Investigating Space Plasma Dynamics over Namibia Using
Signals from Global Navigation Satellite Systems like GPS.
Ben Opperman2,3, Smita Francis1, Robert Van Zyl3, Pierre Cilliers2,3 and John-Bosco Habarulema2
1Polytechnic of Namibia, Windhoek, Namibia
2 Space Science Directorate, South African National Space Agency, Hermanus, South Africa;
3 FSATI, Cape Peninsula University of Technology, Cape Town, South Africa.
[email protected]
Locations of GNSS receivers in
Namibia
◊ Installed
◊ Planned
2. Introduction
The project introduces remote sensing of the
atmosphere by using two novel techniques
involving radio signals transmitted by Global
Navigation Satellite System (GNSS) and by
radio beacons in low Earth orbit.
GNSS signals: The use of GNSS signals for
ionospheric characterization is a well-proven
and operationally-used technique not yet applied
in Namibia for lack of required instrumentation.
The project will see the installation of a number
of SANSA-sponsored dual-frequency GPS
receivers from which timely high resolution 2D
and 3D images of the invisible ionised upper
atmosphere and 2D water vapour content
images from the lower atmosphere will be
derived for the whole country.
Remote sensing of the atmosphere using radio
signals is a technique developed through the NASA
space program of the 1970s to obtain atmospheric
parameters of planets when an interplanetary
probe/satellite’s radio communication signals to Earth
are eclipsed by the planets’ atmosphere.
This technique has been successfully extended to
Global Navigation Satellite Systems (GNSS) such as
the widely-used GPS, to extensively observe Earth’s
neutral and upper atmospheres remotely. This
technique has matured to a very active and exciting
research field with many possibilities, especially in
Africa.
All radio signals passing through the Earth’s lower
and upper atmosphere are changed to some degree
due to refraction caused by the ionised plasma or the
presence of precipitable water vapour (PWV)
(moisture) in the atmosphere. With GPS signals this
refraction results in a measured signal path length
which is 8-10 metres longer than the actual path
length between the satellite and ground receiver.
As a GPS receiving device (e.g. car navigation
systems like Garmin, Tomtom, Nuvu etc.) uses this
signal path length of one carrier frequency to calculate
position, the corrupted signal path length results in
inaccurate position measurement of 5-30 metres.
To eliminate this refraction error, GPS satellites
orbiting Earth at about 20 000 km transmit navigation
signals on two carrier frequencies of about 1.2GHz
and 1.6GHz.
A corrected path-length is then
calculated from the difference between the two
signals’ respective path lengths and an improved
position accuracy position of < 1 m is calculated from
the corrected path length.
The interferometers will determine the direction of
the signal from the satellite transmitter beacon.
Correlating this with the actual position of the satellite
will provide information about the constitution of the
ionosphere at that time.
Geographic location of 350km -high Ionospheric Pierce Points
observed from three Namibian GPS receivers over 24-hours
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Latitude
Earth’s Ionosphere extends from 50 to 1 200
km above its surface and comprises an
electrically-conducting space plasma composed
of negatively charged electrons and positively
charged ions of varying density. Ionisation
results from the upper neutral atmosphere’s
dissociation by hard X-ray and Extreme
Ultraviolet (EUV) solar radiation [1].
The ionosphere is a dispersive medium,
affecting all radio signals propagating through it
by delaying the group and advancing the phase
with a time increment proportional to the
ionospheric Total Electron Content (TEC) and
inversely proportional to the square of the carrier
frequency. Lower frequency signals are refracted
more than higher frequency signals, leading to a
slightly longer refracted path length through the
ionosphere.
3. Scientific and
Technological background
TSUM
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WHUK
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KEET
Radio Beacons: Signals transmitted from radio
beacons on low earth orbiting satellites and
received by a direction finding (DF) antenna
array on the ground can be used to characterize
the ionospheric refraction.
