acquisition scheduling to maximize multi-constellation RF detection and localization accuracy.

High Accuracy Measurement of Geostationary Satellite Location Uncertainty using TWSTFT via NMI’s Time and Frequency Laboratories

A dense distribution of space debris is present in geostationary orbit (GEO). To avoid collision with debris, maneuvers to change the satellite's position are necessary. The uncertainty of the satellite's location defines the region that must avoid the passage of debris. The larger this uncertainty region of the satellite's location, the greater the number of maneuvers that must be performed. Therefore, a reduced uncertainty region, from a few kilometers to a few meters, means reducing the number of maneuvers, reducing fuel consumption and increasing the satellite's lifetime, revenue and safety. Two-way Satellite Time and Frequency Transfer (TWSTFT) is one of the most accurate methods to compare and synchronize time scales between remote stations connected by a geostationary satellite whose clocks are references as time and frequency standards, most of them contributing to UTC. The characteristics of TWSTFT, such as high accuracy of time measurement, time difference almost in real time, multichannel modems, among others, allow that this technique be used to measure the uncertainty of the GEO satellite's location with high accuracy. The time and frequency laboratories (TF-Labs) that contribute to the UTC have highly accurate timescales and interest in intercomparison with other laboratories using the TWSTFT method. Such laboratories, located in national metrology institutes (NMI) and designated institutes (DI), are ideal candidates for implementing a system for determining the uncertainty of the location of geostationary satellites with high accuracy which will reciprocally provide a network for intercomparison among them. These TF-Labs are located in different countries, separated by large distances, which is the ideal characteristic to increase the accuracy of the GEO satellite location. In 2022, my article (DOI: 10.1109/LATINCOM56090.2022.10000557) developed a study to reduce the location uncertainty of the Brazilian geostationary satellite through TWSTFT links. The stations used were TF-Labs from some NMI in Latin America. The results indicated that it is possible to accentuate the reduction of PDOP (position dilution of precision) parameter and, consequently, reduce the location uncertainty of the geostationary satellite to a value below 4.3 m with the current carrier phase TWSTFT technology. A future evolution would be to use the ‘Binary Offset Carrier’ technique in TWSTFT systems to increase accuracy and reduce bandwidth used in geostationary satellites. A new need in metrology is to adopt the Optical Frequency Standard (OFS) for the redefinition of the second. This depends on remote comparisons between OFS located at different NMI/DI. An intercomparison network using TWSTFT represents a step forward to meet this need, considering two-way system has an expected evolution to reach the same accuracy of OFS. The proposed network could be addressed by BIPM CCTF-WGTWSTFT study group that would develop specific studies for the use of TWSTFT on selected GEO satellites, as described in my article. A proposal would be presented to satellite operators for the joint development of the TWSTFT system with a dual purpose: (1) to increase the revenue (lifetime) and safety of satellites and (2) to implement an intercomparison network between NMI/DI.

Simultaneous Train Localization and Radio Transmitter Mapping with TDoA and Magnetic Field Measurements