International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 2

Number 4

December 2008

351

GNSS for an Aviation Analysis Based on EUPOS

and GNSS/EGNOS Collocated Stations in PWSZ

CHELM

A. Fellner, J. Cwiklak, H. Jafernik, P. Trominski & J. Zajac

The State School of Higher Education in Chelm

K. Banaszek

PANSA – Polish Air Navigation Services Agency

ABSTRACT: Under the umbrella of PWSZ Chelm, taking account of future implementation of navigation

using EUPOS and GNSS based on EGNOS several planned actions were carried out in the 2005-2006. The

actions in particular contribute to:

1. ICAO and EGNOS requirements and coverage area (Chelm Town located near Polish-Ukrainian border is

also at the east border of planned EGNOS coverage for ECAC states).

2. Preparatory activities to establishing the EUPOS station in PWSZ Chelm. Cooperation of PWSZ Chelm

and ULC (Polish Aviation Regulator) in the frame of conventional NAV aids use and GNSS

implementation in aviation.

3. Analysis of ICAO requirements and methods of testing SIS (Signal In Space) needed to certify GNSS in

Poland for use for an aviation.

4. Preparatory activities to establishing the EGNOS SIS monitoring station based on EUROCONTROL

Pegasus software and GNSS/EGNOS receiver Septentrio PolaRx2e.

5. Analysis of methods for exchange of information between EUPOS and EGNOS SIS station to initiate the

application of satellite positioning systems to air navigation in Poland.

The project EUPOS is a European initiative aiming at establishment of a uniform DGNSS (Differential Global

Navigation Satellite System) basis infrastructures in Central and Eastern European countries including Chelm

Town where PWSZ is localized playing vital role in GIS/GNSS implementation in the region and Polish

aviation.

1 ICAO STANDARDS FOR GNSS

1.1 Overview of SBAS

SBAS, as defined in the SARPs, has the potential to

support en-route through Category I precision

approach operations. Initial SBAS architectures will

typically support operations down to APV. SBAS

monitors GPS and/or GLONASS signals using a

network of reference stations distributed over a large

geographic area. These stations relay data to a

central processing facility, which assesses signal

validity and computes corrections to each satellite’s

broadcast ephemeris and clock. For each monitored

GPS or GLONASS satellite, SBAS estimates the

errors in the broadcast ephemeris parameters and

satellite clock, and broadcasts corrections. Integrity

me sages and corrections for each monitored GPS

and/or GLONASS ranging source are broadcast on

the GPS L1 frequency from SBAS satellites,

typically geostationary (GEO) satellites in fixed

orbital slots over the equator. The SBAS satellites

also provide ranging signals similar to GPS;

however, these ranging signals cannot be received by

Basic GNSS receivers. SBAS messages ensure

integrity, improve availability, and provide the

performance needed for APV and Category I

precision approach operations. SBAS uses two-

frequency range measurements to estimate the

ranging delay introduced by the Earth’s ionosphere,

and broadcasts corrections applicable at

predetermined ionospheric grid points. The SBAS

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receiver interpolates between grid points to calculate

the ionospheric correction along its line-of-sight to

each satellite. In addition to the clock, ephemeris and

ionospheric corrections, SBAS assesses and

broadcasts parameters that bound the uncertainty in

the corrections. The User Differential Range Error

(UDRE) for each ranging source describes the

uncertainty in the clock and ephemeris corrections

for that ranging source. The Grid Ionospheric

Vertical Error (GIVE) for each ionospheric grid

point describes the uncertainty in the ionospheric

corrections around that grid point. The SBAS

receiver combines these error estimates with

estimates of the uncertainties in its own pseudorange

measurement accuracy and in its tropospheric delay

model, to compute an error model of the navigation

solution.

