Global Navigation Satellite System (GNSS) applications have a continuously increasing need for low-complexity signals that enable fast acquisition, with minimal receiver computational load and power consumption, as well as short time-to-first-fix (TTFF). In this context, Galileo Evolution activities have resulted in the design of Quasi-Pilot (QP) signals that aim to enable simplified acquisition and handover to Galileo legacy signals, facilitating access to Galileo’s high-performance capabilities. In order to ensure timely provision of this essential capability, ahead of the foreseen inclusion of QP signals in Galileo Second Generation (G2G) satellites, a dedicated QP signal – the E5a-QP component – has been designed for inclusion in the Galileo First Generation (G1G) signal plan. Most recently, the deployment of the E5a-QP signal on G1G full-operational capability (FOC) satellites has started.

The specifications of the new E5a-QP signal component were recently published with the release of the new Galileo Open Service Signal in Space Interface Control Document (OS SIS ICD, issue 2.2).

This paper presents results of the test campaign performed as part of the deployment of the E5a-QP signal on Galileo FOC satellites. In particular, the signal-in-space (SIS) characteristics are assessed for both the novel E5a-QP and legacy signal components – both prior to and following satellite reconfiguration. The presented results confirm nominal performance, for both the E5a-QP and legacy signal components, as well as successful tracking on deployed test user receivers. The paper also confirms, based on In Orbit Test (IOT) results, that the minimum power on ground commitment as per OS SIS ICD is maintained for legacy signals also after the reconfiguration.

Further on, the paper reports also on the performance of the E5a-QP signal component in realistic environment (including urban) based on upgraded receivers available to the Galileo programme. These results provide evidence on the improvement in terms of acquisition time and acquisition complexity of E5a-QP compared to a direct acquisition of E5a-Q.

The paper also gives an outline on the signal characteristics of the next evolution of the Galileo QP signals, the Galileo Second Generation Quasi-Pilot Signal Component. This signal introduces also means for fast time ambiguity resolution by means of a Time Delivery Scheme (TDS), which is provided as ultra-low and fully predictable data modulated onto the QP signal. The planned deployment strategy for E5a-QP and G2G QP is further detailed in the paper, as well as a brief description of the Fundamental Elements Projects developing new receiver prototypes.

The spoofing threat has been recognized within the GNSS community for over two decades, and recent years have seen this risk manifest in operational settings. GNSS vulnerabilities are evident both at the data layer and at the pseudorange level. To address these challenges, Galileo introduced the Open Service Navigation Message Authentication (OSNMA), launched operationally in 2025, enhancing data layer authentication capabilities. Presently, Galileo is developing the Signal Authentication Service (SAS) to provide pseudorange authentication.
SAS leverages the Galileo E6-C encrypted signal, which effectively prevents attackers from pre-generating valid signals. Users can download forthcoming Re-encrypted Code Sequences (RECS), where E6-C Encrypted Code Sequences (ECS) are further secured using the OSNMA TESLA key, allowing autonomous operation for up to several days without connection to a server. This process enables distribution of protected sequences to users ahead of transmission. Upon release of the OSNMA key, users can decrypt the RECS and utilize the ECS to correlate against the genuine Galileo E6-C encrypted signal.
While this method provides robust protection against spoofing attacks targeting open service signals, it remains susceptible to replay attacks, including meaconing attacks and more advanced variants allowing adversaries to manipulate the spoofed PVT. [IF1.1][JW1.2]Consequently, replay attacks should be regarded as significant threats.
By making the following two assumptions, a defence strategy can be constructed to mitigate the replay threat:
1. The authentic signal is received.
2. The authentic signal always arrives before any inauthentic signal.
A detailed justification for these assumptions will be provided in the paper; however, to summarise, the first assumption requires active enforcement, whereas the second [IF2.1][JW2.2]follows from the cryptographic mechanism.
Based on the aforementioned assumptions, enhancements can be implemented. One potential risk to the first assumption is an adversary injecting substantial noise into the user’s frontend, thereby obscuring the genuine signal and rendering the replayed signal as the only detectable one. To mitigate this threat, the receiver may directly assess the incoming noise level, commonly by monitoring the Automatic Gain Control (AGC).
To verify that the detected signal is also the earliest, a Vestigial Signal Search (VSS) may be performed. This process involves an exhaustive search across the receiver's time-frequency uncertainty domain. While resource-intensive, several strategies exist to address these challenges, such as employing FFT-based methods or minimizing time-uncertainty through secure synchronization techniques (e.g., NTS), and others which will be defined and examined further in this paper.
The paper will discuss and evaluate different working modes of a possible SAS receiver, including the following:
- A receiver that is simply processing ECS to generate pseudoranges used in an E6-C-based authenticated position and time solution calculated periodically.
- A receiver that is integrating the above with the AGC and VSS checks abovementioned.
- A receiver that is integrating E6-C PVT and AGC/VSS checks with other checks, including C/N0, consistency checks, and possibly INS, allowing position coasting between E6-C-based positions and reduction of authentication latency.

The Galileo High Accuracy Service (HAS) has been operational since January 2023, offering good and stable performance. The next phase of Galileo HAS is currently being implemented, offering enhanced performance and new functionalities. Its full service declaration is foreseen for Q1 2027.

One of the improvements in HAS Phase 2 will be the transmission of ionospheric parameters to users in the European Coverage Area (ECA). These parameters can be used to increase positioning accuracy for single-frequency users and to reduce convergence times for both single-frequency and multi-frequency users. The new Message Type 2 will contain an Iono Mask block, an Iono Vertical Delays (IVD) block and an Iono Vertical Accuracies (IVA) block. The content of these new data blocks will be presented in this contribution.

Ionospheric vertical delays and their associated accuracies will be provided for Ionospheric Grid Points (IGPs) which receivers in the ECA region can see down to 10° of elevation. Data for two ionospheric layers (at heights of 270 km and 1600 km) is planned to be provided, for a total of 899 IGPs. Apart from the ECA, other IGP masks can also be supported.

The IVD block basically provides a two-layer TEC map covering the ECA. However, covering almost 900 IGPs requires a significant amount of data. To conserve bandwidth, an algorithm for lossless compression of the IVD block has been developed. The compression algorithm, which can be described as a hybrid predictive-statistical compression technique, will be presented in detail. Implementation aspects of both the encoding and decoding process will be discussed.

The design and evaluation of the IVD block compression algorithm was supported by a large historical dataset of IONEX (Ionosphere Map Exchange) files. Because the compression ratio is influenced by the variability of the original values, the dataset covers periods of both solar minimum (Jan−Dec 2017) and solar maximum conditions (Jan−May 2024). Compared to encoding the IVD blocks using a fixed number of bits, the results show a median size reduction of about 46% and 33% under solar minimum and maximum conditions, respectively.

The present contribution will allow the community to anticipate the planned evolutions which will eventually be published in the Phase 2 Galileo HAS SIS ICD. The new blocks are currently being implemented in the High Accuracy Data Generator (HADG), which is the Galileo ground segment module that generates the HAS data. The compression approach will be evaluated further as part of the HAS Phase 2 implementation activities, during which it will be exercised using real-time data.