A GR redshift test between optical lattice clocks on the peaks and plains of Colorado 

We describe an ongoing test of the gravitational redshift using an optical lattice clock at high altitude. We briefly summarize the performance and benchmarking of the NIST transportable Yb optical lattice clock (OLC) and associated fieldable clock laser. Transportable optical clocks enable many new measurement opportunities including international clock comparisons, geodetic surveying via clock shift, and tests of general relativity. We present a preliminary measurement campaign from 2025 using optical two-way time transfer to observe general relativistic clock frequency shifts between a laboratory OLC and an OLC located at the summit of a 4300 m (14,100 ft) mountain. We will also present progress towards high accuracy clock comparisons between a laboratory OLC and a transportable clock deployed to a local and well-geodetically mapped mesa.