Challenge
RESEARCH
Using GPS Satellites to Study
Plasma Irregularities
Klemens Hocke and Toshitaka Tsuda
Kyoto University, Radio Science Center for Space and Atmosphere
Each time a GPS
satellite rises or sets
on the horizon, as
viewed by another
satellite, its signal path
slices vertically
through the atmosphere
and is refracted. The
resulting propagation
delay allows scientists
to study plasma
irregularities in the
lower ionosphere,
which can strongly
disturb radio
communication and
navigation.
Toshitaka Tsuda obtained a Ph.D. in
Engineering in 1982 from Kyoto University.
Since 1995 he has been a professor of Radio
Atmospheric Sciences at the Radio Science
Center for Space and Atmosphere (RASC),
Kyoto University. His main research subjects
are related to development of advanced
remote-sensing measurements by means of
radio and optical techniques, and observations of middle atmosphere dynamics.
Klemens Hocke obtained a Ph.D. in physics
in 1994 from the University of Göttingen and
the Max-Planck Institute for Aeronomy. From
1994-2001 he worked at several research
institutes in Germany, Austria, and Japan on
data analysis and remote sensing of Earth's
atmosphere and ionosphere by ground- and
space-based radio techniques. The present
study originates from a recent stay as visiting
professor at RASC. He is currently working for
the Telecommunications Advancement
Organization at Communications Research
Laboratory in Tokyo and is co-chairman of the
special study group “Spaceborne GNS
Atmosphere Sounding” of the International
Association of Geodesy.
34
GPS World July 2001
For over 30 years the National Aeronautics and
Space Administration (NASA) has been using
radio occultation to study the atmospheres of
major planets and moons in the solar system.
More recently, scientists have begun applying
this technique to study Earth’s atmosphere by
placing a GPS receiver on a satellite in low Earth
orbit (LEO). This article describes the use of
GPS occultation to map plasma irregularities in
the lower ionosphere, which can strongly disturb radio communication and navigation.
Radio Occultation
Stanford University and Jet Propulsion Laboratory
(JPL), in Pasadena, California, developed
the radio occultation technique (see Figure 1),
mainly for bistatic radar sounding of planetary
atmospheres and ionospheres. GPS radio occultation uses the excess phase path due to atmospheric refraction of the GPS signal. This longer
path of the GPS-LEO radio link is determined
by the phase data of the GPS receiver on the
LEO satellite and by the precise (ephemeris)
positions of the GPS and LEO satellites.
GPS/MET. The GPS Meteorology Experiment
(GPS/MET) has demonstrated the unique value
of this measurement technique for remote sensing of Earth’s atmosphere and ionosphere. The
University Corporation for Atmospheric Research
(UCAR), in Boulder, Colorado, operates GPS/MET
in collaboration with NASA, JPL, the University
of Arizona, Orbital Sciences Corporation, and
Allen Osborne Associates, Inc. The National
Science Foundation, the Federal Aviation
Administration, and the National Oceanic and
Atmospheric Administration have sponsored
the mission.
The Microlab-1 satellite, launched in April
1995 from Vandenberg AFB in a 730 kilometer
circular orbit with a 70 degree inclination, carries the GPS/MET payload—a 2 kilogram, spacequalified, dual-frequency GPS receiver and a
ceramic patch antenna.
GPS Processing Technique. The tracking software of Microlab-1’s 8-channel GPS receiver
selects the radio links to GPS satellites setting behind Earth’s disc. During such a GPS
radio occultation, the tangency point of the
GPS-LEO radio link moves downward with a
geocentric radial velocity of around 2.5 -3 kilometers per second and scans the atmospheric layers at Earth’s limb from the ionosphere to
the surface. When the radio ray meets a thin
ionization layer or a plasma irregularity at
Earth’s limb, the phase path excesses of the
GPS L1 and L2 signals show strong fluctuations.
Due to ionospheric dispersion of the L1 and
L2 radio waves (d1 = 19 centimeters, d2 = 24.4
centimeters), the difference of the L1 and L2
phase path excess (S12) is proportional to the
number of free electrons along the ray path
(total electron content, or TEC, in Figure 1).
Thus, the small scale variations (vertical scales
less than 7 kilometers) of the observed S12
profile are approximately proportional to
the small scale plasma fluctuations at Earth’s
limb (electron density fluctuation ∆ne).
