International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 2

Number 4

December 2008

345

GNSS for an Aviation

M. Grzegorzewski, J. Cwiklak & H. Jafernik

Air Force Academy, Deblin, Poland

A. Fellner

The State School of Higher Education, Chelm, Poland

ABSTRACT: In Polish aviation on-board GPS units are used for enroute procedures mainly. The use of

GNSS for approach and landing procedures requires overcoming a lot of obstructions, including both

organizational and technical ones. The paper presents information connecting with GNSS implementation in

aviation.

1 OVERVIEW OF GNSS-BASED OPERATIONS

The Global Air Navigation Plan for CNS/ATM

Systems (Doc 9750) recognizes the Global

Navigation Satellite System (GNSS) as a key

element of Communication, Navigation,

Surveillance and Air Traffic Management

(CNS/ATM) systems and a foundation upon which

States can deliver improved aeronautical navigation

services. Standards and Recommended Practices

(SARPs) for the Global Navigation Satellite System

(GNSS) were developed by the Global Navigation

Satellite System Panel and introduced in ICAO

Annex 10, Volume I in 2001 as a part of

Amendment 76 to Annex 10. Guidance material in

Attachment D to Volume I provides extensive

guidance on technical aspects and application of

GNSS SARPs that provided, at the publication date,

for satellite-based en-route through Category I

precision approach operations.

GNSS service can be introduced in stages as the

technology and operational procedures develop. The

staged implementation of GNSS service may be

affected by various factors, including:

− the existing navigation services;

− availability of GNSS procedures design criteria;

− level of air traffic services supporting GNSS

operations;

− aerodrome infrastructure;

− extent of aircraft equipage;

− completeness of relevant regulations.

Depending upon these factors, States may adopt

different implementation strategies and derive

different benefits from the various stages of

implementation.

The introduction of augmentation systems

enhances service and eliminates most limitations.

Based on traffic volume and airspace structure,

States can choose their level of involvement in the

development and implementation of ABAS, SBAS

and/or GBAS. These implementation efforts require

a high level of cooperation among States to deliver

maximum operational advantages to aircraft

operators.

1.1 Operations using Aircraft-Based Augmentation

System (ABAS)

In the early 1990s, many aircraft operators were

quick to adopt GNSS because of the availability of

relatively inexpensive GPS receivers. Operators used

these early receivers as an aid to VFR and IFR

navigation. They quickly saw the benefits of having

a global area navigation (RNAV) capability, and

demanded avionics that could be used for IFR

navigation. The core satellite constellations were not

developed to satisfy the strict requirements of IFR

navigation. For that reason, GNSS avionics used in

IFR operations should augment the GNSS signal to

ensure, among other things, its integrity. The

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aircraft-based augmentation system (ABAS)

augments and/or integrates GNSS information with

information available onboard the aircraft to enhance

the performance of the core satellite systems.

The most common ABAS technique is called

receiver autonomous integrity monitoring (RAIM).

RAIM requires redundant satellite range

measurements to detect faulty signals and alert the

pilot. The requirement for redundant signals means

that navigation guidance with integrity provided by

RAIM may not be available 100 per cent of the time.

RAIM availability depends on the type of operation;

it is lower for non-precision approach than for

terminal, and lower for terminal than for en-route. It

is for this reason that GPS/RAIM approvals usually

have operational restrictions. Another ABAS

technique involves integration of GNSS with other

airborne sensors such as inertial navigation systems.

Many States have taken advantage of GPS/ABAS to

improve service without any expenditure on

infrastructure. The exploitation of GPS/ABAS is a

worthwhile first stage in a phased transition to

GNSS guidance for all phases of flight. Initial

approvals covered en-route, terminal and non-

precision approach operations.

Many service providers have designed new GPS

stand-alone approaches that offer significant benefits

because they can be designed to provide the most

effective flight path to the runway, do not require a

course reversal and provide the pilot with precise

position information throughout the procedure. Most

GPS stand-alone approaches provide straight-in

guidance, so they are considerably safer than circling

approaches. In some States, pilots are authorized to

fly suitable VOR, VOR/DME, NDB and NDB/DME

non-precision approach procedures using GPS

guidance. These are termed “GPS overlay”

approaches and allow operators to benefit from

better accuracy and situational awareness without the

need for the service provider to design a new

approach.

This is seen as an interim step to bring early

benefits to users. Using GPS guidance, pilots follow

the path defined by the traditional NAVAIDs, and

comply with the visibility and minimum descent

altitude associated with the traditional approach.

