International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 5

Number 2

June 2011

213

1 INTRODUCTION

EPISOL has been operated in two Phases: Phase 1 in

2008 before Athens RIMS installation and Phase 2

in 2010 after its deployment and integration in

EGNOS ground station network. It has been de-

signed and operated in the Aegean Sea to provide

important information about EGNOS performance at

the edge of the system's service area.

Figure 1: Concept of EPISOL project.

More specifically, EPISOL main objective is the

validation of EGNOS relative position accuracy

achieved in the Aegean Sea and, in the sequel, the

demonstration of alternative methods for redistribu-

tion of EGNOS messages in order to overcome

EGNOS SiS coverage limitations. In this frame, re-

sults from both phases concerning system perfor-

mance and conclusions for system improvement and

future applications in the area, have been drawn.

2 EXPERIMENTS DESIGN

As noted, EGNOS performance is mainly related to

the achieved accuracy on the position domain.

Therefore, the project has included a significant

number of trials and collection of a large amount of

data on vessels that sail towards very popular island

destinations of the national cabotage, tactically. All

routes were carefully chosen in reference with the

highly demanding environment of the Aegean Sea

and designed for trials in the open sea, as well as for

trials for canal, coast and port approach navigation.

Additionally, EPISOL analysis presents the achieva-

ble system integrity performance in Greece adapting

EGNOS standards to International Maritime Organi-

sation (IMO) requirements.

EGNOS Performance Improvement in Southern

Latitudes

L. Panagiotopoulou & K. Frangos

Geotopos S.A., Athens, Greece

ABSTRACT: This paper intends to provide results from “EGNOS Performance Improvement in Southern

Latitudes” (EPISOL) project. EPISOL is performed by the Greek company GEOTOPOS S.A. under a con-

tract with the European Space Agency (ESA). EPISOL project aims at analysing, testing and validating the

European SBAS EGNOS (European Geostationary Navigation Overlay Service) performance, by outpointing

advantages and limitations. It has been designed and operated in a very complicated and demanding environ-

ment, the Aegean Sea (Greece, Southern Europe), in which a huge amount of commercial and cruise vessel

routes are scheduled daily. Technically, EPISOL also exploits the possibility of EGNOS data collection

through other means than the direct Signal in Space (SiS), such as SiSNet (Signal in Space through interNet).

Results from this project will form a solid basis towards navigation service improvements and safety en-

hancements for highly demanded maritime applications, providing important information about EGNOS per-

formance at the edge of the system's service area. In this frame, EPISOL includes a significant number of tri-

als and collection of a large amount of data on coasting vessels in the Aegean before and after the operation of

EGNOS Ranging and Integrity Monitoring Station (RIMS) in Athens. As EGNOS data analysis illustrates the

European SBAS performance, arguably well-established GNSS navigation techniques, such as GPS RTK, of-

fer reference trajectories for direct comparisons on the position domain.

214

EPISOL analysis also illustrates the continuity of

EGNOS SiS and the need to complement with other

means of signal transmission. SiSNet combines the

powerful capabilities of SBAS navigation and web

technologies and thus, EGNOS SiS messages are

transmitted via the internet in real time. Figure 1

shows the concept of EPISOL project.

To validate EGNOS performance, the recently es-

tablished Hellenic Positioning System (HEPOS) has

provided GPS RTK reference trajectories with re-

spect to HEPOS network coverage and HEPOS

NTRIP RTCM corrections transmission due to the

local GPRS network coverage limitations.

Map 1: EPISOL trials in the Aegean Sea.

Considering these limitations and in accordance

with the project's demands, Map 1 shows seven

routes to famous Greek islands, that cover a major

part of the Aegean Sea, selected to carry out EPI-

SOL trials.

3 DATA COLLECTION PLATFORM

ARCHITECTURE

EPISOL data collection platform is described in

Figure 2:

Figure 2: System Architecture.

Two individual Septentrio Polarx2e_SBAS

GNSS dual frequency (L1/L2) receivers were in-

stalled on board. The first receiver was logging

SBAS messages transmitted from both PRN 120 and

PRN 126 EGNOS geostationary satellites. PRN 120

broadcasts EGNOS Operational Signal which pro-

vides the fully tested system service and PRN 126

broadcasts EGNOS Test Signal, including the latest

healthy Athens RIMS data in the system's status

configuration. Currently, Athens RIMS is gradually

integrated in the system network and the latest sys-

tem status is continuously tested before its official

broadcast. The second receiver accessed EGNOS

messages exclusively through SiSNet and the SBAS

PVT (Position – Velocity – Time) solution was be-

ing internally calculated by the receiver's software.

