Total solar irradiance (TSI), a measure of how much solar radiative energy hits the entirety of Earth, is the principle energy input to the global climate system. Collecting accurate TSI data spanning multiple years helps scientists understand how much solar radiation is deposited in the atmosphere and at the surface and thus how much energy is available to influence weather, climate, the cryosphere, atmosphere dynamics, and ocean currents.
Because of TSI’s relevance for natural climate change, TSI has been identified within the president’s National Plan for Civil Earth Observations [Holdren, 2014] as a vital observation for determining the Earth’s net energy balance. Similarly, within the National Oceanic and Atmospheric Administration (NOAA) Climate Data Record program, TSI is recognized as an important long-term measurement for a robust, sustainable, and scientifically defensible approach to climate change research.
Unfortunately, the main contributors to the TSI record have terminated their observations or are suffering degraded performance. These include the NASA Active Cavity Radiometer Irradiance Monitor Satellite (ACRIMSat), whose mission ended in December 2013 due to battery problems after nearly 14 years in orbit; the Swiss Precision Monitor Sensor (PREMOS) instrument aboard the French Picard satellite, whose mission ended in March 2014; and the Variability of Solar Irradiance and Gravity Oscillations (VIRGO) aboard the European Space Agency/NASA Solar and Heliospheric Observatory (SOHO), which is still operational, albeit with degraded performance, after 18 years in orbit.
To ensure the longevity of the TSI record without data gaps, scientists had to work fast. Extraordinary teamwork between NASA, the Laboratory for Atmospheric and Space Physics at the University of Colorado (CU-LASP), NOAA, and the Air Force over the last few years has enabled the extension of TSI measurements. This cooperation has included revitalizing the Solar Radiation and Climate Experiment (SORCE, see Figure 1), an aging NASA satellite launched in 2003; the launch and initial operations of the TSI Calibration Transfer Experiment (TCTE), a NOAA/NASA irradiance instrument aboard an Air Force satellite; and identifying a new opportunity for deployment on the International Space Station (ISS) in 2017 of the NOAA Total and Spectral Solar Irradiance Sensor (TSIS).
The combination of these and earlier TSI satellite measurements has established a continuous record of solar irradiance variations since 1978.
Repackaging and Launching Old Equipment
Overlapping measurements between the old and new missions are needed to track trends in the Sun’s output energy and to keep uncertainties in TSI measurements (currently at 0.03%) low. One of these old missions was SORCE, which has been making daily measurements of TSI since March 2003. SORCE also monitors daily variations in solar spectral irradiance (SSI)—a measure of the irradiance at individual wavelengths—from 115 to 2400 nanometers. SSI is also an essential climate variable because different wavelengths of sunlight are absorbed at different levels in the atmosphere and oceans.
The satellite, however, reached the end of its planned 5-year mission in March 2008. The original plan established by NOAA and NASA for continuing TSI and SSI observations past the mission lifetime of SORCE was to have TSIS flown on the National Polar-orbiting Operational Environmental Satellite System (NPOESS). But as NPOESS plans changed and its launch was delayed, NASA’s Glory mission, which included a new version of the Total Irradiance Monitor (TIM) from CU-LASP, was planned to fill the potential gap in TSI measurements between SORCE and NPOESS. After the federal government discontinued the NPOESS program in 2010, TSIS was designated to fly as part of the Joint Polar Satellite System (JPSS).
Delays in NPOESS and JPSS raised the risk that Glory would have to carry the burden of ensuring no gap in the Sun-climate record for longer than scientists anticipated. Exacerbating this risk, Glory did not survive its launch on 4 March 2011 because of the failure of the Taurus XL fairing system. After this launch failure, the risk of a gap in the TSI record, which has been uninterrupted since its beginning in 1978, continued to rise and was compounded by further delays in the JPSS program. Luckily, SORCE—now well past its 5-year design life—was still collecting data, buying scientists some time to find a way to ensure a continuous TSI record.
In 2012, CU-LASP developed a concept to refurbish an existing ground-based instrument that monitors total irradiance, a residual from the SORCE mission that was intended only for ground tests, with modern space-qualified electronics to fly on the Air Force’s Operationally Responsive Space (ORS) Space Test Program Satellite-3 (STPSat-3, see Figure 1). With support from NASA and NOAA, CU-LASP accomplished the implementation in 6 months.
Ball Aerospace & Technologies Corporation (BATC) integrated the refurbished instrument package, now called TCTE TIM, onto the STPSat-3 in 30 days, and the mission was successfully launched on a Minotaur 1 rocket on 19 November 2013 from Wallops Island, Virginia. TCTE TIM initially took daily measurements of TSI, although these have since been scaled back to weekly measurements because of budget constraints.
Updating a Satellite
As scientists and engineers scrambled to ready the launch of TCTE TIM, the SORCE satellite’s battery power declined to a level too low to maintain instrument power for solar observations. This resulted in SORCE going into its safe hold mode, temporarily ceasing science operations, including the collection of TSI measurements, on 30 July 2013.
Following a 5-month gap in SORCE daily solar measurements, new flight software was developed by Orbital Sciences Corporation (OSC) and CU-LASP, and it was installed via uplink radio commands in time for a special campaign in the last week of December 2013 to ensure overlapping measurements between SORCE and TCTE TIMs. Additional SORCE flight software deployed in late February 2014 enabled a “hybrid” operation mode that has allowed SORCE to resume making daily solar observations. The overlap between SORCE and TCTE reached 50 days at the end of April 2014, the minimum deemed necessary for TSI continuity, and the overlap continues with daily measurements by SORCE and weekly measurements by TCTE.
Another important result in the recovery of SORCE is that the SSI record may now continue until TSIS begins operations in 2017.
Ensuring Future TSI Records
TCTE does not acquire SSI measurements, and the STPSat-3 mission is not guaranteed beyond 1 year, so it remains critical to begin operation of the next generation of TSI and SSI instruments that are at the heart of the TSIS mission.
The NOAA TSIS-1 instruments have been built and calibrated at CU-LASP and have been ready for flight since late 2013. In March 2014, NOAA approved TSIS-1 to fly on ISS in 2017, and NASA plans to assume responsibility for future climate observations including TSIS-2, the next-generation TSIS that would overlap with TSIS-1. The budgets for TSIS-1 implementation and the build of the future TSIS-2 instruments are currently awaiting approval by Congress.
With new SORCE flight software to handle the aging battery and TCTE operating in orbit, the TSI record is currently being continued, but there is a serious risk of a gap in this record unless TSIS-1 is launched in the next 3 years. Thus, it is imperative for Congress, NOAA, and NASA to implement the launch of TSIS-1 on ISS or to quickly find an alternative launch solution for TSIS-1, such as a ride of opportunity or new free-flyer satellite dedicated for TSIS.
In addition, satellites like SORCE or STPSat-3, or even smaller satellites, could be cost-effective solutions for flying TSIS-2 to continue the Sun-climate record. In the meantime, scientists will continue to attempt to extend the SORCE and TCTE solar observations much beyond their expected mission lifetimes.
The rapid cross-agency cooperation to ensure the longevity of the TSI record naturally encountered many organizational challenges, which scientists and engineers were able to overcome to make certain that the Sun-climate research community did not lose continuous data. In this current era of tight budgets, such cooperation may become the new normal for ensuring that observations from space continue gap free.