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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 7, July 2012)

186

A frame Work for the Integrity Analysis of InstrumentLanding System

Ch Mahesh1, Dr. K Ravindra2, Prof. V Kamakshiprasad31Manager (CNS), Airports Authority of India, RGI Airport , Hyderabad

2Principal, Malla Reddy Institute of Technology, Hyderabad3Professor, SIT, JNTU Hyderabad

Abstract As per International Civil Aviation

Organization (ICAO) Annex 10 Standards And

Recommended Practices (SARPs) the Instrument Landing

System (ILS) shall meet the stringent requirement for

Integrity and Continuity services for adequate assurance

against failures. The guidance provided by ILS must be high

Integrity to ensure that each landing has a very highprobability of success. The analysis of transmitter Integrity

and Continuity are the fundamental values for determining

the overall Integrity and Continuity provided by the ILS. This

paper demonstrates the Integrity requirements,

considerations and analysis of ILS of Hyderabad

International Airport Limited(HIAL). Finally, a method of

calculation has been proposed here which is very much useful

for analysis of ILS Integrity.

KeywordsInstrument Landing System, Integrity, MTBF.

I. INTRODUCTIONThe ILS provides precision guidance to an aircraft

during the final stages of the approach. The radiated signalseither can be interpreted by the pilot from the onboard

equipment or input to the flight management system and

Autopilot system. For safe landing of an aircraft, the

available signal in space (SIS) must be accurate. The ICAO

Annex 10 SARPS and DOC 8071 Guidance Material

provide the necessary requirement to be followed [1]. The

radio navigational aid used by the aircraft shall be subject

of periodic ground and flight check. It is required that the

service a provider has to demonstrate their system

operation services will comply the ICAO laid requirements.

Further, the service provider has to demonstrate the four

primary areas: Integrity, Continuity, Availability and

Accuracy.Even though all parameters are critical, here we

analyzed Integrity of ILS only. Integrity is the quality

which is relates to the trust which can be placed in the

correctness of the information supplied by the facility. The

level of integrity of the localizer or the glide path is

expressed in terms of the probability of not radiating false

guidance signals. To ensure the operational integrity of the

ILS facility, it is required continuous monitoring, ground

testing and proper maintenance of the equipment.

So that, the ILS signal in space (SIS) will be within the

specified tolerances. Flight checks verify the correctness of

the setting by correlating the ground monitoring and the

airborne measurements.

II. ILSINTEGRITY REQUIREMENTSIntegrity has been defined as the probability that a

system will not radiate incorrect navigation guidance

information. It is an assurance provided to the user for

corrective information supplied by the facility. Integrity

requirements are based on the requirements of safe

navigation and ability to overcome the potential hazardous

situations. Integrity is needed to ensure that an aircraft on

approach will have a low probability of receiving false

guidance [1, 2]. The acceptable probability of hazardous

event and allowed time delay before warning is issued are

the key integrity parameters. The integrity requirements are

expressed as risk In any one Landing. The risk is the

probability to lose integrity over the time intervalappropriate to the failure mode the time interval is depends

on failure mode being considered and one of the

component for measuring the integrity. It is generally

accepted that the average rate of fatal accident during

landing, due to failure or shortcomings in the whole

system, the aircraft and the pilot should not exceeds 110-

7.This criterion is called Global Risk factor

TABLE I

Integrity and continuity of service objectives

Level Integrity ContinuityMTBO(Hours)

Up to 200 ft

1

Not demonstrated, or less than required for level2

Up to 100 ft

2

11 x 10-7 in

any one

landing

14 x 10-6

in any

period of 15

seconds

1000

Touch down

3

10.5 x 10-9 in

any one

landing

12 x 10-6

in anyperiod of 15

2000

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seconds

Touchdown &Roll out

4

10.5 x 10-9 in

any one

landing

12 x 10-6

in any

period of 30seconds

(LOC)

15 seconds(GS)

4000 (LOC)

2000 (GS)

Table 1 shows the required level of integrity and

continuity of the ILS given by ICAO for different levels

[2].

