Assessing the Impact of TEC Fluctuations on

ALOS-PALSAR Images

Luca Spogli

1,2

, Elvira Musicò

, Claudio Cesaroni

, John Peter Merryman Boncori

Giorgiana De Franceschi

and Roberto Seu

Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Rome, Italy

SpacEarth Technology, Rome, Italy

Electronics and Telecommunications, Sapienza University of Rome, Via Eudossiana 18, 00184 Rome, Italy

luca.spogli@ingv.it, elvira.musico@ingv.it, claudio.cesaroni@ingv.it, john.merryman@ingv.it,

giorgiana.defranceschi@ingv.it, roberto.seu@uniroma1.it

Keywords: Ionosphere, SAR, InSAR, Total Electron Content, GNSS, Mid-latitude Ionosphere.

Abstract: Trans-ionospheric waves experience delay proportional to the Total Electron Content (TEC), being the

number of free electrons present along a satellite-receiver ray path. TEC is a highly variable quantity,

influenced by different helio-geophysical parameters, such as solar activity, season, time of the day, etc.

Such large variability may lead to TEC spatial and temporal fluctuations over different scales, affecting the

quality of the Synthetic Aperture Radar (SAR) signals and, in turn, limiting further developments of

interferometric techniques, such as InSAR (Interferometric SAR). In the specific, the need of catching

qualitative and quantitative correspondences between TEC fluctuations and InSAR image streaks is a key

point to drive the development of future mitigation techniques to improve the quality of the SAR imaging.

In this paper calibrated TEC values, derived from the RINEX data provided by the RING (Rete Integrata

Nazionale GPS) network of GPS receivers, are analysed to assess the ionosphere conditions during the

ALOS (Advanced Land Observing Satellite) – PALSAR (Phased Array type L-band Synthetic Aperture

Radar) passages over central Italy.

1 INTRODUCTION

The InSAR is among the most used techniques for

geophysical applications, because it is able to

provide high-resolution imaging, independently on

all tropospheric weather conditions (Bürgmann et

al., 2000). SAR signals emitted by ALOS-PALSAR

in L-band frequency range cross the bulk of the

ionosphere, because they are emitted at about 700km

altitude, i.e. in the topside ionosphere. By

consequence, such signals experience changes in

their phase and group velocity induced by the

ionosphere. Changes are proportional to the

distribution of free electrons encountered during

their travel from SAR satellite to ground and back.

The InSAR technique extracts the position and/or

the displacement of a surface by calculating the

phase difference between two SAR images acquired

in different days, but at the same local time and on

the same geometrical area (Ferretti et al., 2007).

Thus, to ensure the performance of the InSAR

imaging and to assess the ionospheric impact on it,

the status of the ionosphere in both passages must be

taken into account, as it is known to be responsible

of the appearance of streaks on SAR co-registration

images. In the specific, the shift of the co-

registration is proportional to the ionospheric TEC

gradients. (Chen & Zebker, 2014).

TEC can be obtained by means of GNSS dual

frequency receivers located at ground. GNSS slant

TEC (STEC) is defined as the total number of free

electrons within a cylinder with a cross section of 1

and height equal to the signal ray path, from

satellite to ground (in the GNSS case). STEC is then

a measure of the integrated electron density Ne

according to the formula:

Spogli L., MusicÃš E., Cesaroni C., Boncori J., Franceschi G. and Seu R.

Assessing the Impact of TEC Fluctuations on ALOS-PALSAR Images.

DOI: 10.5220/0006226700150021

In Proceedings of the Fifth International Conference on Telecommunications and Remote Sensing (ICTRS 2016), pages 15-21

ISBN: 978-989-758-200-4

𝑆𝑇𝐸𝐶 =∫𝑁𝑒

(

𝑠

)

𝑑𝑠,

(1).

where s is the ray path of the satellite-receiver link.

Being GNSS satellites located at an altitude of about

20200 km, TEC measured with suitable ground

receivers provides information also above the ALOS

altitudes. However, the contribution to TEC of the

ionosphere above 700 km is of few %’s (Kelley,

2009).

TEC is a highly variable quantity influenced by

different helio-geophysical parameters such as the

solar activity, the season and the time of the day.

