Design of a High Accurate Aircraft Ground-based Landing System

8/22/2019 Design of a High Accurate Aircraft Ground-based Landing System

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I nternational Journal of Engineering Trends and Technology- Volume4Issue3- 2013

ISSN: 2231-5381 http://www.internationaljournalssrg.org Page 415

Design of a High Accurate Aircraft Ground-based

Landing SystemAhmad Abbas Al-Ameen Salih1,a and Amzari Zhahir2,b

1,2Department of Aerospace Engineering, Faculty of Engineering, University Putra Malaysia

AbstractThe rapid increase in aviation industry requires parallel

effective plans, programs and designs of systems and

facilities nationwide to fulfill the increasing needs for safe air

transportation. Aircraft landing remains a problem for a longtime all over the world. Systems that aircraft rely on in

landing are unreliable to perform a precise guidance due to

many limitations such as inaccuracy, unreliability and

dependency. In low visibility conditions, when pilots areunable to see the runway, the aircrafts are diverted to another

airport. However, low visibility can also affect all airports in

the vicinity, forcing aircrafts to land in low visibility

conditions depending on Instrument Flight Rules (IFR).

Aircraft approach and landing are the most hazardous

portions of flight; accidents records indicate that

approximately 50 percent of the accidents occur duringaircraft landing. Aircraft landing Category III C is not yet in

operation anywhere in the world. It requires landing with no

visibility or runway visual range.

Currently, Global Positioning System (GPS) is the

main navigation system used all over the world for aircraft

navigation, approach and landing. However, in aircraft

approach and landing phase, the accuracy of GPS is not

sufficient to perform a perfect landing due to the possibility

of aircraft to be drifted out of the runway. The accuracy ofGPS could be improved to 3 meter by sending correction.

Improved accuracy has not been able to meet ICAOstandards for aircraft automatic landing. In this paper, aircraftlanding systems characteristics, performance and accuracies

have been studied and compared for the purpose of assessing

limitations and drawbacks. An aircraft landing system with

improved performance is proposed to meet ICAO standards

for all-weather aircraft landing required and recommended

practices with high accuracy to perform full automatic

landing for aircrafts.Keywords: Landing System, Trilateration, Triangulation, Ground-based System, Accurate positioning, ILS, MLS, Positioning, GPS,and DGPS.

1.

INTRODUCTION:1.1. Background:The rapid increase in aviation industry needs parallel

effective plans, programs and designs of landing systems and

facilities nationwide. Generally, in low visibility conditions,aircrafts are always diverted to another airport if the visibility

level is below the allowable limit or when pilots cannot see

the runway. As it approach runway more accuracy is required

since the limit for mismatching the touch point should not

exceed meter level. Aircraft approach and landing are of the

most hazardous portions of flight. Accidents records indicatethat approximately 50 percent of the accidents occur during

aircraft landing (Lisrary & Afb, 2003). International Civil

Aviation Organization (ICAO) has divided landing systems

into three categories according to decision height, visibilityand runway visual range. Category III C is not in operation

yet anywhere in the world because of systematic limitations

of landing systems in service. It requires landing with no

visibility or runway visual range. Currently, the limits of

integrity and accuracy of ground equipments have not been

able to match ICAO standards and recommended practices.

Nevertheless, they are still in use due to the lack of betteralternatives. The main current equipments limitations are:

inaccuracy, unreliability, vulnerability to multipath,

obstruction in signal broadcasting, cause ground service

cognition, lack of integrity and high cost.

In this paper, a ground-based positioning stations

based on concept of trilateration has been designed in order

to reduce or even to eliminate landing systems errors and to

achieve higher accuracy for landing of aircraft can meet CAT

III C. system specifications have been calculated, designedand simulated using Matlab. A simulated design has been

performed using Matlab Simulink to simulate transmissionand reception of data for accurate aircraft positioning and

precise guidance to runway touch point. Positioning error

sources have been considered and eliminated providing all

weathers high accurate aircraft landing system.

1.2. Problem statement:The level of accuracy for aircraft positioning has

been achieved has not been able to match ICAO standard for

CAT III C and it is not sufficient to fulfill aircraft automaticlanding. ICAO favored GPS over MLS, however, GPS is not

highly accurate and has many limitations make such system

not feasible to be used for aircraft automatic landing. GPS

accuracy could be improved by receiving differentialcorrection messages (DGPS) to 3-5 meters. Hence, this error

may lead to drift aircraft out of runway and crash. Beside the

lack of high accuracy, the other GPS limitations are: Satellite

unavailability, Satellite Geometry, Low vertical accuracy,
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Satellite signal broadcasting travel time is longer than signals

transmitted from ground surface, GPS receiver update rate is

low, Signal weakening and degradation, Ionospheric effects,GPS lack of high accuracy. In order to overcome GPSlimitations, even though many systems have been designed to

augment GPS and improve the accuracy, no system can be

relied on to achieve a high accuracy in a range of less than 1

m for high speed applications with high integrity andreliability. Table (1) compares between aircraft landing

systems accuracies.

Table 1: Aircraft landing systems accuracies:Accuracy(feet)Landing system

1200DME with ILS

100DME/P

30GPS

25WAAS

10DGPS

2. Literature review:Aircraft landing remains a problem for a long time all over

the world. Systems that aircraft rely on in landing areunreliable to perform complete automatic landing due to

many limitations. As flying aircraft approaches runway, more

accuracy is required since the limit for mismatching the touchpoint on the runway should not exceed meter level.

