National Satellite Test Bed (NSTB) Observations of the Effects of Ionospheric Storms on a Prototype Wide Area Augmentation System

Thomas Dehel, Kristy Pham, Joe Sheftic, David Nelthropp

Abstract:	The National Satellite Test Bed (NSTB) is an FAA system that has been used as a research and development tool to support the development of the WideArea AugmentationSystem(WAAS). One of the goals of the NSTB is to eolkct data on the impact of ionospheric activity and the effect of the ionospheric stormson the prototypeWAASsystem. This paper will deaeribetheobaemations andtheeftkcts of the most severeionosphericstorms observedin 1998, The WAAS is a system which provides correctionsto the GPSsystemto enable aviation users of the Standard Positioning Semite (SPS) GPS capabilitiesto achieve aecuraey, integrity, and availability in all phases of flight horn En Route through Category I precision approach. One of the corrections provided by the WMS to enable aeeuracy improvements is the correction for the signal delay due to the ionopshere. Users of the SPS GPS system are currently limited to the L1 frequency,and are not dirmtly able to measure the delay of the signal due to the ionosphere. The WAASprovides this delay tiormation in the form of a grid of data which is transmitted to the user as part of the corrections; the WAAS also provides an error bount called the Grid Ionospheric Vertical Error (GIVE) at each of these grid points. The user then interpolates the delay for each satellite in view from this grid of data and uses the GIVEvalues as an input to a function which estimates position error. The estimate of psition error is then used to determine whether WAAS guidance is accurate enough to be considered“available”. The NSTB has demonstrated WAASfunctionalityand performancein many testa over the past 5 yearn. One question of the WAAS is how well will the WAAS performunder periods of ionosphericstorms,especially in auroral regions. Also, the next maximum in the solar sunspot cycle is expectedin the years 2000-2001, which will increase the number and intensity of ionospheric storms. This concern has driven the NSTB to expand installations to Alas@ and Iceland, which are -ed by ionospheric storms much more frequentlythan the continentalUS. Data collectedby the NSTB has shown some effectsin auroral regions during ionospheric storms. Signhicant ionospheric storm dates in 1998 include May 4*, August 27ti, and November 8* and 9*. Specifically, data collected in Ala@ Cana@ and Iceland during these stormshas shownthat: (1) (2) (3) the ionospheric delay over a short time (e.g., over 30 seconds)increases and is easily quantified; in the WAASprototypeused these short term fluctuations generally result in small increases in the GIVE error bound component and the periods of rapid phase fluctuation (again as measured over the 30 second interval) are also accompanied by short _ in W NSTB smvey grade receivers of the L2 signal. This paper provides data which quantify the observations listed above; the conclusions as to the impact on operational WAAS (cmmntly under testing in CONUS)are still being examinedbut are expectedto be Small.
Published in:	Proceedings of the 1999 National Technical Meeting of The Institute of Navigation January 25 - 27, 1999 Catamaran Resort Hotel San Diego, CA
Pages:	779 - 788
Cite this article:	Dehel, Thomas, Pham, Kristy, Sheftic, Joe, Nelthropp, David, "National Satellite Test Bed (NSTB) Observations of the Effects of Ionospheric Storms on a Prototype Wide Area Augmentation System," Proceedings of the 1999 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 1999, pp. 779-788.
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