SURREY SATELLITE TECHNOLOGY US LLC (SURREY) PARTICIPATED IN THE FUNDED STUDY TO CREATE A LANDSAT DATA CONTINUITY MISSION (LDCM) COMPLIANT INSTRUMENT WHICH COULD BE FLOWN ON A SMALLER ESPA-CLASS SPACECRAFT BUS. IN THE COURSE OF THE STUDY, SURREY PROPOSED FURTHER INVESTIGATION INTO A "PIGGY-BACK" RADIOMETER AS A POSSIBLE ALTERNATIVE TO USE INSTEAD OF SUN-ILLUMINATED DIFFUSERS. THE "PIGGY-BACK" IS A TRANSFER RADIOMETER FLOWN ON THE SAME PLATFORM AS A MAIN INSTRUMENT. IT HAS THE SAME FIELD OF VIEW AS THE MAIN INSTRUMENT AND THE SAME SPECTRAL RANGE. HOWEVER, IT WOULD HAVE A GSD LARGER BY AN ORDER OF MAGNITUDE. THIS MEANS THAT IT CAN HAVE AN APERTURE SMALLER BY UP TO AN ORDER OF MAGNITUDE SO THAT IT WILL BE MUCH SMALLER PHYSICALLY. THE PIGGY-BACK IS CALIBRATED USING A VERY SMALL DIFFUSER SYSTEM AND POSSIBLY SMALL BLACK BODIES FOR TIR BANDS. THE MAIN INSTRUMENT IS THEN CALIBRATED BY COMPARISON OF EARTH IMAGE DATA AVERAGED OVER LARGE EARTH AREAS SELECTED FOR LOW SPATIAL NON-UNIFORMITY. THE PIGGY-BACK RADIOMETER COULD BE A MULTISPECTRAL SYSTEM WITH THE SAME FILTERS AS THE MAIN INSTRUMENT. IT COULD USE THE SAME DETECTORS, THOUGH IN THIS CASE ONLY A SINGLE MODULE IS REQUIRED IN EACH FOCAL PLANE IN PLACE OF AN ARRAY OF 10. AN OPTIONAL CONSIDERATION IS TO USE A SPECTROMETER DESIGN FOR THE PIGGY-BACK RADIOMETER. A SPECTROMETER IS NOT SERIOUSLY CONSIDERED FOR A MAIN INSTRUMENT SINCE THIS WOULD NOT BE CONSISTENT WITH THE TDI OPERATION THAT IS NEEDED FOR SMALL SYSTEM SCALE AT THE REQUIRED SNRS. HOWEVER, A SPECTROMETER BECOMES REALISTIC WITH A GSD AND DWELL PERIOD INCREASED BY AN ORDER OF MAGNITUDE. IT COULD PROVIDE THE FULL VSWIR RANGE USING A SINGLE OPTICAL CHANNEL WITH PRISM DISPERSION. A BROAD-BAND MCT AREA ARRAY DETECTOR LIKE THE SOFRADIR NGP COULD COVER THE WHOLE SPECTRAL BAND. A SPECTROMETER SOLUTION FOR A PIGGY-BACK RADIOMETER COULD BE OF INTEREST IN PROVIDING CONTINUOUS SPECTRAL COVERAGE OVER A BLUETHROUGH- SWIR RANGE AT SPECTRAL RESOLUTION COMPARABLE TO THE MAIN INSTRUMENT. IT ALSO SOLVES A POTENTIAL PROBLEM OF SIMULTANEITY. THIS PIGGYBACK WOULD HAVE THE POTENTIAL TO REDUCE OVERALL SYSTEM ENVELOPE AND MASS BY REMOVING THE NEED FOR RELATIVELY LARGE CALIBRATION SOURCES AND SCENE-SELECT MECHANISMS. FOR A BASELINE VSWIR TELESCOPE DESIGN, IT WOULD REMOVE THE NEED FOR AN EXTERNAL ENTRANCE PUPIL. THIS WOULD ALLOW A MUCH EASIER TMA DESIGN WITH SMALLER MIRRORS, AND AN INCREASED SWATH WIDTH COULD BE CONSIDERED. THE SMALL APERTURE OF THE PIGGY-BACK COULD ALSO ALLOW CONSIDERATION OF MORE-ELABORATE ABSOLUTE CALIBRATION TO IMPROVE CONFIDENCE IN LONG-TERM STABILITY. FOR EXAMPLE, AN ADDED MECHANISM COULD BE USED TO CONSTRUCT A RATIOING RADIOMETER, WHICH PROVIDES A DIRECT MEASUREMENT OF DIFFUSER REFLECTANCE. ALL OF THESE ITEMS COULD BE CONSIDERED AS TECHNOLOGY ENHANCEMENT TO ENABLE FURTHER REDUCTION IN VOLUME, WEIGHT, AND POWER OF THE MAIN INSTRUMENT.