Vol50,3,2007


ANNALS  OF  GEOPHYSICS VOL.  50, N.  3, June  2007



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447

ANNALS  OF  GEOPHYSICS, VOL.  50, N.  3, June  2007

Key  words ionosphere – GPS – real-time imaging
– TEC – tomography

1. Introduction

Tomography is a mathematical technique to
reconstruct two-dimensional images and it is
best known for the reconstruction of the human
body from X-ray measurements (Hounsfield,
1972) in the Computer-Aided Tomography
(CAT). In 1988 Austen and co-authors (Alfonsi
et al., 1988) proposed imaging the Earth’s ion-
osphere with tomography to determine the dis-
tribution of free electron concentration from
satellite-to-ground radio signals.

Ionospheric tomography is well established
but from an application viewpoint has two crit-
ical drawbacks. First, tomographic images are
only two dimensional (latitude versus altitude
slices), whereas full three-dimensional images
are usually needed. Second, conventional iono-
spheric tomography used data from Low-Earth-
Orbit (LEO) satellites that are not continuously
in view. This meant that images can only be
produced every few hours, at irregular intervals
when a LEO satellite passes over the region of
interest. This is a problem because the iono-
sphere can undergo significant changes on
timescales of minutes, far shorter than the inter-
val of time between consecutive tomographic
images.

Since the availability of GPS satellites start-
ing from early nineties, phase delay and pseu-
do-range measurements from a relevant num-
bers of ground stations have become available
for research. These measurements furnish TEC
evaluation along the huge amount of ray-path

MIRTO: a prototype for real-time
ionospheric imaging 

over the Mediterranean area

Lucilla Alfonsi (1), Cathryn N. Mitchell (2), Vincenzo Romano (1) and Paolo Spalla (3)
(1)  Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy

(2)  Department of Electronic and Electrical Engineering, University of Bath, U.K.
(3)  Istituto di Fisica Applicata «Nello Carrara» (IFAC), CNR, Sesto Fiorentino (FI), Italy

Abstract
MIRTO (Mediterranean Ionosphere with Real-time TOmography) is a collaborative project between Istituto
Nazionale di Geofisica (INGV) of Rome, the University of Bath (U.K.) and the Istituto Fisica Applicata «Nello
Carrara»-Consiglio Nazionale delle Ricerche (IFAC-CNR) of Florence. The goal of the project is the develop-
ment of a prototype for real-time imaging of the ionosphere over the Italian region with extension to the Mediter-
ranean Sea. MIRTO uses an original imaging technique developed at the University of Bath and upgraded for
real-time use in cooperation with IFAC. The prototype makes use of the data acquired by the real-time ionos-
pheric and geodetic instrumentation operated by INGV. Such measurements drive the imaging algorithm to pro-
duce the image of electron density as well as maps and movies of the Total Electron Content (TEC) over the con-
sidered area. 

Mailing address: Dr. Lucilla Alfonsi, Istituto Naziona-
le di Geofisica e Vulcanologia, Via di Vigna Murata 605,
00143 Roma (Italy); e-mail: lucilla.alfonsi@ingv.it



448

Lucilla Alfonsi, Cathryn N. Mitchell, Vincenzo Romano and Paolo Spalla

station-satellite, and are the basis for 3D and
3D time-dependent imaging of the ionosphere.

In 2001 Spencer and Mitchell published re-
sults from a new type of ionospheric imaging

based on GPS (Spencer and Mitchell, 2001),
which overcame the limitations of the classic
(LEO) ionospheric tomography. The GPS satel-
lites are monitored by a network of dual-frequen-

Fig. 1. Stations included in the MIRTO system.

Table I.  Location and type of instrumentations of the MIRTO sites.

Site Station Code Latitude Longitude Station

Stuetta STUE 46°28l19.9mN 9°20l50.3mE GPS
San Benedetto Po SBPO 45°03l03.6mN 10°55l11.2mE GPS

Milazzo MILA 38°16l14.1mN 15°13l50.6mE GPS
Craco CRAC 40°22l53.0mN 16°26l06.6mE GPS

Ventotene VENT 40°47l40.9mN 13°25l17.8mE GPS
Leonessa LNSS 42°36l0.3mN 13°02l24.9mE GPS

Pesaro PESA 43°56l27.7mN 12°50l25.8mE GPS
Gibilmanna GBLM 37°59l25.2mN 14°01l34.1mE GPS

S. Elia a Pianisi CIGN 41°39l15.1mN 14°54l17.3mE GPS
Chania Chania 35°31l8.0mN 24°02l32.6mE GISTM

Montelibretti Montelibretti 41°06l0.0N 12°36l0.0mE GISTM
Cartagena Cartagena 36°42l00mN 02°30l00mE Virtual GPS

Sofia Sofia 43°0l0mN 25°0l0mE Virtual GPS
El Arenosillo Areno dps 37°6l00mN 06°42l00mW Digisonde

