11. GPS Single-Frequency: From First cm-POD to Single Frequency GNSS-RO/R

In this section we introduce what we call “Positive Code-Phase” linear combination or the LP linear combination (phase and code added) to eliminate the first-order ionosphere effect and estimate LEO orbits using single-frequency GPS measurements, (see Švehla and Rothacher 2003a; 2005b). We do not smooth code measurements with the linear model as proposed by the GRAPHIC (Group and Phase Ionospheric Calibration) linear combination in (Yunck 1993; Gold et al. 1994; Muellerschoen et al. 2004). We show that in the case of the GRACE-B satellite it is possible to estimate LEO orbits to an accuracy of 2–3 cm RMS (1.3 cm radial) using single-frequency GPS measurements only, (see also Svehla et al. 2010a). This is similar to the orbit accuracy of 1–2 cm one can typically achieve with dual-frequency carrier-phase measurements. This is possible due to the very low noise level of the code measurements from the GRACE-B satellite and recent gravity field models from the GRACE and GOCE missions that provide very accurate gravity field coefficients up to degree and order 120 allowing an orbit parameterization with a very modest number of empirical parameters. In addition, thanks to the excellent precision of the real-time GPS satellite clock parameters provided by the IGS, we show that this cm-orbit accuracy can be achieved even in real-time. Subsequently, we introduce an estimation of the group delay pattern of GNSS satellite antennae based on the LP linear combination. We show that the LP linear combination can be used to estimate single-code group delay variations (GDV) for GNSS satellite antennae at the single-frequency level and present the first GDV pattern based on GPS measurements from the GRACE-B satellite. The GDV pattern based on LP linear combination is related to a single code observable and not to an ionosphere-free linear combination, a strong advantage in the presence of multi-GNSS data. After that, we present the concept of using single-frequency GPS radio-occultations (RO) as a very promising alternative to standard GPS-RO based on dual-frequency measurements. The advantage of this approach is that carrier and code measurements on the same GPS frequency follow the same path in the ionosphere. This is not the case for the bended carrier-phase GPS-RO measurements on different GPS frequencies that can reach a vertical separation of up to 500 m in some cases. Since the antenna used for GPS-RO is typically a high-gain antenna, the noise level of the code measurements is very low and, with an additional smoothing, this approach could be used for GPS-RO with SBAS satellites in GEO. The same approach could also be applied to GNSS reflectometry (GNSS-R).