1. Indian Journal of Radio & Space Physics
Vol 41, April 2012, pp 162-167
Radio Beacon for Ionospheric Tomography (RaBIT) onboard YOUTHSAT:
Preliminary results
T K Pant1,$,*, P Sreelatha1, N Mridula1, S Trivedi2, R M Das3, S Koli4, R Sharma5, J Girija1, Arun Alex1, K K Mukundan1,
S B Shukla1, P Purushottaman1, J N Santosh1, Biju Thomas1, M Srikant6, R Sridharan7, K Krishnamoorthy1, Ratan Bisht8,
D V A Raghavamurthy6, M P T Chamy8 & J D Rao9
1
Vikram Sarabhai Space Centre, Trivandrum 695 022
Space Science Office, ISRO, Antariksh Bhavan, New BEL Road, Bangalore 560 231
3
National Physical Laboratory, Dr K S Krishnan Marg, New Delhi 110 012
4
National Balloon Facility, TIFR, P B No 5, ECIL P O, Hyderabad 500 762
5
Master Control Facility, Ayodhya Nagar, N Sector, Bhopal, 462 041
6
ISRO Satellite Centre, PB No 1795, Vimanapura Post, Bangalore 560 017
7
Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, Gujarat
8
Space Application Centre, Jodhpur Tekra, Ambawadi Vistar P O, Ahmedabad 380 015
9
ISRO Satellite Tracking Centre, Bangalore 560 058
$
E-mail: tarun_kumar@vssc.gov.in
2
Received: November 2011; accepted 23 April 2012
This paper presents, for the first time, a few tomograms obtained using India’s own Radio Beacon for Ionospheric
Tomography (RaBIT) onboard YOUTHSAT, a small satellite dedicated for the terrestrial upper atmospheric studies. The
tomograms presented here, were obtained during the beginning of the solar cycle 24, and clearly demonstrate the potential of
the tomography technique to investigate the large scale ionospheric processes over the Indian longitude region.
Keywords: Ionospheric tomography, Radio beacon, Equatorial ionization anomaly (EIA), Equatorial spread-F (ESF), Total
electron content (TEC)
PACS No.: 94.20.dt
1 Introduction
The terrestrial upper atmosphere over the low and
equatorial latitudes is replete with large scale
plasma/neutral processes such as the equatorial
ionization anomaly (EIA) and the equatorial spread-F
(ESF), which significantly influence the distribution
of the ionization as a function of altitude, latitude,
longitude and time1. Understanding these processes
and modeling them pose challenges due to their
highly dynamical nature and large spatial and
temporal variability even during geomagnetically
quiet
conditions.
The
geomagnetic
storms
significantly alter the background ionospheric and
thermospheric structure, energetic and dynamics and
as a consequence modify the major equatorial
ionospheric processes2.
In the recent years, it has become clear that the
understanding of the ionosphere and the processes
therein, especially those over the low and equatorial
latitudes, is central to the design of many modern
communication, navigation and positioning systems.
This is because the position accuracy achievable from
navigation satellites is largely affected by the
intervening ionosphere. The range error is directly
proportional to the total electron content (TEC) along
the ray path. With the increasing use of satellites for
navigation and positioning (GPS, GLONASS, etc.),
characterization and modeling of the ionosphere
(its spatial and temporal variability) has become
extremely important. This includes understanding /
modeling of the processes of the ionospherethermosphere system and its response to the various
external forcings so as to reach a level of predictive
capability3. This requires the knowledge regarding
how the ionospheric plasma distribution occurs as a
function of altitude over a large latitude region and

2. PANT et al.: RADIO BEACON FOR IONOSPHERIC TOMOGRAPHY
evolves with time. The traditional ground-based
experiments like the ionosonde have a limitation in
addressing this aspect.
In this context, it has been unambiguously
established that the tomographic techniques4,5 are very
effective and useful. The effectiveness of this
technique in investigating the large-scale structures
over low and equatorial latitudes, for example,
equatorial ionization anomaly (EIA) has been amply
demonstrated6,7. The low-latitude ionospheric
tomography network (LITN), which is the first
network of six receivers from 14.6°N to 31.3°N
(geographic latitudes) along the 121°E meridian, has
added significantly to the understanding about the
motion of the anomaly crest of the EIA8, the structure
and symmetry of its core and the low-latitude
ionospheric response to magnetic storms9.
2 Indian Radio Beacon Experiments
The Indian Coherent Radio Beacon Experiment
(CRABEX) had also been initiated mainly to address
the aforementioned aspects regarding the large-scale
processes over equatorial and low latitudes over the
Indian longitudes. This experiment consists of five
radio receivers stationed at Trivandrum (8.5°N,
77°E), Bangalore (13°N, 77.6°E), Hyderabad
(17.3°N, 78.3°E), Bhopal (23.2°N, 77.2°E), and Delhi
(28.8°N, 77.2°E) that are capable of receiving
150 and 400 MHz beacon transmissions from the Low
Earth Orbiting Satellites (LEOS) like the Navy
Ionospheric Monitoring Satellites (NIMS) of USA.