Some VOR and NDB-based procedures are not

suited to the overlay programme because certain

approach legs cannot be adapted to the RNAV data

coding system. GPS overlay approaches are not ideal

from the pilot’s perspective, because the original

procedure was not intended to be flown using an

RNAV system. An overlay approach should be

removed from State Aeronautical Information

Publication (AIP) when a GPS stand-alone approach

is designed for the same runway to avoid the

potential for confusion between two approaches to

the same runway. Certain operational restrictions

were deemed necessary for the implementation of

GPS-based NPA procedures. The reasons for and

nature of these restrictions varied by State including:

the effects of GPS outages in large regions; the

availability of traditional NAVAIDs as a backup;

traffic density; and regulations for avionics

redundancy.

A common operational restriction is that the pilot

shall not take credit for GPS approaches at an

alternate aerodrome when determining alternate

weather minima requirements. Some States have

also approved the use of GPS as the only navigation

service in oceanic and remote areas. In this case

avionics should not only have the ability to detect a

faulty satellite (RAIM), but should also exclude that

satellite and continue to provide guidance. This

feature is called fault detection and exclusion (FDE).

Under such approval, aircraft carry dual systems and

operators perform pre-flight predictions to ensure

that there will be enough satellites in view to support

the planned flight. This provides operators with a

cost-effective alternative to inertial navigation

systems in oceanic and remote airspace. Some

aircraft with existing inertial navigation systems

have used another ABAS technique which involves

integration of GNSS with the inertial data. The

combination of GNSS FD, or FDE, along with the

short term accuracy of modern inertial navigation

systems provides enhanced availability of GNSS

integrity for all phases of flight. As long as pilots

rely on map reading and visual contact with the

ground, this use of GPS can increase efficiency and

safety.

1.2 Operations using Satellite-based Augmentation

System (SBAS)

An SBAS augments core satellite systems by

providing ranging, integrity and correction

information via geostationary satellites. The system

comprises:

− a network of ground reference stations that

monitor satellite signals;

− master stations that collect and process reference

station data and generate SBAS messages;

− uplink stations that send the messages to

geostationary satellites; and transponders on these

satellites that broadcast the SBAS messages.

By providing differential corrections, extra

ranging signals via geostationary satellites and

integrity information for each navigation satellite,

SBAS delivers much higher availability of service

than the core satellite constellations with ABAS

alone. SBAS, in certain configurations can support

approaches with vertical guidance (APV). There are

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two levels of APV: APV I and APV II. Both use the

same lateral obstacle surfaces as localizer, however

APV II may have lower minima due to better vertical

performance. There will be only one APV approach

to a runway end, based on the level of service that

SBAS can support at an aerodrome. The two APV

approach types are identical from the perspective of

avionics and pilot procedures. In many cases, SBAS

will support lower minima than that associated with

non-precision approaches, resulting in higher airport

usability. Almost all SBAS approaches will feature

vertical guidance, resulting in a significant increase

in safety. APV minima (down to 75 m (250 ft) DH

approximately) will be higher than Category I

minima, but APV approaches would not require the

same ground infrastructure, so this increase in safety

will be affordable at most airports. SBAS

availability levels will allow operators to take

advantage of SBAS instrument approach minima

when designating an alternate airport. An SBAS

approach does not require any SBAS infrastructure

at an airport. SBAS can support all en-route and

terminal RNAV operations. Significantly, SBAS

offers the promise of affordable RNAV capability

for a wide cross section of users. This will allow

States to reorganize airspace for maximum

efficiency and capacity, allowing aircraft to follow

the most efficient flight path between airports. High

availability of service will permit States to

decommission traditional NAVAIDs, resulting in

lower costs.

There are four SBASs being developed: the

European Geostationary Navigation Overlay Service

(EGNOS); the Indian GPS and GEO Augmented

Navigation (GAGAN) System; the Japanese Multi-

functional Transport Satellite (MTSAT) Satellite-

Based Augmentation System (MSAS); and the

United States Wide Area Augmentation System

(WAAS). Geostationary satellite footprints define

the coverage area of an SBAS. Within this coverage

area, States may establish service areas where SBAS

supports approved operations. Other States can take

advantage of the signals available in the coverage

area in two ways: by fielding SBAS components

integrated with an existing SBAS or, by authorizing

the use of SBAS signals. The first option offers

some degree of control and improved performance.

The second option lacks any degree of control, and

the degree of improved performance depends on the

proximity to the service area of the host SBAS.

In either case, the State, which established an

SBAS service area, should assume responsibility for

the SBAS signals within that service area. This

requires the provision of NOTAM information, as

described in Section. If ABAS-only operations are

approved within the coverage area of SBAS, SBAS

avionics will also support ABAS operations and in

fact better meet availability-of-service requirements.