This must be taken into account when considering

the ionospheric conditions of the days to which the

images obtained by InSAR techniques refer. Many

researches about ionospheric effects on SAR have

been carried out, most of which are based on

numerical simulations, use of global ionospheric

models like WBMOD (Meyer & Agram, 2015) or

large-scale interpolated ionospheric maps that do not

have the spatial resolution needed for InSAR

(Hanssen, 2001). Currently, only few studies

investigated the relationship between ionospheric

variability and SAR/InSAR imaging by means of

independent measurements (like GNSS) and, in

general, a final word has not been told about the

ionospheric influence on real SAR images (Zhu el al

2016). For this reason, high-resolution determination

of TEC and of its spatial-temporal fluctuations is

needed. In this work, we show the preliminary

results obtained by comparing ALOS-PALSAR

images of central Italy and TEC measurements

obtained by means of the Rete Integrata Nazionale

GPS (RING, http://ring.gm.ingv.it/) network of

GNSS receivers, managed by the Istituto Nazionale

di Geofisica e Vulcanologia (Italy).

The paper is organized as follows: section 2

presents the ALOS-PALSAR and GNSS data and

how they have been treated to obtain the results,

discussed in section 3. Then, conclusions are

provided in section 4.

2 DATA AND METHODS

2.1 ALOS-PALSAR

InSAR is an imaging technique that evaluates the

pixel-to-pixel phase difference between two SAR

images, acquired over the same area, to produce an

interferogram. In this work, two SAR images (a

“master” and a “slave”) are acquired with a single

receiver in two different epochs with almost the

same incidence angle and on the same area, i.e. in

the so called “repeat-pass interferometry mode”.

Since the orbital positons of the two SAR passages

are slightly different, it is necessary to coregistrate

the master and slave images, with the accuracy of

sub-pixel, to calculate the phase difference of two

corresponding pixels and, then, obtaining high

quality InSAR images. In the present work, we

concentrate on the results of the co-registration,

without looking at the final InSAR product

(interpherometric phase), with the aim to minimize

the tropospheric error affecting phase measurements

(Hassen, 2001) and to focus on the ionospheric

contribution only.

Figure 1: ALOS ascending tracks over central Italy

We consider the azimuth shifts obtained by the

coregistration of three images acquired by ALOS-

PALSAR over Italy. The observed area is that

covered by the ground track 638 of ALOS, which is

located over central Italy. Shifts are estimated by

using the Multiple Aperture InSAR (MAI) method.

The MAI method, based on split-beam InSAR

processing, is able to extract along-track

displacements from InSAR data more efficiently

than the pixel amplitude correlation (Bechor &

Zebker 2006).

Fifth International Conference on Telecommunications and Remote Sensing

2.2 GNSS

As mentioned before, GNSS signals passing through

the ionosphere experience transmission delay

proportional to TEC. Starting from RINEX data,

dual frequencies GNSS receivers are able to

calculate TEC along the slant path, according to the

following formula:

 



















 











 





 

(2)

where 1 (1575.42 ) and 2 (1227.60 MHz) are

the two frequencies of the signal transmitted, 1 and

2 are the corresponding pseudoranges and ε

represents the biases induced by the receiver,

satellite, multipath etc. In order to minimize the

biases, it is necessary calibrate the STEC (Cesaroni

et al., 2015a).

To such scope, the GOPI calibration software

(http://seemala.blogspot.it/2011/04/rinex-gps-tec-

program-version-22.html) has been used. The GOPI

software provides TEC values projected to the

vertical, under the assumption that the ionosphere

can be represented by a single, thin, ionized layer

located at 350 km (Mannucci et al., 1998) that is

suitable for a quiet mid-latitude ionosphere.

Hereafter, we refer to calibrated and verticalized

TEC as TECcal. The projection to the vertical allows

having TEC values not dependent on the position of

the GNSS receivers at ground. Since ALOS

ascending passages of T638 over the area of interest

occurred during night, it is reasonable to assume that

a mid-latitude ionosphere in the nighttime is

“frozen”, i.e. not meaningfully changing, during a

time interval of 5 minutes around the two passages

of ALOS.

To find the portion of the ionosphere crossed by

the SAR signal, some geometrical consideration are

here reported and sketched in figure 2.

The distance D between the point P at ground

and the projection of the point P_i (which is the

intersection of the SAR signal and the ionosphere) at

ground is approximately given by H_i*tan (α)

(Reuveni et al 2015), where the look angle, α, is

related to the incidence angle, θ, through the

following formula:











; (3)

In which Re is the Earth’s radius and H_i is the

altitude of the thin layer approximated ionosphere.