A system providing GNSS satellite status requires

a few reference stations and simple master stations

that provide integrity only. Providing basic

differential corrections requires more reference

stations and a more complex master station to

generate clock and ephemeris corrections. Providing

precise differential corrections requires more

reference stations in order to characterize the

ionosphere and provide ionospheric corrections. The

four SBASs under development (EGNOS, GAGAN,

MSAS, WAAS) all provide precise differential

corrections. Ranging, satellite status and basic

differential correction functions are usable

throughout the entire GEO coverage area, and are

technically adequate to support non-precision

approaches by providing monitoring and integrity

data for GPS, GLONASS and SBAS satellites. The

only potential for integrity to be compromised is if

there is a satellite orbit error that cannot be observed

by the SBAS ground network and that creates an

unacceptable error outside of the SBAS service area.

This is, however, very unlikely for en-route, terminal

and non-precision approach operations. For a service

area located relatively far from an SBAS ground

network, the number of visible satellites for which

that SBAS provides status and basic corrections will

be reduced. Since SBAS receivers are able to use

data from two SBASs simultaneously, and to use

autonomous fault detection and exclusion when

necessary, availability may still be sufficient to

support approval of some operations.

A State may obtain SBAS service by either:

cooperating with another State (called the SBAS

service provider) that has developed and deployed an

SBAS; or, by developing its own SBAS. A State

might choose the former if its airspace is within the

service provider’s coverage area. It would then have

to negotiate an agreement with the SBAS service

provider covering such aspects as the type of service

and compensation arrangements. A State adjacent to

the SBAS service area could possibly extend the

SBAS service area into its airspace without hosting

any SBAS infrastructure, or it could field reference

stations linked to the SBAS service provider’s

master stations. In both cases the SBAS service

provider’s GEO satellites would broadcast data that

would cover the SBAS service areas of both States.

In any case, it is a State’s responsibility to monitor

the performance of the SBAS within its airspace, and

provide a status monitoring and NOTAM service.

1.2 SBAS-EGNOS

The objective of the EUROCONTROL SBAS

project is to support EUROCONTROL member

States in achieving the operational approval for the

use of GPS augmented by a Satellite Based

Augmentation System. The SBAS system covering

Europe is called the European Geostationary

Navigation Overlay Service (EGNOS). This project

provides a co-ordination platform for all issues

related to the operational validation of SBAS

systems, supporting member States and encouraging

a harmonised approach to operational approval

throughout ECAC. It covers both the operational

validation and the safety assessment.

EGNOS is being developed by the European

Space Agency (ESA) in co-operation with the

European Union and Eurocontrol. The system

provides additional signals to users of satellite

navigation services, broadcast through geostationary

satellites guaranteeing the integrity of GPS so that it

can be used in support of safety-of-life services such

as civil aviation. The various Member States that are

investing in EGNOS intend to offer air navigation

services and operational procedures that make use of

the system. In order to obtain maximum benefits

from EGNOS, operational approvals need to be

achieved as early as possible. The key goal of this

project is to expedite the approval process by

identifying all the tasks that must be carried out, who

should perform them and ensuring that they are

done. Each State offering EGNOS services will have

to go through a safety assessment and operational

approval process. A harmonized approach to

operational approval throughout ECAC will be most

efficient and is preferred. ESA will perform an

extensive EGNOS verification campaign but this

will focus on the signal-in-space as seen by a

network of independent reference stations. Within

the particular environment of an aircraft performing

an operation, ESA will perform demonstrations but

the results of these will not be applicable to the

industrial consortium building EGNOS. As a result

additional validation activities will need to be

performed within the EUROCONTROL SBAS

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project to demonstrate compliance with the EGNOS

Mission Requirements Document for Civil Aviation.

This part of the SBAS project is called GNSS-1

Operational Validation (GOV). The EGNOS Safety

Case Team (ESCT), developing the EGNOS Safety

Case, require the output from the operational

validation activities for use in the assessment of the

safety of operations performed using EGNOS. GOV

will need to provide the evidence that EGNOS meets

all the necessary performance requirements. The

Safety Case will show that the use of EGNOS is safe

for its intended operations based on an agreed set of

assumptions. Whereas the initial focus of GOV will

be on EGNOS, the project will provide knowledge,

experience and tools that will be used to support

future GNSS validation activities for Ground-Based

Augmentation Systems for Category I, II and III

precision approach and landing. The experience will

also contribute to the validation of the second-

generation of satellite navigation systems, in

particular GPS Block IIF and Galileo.

