We detail the the REACH radiometric system designed to enable measurements of
the 21-cm neutral hydrogen line. Included is the radiometer architecture and
end-to-end system simulations as well as a discussion of the challenges
intrinsic to highly-calibratable system development. Following this, we share
laboratory results based on the calculation of noise wave parameters utilising
an over-constrained least squares approach demonstrating a calibration RMSE of
80 mK for five hours of integration on a custom-made source with comparable
impedance to that of the antenna used in the field. This paper therefore
documents the state of the calibrator and data analysis in December 2022 in
Cambridge before shipping to South Africa.Comment: 30 pages, 19 figure