GPS receivers must compensate for the delay a GPS signal experiences as it passes through the ionosphere in order to accurately determine the position of the receiver. Receivers limited to terrestrial operation may utilize the Klobuchar parameters transmitted by the GPS satellites to model the ionosphere and remove much of this delay. However, as a GPS receiver passes through the ionosphere, such as in a spacecraft or low-Earth orbit space station, the Klobuchar model no longer adequately approximates the correction to be applied. Other models exist, particularly the IRI 2007 model created by NASA et al., but these are too computationally complex to be performed in real time by common hardware available for space implementations. Moreover, although the IRI model provides extensive insight into the historical characteristics of the ionosphere, it is purely predictive for times beyond the publication date of the model. Still other models exist that can be used during post-processing but are also not available in real time.

This invention uses the IRI model to compute the historical ratio of the vertical ionospheric delay at the point where the line of site vector between the user position and a GPS satellite passes the midpoint of the ionosphere above the receiver, the “pierce point,” to the vertical delay at a point on the ground immediately below the pierce-point. This ratio is then used to scale the ionospheric delay computed using the real-time Klobuchar data provided by the GPS satellites.

Because the evaluation of the IRI model is constrained to only the vertical delay at these two points, the computational burden is significantly reduced. In addition, use of the real-time Klobuchar data improves the response of this model to ionospheric events (e.g., solar storms) that are not modeled by the IRI.

This work was done by Hui Zhao and John Savoy of Honeywell International, Inc. for Johnson Space Center.

NASA Tech Briefs Magazine

This article first appeared in the October, 2014 issue of NASA Tech Briefs Magazine.

Read more articles from this issue here.

Read more articles from the archives here.