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Orbiting Carbon Observatories
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Instrument Calibration

On Board Calibrator
The area highlighted in the blue circle provides a physical overview of the On Board Calibrator that overlays the instrument telescope/collimator assembly.

on orbit data
On orbit data collections in support of calibration

Calibration parameter summary
Calibration parameter summary

Calibration

Calibration is a critical step for all remote-sensed data. For OCO, the calibration steps ensure that the spectral data are converted into accurate radiance observations for science and application purposes.

To accurately retrieve the O2 and CO2 concentrations, it is necessary to know:

  1. What colors of reflected sunlight are absorbed by the atmosphere (spectral calibration)
  2. How bright is the Earth (radiometric calibration)
  3. Where on Earth the light came from (geo-location calibration)
  4. How the light is polarized (polarimetric calibration)

Approach

Continuing calibration exercises during space operations ensure OCO obtains bias-free radiance measurements. Accurate radiance measurements are crucial to retrieve XCO2 with the accuracy and precision (<0.25% or 1 ppm) needed.

The OCO instrument detectors (focal plane assemblies) record the "brightness" of the incident spectral radiances as raw data numbers (dN), ranging from 0 to 65535 (note they have no units). These data numbers are converted into physical units of photons per square meter per steradian per second per micron through radiometric calibration.

OCO-2 Data Conversion Diagram
OCO-2 Data Conversion Diagram

The conversion of instrument output into physical measures generated by the calibration process are the central data reported in the OCO Level 1B Product.

Data Processing flow chart from Level 1A to L1B
Data Processing flow chart from Level 1A to L1B

Detailed laboratory tests that were performed pre-launch include:

  • Variable intensity light source (radiometric)
  • Variable wavelength laser source (spectral)
  • Movable light sources (geometric)
  • Rotated polarizer (polarimetric)

The characterization of the instrument on the ground before launch yielded the initial set of parameters required to convert instrument data numbers into incident radiances. Once the instruments were in space, data is routinely collected to track changes with time, including:

  • On board lamps (radiometric)
  • Solar Calibration (spectral)
  • Lunar calibration (radiometric, geometric)
  • Coastline crossings (geometric)
  • Vicarious calibration (radiometric)
  • Cross calibration (radiometric)

Inflight Calibration Chain
Inflight Calibration Chain

To Learn More:

Both OCO-2 and OCO-3 were extensively calibrated on the ground prior to deployment.

Read about the OCO-2 calibration campaign here

Read about the OCO-3 calibration campaign here