National Physical Laboratory



The proposed Traceable Radiometry Underpinning Terrestrial- and Helio- Studies (TRUTHS) mission offers a novel approach to the provision of key scientific data with unprecedented radiometric accuracy for Earth Observation (EO) and solar studies and in particular for decadal climate change.

TRUTHS provides 'benchmark' measurements of key 'radiative forcing feedbacks' such as clouds and albedo (in the solar reflective spectral domain) with uncertainties small enough that future change, from a background of natural variability can be detected.

Uniquely, TRUTHS can also upgrade the performance of the Earth observing system as a whole: satellite and ground, through the provision of 'reference calibrations' of high SI traceable accuracy, in essence facilitating the upgrade of existing and planned 'operationally' focused missions to become the building blocks of a global climate observatory. In essence TRUTHS becomes a 'standards laboratory in space'.

TRUTHS is a concept conceived and designed at NPL, the UK's national measurement institute, and has wide support in the international science community.

The Need for TRUTHS

The need for such a mission has been specifically highlighted by the United Nations GCOS (Global Climate Observing System), WMO GSICS (Global Satellite Inter-Calibration System) committees and Committee on Earth Observation (CEOS), culminating in the call for a specific mission by the US Academy of sciences called CLARREO (Climate Absolute Radiance and Refractivity Observatory). TRUTHS is highly complementary to CLARREO and could, through partnership be considered as part of an 'international calibration and climate constellation', with TRUTHS delivering the key traceability for the solar reflective spectral domain.

Calibrating TRUTHS

TRUTHS is the first satellite mission to calibrate its EO instrumentation directly to an SI 'primary standard' in orbit, overcoming the usual uncertainties associated with drifts of sensor gain and spectral shape by using an electrical rather than an optical standard as the basis of its calibration.

The range of instruments flown as part of the payload will also provide accurate input data to improve atmospheric radiative transfer codes by anchoring boundary conditions, through simultaneous measurements of aerosols, particulates and radiances at various heights. It will also make multi-angular spectrally resolved measurements of the ocean and land surfaces to support studies of the carbon cycle. In this way TRUTHS will provide the basis to significantly improve the performance and accuracy of all EO optical missions whether they are global, operational or have focused specific scientific objectives.

TRUTHS has many strategies for providing 'reference calibrations': establishing dedicated Earth based reference standard sites such as those of CEOS, near simultaneous observations of 'targets' at orbital cross-overs and through calibration of the Moon.

These reference calibrations not only improve synergy between missions by reducing errors due to different calibration biases but also offer cost reductions for future missions by reducing the demands for on-board calibration systems. Such improvements are important for the future success of strategies such as Global Monitoring for Environment and Security (GMES) and GEO and the implementation and monitoring of international treaties such as the Kyoto Protocol. TRUTHS will achieve these aims by measuring the geophysical variables of solar and lunar irradiance, together with both polarised and unpolarised spectral radiance of the Moon, Earth and its atmosphere.

Virtual TRUTHS

TRUTHS is the subject of an NPL exhibition in the 3D virtual world of Second Life® and can be viewed at NASA's CoLab.
(If you are new to Second Life then please read the instructions, check the system requirements and register at This will direct you to download and run the Second Life viewer and takes you through a special 15-minute Orientation process in Second Life. At the end of the Orientation, there will be a teleport point straight to the TRUTHS exhibition.)

View Nigel Fox's lecture on 'Seeking the TRUTHS about climate change'.

TRUTHS Mission Goals

  • Measure the radiation - Sun, Earth and Moon - 10 times more accurately.
  • First satellite to calibrate itself - directly traceable to SI units via a primary standard - in orbit.
  • Provide absolute high accuracy calibration for other EO missions.
  • Diagnosis of reflection from the Earth to give details of the atmospheric chemistry (esp. in UV and near IR).

Assessments of climate change depend on data collected over decades in order to detect very small often subtle changes. Scores of Earth Observation (EO) satellites are used to provide inputs to, and allow the development of models, that advise policy makers. Satellite surveys are used to quantify the interactions of complex environmental factors involving the atmosphere, the Sun, oceans, pollution, volcanoes, vegetation, and deforestation.

Satellites should provide enough information to support national and international legislation. However, in most cases they are not accurate enough to distinguish between the predictions of different environmental models, and scientists cannot agree on their conclusions. The last IPCC report highlights this uncertainty- the ensemble of climate models forecast a range of potential temperature rises of the Earth from 2 to 10 °C by 2100. The vastly different consequences associated with this wide range is clearly unacceptable to society. For policy makers to accept and act upon the data with any degree of confidence, it must be referenced to a traceable and internationally accepted standard and have significantly narrower range in potential outcomes. Traceable measurements of unprecedented accuracy, would be provided by the proposed TRUTHS calibration satellite. allowing the models to be constrained and tested so that decisions on their likely validity can be made in a decade as opposed to decades with current uncertainty levels. In addition to establishing benchmark measurements of the Earth itself, the TRUTHS mission plans to establish a set of internationally accepted SI traceable reference targets - Sun, Moon and Earth deserts that could be used to provide calibrations for EO data collected by other satellites. Such reference targets are highly stable, and so could be used to back correct archived data. In addition, use will be made of Simultaneous Nadir Overpass (SNO) where the Earth viewing imager of TRUTHS can simultaneously (or within a few minutes) view the same part of the Earth as another in-flight satellite. The TRUTHS mission is unique in establishing high accuracy SI traceable data in-flight - a 'calibration laboratory in space'. On board the small satellite, a suite of instruments will measure the input solar irradiance and solar reflected radiance (Figure 1).