As these radio signals propagate through the
ionised upper atmosphere they are refracted
through its various layers, similar to light
refracted through water or glass. By comparing
the difference between the beacon signal’s DF
measured arrival angles and the actual line-ofsight angles to the satellite, important
information of the ionosphere and wave
propagation through the ionosphere is derived.
Acknowledgements
The project is jointly funded by the National
Commission on Research Science and
Technology (NCRST), Namibia and the National
Research Foundation (NRF), South Africa.
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6. Significance
The mid-latitude upper atmosphere forms an
important transition zone for upper atmosphere
dynamics of which little is known over this
region.
The data collected Namibian
observations will fill an important data gap and
supplement data.
Present theory about the propagation of upper
atmospheric phenomena is unclear about some
aspects. PWV measurements obtained with
GNSS will significantly supplement the sparse
atmospheric observations obtained (if any at all)
with
expensive
radiosondes.
PWV
measurements give an indication of moisture
content in the atmosphere, a climate-dependent
parameter which changes with time and
atmospheric conditions and could potentially be
used in Numerical Weather Prediction (NWP)
systems.
The satellite-borne HF beacons and
interferometer antenna arrays will contribute to
the monitoring of the density and movement of
the polar and high-latitude ionosphere. It will
enhance ionospheric forecasts for HF
communications, which has critical relevance to
users of long distance HF communications, such
as the Defence Force.
Diurnal GPS-derived Ionospheric TEC observed at
3 Namibian locations on 28 August 2014
35
Keetmanshoop
30
Windhoek
Tsumeb
25
TEC [TEC Units]
1. Abstract
20
15
10
5
0
0
3
6
9
12
15
Time [UTC Hours]
18
21
24
6. Conclusion
The project is of significance to monitor the
ionosphere over both Namibia and Southern
Africa.
The project will assist in the European Global
Navigation satellite Systems
navigation
correction system’s proposed expansion into
Southern Africa.
The project will serve as a catalyst in human
capital development in Space Science and
Space Technology in Namibia.
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5
10
15
20
25
30
35
40
Longitude
4. Materials and Methods
• A training workshop will be held for skills transfer and
training in the use of existing software, data analyses
techniques and the development of new algorithms.
• A central computer will be set up at the Polytechnic
of Namibia in Windhoek for centralised data storage
and dissemination.
• GNSS and ionospheric scintillation receivers will be
installed at Tsumeb, Keetmanshoop, Windhoek, and
other locations where PoN has satellites campuses.
• A 7-element cross-dipole L-shape HF interferometer
array will be constructed at two sites, one at SANSA
in Hermanus, and the other at a suitable location in
Namibia to determine the direction of the signal from
the satellite transmitter beacon.
• Results and data products supportive of the
objectives will be derived and disseminated.
7. References
Opperman, BDL.,PJ.Cilliers, LA. McKinnell, and R. Haggard
(2007), Development of a regional GPS‐based ionospheric
TEC model for South Africa, Adv. Space Res., 39, 808-815.
Van Zyl, R. R., D. F. Visser, P. J. Cilliers, and B. D. L.
Opperman. ZACUBE-1 space weather mission: Characterize
the SuperDARN HF Radar antenna array at SANAE-IV.
Space Weather, doi:10.1029/2012SW000890. 2013.
Opperman BDL, R.R van Zyl, D.F Visser, P.J. Cilliers and D
Agaba. Investigating HF wave propagation through the mid
and high latitude ionosphere using signals received from the
ZACUBE-1 HF beacon at two ground-based direction-finding
interferometers. In Proceedings: International Beacon
Symposium, University of Bath, July 2013.
Habarulema J.B., Z. T. Katamzi, and E. Yizengaw (2014), A
simultaneous study of ionospheric parameters derived from
FORMOSAT‐3/COSMIC, GRACE, and CHAMP missions over
middle, low, and equatorial latitudes: Comparison with
ionosonde data, J. Geophys. Res. Space Physics, 119,
doi:10.1002/2014JA020192.