2 ASSUMPTIONS FOR TESTS

According to ICAO requirements it is necessary to

use applicable equipments such as a hardware and

software.

The monitoring station consists of:

− An antenna (PolaNt (L1/L2);

− GPS receiver Septentrio PolaRx2;

− Personal computer;

− Software (RxControl Septentrio).

We use for PolaRx2 instrument mode with 15

dual-frequency GPS channels and 3 single frequency

SBAS channels. We collect measurements on C/A-,

P1-, P2-code and L1-, L2-carrier phase and Doppler

counts in 1 Hz output rate. The 1 seconds output was

decimated to 30 seconds output rate. The time of

measurements is synchronized with true GPS time in

range of 1 ms.

PolaRx2@ is a versatile multi-channel, dual-

frequency GNSS receiver that can be connected to

up to 3 antennas. As part of the PolaRx2 family of

high-end satellite navigation receivers, it uses an

advanced GNSS chipset and tracking and

positioning algorithms, resulting in low noise

performance and hi gh tracking stability.

Fig. 1. The antenna used for tests (mounted on PWSZ building)

Implemented on a single Euro-card size board, it

brings heading/attitude and other multi-antenna

applications within economical and practical reach

with a possibility changing dates via RS 232 in

RINEX. The PolaRx2 receiver was connected to the

PolaNt (L1/L2) antenna with conical radome during

the experiment.

For collecting PolaRx2 data we use the

RxControl program with our own superstructure.

Main disadvantage for continuous running of

receiver on any permanent station is that the

RXControl software doesn’t start logging of

measurement to files automatic after the on-site

computer starts.

Fig. 2. The software used for tests

Fig. 3. The test equipment configuration

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3 MESSAGES FOR ANALYZES

The table mentioned below depicts EGNOS

messages connected with integrity. It is crucial to

analyze 1,38 × 109 trials to carry out a validation of

RNP for the integrity during non-precision

approaches and 1,38 × 109 during precision

approaches Cat. I.

Fig. 4. EGNOS messages

The Horizontal and Vertical Protection Levels

(XPL),which are computed from broadcast EGNOS

messages, to protect users from potential degradation

of the GPS system, expressed in terms of Horizontal

and Vertical Navigation Error (XNSE) above a

certain user level, called the Alert Limit (XAL).

Several cases for the relation between XNSE, XAL

and XPL exist, however, two cases are very

important from a safety perspective:

1 XPL<XNSE<XAL: System is available but not

safe, not leading to a hazardous situation, called

Misleading Information (MI).

2 XPL<XAL<XNSE: System is available but not

safe and leading to a hazardous situation, called

Hazardous Misleading Information (HMI).

Both cases are considered as an SBAS out of

tolerance condition, and are assumed in EGNOS as

non-integrity events. The EGNOS system will

guarantee that the probability of occurrence of those

events is below 2×10

-7

in 150 seconds. Potential

error sources that may provoke these out of tolerance

conditions include:

− Fast and Slow correction / User Differential

Range

− Error (UDRE) mismodelling Grid Ionospheric

− Vertical Delay (GIVD) / Grid Ionospheric

Vertical

− Error (GIVE) mismodelling

− Extensive local errors (multipath and/or receiver

− noise (due to interference))

It is assumed here that the contribution to XPL

out-of tolerance of tropospheric under bounding

errors at the receiver is negligible.

It is important to note that the receiver recorded

some parameters as:

− MI – Misleading Information (XPE>XPL)

− HMI–Hazardous-Misleading Information (XPE>

XAL>XPL)

These parameters have to analyze for each

approach and landing procedures.

4 RESULTES OF EXPERIMENTS

4.1 The monitoring station

It is important to distinguish between the SBAS

coverage areas and service areas. The SBAS

coverage area is defined by GEO satellite signal

footprints. Service areas for a particular SBAS are

established by a State within an the SBAS coverage

area. That is why it is necessary carrying out trials in

Chelm, because this city is situated on the border of

EGNOS coverage area.

The localization of the monitoring station on the

roof of the State School in Chełm is a good choice.

