The TIM instrument measures the total solar irradiance (TSI), the spatially and spectrally integrated solar radiation incident at the top of the Earth’s atmosphere. TIM continues a solar climate data record, which began from space in 1978 and is used to determine the sensitivity of the Earth’s climate to the natural effects of solar forcing. The TIM TSI measurements monitor the incident sunlight to the Earth’s atmosphere using an ambient temperature active cavity radiometer. The TIM instrument has been on board multiple missions, including Glory, SORCE, TCTE, and most recently TSIS-1. A TIM instrument is currently in development for the future TSIS-2 mission.