Rome Rome dps 41°48l0mN 12°30l0mE Digisonde



449

MIRTO: a prototype for real-time ionospheric imaging over the Mediterranean area

cy receivers, recording the phase and time delay
of each signal. Such networks (for instance,
IGS), spread all over the world, are open for da-
ta downloading providing a valuable source of
information on the ionosphere in the form of ray-
path integrations of electron concentration. Nev-
ertheless, these measurements are rather uneven
in distribution and coverage. The new imaging
technique, known as MIDAS (Multi-Instrument
Data Analysis System), is unique in its approach
to ionospheric imaging, because it performs a
four-dimensional (i.e. 3D time-dependent) inver-
sion (Mitchell and Spencer, 2003). It offers two
key advantages: i) they are fully three-dimen-
sional and ii) a novel time-dependent imaging
technique allowed the ionosphere to be studied
on time-scales of minutes. The University of
Bath performed many studies as, more recently,
imaging during extreme space-weather events
known as storms has shown dramatic changes in
the height of the ionosphere to occur over very
short time-scales (Yin et al., 2004; Mitchell et al.,
2005; De Franceschi et al., 2007). These sudden
events are very important for radio communica-
tions because they cause radio signals to deviate
from those paths predicted by ionospheric radio-
propagation models. Storms also cause the ab-
solute values of electron concentration to de-
crease during the negative storm phase, thus low-
ering the range of frequencies possible for HF
communications. Real-time imaging of the iono-
sphere could assist in communications planning,
reducing potential communications outages in
storm times.

MIDAS is now under development for two
aspects: first, its extension to polar regions (Spen-
cer and Mitchell, 2007); second, real-time imag-
ing. MIRTO, developed in cooperation with
IFAC and INGV, represents the first step in this
direction. The MIRTO project sets the aim of the
realization of a prototype that produces images
over Italy in near real-time making use of data ac-
quired by the real-time ionospheric and geodetic
instrumentation operated by INGV. In order to
cover the western and eastern edges of the area
defined by the system, a GPS receiver has been
recently installed at Chania (Crete, Greece) and
another one is being to be deployed in Spain
(Huelva, El Aeronosillo Observatory) (fig. 1,
table I). MIRTO technique uses ionosonde densi-

ty profiles given in near real-time by some sta-
tions coordinated in the DIAS framework (Be-
leakhi et al., 2005). During the development of
MIRTO when no data have been available at
some stations, artificial data from IRI were used.

In this paper the MIRTO prototype is intro-
duced and described underlining its potentiali-
ties in view of the realization of a useful tool for
scientific aims and space weather applications.
After a few hints on the theory of the inversion
problem, the second section of the paper is
mainly addressed to the description of the
method used by MIRTO for imaging the iono-
sphere; the first results of the prototype and the
next steps of the project are described and dis-
cussed in the third and last section.

2. Method and development 

The total electron content is defined as the
line integral of the electron concentration along
a path from a satellite to a receiver. Dual fre-
quency radio signals that propagate through the
ionosphere are subject to a differential phase
change due to the dispersive nature of the plas-
ma. As a first order approximation the change
in the differential phase shift is directly propor-
tional to the change in TEC between the trans-
mitter and receiver. 

An outline of the theory of the inversion prob-
lem can be found in Mitchell and Spencer (2003)
and references therein. Here we only recall the
crucial steps of the inversion approach applied to
GPS signals, which are: a) set up a three-dimen-
sional grid of j voxels (i.e. volume pixels), each
bounded in latitude, longitude and altitude; b)
compute the length of all elements of every i-th
satellite-to-receiver ray-path through each inter-
sected voxel. The unknown electron concentra-
tion x is defined to be constant within each voxel.
The problem may now be expressed as 

(2.1)

where A is an i × j matrix of the path lengths
within each voxel and b are the i observed
TECs. This cannot be solved directly as the ma-
trix A is rectangular, highly singular and incor-
porates no prior information as to the likely so-

Ax b=



450

Lucilla Alfonsi, Cathryn N. Mitchell, Vincenzo Romano and Paolo Spalla

lution. To overcome this difficulty a mapping
matrix, X, is used to transform the problem to
one for which the unknowns are n coefficients
of orthonormal basis functions, the combina-
tion of which will give the final image of elec-
tron concentration. Here the basis functions (X)
were generated using a spherical harmonic ex-
pansion to represent the horizontal variation
and Empirical Orthonormal Functions (EOFs)
for the radial variation in electron concentration
(Mitchell and Spencer, 2003). For MIRTO, the
radial dependence in EOFs is derived from den-
sity profiles provided by the ionosondes (Ma-
terassi and Mitchell, 2005). After these inver-
sions, MIRTO produces density profiles images
over Italy and Mediterranean area in close to re-
al-time, i.e. with a latency of 15 min. According
to the different information needed by the ap-
plications, the system can provide TEC maps,
TEC gradients, ionization sections, electron

density profiles, peak density and height over
the region of interest (fig. 2).