Using Abel transform, it is possible to invert
radio occultation data into height profiles of
fundamental parameters such as refractivity,
temperature, and electron density.
Contrary to other space remote sensing techniques, the radio occultation technique can measure small vertical structures of atmospheric refractivity (0.1-1 kilometers) caused by inversion and
turbulence layers, atmospheric waves, water vapor
clouds, or thin ionization layers. Individual excess
phase path measurements are accurate to about
0.1 millimeters for a 1 second average, corresponding to a temperature retrieval error of
less than 1 Kelvin in the lower stratosphere.
GPS
TEC
LEO
h
Earth
FIGURE 1 Application of the radio occultation technique, using a GPS receiver on
board a low-earth orbit (LEO) satellite.
www.gpsworld.com
RESEARCH Challenge
been called sporadic E.
Strong sporadic E layers can
be also caused by a combined effect of metal ions
from meteor, neutral wind
shear, particle precipitation,
and/or electric field of magnetospheric origin.
Sporadic E
Generally the mechanisms
of sporadic E formation
have been well studied by
ground-based observations,
theory, and simulations.
Ground-based obser vaFIGURE 2 Plasma irregularities (vertical scales less than 7 kilome- tions, however, cannot proters) in the lower ionosphere at h =105-110 kilometers observed
vide a comprehensive picby GPS/MET. A dot corresponds to a radio occultation event. The
ture of the global sporadic
dot radius is proportional to the electron density enhancement
E distribution. Our current
∆ne of the sporadic E layer. Red dots are observed in June/July
knowledge can be biased
1995, green dots are in October 1995, and blue dots are in
by the fact that rocket and
February 1997.
ground-based observations
of sporadic E are mainly obtained over or near
Plasma Irregularities
the continents, but not over the oceans.
The global distribution, seasonal behavior and
GPS/MET’s new results show a land/sea conother characteristics of plasma irregularities
trast of plasma irregularities in the lower ionosof the lower ionosphere can tell us much about
phere, probably due to variable energy and
atmosphere dynamics, electrodynamics, and
momentum flux of atmospheric gravity waves,
coupling processes of neutral atmosphere,
tides, and mountain waves from below. The
ionosphere, and magnetosphere. Plasma irregatmospheric gravity waves are generated by
ularities can strongly disturb radio commuorographic effect (interaction of surface wind
nication and navigation. From analysis of diswith Earth’s topography), jet stream variabilturbed radio links between GPS satellites and
ity, and convective clouds in the lower atmosMicrolab-1 we estimated the three dimensional
phere. Under favorable conditions, depending
fluctuation field of Earth’s iono sphere.
for example on the background wind profile,
Atmospheric waves originating from source
atmospheric gravity waves can propagate
regions near to Earth’s surface seem to cause
up into the lower ionosphere. Atmospheric
plasma irregularities in the lower ionosphere,
tides due to solar heating of tropospheric water
since the observed occurrence of plasma irregvapor and ozone also transport energy and
ularities shows a close relationship to Earth’s
momentum from the lower to the upper atmostopography.
phere. Especially the short vertical wavelength
Thin ionization layers and plasma irregumodes of the semi- and terdiurnal tides are
larities are typical features of the lower ionoseffective in producing tidal ion layers in the
phere. In the dynamo region, at heights around
lower ionosphere by wind shear effect.
90-120 kilometers, neutral winds are most effective in producing electric currents. Due to
ion-neutral collisions, the ions move with the
Results
neutral air, while the motion of the electrons is
The global distribution of sporadic E is estiinfluenced more strongly by the geomagnetic
mated in Figure 2 by the maximum of ∆ne withfield. The consequence is an electric polarizain the height range 105 to 110 kilometers.
tion field and a plasma drift perpendicular to
The dots show the locations of the radio occulthe electric and magnetic field vectors (E x B
tation measurements. The radius of a dot is lindrift). Rocket, radar, and meteor trail obserearly proportional to ∆ne. Areas with large
vations give evidence for strong wind shears of
dots are areas of enhanced sporadic E. The
the zonal neutral wind in the lower ionosphere
GPS/MET data base has three time intervals of
caused by atmospheric gravity waves and tides.
about two weeks, when GPS’anti-spoofing has
A zonal wind shear induces a drift of plasma parbeen turned off, so that the GPS/MET obserticles from a large volume into a thin layer of
vations have highest accuracy during these
high plasma density which is located in the
times. The red dots in Figure 2 correspond
region between eastward and westward neuto 1,900 radio occultation events collected durtral wind flow. Since the beginning of radio coming the period June 19 to July 10, 1995;the green
munication these plasma irregularities have
dots correspond to 1,540 events during the periwww.gpsworld.com
od October 10-25, 1995; and the blue dots represent 2,690 events during the period February
2-16, 1997.