Commonly, aircrafts are diverted to alternate airport in low

visibility conditions when the visibility is below the

allowable limit. Until the mid-1950s, only visual landing

procedures were possible. In 1958 the first Instrument Flight

Rule (IFR) landing system developed. Currently, the standard

radio landing guidance system used worldwide is the ILS (Ackland et al., 2003) it was selected by (ICAO) in 1946 asthe international all-weather aircraft landing aid [3]. In order

to overcome the operational and technical problems of ILS,ICAO has formulated guidelines for futuristic system thatwill replace ILS. After evaluation the various systems,ICAO has accepted Microwave Landing System (MLS) for

world-wide use [4]. Both ILS and MLS have many

limitations and they are not highly accurate relatively (Table

2). They are unable to provide navigation service for aircraft

flying in conditions of low visibility [5]. Hence, this is where

a GPS-based landing system has the potential to complement

landing systems, or even replacing them completely. Earlier,

several papers described about GPS-based precision approach

and landing [68]. The studies indicated that GPS was a

revolution never dreamed possible that has many advantages

over other navigation and landing systems.Since the introduction of GPS, most existing MLS

systems have been turned off in North America. FAA

favored GPS over MLS [9]. The major issues with GPS,

guidance accuracy near the runway threshold and the

integrity of the system has not been able to match ICAO

standards practices. GPS integrity and availability was

enhanced by adding differential GPS (DGPS) to support newapplications, such as aircraft precision approach and precise

positioning. Therefore, the accuracy can be improved to

about 3 m. (Brown et al., 1996) performed flight tests to

examine DGPS as CAT III B. Results showed that accuracy

could be achieved are in meter level. (J. H. Rye et al, 2004)used DGPS to increase the accuracy of GPS receiver for low

speed movement. The results showed that positioning errors

were considerably reduced. The maximum distance error was

0.546 m. As speed increases, accuracy decreases.

To improve GPS accuracy, many augmentation

systems have been used such as the Differential GPS, Wide

Area Augmentation System (WAAS) and Local AreaAugmentation System (LAAS). (Lawrence et al., 1996)

designed integrated system of Wide Area DGPS with local

Area Kinematic DGPS to improve accuracy. Results showed

that messages were lost and real time correction messages

were disappointing. [11] studied Real-time GPS navigation

accuracy during approach and landing of ultra-light aircraft

using simulated LAAS. The study indicated that accuracy isimproved from 4.57m to 3.12m in dynamic tests.

Stanford University has developed the Integrity

Beacon as a means of augmenting GPS to provide the

performance required to achieve the specifications of

Category III. Results showed that, all the touchdown pointsare within this 95 percent touchdown box [12]. FAA has

established an evaluation program to test the technical

feasibility of using DGPS based technology for CAT IIIB

precision approach and landing applications. Results

indicated that, all of the touchdowns were accomplished with

the outboard landing gear position less than 145 ft away from

the runway centerline [13]. However, the error of 145 ft is

not acceptable especially in low visibility conditions.In 2012, FAA has published Next Generation

Implementation Plan (Next Generation GPS OperationalControl Segment, 2012). FAA plan to use Ground BaseAugmentation System (GBAS) for civil aviation localaugmentation to support all flight phases including aircraft

approach and landing. In 2011, a contract was awarded to

produce Category II and III LAAS ground facility prototype.

The plan expected capitalizing on satellite technology to

implement landing procedures during periods of low

visibility. By 2012, aircraft can land in low-visibility

conditions. In 2012, at Bremen Airport, DFS Deutsche

Flugsicherung was the first air navigation service provider in

the world to operate GBAS for CAT I precision approachesfor regular air services (Satellite-based landing system

certified, 2012.). By the middle of this decade, certificationfor GBAS operations under all-weather operations CAT II

and CAT III is expected.


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The use of GPS-based system for aircraft landing

did not satisfy researchers and operators as well and the

results have proved the lack of reliability and inaccuracy ofthe system (Mitelman, 2004; Lawrence et al., 1996; David etal., 1995; B. Pervan et al., 1997). Landing CAT III C is not

in operation yet anywhere in the world due to unreliability,

inaccuracy and dependence of used systems. Table 2

compares between aircraft landing systems accuracies. In thisstudy, an autonomous aircraft landing system will be

designed to overcome GPS limitations and provide a better

alternative high accurate landing system.

3. Aircraft Landing System:The civil aviation industry is developing rapidly to occupy

the increasing needs for faster, comfortable and safetransportation. Aircraft landing is a critical phase and high

accuracy in required especially when flying under low

visibility conditions. The zero accident policy announced by

FAA requires airliners to have essentially perfect navigation

from take-off to landing (Aviation Safety Action Plan, 1995).

ICAO has divided landing systems into three categories

according to decision height, visibility and runway visualrange [18]. Category IIIC operation requires precision

instrument approach and landing with no decision height andno runway visual range limitations.

3.1.Aircraft Ground-based Landing System:A perfect navigation in landing phase needs a high accuracy

to enhance safe aircraft landing in all-weather situations.

Currently, no system has the capability to achieve aircraft

landing CAT III C which enables the aircraft to land in all-

weather conditions and when the visibility level is low. Inthis paper, a high accurate ground-based aircraft landing

system specifications will be designed to overcome previous

aircraft landing systems limitations and to achieve highaccurate guidance for aircrafts with improved capabilities and

performance to meet ICAO CAT IIIC.

3.1. System components:

4 signal transmitter. Signal receiver on aircraft with 4 channels. 5 clocks; one in each transmitter (4 transmitters),

and one in receiver.

The figure below shows the system components and the

distribution of transmitters towers around the runway.

Figure (1): Ground-based aircraft landing system transmitters

3.2.System theory of operation:This system is based on a simple mathematical principle

called Trilateration. Four base stations surveyed and locatedprecisely beside the runway are used to broadcast radio signal

to aircrafts receiver (figure 4.1). The system is totally

autonomous. It broadcasts its own signals in specific

frequencies via 4 channels. In aircraft landing positioning, 3dimensions are required (longitude, latitude and altitude).

The use of 3 transmitters provides a positioning with 2dimensions (latitude and longitude) while 4 transmitters

provide a full positioning with 3 dimensions since altitude is

significantly needed for aircraft landing system with high

accuracy.