Although the architectures of the various SBASs are

different, they broadcast the standard message

format on the same frequency (GPS L1) and so are

interoperable from the user perspective. It is

anticipated that these SBAS networks will expand

beyond their initial service areas. Other SBAS

networks may also be developed. When SBAS

coverage areas overlap, it is possible for an SBAS

operator to monitor and send integrity and correction

messages for geostationary satellites of another

SBAS, thus improving availability by adding ranging

sources. This system enhancement should be

accomplished by all SBAS operators.

1.3 Operations using Ground-Based Augmentation

System (GBAS)

GBAS ground sub-systems are intended to provide a

precision approach service and optionally may

provide a GBAS positioning service. The precision

approach service is intended to provide deviation

guidance for final approach segments, while the

GBAS positioning service provides horizontal

position information to support 2D RNAV

operations in terminal areas. A ground station at the

airport broadcasts locally relevant corrections,

integrity parameters and approach data to aircraft in

the terminal area in the 108 MHz - 117 MHz band.

A GBAS installation will typically provide

corrections that support approaches to multiple

runways at a single airport. In some cases, the data

may be used for nearby airports and heliports as

well.

GBAS infrastructure includes electronic

equipment, which can be installed in any suitable

airport building, and antennas to broadcast data

broadcast and to receive the satellite signals.

Antenna location is independent of the runway

configuration, but requires the careful evaluation of

local sources of interference, signal blockage, and

multipath. Sitting of the VHF data broadcast antenna

should ensure that the coverage area is sufficient for

the intended operations. The complexity and

redundancy of GBAS ground station installation

depends on the service provided. The cost and

flexibility of GBAS will result in more runway-ends

having qualified electronic precision approach

guidance, resulting in significant safety and

efficiency benefits. Such runways, however, should

meet standards for physical characteristics and

infrastructure.

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2 GNSS IMPLEMENTATION FOR POLISH

AVIATION

The implementation of GNSS operations requires

that Polish aviation authority consider a number of

elements including the following:

− planning and organization;

− procedure development;

− air traffic management (airspace and ATC

considerations);

− aeronautical information services;

− system safety analysis;

− certification and operational approvals;

− anomaly/interference reporting;

− transition planning.

Considering the complexity and diversity of the

global airspace system, planning can best be

achieved if organized regionally and/or in wide areas

of common requirements and interest, taking into

account traffic density and level(s) of service

required. Planning and implementation is a State’s

responsibility within FIRs where it provides air

traffic services, unless States have agreed to jointly

plan services in an area covering more than one

State. Owing to the global nature of GNSS signals, it

is important to coordinate the planning and

implementation of GNSS services to the greatest

extent possible.

While this objective is normally pursued through

ICAO and its regional bodies, it should be

supplemented by bilateral and multilateral

coordination where necessary. The latter

coordination should address detailed aspects not

covered within the ICAO framework. Experience

has shown that the decision to implement GNSS

within States should be made at the highest level and

coordinated regionally within the ICAO Regional

Implementation Planning Groups. Successful

implementation programmes usually involve

cooperative efforts that include all departments

and/or individuals who are affected by the possible

outcomes, who will have the authority for

committing resources to ensure completion of the

programme.

There is a need for users, including air carriers,

general aviation, and the military, to be included in

the GNSS implementation team to allow them to

communicate their specific requirements. Users will

then be able to assist State authorities to develop an

effective and efficient GNSS implementation

strategy. A technical committee could be formed and

given the responsibility for defining requirements

and executing the implementation plan. Team

composition may vary by State, but the core group

responsible for the GNSS programme should include

members with operational expertise in aviation, and

could include:

− Operations (persons responsible for operational

approvals, pilot training, and flight procedures);

− Airworthiness standards (persons responsible for

approving avionics and installations);

− Aviation standards (persons responsible for

developing instrument approach procedures and

developing obstacle clearance criteria, etc);

− Aeronautical information service (persons who

are involved in NOTAM, procedure

− design, databases etc);

− Air traffic services (persons responsible for

developing ATC procedures and controller

− Training);

− Aerodrome operator (persons responsible for

developing aerodrome infrastructure to

− support approach operations);

− Engineering (engineers responsible for the design

of systems and equipment);

− Airline representatives (personnel from flight

operations and flight crew training);

− Other user groups (representatives of general,

business, commercial aviation, unions, as well as

other modes of transport that may use GNSS;

surveyors, GNSS receiver manufacturing

representatives etc);

− Military representatives;

− Other foreign civil aviation or ICAO officials (for

educational purposes).

The plan should identify capabilities that should

be in place in order to meet various requirements for

each approval stage and steps needed for

implementation, and should consider regional and

global planning for CNS/ATM systems. The GNSS

plan should include the development of a business

case. The adoption of CNS/ATM systems has major

economic and financial implications for service

providers and airspace users. Business case

development at the State level is essential in

determining the effect of GNSS and also to choose

the most cost-effective implementation strategy.

