3. The Low-Latitude Ionospheric Tomography Network (LITN)
The LITN consists of a chain of six stations.
Each station receives and records signals transmitted by the Naval Navigation Satellite System (NNSS). The receiver measures the Doppler shifts of the 400 and 150 MHz signals from NNSS due to the ionosphere,
from which total electron content (TEC) can be deduced.
As presented in Figure 2, it shows the geographic location of the six stations.
The six receiving stations are Manila
(121
°E, 14.6
°N), Baguio
(121
°E,
16.4
°N), Kaohsiung
(121
°E, 22.5
°N), Chungli
(121
°E,
25
°N),
Wenzhou (121
°E,
28.0
°N),
and Shanghai (121
°E,
31
°N).
The chain spans a range of 16.4
°in latitude within
1
° of
121
°E longitude or a distance of more than 1800 km along the surface of the Earth. Geomagnetically, the visible region extends from 25° in the north and to just south of the magnetic equator in the south, with the northern equatorial anomaly region completely nested inside.
Referring to Figure 2, for any given path p at any station, the measured phase difference ? between the signals at the two frequencies is related to the slant
TEC C
s for that path by (Leitinger et al., 1975)
where N
e is the electron density,
F0 the unknown initial phase for a given receiver,and D is a proportional constant.
Only when F
0 is found, can one obtain the absolute TEC from the measured data
Y.
To determine
F0 [Leitinger et al. 1975] proposed a two-station procedure.
Figure 2. A vertical cross section depicting scanning of radio rays as one Naval Navigation Satellite System (NNSS) satellite passes overhead of six receiving stations on the ground.
4. The MART algorithm for CIT reconstruction and their results
With the initial constants determined, the absolute slant TEC can be obtained from the data. We start by considering the slant TEC along any path p between a transmitter and a receiver. For tomographic applications, the TEC along some path p is approximated by a finite sum of segment of the integral
The TEC along some path p is approximated. This is carried out by dividing the two dimensional ionosphere into a set of n pixels and denoting the electron density in the j-th pixel by
x
j. Then for the i-th path, equation (1) can be approximated by
or in matrix notation:
C=AX, (2)
where A is a matrix whose element denote the length of the path-pixel intersections for each path. Note that C and X are column vectors for absolute slant TEC and electron density. A is an m n matrix where m is the number of TEC values from all paths at all receiving sites. The elements in A depend on the geometry of the paths and can be computed once the experimental configuration is fixed. The task of CIT is to invert the equation (2) to obtain the electron density vector X.