To locate itself, a receiver must find the distance to four

transmitters of known position. If the receiver finds the

distance from one transmitter, it knows that it must be

somewhere on an imaginary sphere, with the transmitter as

the center a radius of calculated distance between transmitterand receiver. If the receiver can generate spheres for two

transmitters, it knows it can only be located in the surfacesintersection of the two spheres. The two spheres overlap in a

ring of possible receiver positions. By generating a sphere fora third transmitter, the receiver narrows its possible position

down to two points. When the coordination is performed

using three transmitters, the receiver dismisses the point

located in space leaving only one possible position assuming

that the receiver is at mean sea level. By using the fourth

transmitter, altitude can be determined where the fourth

sphere intersects with in one of the two points.

3.3.Aircraft position calculation:A receiver on the aircraft receives the transmitted radio

signals from ground stations transmitters and measures the

time delay that the signal takes between transmission and

reception. Both transmitters and receiver have a precise

clock. The signal transmitted contains information about

signal transmission time, so the receiver uses its clock to

compare time of transmission in the transmitted code withtime of reception to calculate time difference.


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Clocks are synchronized between receiver and transmitter. As

receiver receives signal containing time code, it generates its

time code internally and uses it to compare between twocodes. The receiver determines the range between aircraftand each one of the four stations (figure 2).

Range = (speed) . (time) (1)

Where the speed of the signal is the speed of light which

travels at 299 792 458 m / s and time is the time that thesignal takes between transmitter and receiver. The receiver

then calculates the coordinates (latitude, longitude and

altitude) of the aircraft depending on the ranges to the four

known reference stations by using positioning equations.

Equation (2), (3), (4), and (5).

Figure (2): Time difference calculation

3.4.Positioning equations:Equations of Pseudorange are used to calculate distance to

each signal transmitter. The four ranges are used to calculate

the accurate position of aircrafts.

1= (x1X)2

+(y1Y)2

+(z1Z)2 (2)

2= (x2 - X)2 +(y2 - Y)

2 +(z2 - Z)2 (3)

3= (x3 - X)2

+(y3 - Y)2

+(z3 - Z)2 (4)

4= (x4 - X)2 +(y4 - Y)

2 +(z4 - Z)2 (5)

Where X,Y,Z are the coordinates of ground station and

xn,yn,zn are the coordinates of aircraft.3.5.Receiver equations solutions:Aircraft receiver solves the 4 equations of ranges from 4

stations transmitters simultaneously to calculate the position

of the aircraft accurately. Since the locations of thetransmitting stations are known, they can be used as a

reference points to calculate aircraft position. All ranges to

stations are calculated from time difference of signal

transmission and reception. These values are used by receiver

in pseudorange equation to calculate the position. The four

equations derived from the four stations are used to find out

the values of (x,y,z) of the aircraft.

3.6.System advantages:This system is proposed to overcome the majority of aircraftlanding systems limitations. It has many advantages over

current systems. The main purpose of this study is to design a

system to reduce some positioning errors and eliminate

others. This study aims to meet ICAO standard for CAT III C

which gives aircraft the ability to land in low visibility

conditions and even when the visibility does not exist at alland there is no runway visual range.Compared with the use of GPS for aircraft landing, Aircraft

Ground-based Landing System provides solutions for many

GPS error sources; the ionosphere and troposphere causes the

largest error for GPS signal. Ionospheric and troposphericerrors considerably could be reduced because signal is

transmitted from earth surface and does not travel through

these layers. This will put troposphere and ionosphere out of

the equation.

In addition, this system gives the real and accurate altitude of

the aircraft since it refers to the real height of the runway, not

the mean sea level nor the earth ellipsoid like GPS.Moreover, the signal broadcasted from satellites takes about

0.07 second to reach to earth whereas it takes about 0.000004

second if the signal is transmitted from earth surface. Beside

that, signal transmitted from Ground-based system will be

stronger. It can penetrate construction in vicinity of airport.

Additionally, satellites unavailability due to satellites

distribution or construction blockage is a serious problem incritical stages of landing, this system is designed to beavailable continuously within the approach and landing area.

Also, due to the distribution of satellites the vertical accuracy

is always less than horizontal accuracy. More than that it

causes an accuracy reduction when the visible satellites areall clustered together in a single quadrant, this narrows

satellites visibility angle and consequently reduce the

positioning accuracy. In this system, transmitters stations are

distributed is precisely surveyed location with different

towers height to give a wide visibility angle and provide

accurate altitude (figure 1). Furthermore, this system is

completely autonomous. It does not depend on GPS satellites

such as DGPS where GPS errors are inherited and involvedin calculation, nor any other system. Finally, no need for

sending differential correction messages from DGPS stations.This will eliminate data link problems and errors.

3.7.System specifications calculation:Signal transmitted plays the major rule in the whole operation

of accurate positioning of aircraft and enhances to improve

the safety of landing automatically. A small signal problem

could lead to disaster due to sensitivity of stage.

4.7.1. Covered area:Normally, in Instrument Landing System the Outer Marker

(OM) beacon is located at 7.4 to 13 kilometers from the ILS

threshold to mark the point at which glide slope altitude is

verified or at which the aircraft descent (Terminal Area

Separation Standards: Historical Development, 1997). The

first signal that aircraft receives from ILS is from outermarker when it is about 7-13 km from runway threshold.


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Aircraft Ground- based Landing System is designed to

transmit signal in a range of 13 km from runway threshold

for aircraft approaching and landing.

3.7.2. Earth curvature effect:Earthhas a curvatureat the approximate rate of 6 feet

for every 9 miles [20]. In this study, the distance is

considered small relatively since the system is used

particularly for aircraft approach and landing so the effect of

earth curvature is neglected. The curvature of the earth

affects signal transmission only over a long distance (Figure3).

Figure (3): Earth curvature effects over long distance.

