... Observatory

The National Radio Astronomy Observatory is operated by Associated Universities Inc., under cooperative agreement with the National Science Foundation.

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... Laboratory

Basic research in radio astronomy at the Naval Research Laboratory is supported by the Office of Naval Research.

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...$\phi_{\rm G}$

For this calculation the gradient is taken to be positive in the direction of increasing ionospheric density.

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... OP

Ionospheric refraction at GPS frequencies is negligible for this calculation.

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... TECOR

These values of the TEC are based on data taken from a global grid of up to 100 GPS receivers. The vertical TEC from each is calculated in a solar-geomagnetic reference, using bi-cubic splines on a spherical grid. A Kalman filter is used to solve for instrumental biases and the vertical TEC. The data model is on a 2 $.\!\!^\circ$5 grid in latitude, and a 5$^\circ$grid in longitude, and is archived once every other hour.

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