Spectroscopic Inline Thermometry

In optical distance measurements it is essential to know the refractive index of air with high accuracy. Commonly, the refractive index of air is calculated from the properties of the ambient air using either Ciddor or Edlén equations, where the dominant uncertainty component is in most cases air temperature. The method described in this contribution utilizes direct absorption laser spectroscopy of oxygen to measure the average temperature of air over long distances. The method allows measurement of temperature over the same beam path that is used for the optical distance measurement, which yields spatially well matching data. Indoor and outdoor measurements demonstrate the effectiveness of the developed method in compensating local temperature variations along the measurement path. Our work in the EMRP JRP SIB60 Surveying project aims to extend the measurement distance of oxygen thermometry up to 1 km using a simplified single-laser setup for extra robustness and simplicity. The new setup designed for field applications include larger diameter optics and a separate transmitting and receiving end. The developed instrument was tested at Nummela geodetic baseline in a measurement campaign in Sep/Oct 2015. The goal was to determine the effective temperature of the air with a precision at the 100 mK level using modest averaging (<5 min).