Ž .
ISPRS Journal of Photogrammetry Remote Sensing 54 1999 279–288
Ionospheric effects on differential GPS applications during auroral substorm activity
S. Skone , M.E. Cannon
Department of Geomatics Engineering, The UniÕersity of Calgary, 2500 UniÕersity Dr. NW, Calgary, AB, Canada T2N 1N4 Accepted 29 April 1999
Abstract
Ž .
The use of the Global Positioning System GPS technology has become increasingly incorporated into airborne remote sensing applications over the past decade. While GPS positioning results may prove adequate for several applications at
present, users should expect to experience degraded positioning accuracies over the next few years due to auroral substorm activity. Such degraded accuracies will arise from increased spatial decorrelation of ionosphere range delay errors in
differential GPS applications, as the ionospheric activity increases during solar maximum. In this paper, the spatial decorrelation of ionospheric range delay is estimated during a substorm event and compared with ‘‘quiet’’ time values.
Ž .
Positional errors in both vertical and horizontal measurements in the range 60–80 cm RMSE were observed during a 1997 substorm event that is representative of the activity anticipated at solar maximum around the year 2000. q 1999 Elsevier
Science B.V. All rights reserved.
Keywords: differential GPS; ionospheric effects; airborne remote sensing; auroral region; substorm activity
1. Introduction
Ž .
Global Positioning System GPS is a satellite navigation system which allows the position of a
receiver on the earth to be computed using range information contained in the satellites’ RF signals.
Twenty-four satellites currently provide worldwide coverage and GPS positioning capability. Errors in
the satellite range estimates can degrade positioning accuracies. These errors may be mitigated through
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the use of differential GPS DGPS positioning algo- rithms, where range errors are calculated at reference
stations and transmitted to remote users.
Corresponding author. Tel.: q1-403-220-4984; Fax: q1-403- 284-1980; E-mail: sskoneensu.ucalgary.ca
Differential GPS has been used in support of georeferencing airborne remote sensing applications
Ž for a number of years Anderson, 1989; Baustert et
. al., 1989; Schwarz et al., 1993 . Positioning accuracy
requirements vary according to the application, and Ž
some typical cases are listed in Table 1 Schwarz et .
al., 1994 . Required accuracy, ranging from less than 10 cm for engineering and cadastral applications to
2–5 m for resource applications, can be achieved through proper GPS receiver selection, limitations on
the reference-user receiver distance and the use of
Ž proper data processing algorithms i.e., carrier vs.
. code phase approaches . Under normal operating
conditions, the errors remaining after differential Ž
processing are due to atmospheric effects both tro- .
pospheric and ionospheric errors , multipath and, to
0924-2716r99r - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž
. PII: S 0 9 2 4 - 2 7 1 6 9 9 0 0 0 1 7 - 9
Table 1 Airborne remote sensing positioning requirements
Ž . Application area
Position RMS accuracy m Engineering, Cadastral
0.05–0.10 Ž
. Forestry detailed
0.2–1.0 Cartographic mapping 1:10 000
2–5 Resource applications
2–5
a lesser extent, orbital errors. Multipath, or the re- flection of signals off nearby objects, can be miti-
gated through proper antenna selection and place- ment. Atmospheric errors, on the other hand, can be
Ž rather large depending on the weather conditions in
. the case of the troposphere and the activity of the
ionosphere. Ionospheric error and its contribution to the over-
all aircraft positioning error budget is a particularly important topic given that the next solar maximum is
in the year f 2000. At that time, the ionospheric error will increase in magnitude and the occurrence
of geomagnetic storms will be more frequent, partic- ularly for users in northern and equatorial latitudes.
This may result in poor GPS tracking capabilities due to scintillation effects, as well as poorer differen-
tial positioning accuracies due to decreased spatial correlation of the ionospheric effect. This paper fo-
cuses on describing the ionospheric effects on GPS measurements and provides an estimate of the errors
that can be expected at solar maximum, through analysis of a geomagnetic substorm event which
occurred in April 1997. This analysis also includes a discussion of the spatial decorrelation of ionosphere
range delays and the propagation of these errors into expected DGPS position accuracies.
2. Ionospheric effects on GPS