In order to avoid lever-arm effects, both receivers

were receiving satellite data from one antenna and

an antenna splitter was splitting the signal to the re-

ceiver antenna ports. Finally, two laptops connected

to a 3G/GPRS modem were offering internet access,

providing HEPOS RTCM corrections for the refer-

ence trajectories and EGNOS messages through

SiSNet server when GPRS network was available.

The data collection period of Phase 1 opened at early

May 2008 and it was closed at mid July 2008, of

Phase 2 opened at May 2010 and it was closed at

mid October 2010 while in each Phase almost 70

hours of GNSS/SBAS measurements at 1 Hz rate

have been recorded.

4 PERFORMANCE ANALYSIS AND

EVALUATION

For the scope of this paper, positioning results using

EGNOS from three different routes are displayed,

considering the criterion of the equal geographical

distribution along the Aegean Sea. Thus, the north-

ernmost Route I, the Route G at the central latitudes

of the Hellenic sea area and the southernmost Route

A are selected. The performance analysis is focused

on different evaluation objectives on the position

domain. The main objective that is common to all

selected routes, is the comparison of the perfor-

mance of the achieved position accuracy for both

EGNOS Operational Signal and EGNOS Test Signal

as transmitted from PRN 120 and PRN 126 respec-

tively. The reference trajectory is the provided

HEPOS RTK PVT solution, as long as the vessel

was sailing within the limits of HEPOS and local

GPRS network coverage. All positioning results

from the selected data sets are compared with IMO

requirements for both accuracy alone and accuracy /

integrity, as well.

215

Table 1: IMO requirements.

Navigation

type appli-

cation

System / Service level parameters

Absolute

accuracy Integrity

Availability

per 30 days

(%)

Horizontal

(m)

Protection

level (m)

Alarm

time

(sec)

Ocean /

Coastal 10 25 10 99.8

Port

approach 10 25 10 99.8

Port

2.5

99.8

Table 1 shows IMO requirements for different types

of navigation applications. The system’s integrity

level is defined from the calculated position protec-

tion limits. Positioning results for EGNOS Opera-

tional Signal from Phase 1 and from Phase 2 are il-

lustrated in the analysis performance of Route G.

Finally, positioning from EGNOS SiSNet is provid-

ed and comparisons between EGNOS SiS Test Sig-

nal results and the relative SiSNet results are dis-

played in the analysis performance of Route I.

5 EGNOS POSITIONING IN SOUTH

LATITUDES: RESULTS

Route A

Table 2: Route A EGNOS Performance on the position do-

main.

___________________________________________________

Route A Performance on the position domain

Heraklio VPE EGNOS HPE EGNOS VPE HPE

Piraeus Test Test EGNOS EGNOS

___________________________________________________

Mean (m) 0.20 2.11 1.05 1.64

Standard 2.26 3.47 5.29 2.08

Deviation (m)

2-sigma 4.71 9.05 11.62 5.80

95% (m)

___________________________________________________

Availability 73.0 12.5

(IMO Req) %

___________________________________________________

As noted, EGNOS Horizontal Position Errors

(HPE) and EGNOS Vertical Position Errors (VPE)

that visualize system’s accuracy performance are

calculated using reference position the RTK PVT so-

lution provided by HEPOS. It is remarked that

HEPOS reference position accuracy is perturbed by

all factors that influence RTK positioning. However,

RTK method (when available) offers considerably,

the optimum navigation solution and especially in

maritime applications it is ideally used for position

error calculations. Table 2 shows system’s accuracy

performance for both EGNOS Test and Operational

Status. The general comment from this Table is that

EGNOS Test signal mean values are more than 5

times less than the corresponding values of EGNOS

Operational and the standard deviation on the verti-

cal direction (height accuracy performance) is 135%

improved. Nevertheless, EGNOS Operational Sig-

nal mean values are 25% improved in comparison

with the corresponding values of the Test signal and

the standard deviation is almost 40% improved hori-

zontally (with relevance to the corresponding RTK

solutions).

Figure 3: Route A EGNOS Test HPE/HPL time series.

Figure 4: Route A EGNOS Test VPE/VPL time series.

Figure 5: Route A EGNOS Operational HPE/HPL time series.

216

Figure 6: Route A EGNOS Test VPE/VPL time series.

Figures 3-6 show the time series of EGNOS HPE

and VPE along with the Horizontal Protection Lim-

its (HPL) and the Vertical Protection Limits (VPL).

Figures 3 and 4 correspond to the EGNOS Test sig-

nal time series plots. On Figure 3 the grey dotted

line represent the HP limit and the black dot repre-

sents the horizontal position error whereas, on Fig-

ure 4 the black dotted line represents the VP limit

and the grey dots represent the vertical position er-

ror. Respectively, Figures 5 and 6 correspond to the

EGNOS Operational signal time series plots. It is ev-

ident that EGNOS Test signal offers larger time

spans of protection limits than EGNOS Operational.