The first phase of the project was spent us-
ing the existing data to upgrade and test the re-
al time version of MIDAS, to develop all the
networking software for the GPS data, to inte-
grate the system with other European existing
real time GPS data (SOPAC) and ionosonde re-
al time data and, finally, to produce and flow
the images to the main station to be distributed. 

The data sources of the system are: IGS (In-
ternational GPS Service) for the orbital data, IN-
GV national network RING (Rete Integrata
Nazionale GPS) located in Italy for the L1 and
L2 signals (fig. 1, table I), one GISTM (GPS
Ionospheric Scintillation and TEC Monitor, Van
Dierendonck et al., 1993; De Franceschi et al.,
2006) receiver managed by INGV and located in
Chania (Greece) for TEC data and the DPS-4
digisonde in Rome and El Aeronosillo for the

Fig. 2. Typical output of MIRTO. Clockwise: tracks of all the satellites in view from the MIRTO stations; TEC
map (in TECu=1016 m−2); latitudinal and longitudinal sections of the electron density (in 1011 m−3).



451

MIRTO: a prototype for real-time ionospheric imaging over the Mediterranean area

electron density profiles (SAO format; Reinisch,
1998). Moreover the system makes use of GPS
artificial data (IRI, International Reference
Ionosphere; Bilitza, 2001) from locations of pos-
sible future receivers and of artificial Chapman
profiles (see e.g., Rishbeth and Garriott, 1969) to
contribute to EOF’s construction. The locations
of the MIRTO stations are listed in table I.

The system works according to the following
procedure: it reads the orbit elements from the
data centres, the GPS data from MIRTO and
RING stations and the ionosonde data from the
Rome DPS-4; after the creation of artificial data
from IRI for the virtual stations, it creates input
matrices and then builds the EOF’s using
ionosonde data and model. Finally, the system in-
verts the data producing plots and output files.
Every 15 min (at 5, 20, 35 and 50 min of each
hour, according to the ionosonde sampling) MIR-
TO refreshes the input structures according to the
incoming data (orbits, GPS and ionosonde data),
inverts the data and produces the new outputs. 

A typical MIRTO output is shown in fig. 2: on
the top left are visualized the tracks of all satel-
lites in view from the stations listed in table I; the
right hand (top) map reports the TEC (in TECu)
over the area of interest; the other two plots de-
scribe the longitudinal and latitudinal sections of
the electron density in the same area. 

The MIRTO project is now at the end of its
second phase, essentially a test phase to assess
the reliability of the data flow, the combination of
the data into the MIDAS algorithm and the suc-
cess in streaming of the ionospheric images to the
user-interface. During this phase we also de-
ployed a GISTM receiver in Chania (Crete,
Greece) to cover the eastern edge of the area of
interest. For the coverage of the western border,
we are going to install another GISTM receiver in
Spain (El Aeronosillo Observatory, Huelva).

3. Next steps

The third and last phase of the realization of
the MIRTO prototype will be dedicated to the as-
sessment of images for regions of data sparse,
such as, for instance, North Africa, the assess-
ment of the ionospheric imaging under perturbed
conditions typical, for instance, of magnetic

storms and the assessment of the database sys-
tem. This period will also explore the possible ac-
quisition of supplementary receivers where nec-
essary.

The final aim of MIRTO is to realize a sys-
tem able to produce monitoring of the iono-
sphere over the Mediterranean Sea in true real-
time. To achieve this goal the imaging tech-
nique used by MIRTO is going to be imple-
mented with forecasting tools. At that time the
system will be tested to obtain information on
the reliability of the MIRTO density images
with respect to the real ionospheric scenario.
The upgrading of the prototype will make MIR-
TO a real scientific tool of interest for scientif-
ic purposes and for space weather applications.

A continuous monitoring of the ionosphere
may be of great use even at the mid-latitudes,
monitored by MIRTO, where important «anom-
alous» phenomena are not locally triggered, but
however do happen. Indeed, mid-latitudes can
be affected by geomagnetic and ionospheric
phenomena both because of perturbations com-
ing from the North (e.g., Solar Flares, CMEs),
and because of disturbances coming from the
equatorial regions, due to the sudden variations
in the magnetic and electric fields of the Earth.
All these causes influence the electromagnetic
wave propagation, affecting significantly com-
munications and positioning.

Acknowledgements 

This work was supported by the project MI-
UR RBAP04EF3A.

The authors thank the INGV national GPS
network RING, the Laboratory of Geophysics
& Seismology of the Technological Education-
al Institute of Crete and the COST296 Action.

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(received September 10, 2007;
accepted October 19, 2007)



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