This chapter is concerned with the practical realization of ship-borne thermal infrared (TIR) radiometers used to collect fiducial reference measurements (FRM) for satellite derived sea surface temperature (SST) measurements. Since the early 1980s multichannel TIR satellite instruments have delivered an unbroken record of SST from space. However, the long-term record is composed of measurements derived from satellite instruments having quite different design and performance characteristics and includes some intervals when the SST measurement capability was degraded significantly, for instance by the absence of a dual-view SST measurements. Furthermore, the retrieval algorithms used to derive SST from top-of-the-atmosphere thermal radiance have also evolved complicating the merging of satellite data into a single homogenous well-calibrated climate data record (CDR). Over the last 10years, a new generation of in situ field radiometers has been developed that are required to validate satellite SST measurements with traceability to Systeme International primary standards. These measurements can be used as a reference measurement data set to verify and validate CDRs of satellite derived SST. This paper reviews the successful evolution of field radiometer design choices that have resulted in a modern ship-borne capability to provide the FRM of SST that are needed to demonstrate that the performance of satellite SST retrievals is meeting climate requirements. Requirements are first reviewed from a CDR perspective and the fundamental measurement equations are presented leading to basic ship-borne TIR radiometer instrument design requirements. These are elaborated in the following sections using examples from a variety of instruments that have been deployed in the field. Finally, the chapter concludes with a consideration of innovations expected in the coming years. A companion chapter (Chapter 5.2) presents Field Measurement Campaign design from an end-to-end network perspective and the development of measurement uncertainty budgets.