Therefore and in accordance with Table 1 concern-

ing IMO requirements for different navigation

modes, EGNOS Test delivers significantly better re-

sults. Maps 2 and 3 show dynamic plots for EGNOS

Operational and Test respectively, corresponding to

IMO requirements for accuracy alone. Simple dots

are the epochs where HPE is more than 10m, small

circles represent epochs where HPE is less than 10m

(requirements for ocean, coastal and port approach

navigation), while star shapes are epochs that corre-

spond to HPE less than 1m (port navigation).

Map 2: Route A EGNOS Operational plot for IMO accuracy

requirements.

Map 3: Route A EGNOS Test plot for IMO accuracy require-

ments.

Map 4: Route A EGNOS Operational plot for both accuracy

and integrity IMO requirements.

Map 5: Route A EGNOS Test plot for both accuracy and integ-

rity IMO requirements.

217

Accordingly, Maps 4 and 5 are dynamic plots for

EGNOS Operational and EGNOS Test respectively,

that correspond to IMO requirements for both accu-

racy and integrity on different navigation modes.

Simple dots represent positions where Horizontal

Position Limit value is larger than 25meters. Square

shapes are epochs at which the position horizontal

limit is less than 25 meters and horizontal position

error is less than 10 meters at the same time, condi-

tions that cover IMO requirements for open sea,

coastal and port approach navigation. Star shapes

represent epochs where HPL is less than 2.5m and

Horizontal Position Error is less than 1 meter at the

same time. According to Figures 3 and 5 this route

has not had positioning results where HPE is less

than 1 meters and HPL is less than 2.5 meters at the

same time, conditions required for port navigation.

However, as shown on Maps 2 and 3, both Test and

Operational Signal HPE results meet the IMO re-

quirements for port navigation.

Route G

Table 3: Route G / Phase 1 and 2 EGNOS Performance on the

position domain.

___________________________________________________

Skiathos Performance on the position domain

Volos VPE HPE VPE HPE

EGNOS EGNOS EGNOS

EGNOS

TEST TEST

___________________________________________________

Phase 1 2 1 2 1 2 1

___________________________________________________

Average 1.26 0.27 0.89 1.20 1.22 0.30

2.82 2.23

(m)

Standard 2.30 1.56 0.63 0.63 8.88 3.76

15.95 5.21

Deviation

(m)

2-sigma 5.86 3.39 2.47 2.47 18.99 7.22

34.72 12.65

95% (m)

___________________________________________________

Availability 89 98 89 98 16 41 16 41

(IMO Req) %

___________________________________________________

EGNOS positioning results from Route G have

been selected to outline the system’s performance

improvements on the position domain between the

two Phases of the project. Table 3 shows the basic

statistics of the results. VPE EGNOS Test mean val-

ues is more than 4.5 times improved in Phase 2,

whereas the HPE EGNOS Test mean values are 25%

improved as well. The VPE EGNOS Test standard

deviation is 33% improved in Phase 2 and HPE

EGNOS Test standard deviation is on the same level

for both phases. Accordingly, VPE EGNOS Opera-

tional mean values are extremely improved in Phase

2, whereas the corresponding standard deviation is

almost 60% improved as well. Finally, HPE EGNOS

Operational mean values are almost on the same

level in both phases, when the standard deviation is

almost 3 times improved in Phase 2.

Figure 7: Route G Phase 1 EGNOS Operational HPE/HPL and

VPE/VPL time series.

Figure 8: Route G Phase 1 EGNOS Test HPE/HPL and

VPE/VPL time series.

Figures 7 and 8 display the horizontal and vertical

position errors along with the horizontal and vertical

protection limits time series charts in Phase 1. Figure

7 corresponds to the HPE-HPL / VPE-VPL perfor-

mance through time as provided from EGNOS Op-

erational signal and Figure 8 corresponds to the

HPE-HPL / VPE-VPL time series of EGNOS Test

signal.

Figure 9: Route G Phase 2 EGNOS Test HPE/HPL time series.

218

Figure 10: Route G Phase 2 EGNOS Test VPE/VPL time se-

ries.

Figure 11: Route G Phase 2 EGNOS Operational HPE/HPL

time series.

Figure 12: Route G Phase 2 EGNOS Operational VPE/VPL

time series.

Accordingly, Figures 9-12 display the horizontal

and vertical position errors along with the horizontal

and vertical protection limits time series charts in

Phase 2 Figures 9 and 10 show HPE-HPL and VPE-

VPL diagrams of EGNOS Test, respectively. In the

same manner, Figures 11 and 12 show the HPE-HPL

and VPE-VPL diagrams of EGNOS Operational

Signal. Comparing the integrity performance be-

tween the Operational and Test signal, it is obvious

that system’s integrity performance is enhanced after

the installation and deployment of Athens RIMS.

