The cheapest method for an instrument to perform radiometric monitoring in orbit is to compare its radiometric response from a scene to the known radiance of that same scene. This is known as vicarious calibration. The known radiance of this scene comes mostly from other space instruments. The limiting factors of this vicarious calibration approach arise from differences in the acquisition time and illumination/viewing geometry between the two measurements. Earth scenes may change over time, which limits vicarious calibration to quasi stable scenes. The level of stability of these scenes limits the level of accuracy that can be achieved. Likewise, the bi-directional scattering distribution function (BSDF) of the observed scene is likely to cause differences in observed radiance if the illumination and/or viewing geometry changes. If the observation of the scene is at the same time, stability of the scene is no longer an issue, and if the observation is at the same geometry then BSDF effects will cancel, and direct comparison is possible. This is rarely the case unless the instrument is on the same satellite. In this paper we present the design and measurement concept of such a small, on-board calibration instrument; the Absolute Radiometric Reference Instrument (ARRI). We believe this concept will revolutionize the approach to in-orbit absolute reflectance calibration.