3.7.3. Distance between transmitters:A distance between transmitters is a very important factor

must be taken into consideration in aircraft landing systemdesign. The precisely calculated distribution of towers aroundthe runway provides efficient operation with the best

performance. Figure (4) indicates transmitters distribution

around the runway. As transmitters are separated apart, the

value of PDOP increases providing a better availability and

more accurate positioning. Moreover, the vertical accuracy

will be extremely improved. Overall, the possibility of signal

to overlap will be less.

Figure (4): the distribution of transmitters around the runway

The distance for each of front transmitters from runway

center line is 1 km and for the back transmitters it is 1.5km.

Distance between front transmitters is 2 km and between

back transmitters is 3 km. Distance to runway touch point forfront transmitter is 1 km and for back transmitters is 3.354

km.

3.7.4. Frequency:The travelled signal range depends on frequency of the

signal. This system is proposed to be used for aircraft landing

where the area is considered small relatively, so a very High

Frequency (VHF) band could be used to cover the required

area for aircraft approaching and landing.

The chosen frequency should be in a range where the signalpropagation is not influenced by weather phenomena like,rain, snow or clouds. The characteristics of

VHF propagation are ideal for short distance terrestrial

transmission. VHF radio does not reflected by the ionosphere

and thus transmissions are restricted to the local area. It doesnot interfere with transmissions thousands of kilometers

away. Therefore, it is less affected by atmospheric noise and

interference from electrical equipment than lower

frequencies. Whilst it is easier than HF and lower frequencies

to be blocked by land features, it is less affected by buildings

and other less substantial objects than higher frequencies.

Certain subparts of the VHF band have the same use aroundthe world. Frequency range from 108 to 118 MHz is used for

Air navigation beacons VORand ILS [21]. In this system,

data are broadcasted to receiver via four different channels in

frequencies (110 MHz, 112 MHz, 114 MHz and 116 MHz).

3.7.5. Wavelength:

= v / f (6)In this equation v is speed of light. For VHF range the wave

length is between 1-10 meters. Wavelengths for systemfrequencies are listed in table (2).

Table (2): system channels frequencies wavelengths:

Frequency

(MHz)

Wavelength

(m)

110 2.727

112 2.679

114 2.632

116 2.586

3.7.6. Doppler shift:

Doppler shift is the change in wave frequency andwavelength of observer due to relative movement between

observer and wave source. Scientifically, when the relative

speeds of source and receiver to a medium are lower than the

wave velocity in the medium, received frequency and

emitter frequency could be calculated from the formula:Change in frequency:

f = (v / c) fo (7)

Where: c is the velocity of light, Vris receiver velocity and

Vs is source velocity.

The frequency change in approaching aircraft receiver due to

the relative movement between transmitters and aircraftreceiver could be determined as follows:

For aircraft landing with speed of 100 km/h when sourcefrequency is 110 MHz:

Doppler shift = 36,666.67 HZ = 0.03667 MHz
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Frequency shift is proportional to aircraft speed and also to

transmitter frequency.

3.7.7. Transmitter power:The general rule of transmitter power is it will take four times

the power to double the transmission distance. Transmission

range could be increased by increasing antenna height

without increasing power. Raise the height significantly

increase broadcast distance. Typically, transmission power is

measured in dBm. The greater transmission power, the

greater distances can be achieved. Friis transmission equation(equation 4.15) is used to calculate transmitted and received

power ratio (Pr/Pt). Whereas, equation 10 is used to calculate

the power received.

Pr / Pt = Gt Gr( /4 R)2

(8)

where Gt and Grare transmitter and receiver antennasgains respectively, Pt and Pr are transmitted and received

power respectively, is the wavelength, and R is thedistance.

A typical VHF station operates at about 100,000 watts (80dBm). Transmitter power = 100 KW (100,000 W).

Transmitter power in dB = 10 log 100,000 = 50 dBW =80

dBm

3.7.8. Received power:Power received in dB:

Pr=Pt(dB)+Gt(dB)+Gr(dB) - 20log(4 d/) (9)= - 43.5 dBW = 0.000,0447 W = -13 dBm

3.7.9. The height of the antennas:The height of the antenna is the most important factor to

consider. It is used to calculate covered area. The area

covered could be calculated from the formula:

Distance (km) = 12.746 Am

(10)

Where Am is the height of the antenna in meter. For front

transmitters, the coverage area is about 13 km. so,

13= 12.746 Am (11)Am= 13.259 m

For the two back transmitters, the covered distance includes

the length of the runway. For large aircrafts, the runway is

designed with a length of 3 km. This means for two back

transmitters the covered distance must be taller to cover

approximately 16 km (Figure 5), hence:

16 = 12.746 Am (12)Am = 20.085 m

Transmitter antenna radiates radio wave uniformly and

continuously in all directions. Omni-directional antenna is

used for transmission and reception of signal.

Figure (5): The height of front and back transmitters

3.7.10. Antenna gain: it is a unitless measure that combines

antennas efficiency (Eantenna) and directivity (D):

G = Eantenna . D (13)

In aviation VHF transmission, experiments and theoretical

formula comparison showed that it is a good approximation

for a general model to use the maximum gain of transmitter

and receiver; respectively, they are -4 dB and 2.15 dB (

Roturier & Chateau, 1999; Maschinenbau, 2011).

3.7.11. Path loss:

Free-space path loss is proportional to the distance squaredbetween the transmitter and receiver, and also proportional to

the radio signalfrequency squared. The equation for FSPL is:

FSPL = (4 d / )2 (14)

= (4 d f / c)2

Table (3) and Table (4) shows the FSPL for front and backtransmitters.

Table (3): Free Space Path Loss (FSPL) when range is 13km for differentfrequencies:

Frequency (MHz) Wavelength (m) FSPL (dB)

110 2.727 95.55112 2.679 95.7

114 2.632 95.86

116 2.586 96.01

Table (4.4) determines the level of FSPL when the covered

range is 16 km.

