In addition to its air traffic management support systems NAV Portugal participates in an important number of projects underway in the area of traffic management support technologies:

The GNSS project (Global Navigation Satellite System)
At the development phase, the project aims to apply computer and satellite communication technology to air transport. NAV Portugal is one of the Portuguese partners in this joint venture between the European Union, the European Space Agency and Eurocontrol. 
 


INTRODUCTION


According to EUROCONTROL’s navigation strategy for the region covered by the European Civil Aviation Conference (ECAC), in the next few years there will be a gradual transition to air navigation services based on satellite navigation (Global Navigation Satellite System – GNSS).
The current satellite navigation systems, namely those using the American NAVSTAR Global Positioning System (GPS) do not provide the accuracy and reliability normally required by an air navigation system.
With a view to providing additional services and increased service quality, augmentation systems were defined and these have been classified by ICAO according to the way in which they will transmit additional information to users. 
Of particular note among these systems are the Satellite Based Augmentation Systems – SBAS. The SBAS systems are made up of a ground segment which monitors the navigation satellites, transmitting the necessary corrections and integrity information to the user segment, through a geostationary satellite space segment. 
In addition, information is transmitted on the positions and distances of the geostationary satellites allowing these to be used in the determination of a more accurate navigation solution.   
To ensure the inter-operability of all the SBAS systems, the signal transmitted to the users is coded in a similar way to that used in the GPS signals, with the content being defined by the RTCA-DO-229 standard.

In accordance with the performance requisites for air navigation systems defined by the ICAO, the SBAS systems must comply with the specifications for category I (CAT-I) accuracy approaches in any flight phases and in any weather conditions. 
 

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Three regional SBAS systems are currently being developed, all of these are based on an increase in the NAVSTAR GPS navigation group: the American Wide Area Augmentation System (WAAS); the Japanese Multi-Functional Transport Satellite (MTSAT) Satellite-Based Augmentation System (MSAS); and the European Geostationary Overlay Navigation Service (EGNOS).  The development and implementation of the latter is being looked after by the European Commission (EC), by the European Space Agency (ESA) and by EUROCONTROL, and will cover the region covered by ECAC.


To allow preliminary performance evaluation and to enable the development of resources and procedures to be used for its operational validation, a prototype version of the EGNOS system, known as the EGNOS System Test Bed (ESTB) was available between February 2000 and September 2003. The EGNOS is currently being implemented in a gradual manner.
EGNOS should be fully operational very soon. 
 

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EUROCONTROL is coordinating the testing and operational validation of the EGNOS system through the GNSS-1 Operational Validation Working Group (GOV). This working group includes people from European entities responsible for air navigation services which wish to offer air navigation services based on the use of the EGNOS system, namely those belonging to the EGNOS Operators and Infrastructure Group (EOIG) of which NAV Portugal is a part.

OBJECTIVES


For the testing and validation of the EGNOS system as an air navigation system, NAV Portugal has requested the collaboration of the flight test working group of the “Instituto Superior Técnico” (Higher Technical Institute of the University of Lisbon - GTEV/IST) which is associated with the Portuguese Air Force (“Força Aérea Portuguesa” - FAP), namely through Squadron 504 (ESQ504). All the information obtained was been shared with other European entities involved with the same type of work through the ESTB Data Collection Network which GTEV/IST is a part of.

The testing and validation of the EGNOS system involves static and dynamic measuring campaigns with the carrying out of in-flight tests. With the static measuring tests it is sought to validate the models established for the EGNOS system. In Portugal these tests are normally carried out at Lisbon International Airport; on some other occasions they are carried out at places considered as being relevant, such as Madeira and the Azores. The in-flight tests allow correlation between the static test data and those actually obtained for an aircraft in flight.

TEST PROCEDURES


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The first data collected in the static or dynamic measuring campaigns come from the GPS constellation and the EGNOS system. This collection of data is done by using suitable NovAtel OEM3 and Septentrio PolaRx-2 receivers linked to the respective antennae and using computerised recording means. The data processing is done in deferred time using a programme which emulates an EGNOS receiver called WinGPSAll; the development of this software is the responsibility of EUROCONTROL. The evaluation of the performance of the EGNOS system, resulting from the analysis of the results obtained, involves several parameters which are presented below.

Accuracy, which is quantified by the Navigation System Error – NSE, defined as the difference between the position corresponding to the EGNOS navigation solution and the “true” position of the antennae, being expressed by the Horizontal Position Error – HPE and the Vertical Position Error - VPE. The requisites imposed by the ICAO for CAT-I operations are 16 metres for the HPE and 4 to 6 metres for the VPE, depending on the flight phase.


Integrity, which is defined as the capacity to protect the user, in a timely manner, from inaccurate navigation solutions.  This is quantified through the Horizontal Protection Level – HPL and the Vertical Protection Level – VPL. The requisites imposed by the ICAO for CAT-I operations are 40 metres for the HPL and 10 to 15 metres for the VPL, depending on the flight phase.


Continuity, which measures the system’s capacity to operate without interruptions occurring in the services supplied.


Availability, which is defined as the percentage of time in which the services provided by the navigation system are within the required performance limits. Under the requisites imposed by ICAO, for a satellite navigation system to be useable for a certain type of operation the corresponding availability level must be greater than 99%.

In addition, analysis of all faults found in all the measurement campaigns are analysed. 
A Dassault Falcon 20 aircraft operated by ESQ504 is used for the in-flight tests.  The same aircraft is normally used for the verification and calibration of the navigation aids. For this purpose two antennae and receivers, plus the necessary data collection resources, were installed on this aircraft.
As referred to earlier, to analyse the performance of the EGNOS system it is necessary to know accurately the “true” position of the antenna of the aircraft used. For this purpose various complementary methods are used: the differential correction of the data obtained is done using information obtained in GPS reference stations installed on the ground and the Trimble Total Control (TTC) programme; an accurate navigation solution is generated on the aircraft using another GPS receiver with differential real time correction, through a UHF data link and a ground reference station; and data obtained from the instrumentation console installed on the Dassault Falcon 20 aircraft using an on-board data acquisition system. The use of this instrumented aircraft is also of major importance for testing the EGNOS prototype receivers.  The Septentrio PolaRx-2 receiver which has been used is an example of this.
 

RESULTS OBTAINED ​AND CONCLUSIONS 


All the measurement campaigns carried out up to the present have involved the use of the ESTB system which, in spite of having all the characteristics of the EGNOS system, is limited since, for example, its coverage is insufficient on mainland Portugal and, particularly, on Madeira and the Azores. Nonetheless, the results obtained show that the requisites imposed by ICAO for CAT-1 operations are virtually achieved on mainland Portugal. With the carrying out of the in-flight tests it has been possible to verify that, in general, the performance data obtained in flight are better than those obtained on the ground.
In addition to the obtaining of results, the measurement campaigns undertaken have enabled the development and establishment of the means and procedures which will be adopted for the operational evaluation of the EGNOS system.


With the progressive development and implementation of the EGNOS system, the results obtained with the ESTB lead us to believe that the objectives set will be achieved.
We are now finalising the methodology to be adopted to allow the validation of the EGNOS system as an aerial navigation system. This will be based on the permanent collection of data during one year, in several locales in Europe.  One of these locales will be Lisbon and this work will be under the joint responsibility of NAV Portugal, IST and the FAP.