The Davos Absolute Radiometer DARA on PROBA-3, is an absolute radiometer for measuring Total Solar Irradiance (TSI). TSI is the energy input to Earth (per m2 Earth cross section) and any long-term variation of TSI will directly translate into a change of the terrestrial climate. Measurements of TSI from space are made since 1979 and during this 36 years TSI was on average 1361 W/m-2 and varied in phase with the solar cycle by about ±0.6 W/m-2 (±0.04 %). The long-term trend of TSI was zero within the observational uncertainties of a composite from the records of eight different experiments. The uncertainty of a trend of the TSI composite is very difficult to assess but is probably of the order of about 0.2 to 0.5 Wm-2 over the TSI composite time. It is unknown and controversially discussed in the science community whether the solar irradiance has a long-term trend. The uncertainties mentioned above would not exclude a change of 1 Wm-2 over 100 years. In particular, it is suspected that the cause of past natural climate variation as e.g. the Roman optimum or the little ice age, were due to variations of the solar irradiance. It must be emphasised that a solar influence is suspected, but far from established. This is why the World Meteorological Organization lists TSI as an essential climate variable (see GCOS Essential Climate Variables).
In view of the unknown role of suspected (and predicted) variability of TSI for the terrestrial climate it is mandatory that monitoring of TSI continues on an accuracy level that is capable to capture any variation that could have a significant impact on the terrestrial climate. The main science goal of DARA is to measure TSI with an uncertainty better than 0.3 W/m-2 on an absolute irradiance level and a relative stability of 5 mWm-2/year (0.0005 % of the TSI per year). As the previous experiment PREMOS on PICARD had an absolute uncertainty of its TSI measurements of 0.4 W-2, this instrument performance will be sufficient to measure any climate-significant variation even for the case the continues TSI monitoring record is broken before the launch of PROBA-3.
A science goal of DARA, which is important to the metrology community, is to contribute new observational evidence for the resolution of the past discrepancy of 4.5 Wm-2 between the absolute measurements of the Total Solar Irradiance by VIRGO/SOHO and TIM/SORCE. Both instruments, TIM/SORCE and VIRGO/SOHO, have been fully characterized so they are measuring in absolute terms. Recent investigations have shown that about half of this difference is explained by an underestimated stray light effect in the PMO6 type radiometers and the other half by an offset of the World Radiometric Reference to which VIRGO is traceable. Substantiating the solution by additional measurements is mandatory for metrological claims, as verification by now two – PREMOS and TCTE – fully SI traceably instruments is insufficient. DARA will be SI traceable by carefully pre-flight calibrations in vacuum, based on comparisons with ground-based cryogenic radiometers at different metrological institutes. The instrument scientists of the operating radiometers on VIRGO/SOHO and TIM/SORCE are members of the DARA Science Team, and will actively contribute (e.g. with access to the e.g. TSI radiometer facility, TRF, in Boulder) to the calibration, evaluation, and interpretation of the DARA characterization and measurements.
Management of the DARA experiment is straightforward as the PMOD/WRC is the only hardware institute. In support of the PMOD/WRC hardware team we have set up an international Science Team, who will be in charge of the interpretation and exploitation of DARA data. Operation of DARA needs minimal support: Change of the basic operation mode is only rarely needed and is done by uploading time tagged commands. The reduction of the DARA raw date is done by pipeline routines, which will extract absolutely calibrated level 2 data (as has been done for PREMOS on PICARD). Level 2 data are available immediately after down load and pipeline processing. The analysis of long-term sensitivity changes of the cavities, and the removal of a possible long-term trend in the measurements will require careful analyses of the behaviour of the data from the main and backup instruments. This effort is done primarily by PMOD/WRC and supported by the science team members once an observational record over a longer time interval is available, which is typically once per year. This effort will be lead and coordinated by PMOD/WRC and trend corrected TSI data will be released periodically.