The SIM Time Network - Twenty Years of Operation

The SIM Time Network (SIMTN) was established two decades ago to provide real-time time and frequency comparisons among National Metrology Institutes (NMIs) across the Americas. Since its inception, SIMTN has played a crucial role in strengthening regional time and frequency capabilities by enabling continuous monitoring, promoting traceability to UTC, and supporting the realization of UTC(k) time scales. Currently the SIMTN comprises 26 labs. This paper presents a comprehensive overview of the design, operational evolution, and achievements of the SIMTN over twenty years of continuous operation. Key aspects include the initial system architecture based on GPS common-view methods, the deployment of decentralized data centers, and the development of custom software for data acquisition, processing, and dissemination. We discuss the significant upgrades introduced to enhance reliability, automation, and measurement precision, as well as the network's contribution to international timekeeping activities. Finally, we outline the near future prospects for the SIMTN, including dual-frequency receivers to decode both the L1 (1575.42 MHz) and L2 (1227.6 MHz) GPS signals. This allows a measurement of the actual delay of the signals through the ionosphere instead of relying on calculated models. We demonstrate that this approach significantly enhances positioning accuracy and reduces delay variations caused by diurnal fluctuations, enabling time comparisons within the SIMTN with a combined uncertainty (k = 2) of less than 2 nanoseconds.