TRUTHS satellite concept
Figure 1: TRUTHS satellite concept

TRUTHS Specification

Solar Irradiance

  • Accuracy
    • Total Solar Irradiance (TSI) < 0.01 %
    • Solar Spectral Irradiance (SSI) (UV - IR) < 0.1 %

Earth Spectral Radiance

  • Accuracy 320 - 2450 nm < 0.3 %
  • > 200 (10 nm) channels continuously sampled globally at Nadir
  • Multi-angular spectrally resolved measurements of specific targets
  • 40 m ground resolution (max.)


  • Lifetime up to 5 years
  • Power Consumption < 200 W
  • Mass < 175 kg
  • Small Satellite configuration
  • Designs demonstrated terrestrially
  • Available for flight in < 3 yrs

These instruments will be calibrated directly against an on-board primary standard - a cryogenic radiometer called CSAR (Cryogenic Solar Absolute Radiometer) through a procedure, which mimics that carried out on the ground. The CSAR or its terrestrial equivalent is the primary standard of choice for terrestrial applications. Adapted for use in space, it compares the heating effect of optical radiation with that of electrical power - transferring all the difficulties associated with current space based optical measurements (drift, contamination etc) to more stable electrical SI units.

One aim of the TRUTHS satellite is to transfer its high accuracy to other satellite missions measuring optical radiation. It will do this by calibrating optical radiation from reference targets such as the Moon, or large uniform areas like deserts. Other satellites can then calibrate their instruments against these reference targets. In some cases opportunities to simultaneously view the same target as another satellite will have added benefits.

The Earth imager on TRUTHS would provide high-resolution spectral and spatial measurements of reflected solar radiation at multiple angles on a range of Earth targets. Thus in addition to the calibration of reference targets, the resulting spectral information could be used for specific science studies to help understand the carbon cycle – by identifying and quantifying the type of vegetation on land and in the oceans, as well as sources of pollution and the clarity of the atmosphere.

The Thames, London, during 17th Century mini ice age
Figure 2: The Thames, London, during
17th Century mini ice age

Solar radiation is the driving force of the Earth's climate and small changes in the total output of the Sun can have significant effects on the Earth's surface. It is believed that a 0.3% change in Total Solar Irradiance (TSI) was responsible for the mini-ice-age of the 17th century, Figure 2. The TSI record relies upon the data from many different solar radiometers flown over the last 20 years, whose inherent variability (~0.8%) could affect the prediction of models of global temperature change by as much as 0.8 K.

In Figure 3, a relatively smooth representation of TSI is only achievable through normalisation of different data sets. This emphasises the risk, that any loss in overlap of a mission and thus data set, would result in the loss of climate relevance of TSI data, and our ability to determine if there are any natural changes, which can either offset or enhance anthropogenic induced climate change. The cryogenic solar absolute radiometer (CSAR) of TRUTHS is designed to reduce this uncertainty by more than a factor of ten and establish undisputed, continuous, SI traceable, TSI data for the future with no risk of data gaps. In addition, TRUTHS can make calibrated, accurate, high-resolution measurements of Solar Spectral Irradiance (SSI) from the UV to the IR, with instrumentation that is regularly recalibrated against a primary standard in orbit. Variations in solar irradiance are an essential input for studies of the Earth's climate including:

TSI data sets
Figure 3: TSI data sets
  • Models of global warming
  • Ozone depletion
  • Radiation budgets
  • Atmospheric chemistry
  • Cloud cover
  • Atmospheric transmittance

Predictions of the impact and unequivocal attribution of climate change can only be determined through accurate long time-base studies of the Earth and Sun and their interaction - detecting small signals from a noisy background of natural variability. This requires measurements to be reliable, stable, high accuracy and traceable to SI units. TRUTHS will, for the first time, allow many of these parameters to be firmly anchored to SI units ensuring baseline measurements made now can be reliably compared to those of future generations.

The TRUTHS proposal and its objectives are gathering attention and momentum in the international climate science community, with specific interest coming from the World Meteorological Organization, UN and other international programmes. NASA and National Measurement Institutes in the US and Europe are interested in a collaborative effort to establish an orbiting space measurement laboratory.

Terrestrial vs TRUTHS

Satellite instrumentation is currently calibrated against primary standards through a traceability chain. The terrestrial calibration methodology carried out by an NMI (National Measurement Institute) can be traced back to a primary standard able to achieve uncertainties of 0.001%. However, the number of steps beyond this primary standard rapidly increases the resultant uncertainty. Subsequent storage and then transfer to the relatively harsh environment of space compound the problem and not only increase the uncertainty but, in effect completely break the traceability chain.

By mimicking the terrestrial calibration chain in-flight, TRUTHS would yield an overall SI traceable uncertainty of < 0.3% in spectral radiance - more than an order of magnitude better than any other EO mission. The main difference in the translation of the methodology into space is that TRUTHS will use the Sun as a source for its calibration sequences rather than a laser source or high temperature black body as used on the ground. Since the Sun is only used as a convenient source, with its spectral power measured directly every time it is used by the CSAR (primary standard), any drifts are inconsequential. This step change reduction in uncertainty is similar in magnitude to that obtained when NMIs, led by NPL, started to introduce cryogenic radiometers into their terrestrial calibration chain 25 years ago, and it is perhaps timely that this benefit is now incorporated into the EO sector.

Watch a short explanatory video of TRUTHS

Contact: Nigel Fox

Last Updated: 11 Apr 2013
Created: 12 Aug 2010