Table (4): Free Space Path Loss (FSPL) when range is 16km for different

frequencies:

Frequency (MHz) Wavelength (m) FSPL (dB)

110 2.727 97.35

112 2.679 97.51

114 2.632 97.66

116 2.586 97.81

In Figure (6), curves indicate the path loss in free space for

150 MHz and 200 MHz. it can be observed that, path is

directly proportional to signal frequency and also to distance

between transmitter and receiver.
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Figure (6): the path loss in free space for 150 MHz and 200 MHz.

3.7.12. Aircraft approaching and landing:The distance from the runway where the aircraft starts to

descend differs for airports. Therefore, the best glide speed

and the glide ratio depend on the aircraft. Normal descents

take place at a constant airspeed and constant angle ofdescent (3-4 degree final approach at most airports) [24]. In

this system altitude determination is referenced to the height

of the runway.

Figure (7): Aircraft altitude, distance to touch point and angle of decent

At a distance of 13km from runway touch point with a

descend angle of 4:

cos 4 = 13/h (15)

h= 13.0178

Sin 3 = altitude/h (16)

Altitude= 0.909 km = 909 mThe determination of altitude according to the descend angleand distance to touch point are summarized in (Table 5):

Table (5): aircraft altitude for 4 degree descend angle:

Distance to touch point (km) Aircraft altitude (km)

13 0.909

8 0.559

5 0.350

3.354 0.235

3 0.210

1 0.070

0.1 0.007

The angle between transmitter aircraft receiver:Calculation done showed that, for aircraft landing with a

maximum angle of decent of 4, antennas angle are: = 3.93

for front transmitter and = 3.166 for back transmitters.

However, the use of omni-directional antenna does not need

antennas to be directed to each other because it radiates radio

waves equally and uniformly in all directions.

3.7.14. System coordination:To indicate an object position precisely, many systems have

been developed for this purpose such as Decimal degrees. As

with latitude and longitude, the values are bounded by 90

and 180 respectively. Typically, latitudes are mentioned

before longitude. The other system used for coordination is

UTM where coordinates are expressed as a distance in meters

to the east and a distance in meters to the north. In UTMsystem coordinates are expressed in meters. In this system, anapproaching aircraft positioning has been simulated using

UTM system providing the transmitters heights and

coordinates with respect to runway. Figure (8). Towers havebeen located around the runway according to calculations

have been made determining transmitters height and

locations.

Figure (8): transmitters coordinates around the runway

As can be seen in the figure, transmitter coordinates are (0, 0,

20.085), (500, 3000, 13.2599), (2500, 3000, 13.259), (3000,

0, 20.085). The z values indicate the height of transmitter

antenna.

3.7.15. Bad weather effect:Landing of aircrafts in a bad weather condition when there is

no visibility requires a perfect positioning with highreliability and error possibility in a range not more than 1 m.

currently, there is no landing system has this capability all

over the world. Beside causing visibility reduction, badweather affects the signals transmitted to aircraft which

means signal delay needs significantly to be taken into

account because an error in a signal of 1/100 second would
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lead to an error in the position determination of about

3,000 km.

Generally, weather radar data is used as an indicator oftroposphere delay differences between two locations. Radardata of reflectivity is a good source to determine the effects

due to weather condition, rain rate and suspended water

droplets. The impact of these delay differences was found to

be less significant and has no effect over signal travel time.Studies showed that the effect of rain could be neglected in

signal propagation especially in low frequency and over short

ranges of transmission.

Researches in GPS indicated that, rain, fog, snow, dust have

no effect on GPS signal, and that is why GPS is considered as

all-weather system. More accurate other GPS studies have

shown that troposphere delay is increased due to the presenceof severe weather fronts and heavy rain. (Gregorius &

Blewitt, 1998) showed that troposphere zenith delay can

increase up to 8 cm when a warm weather front approaches

the receiver. Another study showed the significance of

various tropospheric components on GPS signal accuracy is

(Solheim et al., 1999). It has shown that water vapor, cloud

liquid, rain and sandstorms can induce significant delays.According to the information provided in (Solheim et al.,1999), the delay induced due to water vapor is large

compared to various other atmospheric components and can

reach up to 140 mm/km for vapor diameter less than 10-7

mm. On the other hand, delays induced by clouds are lessthan 8 mm/km and drizzle and steady rain can induce surface

delays of 0.2 mm/km and 2 mm/km, respectively, Also, it is

reported in (Solheim et al., 1999) that water vapor has the

largest contribution in the wet delay. The fog is considered as

another form of rain. The importance of attenuation caused

by fog is minor and could be neglected at frequencies lower

than 2 GHz [27]. Scientists assume that, snow attenuation is

less than rain attenuation falling at the same rate. Radiowaves attenuation caused by hailstones is considerably less

than that caused by rain. Figure indicates that in a heavy rain(200 mm/hour) the distance error is about 20 mm/km(Solheim et al., 1999).

Figure (9): The effect of rain in signal delay over distance

Another researches about VHF ground transmitters

propagation effect in bad weather condition found that, signal

is not affected by the presence of rain as it is similar to freespace propagation. The analysis of the radar backscattermeasured signal delay in different rainfall rates does not

show organized and salient time delays ( Roturier & Chateau,

1999).

3.7.16. System multipath:Previously, many researches have been done intending tomitigate the effect of signal multipath. In GPS positioning, it

is one of the major sources of position error. However, many

techniques have been produced to reduce and even eliminate

the phenomenon. There are a wide variety of mitigation

techniques which employ schemes of data processing.

Progress in Electromagnetics research measurements providean efficient reduction of the multipath to centimeter level and

are widely used. Other typical methods are focusing on

taking advantage of SNR measurements, repeatability of

multipath at ground reference stations. Multiple receivers can

be used to cancel spatial correlated multipath. The results due

to adaptive filtering methods are encouraging and significantreduction of error to centimeter level is observed. A choke

ring antenna is originally designed to mitigate multipath.

Many studies compared between different types of antennas,

results showed that the choke ring antenna has the bestperformance in mitigating multipath.

