Session P1a: Activities at National Metrology Institutes

Second Carrier Phase TWSTFT Campaign in Europe Employing the SRS Modem - First Results
Carsten Rieck, RISE Research Institutes of Sweden; Tung Thanh Thai, Ilaria Sesia, INRiM Istituto Nazionale di Ricerca Metrologica; Joseph Achkar, OP LNE-SYRTE Observatoire de Paris – Université PSL, CNRS, Sorbonne Université; Dirk Piester, PTB Physikalisch-Technische Bundesanstalt; Miho Fujieda, and Mamoru Sekido, NICT National Institute of Information and Communication Technology
Location: Seaview A/B
Date/Time: Tuesday, Jan. 28, 2:35 p.m.

Traditional Two Way Satellite Time and Frequency Transfer (TWSTFT) uses the mutual, PRN code phase derived, range between ground stations to estimate the two-way station clock offset with typically a few hundred picoseconds DRMS over arcs of 120 s and chip rates of 2.5 Mchip/s. The utilization of the carrier phase can improve the short term link noise by several orders of magnitude and is potentially surpassing GNSS PPP techniques in performance even for long integration times. The introduction of carrier phase measurements into operational TWSTFT is a necessary step towards supporting the redefinition of the SI-second, as well as the improvement of UTC over the next decade.
The SRS carrier phase TWSTFT modem [1] is a state-of-the-art digital modem developed by NICT in 2019/2020 that operates on 1 Mchip/s code rates for code division multiplexing and signal access and is able to track and extract observations of the carrier phase. A typical noise figure is in the order of 2 x 10-12 at 1 s ADEV, and it has the potential to reach below the 1 x 10-16 level at one day. The modem is regularly used on national baselines in Japan.
A small consortium of European NMIs has agreed with NICT to conduct a measurement campaign with the SRS modem. A total of four modems will for one year be integrated into the existing TWSTFT installations of OP, PTB, INRiM and RISE and will measure alongside the current set-ups with SATRE modems used for TAI links. The main objective is to demonstrate carrier phase measurements using a Telstar T11-N Ku-band transponder and exploit the setup to link optical frequency references.
A previous experiment during 2020 [2] did not reach all its goals due to instrumental limitations and the operational constraint to only use the odd UTC hours, which are reserved for testing and experiments. During the new campaign a truly continuous measurement will fully utilize the capabilities of the SRS modem and will demonstrate the compatibility with the existing infrastructure and the coexistence with other transmissions on the same communication channel. The different inhomogeneous TW installations are a good testbed for the determination of ground station requirements for a wider implementation of TW carrier phase in the future.
This paper reports on the objectives and the goals of the experiment, the requirements on the local installations, and the implemented set-ups. The first results of this new campaign based on performance comparison with IPPP and operational TWSTFT in our networks are reported.
[1] Fujieda M. et al. „DEVELOPMENT OF A NEW DIGITAL TWSTFT MODEM“, DOI:10.6329/CIEE.202008_27(4).0004, International Journal of Electrical Engineering, Vol.27, No.4 pp.141-145, 2020
[2] Thai T. et al. “Code-and-carrier-phase based Two-Way Satellite Time and Frequency Transfer (TWSTFT) Experiment Between INRiM, LNE-SYRTE and PTB”, DOI:10.33012/2021.17793, PTTI2021