Seasonal Behavior. At first we can see a clear
seasonal behavior, since sporadic E is strongest
in the summer hemisphere. In June/July it occurs
mainly in the northern hemisphere, and in
February 1997 in the southern hemisphere.
During spring/autumn (October 1995) sporadic
E is comparatively weak. These results are in
agreement with ground-based observations and
with an article by J.D. Whitehead (see the “Further
Reading” sidebar) who notes that sporadic E is
a phenomenon of the summer hemisphere. A
new result is the dominance of sporadic E over
Eurasia compared to North America, the Atlantic,
and the Pacific (at least during June/July 1995
and at heights of 105 -110 km). Strong sporadic
E appears over the Strait of Gibraltar between
Spain and Morocco and in western France, which
are probably source regions of orographic waves.
Large red dots are following the shape of Japan,
which is known to have strong sporadic E in the
summer. World maps of sporadic E appearance
were derived in 1961 from the huge data set
of the dense network of ground-based ionosondes operated during the International Geophysical
Year 1958. Many characteristics of the sporadic
E distribution observed by GPS/MET (Figure 2)
are already present in these early world maps
of peak plasma frequencies, or foEs.
The Ionosphere
The ionosphere is the region of the Earth’s
atmosphere where ionizing radiation, primarily in the form of solar extreme ultraviolet and x-ray emissions, causes electrons to
exist in sufficient quantities to affect the propagation of radio waves. When the photons
that make up the radiation impinge on the
atoms and molecules in the upper atmosphere, their energy breaks some of the bonds
that hold electrons to their parent atoms. The
result is a large number of free, negatively
charged electrons as well as positively charged
atoms and molecules called ions. Such an
ionized gas is known as a plasma.
The ionosphere is not bound by specific, fixed limits, although the altitude at which
the ionosphere begins to be detectable is
about 50 kilometers and stretches to heights
of 1,000 kilometers or more. The upper boundary of the ionosphere is not well defined
as the electron distribution thins out into the
plasmasphere (or protonosphere, as the dominant positive ions there are protons) and
subsequently into the interplanetary plasma.
[Excerpted from “GPS, the Ionosphere, and
the Solar Maximum” by Richard B. Langley in
GPS World,Vol. 11., No. 7, July 2000, pp. 44-49.]
GPS World July 2001
35
Challenge
RESEARCH
Further Reading
FIGURE 3 Longitude-height section of plasma irregularities of the
lower ionosphere derived from GPS/MET observations in February
1997. The horizontal dashed lines in Figure 2 show the location
of the slice between 458S and 658S. The average is taken by a sliding window of 208 in longitude and with respect to the sign of
∆ne. Negative (positive) longitude corresponds to West (East).
Southern Hemisphere. In the southern hemisphere in February 1997 (blue dots in Figure
2), enhanced sporadic E is centered around the
southern tip of South America. This is probably due to interaction of the eastward jet stream
with the Andean mountain ridge. Observations
confirm the theory that the Andes generate
mountain waves. Ground-based, optical interferometer measurements showed that the temperature of the thermosphere over Peru is sometimes several hundred Kelvins higher than over
the Pacific. Some researchers have suggested
enhanced heating of the thermosphere by orographic waves from the Andes.
Regarding the concentration of irregularities around the southern tip of South America
in Figure 2, the lower ionosphere may be compared to the surface of a river which has curls
and surface waves in the neighborhood of a
river bend, and/or if big rocks are on the river
floor. However the situation is more complicated for the atmosphere since air is compressible, and the mean neutral wind can change
its direction in the middle and upper atmosphere. The change of the plasma density with
location, season, solar cycle, and the effect of
geomagnetic inclination on the plasma drift
also have to be considered. So it was unexpected to find that the sporadic E distribution
detected by GPS/MET is so closely correlated
to Earth’s topography.