A more specialized study about the use of VHF foraircraft transmission has been conducted dedicating tointroduce a general model for fading on the aeronautical VHF

multipath channel ( Roturier & Chateau, 1999). The model

designed is based on previous theoretical and experimental

multipath studies developed in the context of VHF Digital

Link (VDL) system validation. Practically, determining

receiver and transmitter antennas radiation prosperity is themain challenge, however, experiments and theoretical

formula comparison presented in ( Roturier & Chateau, 1999)

showed that it is a good approximation for a general model to

use the axisymmetric radiation diagram functions of avertical half-wavelength dipole and maximum gain Gmax of

transmitter and receiver such as defined in [23], respectively,

they are:

20 log 10 (max ground) = -4 dBi (18)

20 log 10 (max ground) = 2.15 dBi (19)

4. Aircraft Ground-based Landing Systemsimulation:

In previous section, Aircraft Ground-based Landing System

characteristics and specifications have been calculated toenable the system to precisely locate the aircraft position and

enhance it to approach and land safely. In this section, systemspecifications will be simulated to test the system

performance and evaluate the positioning accuracy. This


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system will be simulated using Matlab Simulink. It provides

built-in support for prototyping, testing, and running models

on low-cost target hardware. It provides the possibility of

comparing theoretical results with experimental results.

4.1.Simulation design:The operation of broadcasting data from transmitters to

receiver takes place in sequent steps start by generating of

signals and codes (0s and 1s) to be decoded. Each one of

the four transmitters has its unique code which is used by

receiver to identify transmitters individually. After that signalis modulated and broadcasted through channel. All signalsare received by one receiver in aircraft. Signal are

demodulated, decoded and then displayed. Code received is

used to determine transmitters locations and distance to each

one of them. Figure (10) shows the process of datatransmission and reception.

Figure (10): Aircraft Landing Ground-based Systemsimulation blocks4.2.One transmitter simulation:Data transmission takes place from each transmitter to

receiver individually. One Transmitter/Receiver circuit

(Figure 11) is used to examine time delay,transmitter/receiver error correlation and multipath effects.

Figure (11): Data transmission from one transmitter to receiver


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4.3.Time delay:The broadcasted signal contains a code generated bytransmitters. Broadcasted signal includes: The time of

message transmission and transmitter position when message

is transmitted. Receiver receives the signal and generates

internal code which is compared with transmitted code to

determine lag time to each transmitter since each one has its

unique code. Figure (11) shows transmission operation

simulation. (Figures 12.a and 12.b) show the signal pulsegenerated in transmitter and receiver.

Figure (12.a): Transmitted signal pulse

Figure (12.b): Received signal pulse

Time difference between transmitter and receiver specifies

the distance that the signal takes to reach to receiver (Figure

13). Time differences provide a solution for positioning

equations to locate the aircraft.

Figure (13): Time difference between transmission and reception

Determination of distance between transmitter and receiver is

carried out using the received code since receiver and

transmitters clocks are precisely synchronized. Distance is

continuously computed by receiver to solve trilateration

equations and locate aircraft. A very small clock drift leads to

remarkable position error. Matlab time scope block shows

time difference precisely.

4.4.Attenuation and Multipath:Rayleigh and Rician fading channels are useful models of

real-world phenomena in wireless transmission. These

phenomena include multipath scattering effects, time

dispersion, and Doppler shifts that arise from relative motion

between the transmitter and receiver (Loo & Secord, 1991).

The Communications System Toolbox provides a plottingfunction that helps to visualize the characteristics of a fadingchannel. For direct line-of-sight path from transmitter to

receiverRician Fading Channel is used while for one or more

reflected paths from transmitter to receiver MultipathRayleigh Fading Channel is used.

Figure (14): Rayleigh fading channel multipath

Figure (15) shows Rician fading channel multipath. Relative

motion between the transmitter and receiver causes Doppler

shifts in the signal frequency (Figure 16).

Figure (15): Rician fading channel multipath

Figure (16): Doppler spectrum
http://www.internationaljournalssrg.org/http://www.internationaljournalssrg.org/http://www-rohan.sdsu.edu/doc/matlab/toolbox/commblks/ref/ricianfadingchannel.htmlhttp://www-rohan.sdsu.edu/doc/matlab/toolbox/commblks/ref/multipathrayleighfadingchannel.htmlhttp://www-rohan.sdsu.edu/doc/matlab/toolbox/commblks/ref/multipathrayleighfadingchannel.htmlhttp://www-rohan.sdsu.edu/doc/matlab/toolbox/commblks/ref/multipathrayleighfadingchannel.htmlhttp://www-rohan.sdsu.edu/doc/matlab/toolbox/commblks/ref/multipathrayleighfadingchannel.htmlhttp://www-rohan.sdsu.edu/doc/matlab/toolbox/commblks/ref/ricianfadingchannel.htmlhttp://www.internationaljournalssrg.org/


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The Doppler spectrum is a statistical characterization of the

fading process. The channel visualization tool makes

periodic measurements of the Doppler spectrum (blue dots).The Multipath Rician Fading Channel block implements a

baseband simulation of a multipath Rician fading

propagation channel (figure 15). This block is used to model

wireless transmission systems when the signal travels along a

line-of-sight or direct path (Tsai, 1994).Multipath simulation results showed that the

analytical results match with GPS and VHF transmission

systems results such as in (Hannah, 2001; Roturier &

Chateau, 1999; Hoeher, Haas, & Kiel, 1999; Hajj & Young,

2002; Rost & Wanninger, 2012). Multipath is directly varies

from one transmission environment to another. The number

of signal reflected is different changes as the aircraft moves.The corresponding numerical results have been presented has

shown that the analytical and simulation results match quite

well for both static and dynamic environments. Since these

analytical results are optimal among standard transmission

systems, they can be applied to analyze the performance real

aircraft landing system despite of that multipath effects

depend on transmission environment and number of reflectedpaths and this could be different from one airport to another.Nevertheless, it is easily can be modeled and mitigated.