Then, to highlight the difference (if any) between

the ionospheric features in correspondence with the

two ALOS passages, maps of TECcal values have

been considered. To obtain a fine representation of

the ionospheric features as derived from regional

TEC maps, it is necessary to have a very dense

GNSS network. To the scope, the RING geodetic

network has been selected, being composed by more

than 100 receiver spread all over the Italian territory

(Figure 3). TECcal maps boundaries have been

defined to correspond to the SAR images

boundaries, following the geometry described in

Figure 2.

Figure 2: Ionosphere crossed by SAR signal. P_i is the

intersection between the SAR signal and the ionosphere, D

is the distance between the point P at ground and the

projection of the point P_i on the ground, α and θ are

respectively the look angle and the incidence angle, H_i is

the altitude of the thin layer ionosphere.

Figure 3: Location of the GPS stations of the RING

network

Assessing the Impact of TEC Fluctuations on

ALOS-PALSAR Images

In such maps, TECcal are calculated at each IPP

(Ionospheric Pierce Point) for every satellite in view

by each receiver over a 5-minute interval (frozen

ionosphere) centred in the time of the track. TECcal

are then interpolated with the natural neighbour

method (Sibson, 1981). Such interpolation technique

has been selected according to Cesaroni (2015b) and

to Foster and Evans (2008), in which the authors

demonstrate that the natural neighbour interpolation

is the best choice when regional TEC maps are

considered. To avoid border effect due to the

interpolation, maps have been calculated all over

Italy and, then, restricted to fit the SAR image

boundaries, as mentioned before. To evaluate the

TECcal differences between the two passages, the

values of the maps that correspond to the master and

slave days are subtracted and a map of difference

(ΔTECcal) is produced.

3 RESULTS AND DISCUSSION

The variations of the ionosphere between the

different passes of the SAR cause an azimuth shift

between the positions of the pixels of the master and

slave image. This effect, also known as “azimuth

streaks”, influences the optimal coregistration of the

interferometric pair (Gray et al., 2000).

In this work, we concentrate on 3 ALOS-

PALSAR images of central Italy, whose

master/slave dates and parameters are summarized

in Table 1. For each coregistred image, azimuth

shifts have been determined and reported in figures

4 to 6.

Table 1: Images and relative parameters used in this work,

where θm and θs are respectively the incidence angle of

the master and the slave image, Bt stands for the temporal

baseline and Bp stands for perpendicular baseline.

Image #

Master

01/07/2007

16/08/2007

θm (°)

38.7191

38.7395

Slave

16/08/2007

01/10/2007

θs (°)

38.7395

38.7223

Bt(days)

Bp (m)

279.9401

539.2134

259.2523

In all three cases, the shift varies from -3 m to 3

m. By comparing such figures, a strong similarity

between image #1 (fig. 4) and #2 (fig. 6) can be

noticed. In particular, a similar pattern of the shift

are present, but with opposite sign. The sign shift is

because the day 16 August 2007 is used as slave in

figure 4, while in figure 6 it is used as master.

Figure 4: Shift obtained by coregistration of the master

(01/07/2007) and slave (16/08/2007) images

Figure 5: Shift obtained by coregistration of the master

(01/07/2007) and slave (01/10/2007) images

Fifth International Conference on Telecommunications and Remote Sensing

By looking closely to the red strike located at

around 41.8°N in figure 4 (or the corresponding blue

strike in figure 6), it is possible to note that it divides

into 3 smaller sub-strikes.

Figure 6: Shift obtained by coregistration of the master

(16/08/2007) and slave (01/10/2007) images

In Figure 5, the streaks are broader and less

structured than those in Figures 4 and 6. In the case

of Figure 5, 4 large streaks can be identified:

 a positive shift around 43°N,

 a negative shift between 41.7°N and

42.8°N,

 a positive shift between 41.5°N and 41.7°N,

 a negative shift between 40.7°N and

41.5°N.

Since the SAR image of the 16 August is used

both in Figures 4 and 6 but not in Figure 5, the

ionosphere in 16 august 2007 seems to be the

principal responsible for the streaks appearance.

According to Dst (Figure 7) and Kp (not shown)

indices, 16 August 2007 can be considered quiet

from the geomagnetic point of view (red box of

middle panel). Also 1 July 2007 (red box, top panel

of Figure 7) and 1 October 2007 (red box, bottom

panel of Figure 7) can be considered quiet.