This chain is unique as it covers the crest and trough
regions of the EIA latitudinally, and goes well beyond
the anomaly region. The data obtained using this
chain is used to generate tomograms for
understanding the temporal and spatial evolution of
equatorial and low-latitude ionospheric phenomena
like EIA and ESF, and their interrelationships. A
detailed discussion on the accuracies involved in the
tomogram generation using the CRABEX data is
already presented10.
Even though, the beacon transmissions on board
LEO satellites are useful to provide excellent spatial
coverage, the temporal information offered by these
techniques is largely limited by the number of
satellites available. At present, there are only a few
beacon satellites operational, mainly from NIMS and
COSMOS (Russian) series and over the years their
number has also decreased. As has been mentioned
earlier, it is desirable to have a better temporal
163
coverage along with spatial information during
varying geophysical conditions which necessitates
more number of satellite based beacon payloads.
As a first step in this direction and a logical
extension of CRABEX, India’s first indigenous
beacon, namely the Radio Beacon for Ionospheric
Tomography (RaBIT) payload was conceived and
developed at the Indian Space Research Organisation
(ISRO). The RaBIT is onboard a small satellite,
which is an Indo-Russian collaboration dedicated for
the upper atmospheric studies, called the
YOUTHSAT. The satellite YOUTHSAT was
launched successfully from SHAR (13.7199°N,
80.2301°E), India on April 20, 2011. YOUTHSAT
was placed in an 817 km orbit, with semi-major axis
being 7195.12 km. Along with the RaBIT,
YOUTHSAT has another scientific payload which is
an indigenously developed airglow imager called the
LiVHySI (Limb Viewing Hyper Spectral Imager).
The RaBIT has the equatorial crossing time of
10:30 hrs IST at the descending node (20:30 hrs IST
at the ascending node) and is having an orbit period of
101.35 minutes. YOUTHSAT having an inclination
of 90.7°, the RaBIT has a repeativity of 22 days. The
RaBIT beacons at two radio frequencies that are
150 and 400 MHz. The two frequencies are
transmitted to eliminate the effect of satellite motion
and tropospheric refractive index (which are nondispersive) since the ionospheric effect on radio signal
is frequency dependent. In the conventional satellite
beacons used for ionospheric studies, the coherent
frequencies are generated from a single crystal
oscillator using the multiplier technique. Though this
degrades the phase noise of the signal, it requires
sharp filters to remove harmonics and sub-harmonics
which are generated in this technique. The RaBIT
uses the frequency synthesis technique generating the
beacon signals at desired frequencies using a single
source. This ensures good stability and repeatability.
The RaBIT beacons at 150 and 400 MHz at a power
of 1.0 Watt, each giving very high signal to noise
ratio when received at the ground. For the RaBIT
beacon reception, the ground receiver chain is the
same as the one currently used as part of the
CRABEX.
The basic data for ionospheric tomography is the
line-of-sight total electron content estimated along a
number of ray paths from a chain of ground receivers
aligned along the same longitude. The line-of-sight
total electron content at the receiver is obtained by

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INDIAN J RADIO & SPACE PHYS, APRIL 2012
employing the Differential Doppler technique. Here,
the measured data is the relative phase between
150 and 400 MHz, and is proportional to the relative
slant TEC (STEC) along the propagation path of the
signal as:
φ = C D x STEC
…(1)
where, φ, is measured in radians; STEC is in m-2 and
CD = 1.6132 × 10-15 for NNSS satellites11. Since, the
phase measurements are accurate to < 3° when the
ground receiver is at locked condition and the data
sampling is at 100 Hz, these observations yield
accurate estimates of the relative TEC with errors
< 0.05%. The TEC data obtained at all the ground
stations is kept at the Indian Space Science Data
Centre (ISSDC). The ionospheric tomograms over the
Indian region are generated using the TEC measured
at each station and kept at the ISSDC for users.
3 Results and Discussion
For the YOUTHSAT configuration, the tomogram
covers the ionosphere from ~5° south of Trivandrum
to ~4° north of Delhi depending upon the satellite
elevation. The RaBIT tomography network is,
perhaps, the longest operating network existing
anywhere in the world and is unique, therefore.
Figure 1 shows the YOUTHSAT (RaBIT) tracks
across the globe. The first few tomograms are
presented here representing the ionosphere over the
77°E meridian over India, obtained using the RaBIT
observations made during the beginning of the solar
cycle 24.