III. INTEGRITY CONSIDERATIONSAn integrity fails can occur if radiate signal is outside

tolerances and which is unrecognized by the monitoring

equipment or the control circuit fails to take the action [3].

The probability to transmit such false information to the

user is the product of the probability to generate such

information and probability to not detect by monitor and

associated control system. Neverthlessly we can say, the

ability to achieve high integrity depends on the monitoring

system that receives radiated signals as well as the action

taken by the associated control system. ICAO Annex 10,

SARPS given firm guidelines for monitoring of specific

parameters for both the Localizer and Glide Path of the

ILS. The service providers of contracting states should

follow the guidelines given by the ICAO for monitoring the

specific parameters of the system.

There is no unique way to design a system to achieve the

integrity of service as per ICAO. But there are few factors

can be considered for calculations of integrity. They are : 1)

failure rate of the transmitter, 2) failure rate of the monitor

and associated control system, 3) period of time between

checks on the monitor and associated control systems. Even

though the failure of transmitter is one of the component

for calculation of integrity, but very less remote chance of

failure of ground equipment. Due to this, the important

elements are the monitor and associated control systems

which plays important role for the measure of integrity.

IV. ILSMONITORING PARAMETERSTo provide high level of integrity it is necessary to check

the transmitted information in space as well as inside the

equipment. In order that the integrity of the radiated beams

are maintained in operation, the ILS include

comprehensive monitors that alert operational staff when

any changes occurred in monitored parameters.

The specifications and tolerances for the localizer are:

a) The lateral alignment accuracy required forlocalizer is 10.5m (35ft) for Category (CAT) I,7.5m (25ft) for CAT II and 6m (10ft) for CAT III.

This requirement for the mean course line radiated

beam which represent the runway centerline.

b) The required accuracy of Displacement Sensitivitywill be 0.155 DDM with 17% tolerance

c) The output power is not less than 80% for twofrequency system where as the minimum 50% for

single frequency system.

If any deviation will occur in the above specified limit,

the total period of radiation that can be tolerated outside the

performance limit is 10 seconds, 5 seconds and 2 seconds

for the CAT I,II and III respectively . Figure 2 shows thespecified tolerances of the localizer of the ILS.

Figure 2 coverage area of Localizer

.

Figure 3 Glide Path coverage Area

Figure 2 and figure 3 are shows the typical coverage of

localizer and Glide Path [2].

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The elevation guidance accuracy required for glide slope

is for CAT I, II and III respectively are:

a) The mean glide path angle is 075.0 b) The minimum output power is 50% for single

frequency system and 80% for dual frequency

system

c) The required Displacement Sensitivity for GlidePath is 0.175 with tolerance of 25%.

If any fault will occur, the total period shall not exceeds

6 seconds for CAT I, 2 seconds for CAT II and III as

shown in figure 3. The ILS signals are monitored by

Integral (Built-In), near and Far field monitors. Integral and

near field monitors are used for CAT I, II and III of ILS.

Far field monitor located at the far end of the runway to

monitor the localizer signals for CAT III of ILS [4].

The continuously operating automatic monitor system

receives radiated signals from different sensors and

compares the detected information. If any parameter is

outside the specified tolerance, the automatic monitor

system will provide warnings and take the following

actions:

a) Termination of the radiated signalb) Removal of Navigation and Identification

components from the signal

c) Reversions to a lower category in the case of CATII and III.

Normally, the monitor limits of the ground equipmentwill set based on the available signal in space (SOS) which

is carried by the Flight check.

V. INTEGRITY CALCULATIONSThe level of integrity of the system in any landing can be

calculated from the following formula [5]

PI 1 (1)

2121

21

MM

TTP

For

1T 71011

ILS-27 216003.25 21597.15 2160.11

21597.151.330915

10.9999998713

> 71011

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IJETAE_0712_34