It is important to note that there are not regulation

and certification concerning the utility of GNSS in

Polish aviation. The transition to GNSS represents a

significant change for aviation, so it requires new

approaches to regulations, providing service and

operating aircraft. A successful transition to GNSS

requires a comprehensive orientation and training

programme aimed at all involved parties. This

program should keep pace as GNSS evolves. It is

most important that the decision-makers in aviation

organizations receive a broad appreciation of the

capabilities and potential of GNSS to deliver service.

The GNSS transition path and timetable depends

on a variety of factors, so the information provided

349

to decision-makers should evolve accordingly. Staffs

in regulatory and service provider organizations

require background training to be able to appreciate

how GNSS could affect their area of responsibility.

This should include: the basic theory of GNSS

operations; GNSS capabilities and limitations;

avionics performance and integration; current

regulations; and concepts of operation. This should

be followed by job-specific training to prepare staff

to plan, manage, operate and maintain the system.

For many pilots, GNSS represents the first

exposure to avionics that require programming

instead of simply the selection of a frequency. The

wide variety of pilot interfaces dictates a new

approach to training and the certification of pilots.

Aircraft operators should develop manuals and other

documents aimed at assisting pilots to use GNSS

properly and safely. ATC training should include the

application of GNSS to RNAV to ensure maximum

use of this technology.

3 THE GNSS FLIGHT TEST IN POLISH

MILITARY AVIATION

Polish trainer jet called TS11 Iskra equipped with

GPS hardware was used for the flight tests. The GPS

observations were carried out with Ashtech GPS

receivers (Ashtech Z-Surveyor, Ashtech Z-XII), for

EGNOS corrections Javad Legacy receiver was used.

Four GPS reference stations were taking part in the

experiment, located along the aircraft route. The

reliable - reference positions of the aircraft trajectory

were determined as an average of four positions

calculated independently on the basis of every

reference station.

Fig. 1. Vertical plot of the TS-11 Iskra aircraft as a function of

ellipsoidal height and GPS time

RMS

0.00

5.00

10.00

15.00

20.00

25.00

9:21:00

9:26:26

9:31:30

9:36:34

9:41:38

9:46:42

9:51:46

9:56:50

10:01:54

10:06:58

10:12:02

10:17:06

10:22:10

10:27:14

10:32:18

10:37:22

10:43:06

10:49:37

10:59:17

Fig. 2. Plot of mean geodesic co-ordinate errors during descent

The plot illustrates mean geodesic co-ordinate

errors during descent as well, but it is important to

note, that these errors rose, when the aircraft

executed approach descent flight with changing

flight parameters, such as G-force, the angle of pitch,

banking and the value of angle acceleration.

4 CONCLUSIONS

The analyse of ICAO documents concerning GNSS

implementation for aviation and experiences gained

during the experiment allow to draw the following

conclusions:

− differential real time positioning methods (SBAS,

GBAS) are applicable during the approach and

landing phase;

− mean geodesic co-ordinate errors obtained during

the experiment, when corrections based on

EGNOS were available, were only 0.5 m;

− ellipsoidal height parameter was stable and

equalled 2 m, which is also favourable.

− non-precision landing procedures based on

VOR/DME, NDB could be replaced with

procedures based on GNSS, particularly with

SBAS (EGNOS) and GBAS;

− implementation of GNSS will enhance flight

safety and airspace capacity in area and terminal

operations;

− the use of Polish part of the project EUPOS for

Polish aviation is advisable.

REFERENCES

Galotti W.P., The Future Air Navigation System (FANS),

England 1997.

GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS)

MANUAL, ICAO Version 1.0 AN-Conf/11.

Gunnar Frisk: Gate-to Gate Seamless Aviation, Galleo’s

World, Spring 2000).

Grzegorzewski M., Ćwiklak J., Oszczak S, Ciecko A.,

Popielarczyk D. Determination of Aircraft Flight

Wysokość elipsoidalna samolotu TS 11 Iskra w czasie zniżania

500

1000

1500

2000

2500

3000

3500

1 101 201 301 401 501 601

czas [s]

[m]

350

Trajectory with Radar, GPS OTF and EGNOS Positioning,

The GNSS Conference, Rotterdam 2004.

Oszczak S., Grzegorzewski M., Jaruszewski W., Fellner A.,

Wasilewski A. Preliminary Results of DGPS, DGLONASS

Su-22 and TS-11 “Iskra” Aircraft trajectory

Determinations, The GNSS Conference, Edynburg 2000.

Śledziński J., EUPOS - European network of multifunctional

reference stations, Seminar in The State School of Higher

Education, Chełm 2005.