Fig. 1—Dotted lines indicate the YOUTHSAT tracks across
the globe; pink dots indicate the position of RaBIT receivers
(RaBIT beacon is switched on only when its visible in the Indian
longitude zone)
Three tomograms are discussed here, two
representing the ionosphere during the day and night
on 2 July 2011; the third tomogram representing the
nighttime on 25 October 2011 but for a different
geomagnetic condition.
The tomograms shown in Figs (2 and 3),
respectively
depict
the
altitudinal-latitudinal
distribution of the electron density around the 77°E
meridian over the Indian region at 10:50 hrs IST
(i.e. daytime) and 21:53 hrs IST (i.e. nighttime) on
2 July 2011. The positions of the RaBIT ground
receiving stations are marked in the tomogram as
TVM (Trivandrum), BNG (Bangalore), HYD
(Hyderabad), BPL (Bhopal), and DEL (Delhi). The
curved black lines represent the terrestrial magnetic
field. The distances mentioned are in geocentric
reference frame. As is evident, the magnetic field
lines are horizontal over TVM indicating that this
receiving station is located right over the dip equator.
The presence of two ionization crests at locations
away from the equator and a trough over the equator
is clearly evident in the tomograms. During daytime,
the northern crest of ionization is found to be
extending beyond Hyderabad with overall electron
density being high at all the latitudes; the peak
electron density being 1.12 × 1012 m-3 at an altitude of
320 km just beyond the HYD latitude. The electron
density at the trough location, i.e. over the dip
equator, is found to be almost half of that over the
crest. It is interesting to note that while the overall
electron density is less during the night, the crest is
still prominent and is located closer to BNG, i.e.
closer to the equator. The peak electron density during
the night is 6 × 1011 m-3 at an altitude of 300 km just
beyond the BNG latitude.
The presence of the ionization crests and trough in
the ionosphere, as is seen in the tomograms presented
here, is the typical plasma density distribution over
the low and equatorial latitudes, also known as the
equatorial ionization anomaly (EIA). The day and
night differences in the overall ionization density and
the crest location can be attributed to: (a) solar
radiation and (b) the equatorial electrodynamics. The
evolution of the dynamo electric field over the
equator, along with the conductivity changes, drives
the EIA which in turn modifies the electron density
distribution away from the equator.
Nevertheless, the neutral and electrodynamics
along with the composition over the low and
equatorial latitudes can undergo substantial changes

4. PANT et al.: RADIO BEACON FOR IONOSPHERIC TOMOGRAPHY
165
Fig. 2—Altitude-latitude cross-section of the ionosphere as obtained using the RaBIT beacon signals during daytime (10:50 hrs IST) on
02 July 2011 for the YOUTHSAT pass (position of RaBIT ground stations is marked; vertical black lines represent geomagnetic field
configuration; color indicates the electron density)
Fig. 3—Altitude-latitude cross-section of the ionosphere as obtained using the RaBIT beacon signals during nighttime (21:53 hrs IST) on
02 July 2011 for the YOUTHSAT pass (position of RaBIT ground stations is marked; vertical black lines represent geomagnetic field
configuration; color indicates the electron density)

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INDIAN J RADIO & SPACE PHYS, APRIL 2012
Fig. 4—Altitude-latitude cross-section of the ionosphere as obtained using the RaBIT beacon signals during nighttime on 25 October
2011 for the YOUTHSAT pass (A moderate geomagnetic storm commenced early on this day; position of RaBIT ground stations is
marked; vertical black lines represent the geomagnetic field configuration; color indicates the electron density; the marked changes in the
electron density distribution vis-à-vis Fig. 2 can be clearly seen in this tomogram)
during geomagnetically disturbed periods. As a
consequence, the plasma distribution over these
latitudes show marked difference from that during
quiet periods. This aspect is clearly demonstrated
through the tomogram presented in Fig. 4. This
tomogram depicts the electron density distribution at
21:46 hrs IST on 25 October 2011. In fact, a moderate
geomagnetic storm commenced in the morning hours
(3:30 hrs IST) of 25 October 2011, after a
geomagnetically quiet period. The observed electron
density distribution on this night is markedly different
from what can be seen in the tomogram presented
in Fig. 3.
As can be seen in this tomogram, the electron
density on the top-side of the ionosphere, especially
between latitudes representing BNG and HYD is
significantly enhanced. It must be mentioned that
these changes in the top-side ionosphere can be
observed only through the tomography technique. The
single ionization crest appear to have been modified
in such a way that there are two regions where the
enhancement in the ionization is observed. These
observed changes are attributed to the overall changes
in the electrodynamics due to the storm. These
changes are being investigated in detail. Nevertheless,
a discussion of these is beyond the scope of present
manuscript which intends to present the first few
results and highlight the potential of the RaBIT based
tomography for the Indian region.
Acknowledgements
The significant contributions made by the entire
small satellite and RaBIT team are heartily
acknowledged. Their efforts made the Indian beacon
payload RaBIT a reality.
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