Finally the longitude-height section of sporadic E is calculated by using all occultation
events of February 1997 within the latitude
range 45 8 - 65 8 S as shown by the horizontal
dashed lines in Figure 2. The average ∆ne field
in Figure 3 has an interesting plasma structure
at around 758W or 2758, the location of the
Andes. The multi-layer structure with maxima at around 95, 105, and 115 kilometer height
could be a far-reaching consequence of the dis-
36
GPS World July 2001
c S.D. Eckermann, P. Preusse, Science 286, 1534, (1999).
c K. Hocke, A. Pavelyev, O. Yakovlev, L. Barthes, N. Jakowski,
Journal of Atmospheric and Solar-terrestrial Physics, 11691177, (1999).
c A. Komjathy, J.L. Garrison, and V. Zavorotny, “GPS: A New Tool
for Ocean Science”, GPS World, Vol. 10, No. 4, April 1999.
c R. Kursinski, “Monitoring the Earth’s Atmosphere with GPS”,
GPS World, Vol. 5, No. 3, March 1994.
c J.D. Mathews, Journal of Atmospheric and Solar-terrestrial
Physics 60 , 413, (1998).
c J.W. Meriwether, M.A. Biondi, F.A. Herrero, C.G. Fesen, and D.C.
Hallenback, Journal of Geophysical Research,102 , 20041,
(1997).
c C. Rocken, Anthes, R., Exner, M., Hunt, D., Sokolovskiy, S.,
Ware, R., Gorbunov, M., Schreiner, W., Feng, D., Herman, B.,
Kuo, Y.-H., and Zou, X., Journal of Geophysical Research 102 ,
29849, (1997).
c S. Taguchi, H. Shibata, Journal of the Radio Research
Laboratories 8 , 355, (1961).
c T. Tsuda, M. Nishida, C. Rocken, R. H. Ware, Journal of
Geophysical Research. 105 , 7257, (2000).
c R. Ware, “GPS Sounding of Earth’s Atmosphere”, GPS World,
Vol. 3, No. 8, September 1992.
c J.D. Whitehead, Journal of Atmospheric and Terrestrial Physics.
51, 401, (1989).
turbance of the surface wind
by the Andean mountain
ridge.
Explanation. The forms of
the quasi-stationary plasma
density structures at 21208
to 2208 longitude may indicate neutral air flow of the
upper atmosphere, if one considers that the
plasma is often accumulated in the nodes of
neutral wind shears. In Figure 3 a green, broad
layer of plasma irregularities is present at all
longitudes between 90 and 110 kilometers height.
This broad layer of plasma irregularities could
be explained by the characteristics of tidal ion
layer in ground-based observations which can
monitor the slow downward phase progression
of tidal ion layers at these heights.
Implications
The geographic position, height, time, intensity, and form of sporadic E occurrence are
interesting for more than just optimization of
radio communication and navigation. The global sporadic E distribution tells us about the
coupling of the lower, middle, and upper atmosphere by atmospheric waves, electrodynamical processes and electric conductivity of the
dynamo region. A rather complex system of
interactions exists between the neutral and
ionized atmosphere, magnetosphere, solar radiation, and solar wind. Global observation of
the temporal and spatial field of small-scale
atmospheric fluctuations gives insight into
solar-terrestrial relationships, atmosphere
dynamics, and energetics.
Because of the convincing GPS/MET results
and the economical principle of bistatic GPS
atmosphere sounding, many new GPS meteorology missions are already launched or in
preparation (e.g., Oerstedt, CHAMP, SAC-C,
GRACE, COSMIC). The signal-to-noise ratios
of the new GPS receivers have been significantly improved, and precise monitoring of
Earth’s atmosphere, ionosphere, and ocean
surface is now feasible both when antispoofing is on and when it is off.
Acknowledgments
We are grateful to C. Rocken, principal investigator of the GPS/MET project at UCAR and to the
GPS/MET teams at JPL and UCAR for raw data
analysis and for the data center (http://cosmic.cosmic.ucar.edu/). We thank M. Yamamoto,
of RASC, and T. Ogawa, of the Solar-Terrestrial
Environment Laboratory, in Nagoya, Japan, for
improvements and discussions. This study is supported by the Japanese GPS-Meteorology project
of the Science and Technology Agency (STA). c
Manufacturers
The Microlab-1 satellite is manufactured, owned
and operated by Orbital Science Corporation.
The GPS receiver on board Microlab-1 is a
TurboRogue manufactured by Allen Osborne
Associates. The ceramic patch antenna is manufactured by Ball Aerospace Systems Group.
www.gpsworld.com