Journal of Global Positioning Systems (2002)
Vol. 1, No. 1: 66-67
GPS For Ionospheric Sensing: Space and Ground Based
E. A. Essex
CRC for Satellite Systems, La Trobe University, Vic 3086, Australia
Received: 5 May 2002 / Accepted: 16 June 2002
Bibliography
E. A. Essex, is a senior lecturer in the Department of
Physics at La Trobe University. She obtained a PhD in
Space Physics from the University of New England.
Since obtaining her doctorate, she has worked overseas at
the University of the West Indies, and also at the Air
Force Geophysics Laboratory, Massachusetts. Currently
she is the leader of the GPS Space Science project for the
Australian satellite FedSat.
Space Weather Studies Using GPS
At the frequencies used by GPS, the ionosphere produces
the largest error in position location and timing. The
magnitude of the total electron content (TEC) of the
ionosphere is an important parameter in satellite
measurements such as those obtained from GPS as it is
directly related to timing and positioning determinations.
During extreme space weather effects such as magnetic
storms, the TEC of the ionosphere is often subject to
large spatial and temporal variations over the globe
(Essex et al, 2001(a)).
With the occurrence of increased solar activity of the past
two years, around the peak of the sunspot cycle, interest
in the possibility of severe space weather has increased.
Many organisations around the world are now actively
engaged in monitoring, modelling and predicting the
space weather. One of the consequences of severe space
weather is often the disruption to satellite
communications and navigation. GPS navigation
accuracy is particularly sensitive to changes in the
electron density profiles of the ionosphere. Dual
frequency data from the constellation of GPS satellites
and ground stations are being used to study recent severe
magnetic storms and their effects on the ionosphere,
especially at low latitudes in the equatorial regions and in
the high latitudes of both the Northern Polar regions and
Antarctica. Of particular interest is the reliability of
WAAS during severe ionospheric disturbances.
Occultation Studies of the Ionosphere
NASA originally developed the occultation (limb
sounding) technique for the study of planetary
atmospheres. With the advent of the Global Positioning
System (GPS) satellites accurate occultation
measurements of our own earth's atmosphere are now
possible. As occultation occurs, the effects of the
ionosphere on ray paths as they propagate from the GPS
satellites to a receiver on board a low earth orbit (LEO)
satellite provide information on the ionosphere. These ray
paths provide horizontal slices through the ionosphere.
The information contained in these sets of signals is then
extracted by an inversion technique known as
tomographic reconstruction to yield the vertical electron
density profiles of the ionosphere.
As part of the activities of the Cooperative Research
Centre for Satellite Systems, Australia is planning the
launch of a scientific satellite, called FedSat, in
celebration of the centenary of Federation. One of the
payloads on board is a Blackjack GPS receiver, which
will be used to undertake occultation studies of the
studies of the ionosphere as well as navigation. Particular
emphasis will be directed toward the improvement of the
current models of the ionosphere over the vast expanses
of the Southern Hemisphere where existing data is sparse.
This new, cost-effective technology based on the GPS
constellation, is currently being trialled by space
scientists using data bases from the German satellite
CHAMP and the Argentine satellite SAC-C. With a
constellation of LEOs equipped with on board GPS
receivers providing hundreds of occultations per day,
near real time updates of the state of the ionosphere
would be possible. This remote sensing technique offers
not only a new data source for the upper atmosphere but

Essex: GPS For Ionospheric Sensing: Space and Ground Based 67
also may revolutionize the weather forecasting in the
lower atmosphere.
References
Essex, E.A., Birsa, R. Shilo, N.M., Thomas, R.M., Cervera,
M.A. and Breed, A.M. Global Positioning System signals
under solar maximum conditions, SatNav 2001.
Proceedings of the 5th International Symposium on
Satellite Navigation Technology and Applications. Eds. K.
Kubik and N. Talbot, Canberra, Australia, 24-27July,
2001(a). CDROM.
Essex, E.A., Birsa, R. Shilo, N.M., Thomas, R.M., Cervera,
M.A. and Breed, A.M. Scintillation effects on Global
Positioning Signals under solar maximum conditions,
Invited paper. Proceedings of the International Beacon
Satellite Symposium. Ed. P. Doherty. Boston, USA, June
4-6, 2001(b). CD Rom