Map 6: Route G Phase 1 EGNOS Operational plot for IMO ac-

curacy requirements.

Map 7: Route G Phase 1 EGNOS Test plot for IMO accuracy

requirements.

219

Map 8: Route G Phase 2 EGNOS Operational plot for IMO ac-

curacy requirements.

Map 9: Route G Phase 2 EGNOS Test plot for IMO accuracy

requirements.

Maps 6-9 are the IMO requirements plots for ac-

curacy alone and correspond to EGNOS Test and

EGNOS Operational signal performance for both

Phase 1 and Phase 2. Same, Maps 10-13 are the

IMO requirements plots for both accuracy and integ-

rity for EGNOS Test and EGNOS Operational signal

performance, accordingly. A close examination of

Map 13 shows that the system performance im-

provement in both accuracy and integrity during

Phase 2 is even clearer.

Map 10: Route G Phase 1 EGNOS Operational plot for both

accuracy and integrity IMO requirements.

Map 11: Route G Phase 1 EGNOS Test plot for both accuracy

and integrity IMO requirements.

220

Map 12: Route G Phase 2 EGNOS Operational plot for both

accuracy and integrity IMO requirements.

Map 13: Route G Phase 2 EGNOS Test plot for both accuracy

and integrity IMO requirements.

Route I

Table 4: Route I - EGNOS SiS and SiSNet Performance on the

position domain.

___________________________________________________

Thasos Performance on the position domain

Kavala VPE HPE VPE HPE

EGNOS EGNOS EGNOS

EGNOS

TEST TEST

___________________________________________________

Phase SiS SiS SiS SiS SiS SiS

SiS SiS

NeT NeT NeT

NeT

___________________________________________________

Average 0.13 0.47 1.12 0.47 8.60 1.01

3.63 1.53

(m)

Standard 3.99 4.00 1.54 1.76 11.90 2.96 4.27

0.81

Deviation

(m)

2-sigma 8.12 8.48 4.20 3.98 32.40 6.93

12.16 3.14

95% (m)

___________________________________________________

Availability 97 97 97 97 15 N/C* 15 N/C*

(IMO Req) %

___________________________________________________

(*N/C – Not Computed)

EGNOS performance analysis in the European

south latitudes includes performance comparisons

between SiS and SiSNet, aiming at the evaluation of

alternative means of receiving EGNOS messages

than the direct satellite signal reception. Table 4 is

displaying SiS and SiSNet performance on the posi-

tion domain. It is obvious that EGNOS Test for both

message reception methodologies perform alike on

the vertical position direction, whereas small differ-

entiations are observed on the horizontal position

accuracy. It is also evident, that the levels of the po-

sition accuracy for EGNOS Operational are ex-

tremely improved using SiSNet, however protection

limits for EGNOS SiSNet Operational were not

available. For this reason Figures 13 and 14 display

the horizontal and vertical position errors along with

the horizontal and vertical protection limits time se-

ries charts, for EGNOS SiS Test signal alone and

EGNOS SiSNet Test signal alone, respectively.

221

Figure 13: Route I EGNOS SiS Test HPE/HPL and VPE/VPL

time series.

Figure 14: Route I EGNOS SiSNeT Test HPE/HPL VPE/VPL

time series.

6 CONCLUSIONS

EPISOL is a project mainly concentrated on the per-

formance analysis of the position domain. Namely, it

is focused on the position accuracy and integrity that

can be achieved using EGNOS for maritime applica-

tions. As the project has taken place in two Phases,

before and after Athens RIMS deployment, using

SiSNet as the alternative means for EGNOS mes-

sages reception, the most important conclusion

drawn by the analysis results is the significant sys-

tem improvement after the RIMS deployment. Actu-

ally, and since RIMS data are gradually integrated

into the system’s new configuration, it is anticipated

that EGNOS accuracy and integrity performance at

the south latitudes, shall further be improved at the

time of the complete integration of the RIMS in the

system’s network. Moreover, it has been proved that

EGNOS SiSNet could equally replace SiS reception

in environments and under conditions that SiS recep-

tion is not available. Finally, it has been shown that

even under the current configuration status, EGNOS

can be used as the primer navigation system for

many maritime applications meeting IMO require-

ments for sea navigation.

ACKNOWLEDGEMENTS

The work presented in this paper has been per-

formed and funded by ESA in the framework of a

contract under "ESA - Greece Incentive Scheme - 1

Call for Ideas".

REFERENCES

EGNOS Performance Improvement in Southern Latitudes (EP-

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EGNOS Performance Improvement in Southern Latitudes (EP-

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2, GEOTOPOS S.A., 2009.

EGNOS Performance Improvement in Southern Latitudes (EP-

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"Data Performance Evaluation" of Phase 2. -GEOTOPOS

S.A., 2011.

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