Session P1a: Activities at National Metrology Institutes

RISE New TWSTFT Ground Stations
Gustav Jönsson, Sven-Christian Ebenhag, Carsten Rieck RISE Research Institutes of Sweden
Location: Seaview A/B
Date/Time: Tuesday, Jan. 28, 2:12 p.m.

Two Way Satellite Time and Frequency Transfer (TWSTFT) is an intercontinental microwave domain technique that is used to link National Metrology Institutes and Designated Institutes. TWSTFT has few common failure modes with the easily jammed GNSS and is thus a good complement. It is often used in combination with GNSS to gain performance and TWPPP is today linking a large portion of the TAI clock potential. TWSTFT is currently providing the lowest systematic uncertainties for radio-based time and frequency transfer and with the future use of carrier phase measurements it has the potential to surpass any current GNSS technique in terms of instability for any time of integration.
TWSTFT is for RISE the main method to access UTC. To strengthen the UTC linkage of UTC(SP), RISE has implemented two new collocated TWSTFT ground stations on its Borås campus. Both stations are built upon 2.4 m motorized antenna systems, which are placed on top of the highest building in the area and offer visibility to all geostationary satellites visible at the northern latitude of Borås. The first new station, SP02, will be dedicated to operation of daily comparisons within the European/American Network currently using Telstar T11-N. This setup uses a SATRE modem to support current operations, but also offers to host other modem solutions, such as the NICT SRS modem. The second new station, SP03, primarily intended for research and development, is employing an SDR frontend as its modem. This station can also be used to establish an operational link to Asian timing laboratories in the future. The original station SP01 will be held operational to transfer current calibrations and will in the future be used to develop and test the passive concept for a national time distribution using TWSTFT.
The ground stations are situated at a distance from the time and frequency laboratory not suitable for coaxial cabling. For access and environmental reasons, the transceiving components of the SP02 station are kept close to the UTC(SP) timescale and signal transport to the ground station at about 500 m distance is achieved in Ku-band using a purpose-built optical link. This allows to maintain highest stability of the critical components in the signal chain and is a foundation for stable and maintainable operation. The link also allows to place possible future Ku-band digitizers close to the time scale.
The previously presented bi-directional Ku-band link [1] comprises two modulating assemblies and two receiving assemblies. Each modulating assembly generate carrier laser light using PM-fiber coupled 60 mW, 1550 nm DFB lasers, driven by a LD TEC controller. The optical carrier is coupled by PM-fiber to a 20 GHz intensity modulating Mach-Zehnder interferometer. The operation point of the MZ-modulator is controlled by a dither free bias controller. The RF-modulated optical signal is carried by single mode fiber to the respective receiving assembly consisting of a 30 GHz linear photoreceiver which in turn is connected to a series of RF-amplifiers selected to attain proper signal levels at the respective end. We present the final implementation of the link, and analysis of its performance including temperature induced asymmetries.
In contrast, the SP03 research station places the SDR-sampler and the up- and down conversion at the remote location using reference signals provided by a low noise White Rabbit (WR) link. The SP03 SDR solution currently deployed is based on the SDR Rx software by TL that the participating stations of the CCTF working group on TWSTFT are licensed to use [2]. In addition, RISE has implemented a SDR based transmitter at 70 MHz that allows single SATRE code transmissions without data overlay. This solution is presented, and performance is analyzed.

[1] Jönsson et al. EFTF2024
[2] Zhiheng Jiang et al, “Use of software-defined radio receivers in two-way satellite time and frequency transfers for UTC computation”, 2018 Metrologia 55 685