26To ensure confidence, measurements carried out by imaging radiometers mounted on satellites 27 require robust validation using 'fiducial quality' measurements of the same 'in-situ' parameter. 28 For surface temperature measurements this is optimally carried out by radiometers measuring 29 radiation emitted in the infrared region of the spectrum, co-located to that of a satellite 30 overpass. For ocean surface temperatures the radiometers are usually on-board ships to sample 31 large areas but for Land and Ice they are typically deployed at defined geographical sites. It is 32 of course critical that the validation measurements and associated instrumentation are 33 internationally consistent and traceable to international standards. The Committee on Earth 34 Observation Satellites (CEOS) facilitates this process and over the last two decades has 35 organised a series of comparisons, initially to develop and share best practise, but now to assess 36 metrological uncertainties and degree of consistency of all the participants. The fourth CEOS 37 comparison of validation instrumentation: blackbodies and infrared radiometers, was held at 38 the National Physical Laboratory (NPL) during June and July 2016 sponsored by the European 39 Space Agency (ESA). The 2016 campaign was completed over a period of three weeks and 40 included not only laboratory based measurements but also representative measurements carried 41 out in field conditions, over land and water. This paper is one of a series and reports the results 42 obtained when radiometers participating in this comparison were used to measure the radiance 43 temperature of the NPL ammonia heat-pipe blackbody during the 2016 comparison activities 44 i.e. an assessment of radiometer performance compared to international standards. This 45 3 comparison showed that the differences between the participating radiometer readings and the 46 corresponding temperature of the reference blackbody were within the uncertainty of the 47 measurements but there were a few exception, particularly for a reference blackbody 48 temperature of -30 o C. Reasons which give rise to the discrepancies observed at the low 49 blackbody temperatures were identified. 50 51 1 INTRODUCTION 52 53The measurement of the Earth's surface temperature and, more fundamentally, its temporal 54 and spatial variation, is a critical operational product for meteorology and an essential 55 parameter for climate monitoring (Yoder et al., 2014). Satellites have been monitoring global 56 surface temperature for some time. However, it is essential for long-term records that such 57 measurements are fully anchored to international physical standards as represented by the 58 Systeme International (SI) units. Field-deployed infrared radiometers 1 currently provide the 59 most accurate measurements of the Sea Surface Temperature and are currently used for 60 calibration and validation of Earth observation radiometers (Minnett and Corlett, 2012). These 61 radiometers are in principle calibrated traceably to SI unit...