Support for LEO-PNT Constellation Data in IGS Data and Product Files
"Spaceborne GNSS data" traditionally referred to GNSS raw measurements collected by receivers onboard Low Earth Orbit (LEO) satellites that track the currently existing Medium Earth Orbit (MEO) systems; GPS, Galileo, Beidou, and Glonass. This data is utilized not only in navigation but also in various fields including atmospheric research (e.g., COSMIC), altimetry (e.g., Jason), and gravity missions (e.g. Grace). However, given the rise of new GNSS constellations in LEO orbits, this data will eventually include data collected by receivers able to track these new satellites. Similarly to the MEO counterparts, this LEO-PNT data can be used not only in navigation, but also in other fields, including ionospheric monitoring (see e.g. [Prol et al., 2023]) or geodesy. Consequently, file formats to accommodate this new observable data will need to be extended. In particular, RINEX files with entries for currently existing MEO constellations (designated by already allocated letters such as ‘G’, ‘R’, ‘E’ or ‘C’ followed by a 2-digit ID) could potentially incorporate new designators for LEO constellations (e.g. ‘L’ or other available characters).
However, the proliferation of LEO-PNT satellites, led by companies like Xona and Geely planning launches of hundreds to thousands of satellites, challenges the applicability of the RINEX format as it stands today due to its two-digit satellite ID limitation. In order to address this, a preliminary proposal has been made to extend the RINEX format to accommodate these new constellations, both at the navigation [Garcia-Fernandez, 2023] and the observable files [Garcia-Fernandez, 2024a].
In the case of the navigation file, the proposal leverages the fact that in RINEX 4 format, the “EPH” entries may allow for an arbitrary long satellite designator. In particular, for LEO-PNT constellations, the approach adopted is a single character for the constellation followed by a 5-digit ID number (which could accommodate for instance the NORAD ID number of the satellite). For the navigation file, the extension describes the satellite orbit using the orbital parameters (instead of position, such as Glonass or SBAS). The rationale of this decision is that positioning engines can already leverage the orbit propagators required for GPS and Galileo and could even be used with minor modifications for Hardware-In-The-Loop simulations with COTS components, as done in [Garcia-Fernandez, 2024b].
For observable RINEX files, the full five-digit IDs is then required to maintain consistency with navigation file designations. However this involves a breaking compatibility adjustment in the observable records: a shift of 3 characters will be required (compared to the observable records for the MEO counterparts). One major challenge for observable files is the necessary discussions on codes and attributes to define the observables of each constellation. This is due to the limited public information regarding signal Interface Control Documents (ICDs). While some details on frequency bands of planned LEO-PNT constellations are available through public Federal Communications Commision (FCC) filings, companies operating LEO-PNT systems often keep signal structures confidential. At some point, however, disclosure of this information will have to occur so that receiver manufacturers can develop commercial solutions. Publicly operated constellations like the ESA FutureNav may offer more accessible signal ICDs in the future.
Finally, besides the RINEX files, the SP3 format may also require a modification at some point. Currently, LEO constellations are addressed, and some satellites have unique designations already fixed and standardized in [IAG 2024], however, only 20 slots for the letter ‘L’ are available (at the time of writing this abstract), clearly insufficient for the upcoming constellations. In fact, the letter ‘A’ is being used as a workaround for LEO constellations devoted to atmospheric research (such as COSMIC satellites). In this direction, the same approach adopted for the proposed RINEX modification (letter constellation followed by 5-digit ID) could be used, albeit this represents a major breaking change of the format.
This talk presents an initial array of proposals and illustrative examples regarding modifications to the RINEX and SP3 formats. The goal is to initiate discussions on potential adaptations of these standards to support emerging constellations effectively. The modifications on these formats, which are widely used not only by the scientific community but also by the industry, can pave the way for seamless integration of new constellations into existing GNSS infrastructure and data formats.
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