Precise Point Positioning with Ambiguity Resolution

Charter

Precise point positioning (PPP) is characterized by its undifferenced data processing with predetermined precise satellite products. After resolving undifferenced ambiguities with the aid of code and phase bias products, PPP has the potential to achieve a millimeter-level positioning precision on a global scale, which is consistent with conventional network analysis techniques. Over the past decade, PPP ambiguity resolution (PPP-AR) has been well studied by the GNSS community, and the IGS established the PPP-AR Working Group (WG) in 2018.

The goal of the PPP-AR WG at the first stage is to encourage IGS analysis centers (ACs) to generate phase bias products and further combine them to study their interoperability. At the moment, CODE (Center for Orbit Determination in Europe), GFZ (German Research Center for Geosciences), CNES/CLS (Centre National d’Etudes Spatiales/Collecte Localisation Satellites) and Wuhan University are routinely providing GPS/Galileo phase bias products in the form of observable-specific signal biases (OSBs). Since such code/phase biases are highly correlated with clocks, their cross-validation and combination entail both signal biases and clocks, and an interoperability test has been performed throughout IGS ACs (Banville et al. 2020). In addition, satellite attitudes are also recommended as quaternion products to cope with eclipsing seasons and thus improve the consistency of satellite clocks/biases among ACs (Loyer et al. 2021). In the IGS Repro3, four ACs or institutions (i.e., CODE, CNES/CLS, NRCan, TUGraz) provided OSBs starting from the year of 2000. Their interoperability has been examined and a combined product has been released (Lin et al. 2023). A multi-GNSS satellite phase clock/bias combination software package has also been transferred to the IGS for routine combination tasks.

Based on the achievements above, the goal of the IGS PPP-AR WG for the next stage is to routinely cross-validate each contributing AC’s phase clock/bias products and encourage their continuing efforts in improving phase bias qualities and developing multi-frequency phase clock/bias products.

Goals

Phase 1

• Survey the technical practices and problems from current phase bias estimates.
• Encourage more ACs to produce GPS/Galileo/BDS phase clock/bias products.
• Start a cross-validation task to assess the phase clock/bias qualities and improve the consistency among the contributing products.
• Work with the Bias WG to ensure continuity of clocks/biases at day boundaries by using the integer properties of the combined products.
• Initiate a project to expose the combined clock/bias solution to open testing for the public.
• Develop a phase clock/bias combination software open-sourced across the IGS community.
• Recommend the GB and ACC to introduce a combined clock/bias solution as an official IGS satellite product.

Phase 2

• Work in partnership with the Clock WG on time scale issues associated with continuous “phase” clocks.
• Encourage the RT WG to compute and broadcast real-time phase biases by sharing knowledge on phase bias generation.
• Survey proper models, algorithms, and requirements for multi-frequency phase bias products, and start a pilot project to generate such products.
• Assess the versatility and scalability of multi-frequency phase biases for all conceivable observable combinations and frequency choices, and start a pilot combination task.