The NASA Spaceflight article ICON resets for October launch from the East Coast indicates that the Ionospheric Connection Explorer is scheduled to launch in October 2018 from an airplane! (Pegasus XL).

Wikipedia says:

ICON will be equipped with four instruments: a Michelson interferometer, built by the United States Naval Research Laboratory, will measure the winds and temperatures in the thermosphere; an ion drift meter, built by UT Dallas, will measure the motion of charged particles in the ionosphere; and two ultraviolet imagers built at UC Berkeley will observe the airglow layers in the upper atmosphere in order to determine both ionospheric and thermospheric density and composition. (emphasis added)

According to this NASA page:

MIGHTI: The Michelson Interferometer for Global High-resolution Thermospheric Imaging instrument observes the temperature and speed of the neutral atmosphere. These winds and temperature fluctuations are driven by weather patterns closer to Earth’s surface. In turn, the neutral winds drive the motions of the charged particles in space. MIGHTI is built by the Naval Research Laboratory in Washington, DC. (emphasis added)

I'm just guessing that the Michelson interferometer is used as an imaging Fourier transform infrared spectrometer, or FTIR so it can build up a black body thermal spectrum for each point imaged. While etalons are usually used for the highest resolutions, a Michelson interferometer would work as well.

But I don't understand how MIGHTI will measure wind speed. I can't imagine its resolution is so high that it's making a doppler measurement!

Question: How will the Ionospheric Connection Explorer (ICON)'s Michelson interferometer measure wind speed?


1 Answer 1


As you suspected, MIGHTI uses the Doppler shift to make wind speed measurements. It measures the shift in the red and green emission lines of oxygen using a pair of orthogonal interferometers.

UC Berkeley provides an explanation of MIGHTI's operation:

The MIGHTI instrument consists of two units with orthogonal fields of view, pointed 45° and 135° from the S/C velocity direction toward the port (northern) side of the spacecraft. With this viewing geometry, MIGHTI makes two perpendicular line-of-sight wind measurements of the same air volume as the spacecraft passes by. These line-of-sight wind measurements are calculated from the Doppler shift of emission lines in the Earth’s upper atmosphere. Two emission lines of oxygen – on green (557.7 nm) and one red (630.0 nm) are used by MIGHTI. When air parcels are moving towards ICON along MIGHTI’s line-of-sight, these emission lines shorten in wavelength (become more blue), and when air parcels are moving away, these emissions lengthen in wavelength (become more red). The amount of this shift is related to the speed of the air parcel, relative to the speed of light. Each measurement represents a set of observations of the Earth’s horizon, covering altitudes between 90 and 300 km. The vector combination from the two perpendicular lines of sight provides an altitude profile of wind vectors.

Regarding the interferometer itself:

The interferometer is the heart of the MIGHTI instrument. By splitting the incoming light into two channels, with different path-lengths, and mixing them back together, the MIGHTI interferometer forms what is called an interferogram, which is comprised of dark and bright stripes. The position of these changes with the Doppler shift of the emission line observed, allowing each MIGHTI unit to measure the line-of-sight wind.

Sorry for the large quotes, but the website has better explanations than I could give. Additional information can be found in the introduction of this draft of High efficiency Echelle gratings for MIGHTI, the spatial heterodyne interferometers for the ICON mission. (official, paywalled version https://doi.org/10.1364/AO.56.002090) The spatial heterodyne interferometer design captures the entire interferogram at once, rather than slowly scan the interferometer's path difference using a moving mirror.

More is also available at the eoPortal's ICON (Ionospheric Connection Explorer) Mission.

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above: Figure 2: ICON's observational geometry allows simultaneous in situ and remote sensing of the ionosphere-thermosphere system (image credit: UCB/SSL)

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above: Figure 10: Left panel: Photo of the SHIMMER interferometer; right panel: Design of the MIGHTI interferometer (image credit: NRL)

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above: Figure 11: Design of a MIGHTI optical unit (image credit: NRL)

  • $\begingroup$ This is a great answer, thanks! Nothing wrong with the size of your quotes; they are well credited to their source and well chosen. I'd never heard of a spatial heterodyne interferometer before, that's going to be particularly interesting to read aobut. $\endgroup$
    – uhoh
    Sep 1, 2018 at 0:07

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