Geopotential measurement with a robust, transportable Ca+ optical clock

We present a robust, transportable Ca⁺ optical clock, with a systematic uncertainty of 1.3 × 10⁻¹⁷ limited
by the black-body radiation (BBR) field evaluation and an uptime rate of >75% over a 20-day period. The
clock is then installed in an air-conditioned car trailer, making it more convenient for applications. Referenced
to a stationary laboratory clock, geopotential measurements are made with the transportable clock with a total
uncertainty of 0.33 m (statistically 0.25 m and systematically 0.22 m) and agree with the spirit level measurement.
After being moved >1200 km, the absolute frequency of the Ca⁺ optical clock transition is measured
as 411 042 129 776 400.41(23) Hz, with a fractional uncertainty of 5.6 × 10⁻¹⁶, which is about one order
of magnitude smaller than our previous measurement. The transportable built can be used for sub-meter-level
elevation measurements, comparing intercontinental optical clocks, verifying basic physical theories, etc.