4.5.Transmitters positioning system simulation:As in GPS, in order to determine the aircraft coordinates, at

least, a signal from different four transmitters is required. In

this system simulation, trilateration positioning equations

have been solved using Matlab. Four transmitterscoordinates and distance to each one of them are required to

determine receiver location coordinates. Matlab simulation

showed that distance from three transmitters assumes altitude

is on earth surface so a signal from four transmitters at least

is required. Figure (17) shows two possibilities of altitude forranges from 3 transmitters.

Figure (17) : three transmitters, two possible altitudes.

4.6.Aircraft accurate positioning simulation:Since transmitters are in the same area and they are close to

each other relatively, environmental conditions affect

transmitters equally. Initially, to simplify the explanation of

accurate positioning process, an imaginary system has been

established. Matlab code (figure 18) has been used todetermine aircraft coordinates (x,y,z).

Figure (18): Matlab aircraft positioning code

4.7.High accurate Ground-based Aircraft LandingSystem position calculation error:

Figure (19) indicates the positions on transmitters towers.

Figure (19): transmitters towers positions

Simulation results showed that, when system has an error of20 mm/km from transmitters, the system is feasible; themaximum position error is 4 m. When simulating a landing

system using the same characteristics with error of 1mm/km,the position errors determined in three dimensions does not

exceed 0.2062 m.

Aircraft position has been calculated and simulated taking

into account the position error. Positing errors are the

differences between route coordinates and aircraft position.

The route points coordinates have been calculated according

to distance to aircraft runway touch point and altitude when

the aircraft descending with an angle of 4 degrees. Figure

(20) shows the aircraft approaching and landing to runway

touch point. The aircraft appears as a red dot rounded by a

circle. The dot in the center indicates the route data point orthe accurate aircraft position as there is no position error. The

circle rounded the dot indicates the positioning error since

aircraft could be at any point in the circumference. The

position error identifies the circle radius.


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Figure (20): Aircraft route position and error limits

Generally, the main positioning error factors are: ionospheric

and tropospheric delays, signal multipath, receiver clock drift

and receiver noise. In order to achieve an accurate aircraft

positioning, accuracy reduction factors must be eliminated. Inthis system, weather conditions have no effect due to the use

of VHF band and also because signal traveled distance is

short relatively. The uses of modern techniques reduce

multipath effect to minimum. Researches indicated multipathpositioning accuracy reduction is in centimeter level.Receiver clock drift is corrected by receiving signal from

many separated transmitters clocks. Receiver noise and

receiver malfunctions errors have been reduced using a good

performance and high reliability.

The number and the strength of reflected signals depend on

transmission environment. Signal reflections increase as

the distance between the transmitter and a receiver decreases.

As aircraft approaches, the signal multipath effect increases.

The technology of Trimbles Everest multipath rejectionprovided a high-accuracy solution for positioningapplications. Everest GPS Pathfinder provides up to 50%

higher accuracy than previous systems since multipath

signals are rejected. Another test results from RT2 real-time

positioning system developed by NovAtel indicated the levelof multipath effect is considerably reduced. Dynamically, the

error is 13.3 cm horizontally and 15 cm vertically. Table

(5.8) simulates approaching aircraft positioning with an effect

of multipath of 13.3 cm horizontally and 15 cm vertically.

The reflection of signal depends on the environment of signal

transmission and reception which is totally different from one

place to another, as a consequence, a multipath effect iscontinuously changing as aircraft approaches. Signal

multipath mitigation depends on antenna and receiver design.

In this system, a receiver is expected to have a high

performance and ability to mitigate multipath to low limits.

A receiver noise is another source of error that causes an

accuracy reduction. However, modern and advanced

techniques using integrated hardware and improvedalgorithms reduces this factor to low levels and even it could

be neglected. [34] conducted a research to assess GPS

receiver noise effect. The precision achieved is at the

millimeter level. Studies showed that even for high accurate

positioning, receiver noise can be neglected.

Table (7): Aircraft positioning with multipath effect of 13.3 cm horizontally and 15 cm vertically

Distance to touch

point (km)

Route coordinates

(x,y,z)

Aircraft position

(x,y,z)

Position error

(m)

13 (1500, 16000, 909) (1500.001, 16000.002, 909.02) 0.020

8 (1500, 11000, 559) (1500.02, 11000.05, 559.08) 0.0965 (1500, 8000, 350) (1500.04, 8000.07, 350.11) 0.136

3.354 (1500, 6354, 235) (1500.05, 6354.09, 235.126) 0.163

3 (1500, 6000, 210) (1500.06, 6000.1, 210.13) 0.175

1 (1500, 4000, 70) (1500.087, 4000.1, 70.15) 0.200

Figure (21) shows the relationship between positioning

accuracy and distance to runway touch point. As the aircraftapproaches, the effect of multipath increases and

consequently the accuracy is decreased.

Figure (21): Positioning accuracy as aircraft approaches


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4.8.Ground-based Aircraft Landing System Indicatorsimulation:

ILS indicator simulator has been designed usingVisualBasic.net to simulate data reception and displaying. Asthe receiver receives the signal from ground-based

transmitters, it determines its current location which is

updated continuously. Since the main purpose of this system

is enhancing the aircraft to land with a high accuracypositioning, aircraft drift from route is indicated precisely.

Vertical and horizontal deviations have been displayed in

ILS-style display which has been attached for an easier

indication of the aircraft position with respect to route data.

The vertical deviation has been determined based on the

difference in altitude between determined position and route

data. The horizontal deviation has been calculated usingazimuth between two points.

