Most modernized civil navigation signals offer a data and a pilot channel modulated on the same carrier frequency (e.g. GPS L1C/L2C/L5, Galileo E1 OS/E5a/E5b, BDS B1C/B2a). While the data channel needs to be demodulated to obtain ephemerides, clock corrections, system time, etc., the pilot channel exhibits only predictable symbol transitions to facilitate tracking of the signal’s time of arrival (ToA). Nevertheless, receivers may choose to perform ToA tracking based on a combined tracking (X) of pilot and data channels, pilot channel only (P) or even data channel only (D). While the latter is rarely used in practice, X-tracking offers lower tracking noise than P-tracking, since it combines the power of both channels.
Recent studies have reported that receivers may experience different pseudorange biases depending on whether D-, P-, or X-tracking is used [1,2]. This causes problems for the harmonized usage of receiver network data such as from the International GNSS Service (IGS), since the network essentially needs to be partitioned into separate groups of stations depending on which tracking mode is used [1]. In particular, differential biases between the P- and X-tracking receivers will affect orbit determination and time synchronization (ODTS), hence many applications in the field of surveying, timing, and geodesy. The above references provide an exhaustive overview of differential data-pilot (D-P) as well as differential combined-pilot (X-P) biases of modernized GNSS signals. Measurements were conducted with a set of commercial GNSS receivers.
While such efforts greatly facilitate the usage of IGS network data, the causes and individual contributors for the D-P bias are not yet fully understood. Interestingly, the largest D-P biases (+/- 3 ns) have been observed for GPS L5 signals, which is surprising in view of the similarity of the L5 data and pilot power spectral density (PSD). Moreover, there is reason to assume that the bias is receiver-specific as well as satellite-specific. For the Galileo E1 Open Service, where pilot and data components have slightly different PSD, the reported D-P biases are smaller but still noticeable (up to 0.2 ns), but are not consistent across different receivers. Naturally, the receiver acts as a black box in measurement campaigns such as the above mentioned. Receiver parameters such as correlator spacing, front-end bandwidth, or other implementation details (e.g., type of discriminator) are not usually disclosed by manufacturers, but may have a notable impact on the observed bias.
In this work, we present a model for the D-P and X-P differential biases. Our preliminary results suggest that the observed biases can be mostly traced back to four parameters, two of which are satellite-specific and two of which are receiver-specific. At the satellite, an offset between data and pilot stream as well as the difference in effective chip duration between data and pilot channel were identified as the main causes for D-P biases. These findings are underpinned with measurements from DLR’s 30 m dish high-gain antenna located at Weilheim, Germany. At the receiver, front-end bandwidth and correlator spacing have a noticeable impact on the D-P biases. Record & replay of real-world data to a software receiver and commercial receivers will serve to validate the proposed model.
[1] O. Montenbruck, P. Steigenberger, J. M. Sleewaegen, “Data+pilot biases in modern GNSS signals”, GPS Solutions (2023) 27:112.
[2] J. M. Sleewaegen, F. Clemente, “Quantifying the pilot-data bias on all current GNSS signals and satellites”, IGS Workshop 2018, Wuhan.