A recorded satellite signal has a very good quality.

The graph mentioned below depicts the accuracy of

the antenna.

Fig. 5.. The recorded antenna positions

EGNOS messages transmitted form PRN120 and

PRN126 satellites were recorded. An accurate

analyze recorded dates will be carried out in the near

future.

The figure 5 presents SkyPlot view with marked

satellite, that transmitted EGNOS corrections.

Fig. 6. The SkayPlot view

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Recorded parameter values as MI – 7 and HMI –

236 show us, that EGNOS service does not meet

requirements for APV, especially near the border of

EGNOS coverage area.

Fig. 7. Misleading Information recorded by the monitoring

station

It has to be underlined that our testing system has

recorded many epoches containg System

Unavailable alarm. This means limiting the

availability of system by 74%. This value is to low

to meet ICAO requirements. That is why initial

evaluation of EGNOS do not allows to qualifies for

APV on the border of coverage area.

Fig. 8. Misleading Information recorded by the monitoring

station

5 DATES RECORDING

We are going to analyze SIS with software called

Mat Lab and PEGASUS. PEGASUS (Prototype

EGNOS and GBAS Analysis System Using

SAPPHIRE) is a prototype which allows analysis of

GNSS data collected from different SBAS and

GBAS systems and using only algorithms contained

in the published standards. The tool has been

developed in the frame of the GNSS-1 operational

validation activity defined in the EUROCONTROL

SBAS project and aims to be a first step forward the

development of a standard processing and analysing

tool to be used for the future EGNOS operational

validation. PEGASUS was designed to facilitate the

output data handling and interchange. The tool

provides several functionalities such as computation

of position and GNSS systems attributes like

accuracy, reliability, and availability simulating

MOPS-compliant receivers, computation of

trajectory errors, prediction of accuracy and

availability with the requiredintegrity and simulation

of GBAS Ground Station processing algorithms.

Thanks to these programs it will be possible to

compute HPL, VPL, HPE, VPE parameters.

6 CONCLUSIONS

It is not possible to verify the appearance of the

facts, described by probability from 10

-7

to 10

-9

using traditional methods of testing the system. It is

obvious to carry out the approach to landing in 10

is not enough for our technical and time-consuming

abilities.

For our references to be possible to deal with

requirements of the GNSS to be registered by

monitoring stations parameters it is necessary to

change them (defined as “for operation” or “for the

time of the flight”) to 1/s.

Taking into consideration, described above, all

the requirements for EGFNOS it is necessary to

mention that the certification of parameters of the

EGNOS is possible only and solely thanks to joining

the methods of systematic analysis using the statistic

survey gained by properly constructed monitoring

stations (such as the one that was tested in PWSZ in

Chelm)

The station in Chelm is design for navigation and

real time position determination with accuracy of3 m

up to 0.5 m, dependent on the used rover station

equipment, providing compressed and encoded

DGNSS correction data via: Internet, GPRS/GSM,

VHF radio/radio broadcast.

356

REFERENCES

Raport z 16-stej plenarnej sesji ICAO AWOP, AWOP/16-

WP/756, 04.1997.

ICAO Załącznik 10 Tom 1 Pomoce Radionawigacyjne.

„Required Navigation Performance (RNP) for Precision

Approach and Landing with GNSS Application” R.J. Kelly

i J.M. Davis, NAVIGATION - Vol.41, No.1, 1944.

Fellner A., „ANALIZA SYSTEMÓW NAWIGACYJNYCH I

KONCEPCJA STACJI PERMANENTNYCH RTK DGPS

DLA POTRZEB LOTNICTWA” - Warszawa 1999

ICAO Załącznik 10 Tom 1 Pomoce Radionawigacyjne –

Rozdział 3 Warunki Techniczne dla pomocy

Radionawigacyjnych.

ICAO Załącznik 10 Tom 1 Pomoce Radionawigacyjne –

Rozdział 3 Warunki Techniczne dla Pomocy

Radionawigacyjnych Tabela 3.7.2.4-1.

Dokumentacja Septentrio: PolaRx2/2e User Manual.