Route data are recorded in simulator. As a particular route is

chosen, all data regarding destination will be presented

including distance and time to destination and route drift data

as well. System compares between positioning data and route

data to determine position deviation. As system detect any

drift, landing indicator display the amount of deviation. Thedeviation could be either in horizontal or verticalcoordination or in both of them. The system may be

connected to auto-pilot to follow the route data precisely and

land the aircraft automatically. The system determines the

true altitude of the aircraft above the runway unlike GPSaltitude determination which is based on altitude above mean

sea level not above runway. The simulator indicates current

altitude, route altitude and difference between them. As the

aircraft fly above route recommended altitude, the indicator

will be under the center and pilot must decent until indicator

point to center, and vice versa.

4.8.1. ILS indicator:Tepically, in ILS, two independent indicators are used toguide the aircraft laterally and vertically. In order to correct

position error, the pilot needs to center the two indicators; left

and right for lateral indicator, up and down for vertical

indicator. Figure (22).

Figure (22): ILS indicator

Figure (23) explains the changes of ILS indicator as the

aircraft approaches. In point A, aircraft is in left of runway

centerline. Pilot must turn right. In the same point, theaircraft is lower than recommended landing path. Pilot mustgo up. In point B, aircraft is heading into correct coordinates.

The indicator shows the improvement of lateral and vertical

indication. As the aircraft reaches point C, where it is the

correct coordinates, indicator deviation vanished.

Figure (23): ILS indicator deviation

Touch point coordinates (x,y) are (1500, 3000) in UTM

system. When it is converted to decimal degrees, it isequivalent to (-175.475306, 0.0270586283). The simulationshowed that the azimuth between the two points (1500, 3000)

and aircraft coordinates (1500, 16000) is 360. The aircraft is

in a straight light with runway centerline. When comparing

the two points: runway touch point is (1500, 3000) and

aircraft position is in point (1600, 16000), the azimuth

reading between the two points is 0.431. The runway center

line in on the right to pilot; pilot must turn right. Figure

(23.a). When comparing the two points: runway touch point

coordinates: (1500, 3000) and aircraft position (1400,16000), the azimuth reading is 359.55. The runway center

line in on the left to pilot; pilot must turn left. Figure (23.b).

(a) (b)Figure (5.23): ILS indicator

Figure (24) shows the aircraft landing system display.


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Figure (24): Aircraft landing system indicators

5. Conclusion:Aircraft landing is a very critical stage. The risk for aircraft to

be drifted out of the runway or crash building or other aircraft

is high so a superior accuracy is required to guide aircraft to

runway touchdown point precisely. In bad weather conditionswhen visibility is low or runway visual range does not exist,

aircraft rely on full automatic system to land systematically.

Currently, no system is able to provide this reliability all over

the world. In this study a new landing system has been

proposed to reduce and eliminate many aircraft landing

systems limitations. System characteristics and specificationshave been calculated to enhance aircraft to land in all

weathers safely with high accuracy guidance.

Aircraft landing ground-based system provides solutions for

many landing systems problems and reduces some of the

positioning errors sources and eliminate others; inonsphere

and troposphere effects are out of positioning equation.Signal travel time has been shortened from 0.07 second to

about 0.000004 second. The aircraft altitude determination

has become more reliable since it refers to the real height ofthe runway, not the mean sea level nor the earth ellipsoid like

GPS. Beside that, signal transmitted from Ground-basedsystem is stronger. It can penetrate construction in vicinity of

airport. Satellites unavailability due to satellites distribution

or construction blockage is a serious problem especially

within the approach and landing area. The problem has been

dealt with by accurate and calculated transmitters distribution

around the runway with different towers height to give a

wide visibility angle and provide accurate altitude

calculation. Therefore, vertical accuracy became as accurateas horizontal accuracy. The receiver clock drift error can be

corrected by receiving signal from many transmitters since a

separated clock for each transmitter is used. Overall, this

system is completely autonomous. It does not depend on GPS

satellites such as DGPS where GPS errors are inherited and

involved in calculation, nor any other system.

In this system, the only sources of positioning error are signal

multipath and receiver noise. However, studies showed that

Multipath could be mitigated to centimeter level using newtechniques and antennas. In addition, modern and advanced

techniques use integrated hardware and improved algorithms

can reduce receiver noise to low levels. Studies indicated it

could be neglected even for high accuracy positioning since

its effect is in millimeter.

The calculated system specifications have been simulated toevaluate the performance and test the capability of the system

to achieve aircraft accurate positioning. Trilateration

positioning equations have been solved using Matlab. A code

was established to indicate the aircraft position coordinates.

Aircraft approaching and landing have been simulated with

different positioning error sources. However, the real systemspecifications results a position error of 0.2 m as multipath is

in the highest level. Simulations have been done showed that,

the accuracy of the system is in centimeter level.The aircraft positioning simulations have been performed

comparing the system accuracy with current aircraft landingsystems. Results achieved showed that, this system is far

outperforming other systems. This system is 50 times more

accurate than GPS and 15 times more accurate than DGPS.

Some of GPS and DGPS accuracy reduction factors have

been eliminated and others have been reduced resulting a

high accurate all-weather system capable to achieve ICAO

recommended standards for CAT III C to enhance aircrafts to

land blindly.For an easy access and simple indication of aircraft

approaching the runway, ILS style indicator has been

attached to indicate the aircraft position with respect to

runway and runway touch point. The indicator has been


15/15



simulated using VisualBasic.net. It provides an accurate

aircraft deviation from recommended route points

coordinates enhancing the aircraft to land in all weatherconditions. Furthermore, this system can be used to guideaircrafts to the parking lots in the airport. Finally, the

positioning results of simulation showed the performance of

the system. Compared with GPS, the majority of error

sources have been eliminated. Calculations and simulationhave been done showed that visibility, integrity, availability

and accuracy have been considerably increased.

Acknowledgement:We would like to express our gratitude to Universiti Putra

Malaysia (UPM) for granting the Research University Grant

Scheme (RUGS) with Project No. 05-05-11-1560RU, inwhich this research work is made possible.

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Design of a High Accurate Aircraft Ground-based Landing System

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