In Figures 8 to 10, the ΔTECcal maps

corresponding to the images #1 to #3, respectively,

are reported. All maps presents structures of

ΔTECcal, that are likely responsible for the presence

of streaks in the ALOS-PALSAR images.

Figure 7: Dst index of July, August and October 2007.

(http://wdc.kugi.kyoto-u.ac.jp/dst_final/). Days of the

images are highlighted in athe red boxes

We remind that, to highlight the correspondence

between azimuth streaks and gradients of ΔTECcal,

the ionosphere maps are fitted on SAR area,

according to the method described in Section 2.2. By

comparing ΔTECcal behaviour in the figures, we

can see that the ΔTECcal values show largely

different patterns among the three days, reinforcing

the idea on how variable can be the ionospheric

conditions in different days, even if at mid latitudes.

Figures 8-10 show patterns of ΔTECcal

characterized by large ΔTECcal gradients mainly in

the N-NW direction of all over the maps, and, in

particular, gradients of 1-2 TECu in the latitudinal

range between 42.2°N and 41.5°N, where the

strikes, highlighted in figures 4-6, are present. This

confirm how variation of few TECu’s and below

have a great impact on the SAR imaging.It is

important to note that ΔTECcal map in figure 8 is

obtained by differencing TEC data acquired in the

same season, while the maps in Figure 9 and 10 have

been obtained by differencing summer and fall data.

Considering that the geospace conditions can be

considered quiet for all the 3 days, the seasonal

variation seems to be a key point in understanding

the impact of the ionosphere on the streaks

appearance in the analysed case.

Assessing the Impact of TEC Fluctuations on

ALOS-PALSAR Images

Figure 8: ΔTECcal maps (01/07/2007-16/08/2007) The

color bar shows the differences of the ΔTECcal values in

TECu

Figure 9: ΔTECcal maps (01/07/2007-01/10/2007) The

color bar shows the differences of the ΔTECcal values in

TECu.

Figure 10: ΔTECcal maps (16/08/2007-01/10/2007) The

color bar shows the differences of the ΔTECcal values in

TECu

The spatial scale of the identified ΔTECcal

gradients has been reached thanks to the fine spatial

density of the GNSS receivers at ground. Such

resolution is not reachable with the standard, global,

ionospheric models commonly used.

4 CONCLUSIONS

TEC fluctuation can significantly affect InSAR

applications. To investigate these ionospheric effects

on ALOS-PALSAR images, we analysed 3 maps of

azimuth shift and 3 maps of TEC differences

(ΔTECcal) over central Italy.

The InSAR images present meaningful streaks,

with shift ranging from -3 m to 3 m.

The ΔTECcal maps were obtained by:

 using dual frequency GPS data of the RING

network,

 assuming that the ionosphere is a thin layer

located at an altitude of 350 km above the

Earth’s surface,

 processing the GPS data with a calibration

technique to reduce the errors introduced by

satellite receiver biases

Fifth International Conference on Telecommunications and Remote Sensing

 interpolating the data with natural neighbour

interpolation

In order to show the condition of the ionosphere

in terms of TEC fluctuation in correspondence with

the coregisistration of two L-band SAR images

(Fig. 4-6), the maps of ΔTECcal were projected on

the same area. Results indicate how variations below

1-2 TECu observed with ΔTECcal provides the

appearance of streaks. The map (Fig. 9) in which

 the maximum value of ΔTECcal is larger

than those of the other 2 maps

 the gradient are less and smaller than those

in Figures 8 and 10

corresponds to the case with less strikes.

Furthermore, the results show that the 16 August

2007 seems to be the principal responsible for the

streaks appearance.

These case study suggests how variations of few

TECUs can affect considerably ALOS-PALSAR

images. Further refinement of the work will aim at

compensating for the seasonal variation, aiming at

catching finer ionospheric effects and different

geospace conditions and geographical sectors will be

investigated. A comparison between the results with

the TEC mapping here shown and common

ionospheric models (like GIM, WBMOD) is

currently ongoing. Future refinement of the work

will aim at quantitatively assess the impact of TEC

fluctuation on L-band SAR, by evaluating the TEC

gradient along the azimuth direction. Effect of the

zonal and meridional ΔTECcal gradients will be also

investigated.

ACKNOWLEDGEMENTS

ALOS PALSAR data were provided by the

European Space Agency through Category-1

Proposal 26350. Authors are grateful to Dr. Antonio

Avallone for his support with RING data.

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Assessing the Impact of TEC Fluctuations on

ALOS-PALSAR Images