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Laboratory for Atmospheric and Space Physics


Determining the Sun’s role in climate variability and change will require uninterrupted time series measurements of total and spectral solar irradiance that are of sufficient length, consistency, and continuity. Gaps in the time series of TSI will prohibit the construction of composite data sets inducing large uncertainties that will hamper insight into the true variability of TSI. The uncertainties arise from the use of ambiguous proxy or model data that would be needed to fill a gap and because the instrumental record would rely on individual, instrument accuracies in bridging any breaks in the data.

The TSIS TIM and SIM instruments are designed with the requirements needed to detect possible long-term solar variability in mind. Kopp and Lean [2011] discusses the uncertainties that will be induced in the current total solar irradiance record through undesirable gaps in the continuous measurement record. A TSI Calibration Transfer Experiment (TCTE), where the SORCE TIM “witness” unit that was launched late 2013 helps to bridge the gap between the SORCE and TSIS measurement records albeit with increased uncertainty than TSIS TIM.

Science and Mission Objectives

The overall goal of TSIS is the accurate measurement of total and spectral solar irradiance for better understanding of solar forcing variations and their impacts on the Earth climate system. The TSIS observations are the follow on to the LASP-built instruments of the Solar Radiation and Climate Experiment (SORCE) and the TSI Calibration Transfer Experiment (TCTE) continuing the observations of total and spectral irradiance from 2003 through present day, albeit with higher accuracy, increased precision, and improved stability. The improvements in accuracy, precision, and stability (i.e. the measurement requirements) are driven by the need to:

  • improve our understanding of Earth’s climate response to solar variability,
  • for separating natural from anthropogenic climate forcing effects, and
  • for the proper monitoring and interpretation of the variability in spectrally dependent radiative processes induced by changes in Earth’s surface and atmosphere.


Observing small signals of long-term global climate change places very specific requirements on satellite observing systems. For solar irradiance, variations of less than 0.1% per decade are typical of the kinds of signals that must be extracted from “noisy” time-series measurements. To measure such signals, improved satellite instrumentation calibration is required along with inter-comparison to measurements made by similar instruments. Both high absolute accuracy and high relative stability are necessary for measuring a climate variable. The high absolute accuracy is vital for understanding the climate processes and changes. The high relative stability is necessary for determining longterm changes or trends. For flight (satellite) instruments, in general, accuracy is more difficult to achieve than stability. Three approaches can be used for establishing the response of a flight instrument relative to SI units. One approach involves transferring a calibration from a “known” standard, a second approach makes a measure of the flight instrument response against an “irradiance standard”, and a third approach characterizes the flight instrument as an “absolute sensor”. This final approach has been adopted for TSIS and it involves characterizing each term in the measurement equation and tabulating a list of individual uncertainties and root sum square errors for overall measurement uncertainty.

The TSIS Total Irradiance Monitor (TIM) instrument is expected to be about three times more accurate than the SORCE TIM due to engineering advances in the optical and electrical sensors and to the end-to-end validation of the radiometer at the TSI Radiometer Facility (TRF) at LASP. The improvement in accuracy will allow the TSIS TIM to determine definitively if changes in total solar irradiance (TSI) are of solar origin or instrument artifacts. The original uncertainty and stability of the SORCE TIM instrument were 350 ppm and 10 ppm per year, respectively, and these specifications were met for much of the SORCE mission. However, beginning in November 2012, SORCE TIM timedependent uncertainties have increased and on-orbit stability has decreased due to instrument aging and changes in spacecraft operation such as instrument power cycling which acts to conserve spacecraft batteries during orbital eclipse periods but increases thermal variations in the instrument. Current SORCE TIM uncertainties are 446 ppm ( Instrument stability levels may no longer be achieving 10 ppm per year [Kopp,

Lessons learned from the first-ever measurements of spectral solar irradiance at wavelengths spanning the visible to the near-infrared (400-2400 nm) made by the SORCE Spectral Irradiance Monitor (SIM) have been incorporated by TSIS SIM to meet the measurement requirements. The specific TSIS SIM capabilities over SORCE SIM include reduced uncertainties in the prism degradation correction to meet long-term stability requirements, improved noise characteristics of the electrical substitution radiometer (ESR) and photodiode detectors to meet the measurement precision requirement, and improved absolute accuracy through pre-launch calibration using the novel Spectral Radiometer Facility (SRF).

The Science Objective of TSIS

  1. Measure 4x daily total solar irradiance (TSI) with an absolute accuracy of 100 ppm and a relative accuracy of 10 ppm and to provide calibrated overlap with SORCE TIM, TCTE, and with future missions.
  2. Measure 2x daily spectral solar irradiance (SSI) at variable resolution from 200-2400 nm with an absolute accuracy of 0.2% (2000 ppm), a relative accuracy of 0.01% (100 ppm), and with long-term relative stability of 0.05% per year (for wavelengths shortward of 400 nm) and 0.01% per year for wavelengths longward of 400 nm.
  3. Validate and understand the reasons for the observed irradiance data in terms of solar variability, and assess how the variable irradiance affects our atmosphere and climate. Use this knowledge to improve estimates of past and future solar behavior and climate response.