Sarajevo Airport is located 8 km northwest of the city center. The airport is surrounded by high mountains, except in the northwest, where there is an open field towards the Bosna River valley. This terrain configuration prevents the warm Mediterranean air from entering the valley, which is why we have a pronounced continental climate. The airport is in the vicinity of the slopes of the mountains, and in the winter months, the cold air descends into the valley which causes fog formation. Besides, there are also many rivers with their tributaries, which contribute to increased humidity.
With the significant prevalence
of anticyclone and temperature inversion, low ceiling and fog are present,
which are particularly pronounced between November and the beginning of March.
The most frequent type of fog is radiation fog. Fog development is strongly
associated with aerosols condensations in the air, especially in environments
where we have a large number of pollutants in winter months. The most common
winds are NW-SE and NNW-SSE. Surrounding terrain has a profound influence on
wind direction and intensity. These winds do not significantly affect take-off
and landing operations, however, they do affect the intensity and duration of
the fog, as they allow air stagnation and cooling. All mentioned weather
conditions cause a significant decline in the number of operations during
winter months, especially in the morning and evening hours, when visibility and
ceilings are low. In addition to poor visibility, Sarajevo Airport has issues
with other meteorological events such as heavy snowfall and intense winds from
south (Adriatic Sea).
2.
Current ILS CAT I at Sarajevo International Airport
The first thing we need to analyze is the meteorological parameters that affect traffic operation. As shown in Fig. 2, we see Runway Visual Range data (RVR) visibility data, which is updated hourly. Data shows that the significant low visibility occurrences are most pronounced in the morning hours when we usually have traffic rush hours. Data were taken between 2014 and 2020. We analyzed months of December and January when we had major operation cancellations due to low visibility. The line represents the number of Meteorological Aerodrome Reports (METAR) for a given period and the horizontal axis represents 24 Universal Time Coordinate (UTC) hours’ time frame.
Sarajevo Airport is also certified
for Low Visibility Procedures (LVP). The introduction of LVP procedures allows
the operation to be performed on conditions that are less than the minimum
conditions for CAT I category, primarily concerning RVR parameters. At Sarajevo
airport, Runway (RWY) 29 is suitable for non – guided Low Visibility Take Off
(LVTO), however, for RWY 11 only visual departure is possible. The fixed
minimum required RVR value for LVTO at Sarajevo airport is 200 m for non-guided
LVTO and only conducted when RVR is below 400 m (BHANSA, 2015).
Currently, Sarajevo Airport is
equipped with Thales’ CAT I ILS/DME, (type ILS 381), with identification „BHS“,
Localizer frequency 110.7 MHz / Glide Path (GP) Frequency: 330.2 MHz with
Distance measuring equipment (DME) on Channel 44x, ILS, co-located with the
glide path antenna. The middle marker is located at 900 m from the runway
threshold 11 and the GP approach angle is 3.2 degrees. All Standard Instrument
Departures (SID) from Sarajevo require aircraft to maintain higher than normal
climb rates until above the surrounding terrain. This requirement is introduced
to manage the descent of arriving aircraft to accommodate a suitable profile to
safely establish on the ILS runway 11 (BHANSA, 2015). Due to surrounding
mountainous terrain, a higher approach angle is needed for landing and missed
approach procedures. One limiting factor for higher ILS category implementation
is the pre-threshold terrain of runway 11. There is a defined distance before
the runway threshold, which must be obstacle-free for normal radio altimeter
operation.
The ILS system is installed in
the runway close vicinity and is subjected to multipath effects which place
restrictions on further airport infrastructure development and also limit the
aircraft movements. Spacing under low visibility conditions must be
significantly increased due to potential reflections or distortions of the
guidance signals by preceding aircraft. Due to the ILS signal protection,
sensitive areas become larger and aircraft entering the runway areas are
required to hold on the CAT III holding points as opposed to CAT I holding
points, which are closer to the runway. This results in restricted ground
movements and greater final approach spacing margins between aircraft to
accommodate the subsequently longer Runway Occupancy Times (ROT). As shown in
Fig. 3, higher than ILS I category will require creating larger sensitive areas
in already limited space, due to residential buildings in the pre threshold
terrain. Another important factor is a current approach lighting system that
has to be upgraded as a requirement for higher precision categories. The
current system is Approach Light System, with Sequenced Flashing Lights -ILS
Cat-I (ALSF -I) configuration, as shown in Fig. 3. Essentially, for higher ILS
categories the lights should be installed inside the urban area in the
pre-threshold area of the runway 11, which is another limiting factor.
Higher ILS category enables lower minimums, however, the problem would appear in the Missed Approach segment. The Missed Approach Climb Gradient (MACG) would have to be larger, which is restrictive since we have high terrain immediately after the runway in the missed approach procedure. Another issue of Sarajevo Airport is traffic nature, take-offs and landings are conducted in the opposite direction (departing from runway 29 and landing on runway 11), which significantly reduces the runway capacity. To look at the benefits of implementing a GBAS system, it is necessary to look at the factors that affect visibility. Based on the number of flights cancelled due to reduced visibility, we can conclude that the highest number is in December. Figure 4 shows all cancelled flights that match meteorological conditions where visibility was below 1000 .m and the ceiling was overcast or broken below 1.000 ft. On the right figure, we notice a significant number of cancelled flights during the morning, afternoon, and evening hours.
2.1.
Possible Installation of Higher ILS Categories
To introduce higher ILS
categories, it is necessary to have certain conditions on the runway and in the
pre-threshold terrain profile. While terrain under the approach path should be
relatively leveled concerning the runway surface, there will usually be irregular
contours ahead of the threshold. Radio altimeters operate on the principle of
measuring the time interval required for electromagnetic waves emitted from
aircraft to reach the ground, bounces back, and return to the aircraft. Due to
the importance of radio altimeters function in determining the decision height,
several conditions must be met. The terrain should be free of significant
obstacles to ensure the proper function of the radio altimeter during a
critical phase of descent, from a distance of at least 3000 ft ahead of the
threshold, and a width of 100 ft on either side of the extended runway
centerline. Gentle terrain changes may be acceptable up to +-5 ft, or isolated
sudden changes up to 10 ft. However, any repetitive abrupt changes should be
restricted to less than 3 ft in distance and preferably should be avoided.
Single buildings of heights up to 10 ft can be tolerated if their length
is less than 50 ft measured parallel to the centerline. We can conclude that
the Sarajevo airport pre-threshold area has a significant number of obstacles
in the form of solid buildings, which certainly disrupt a normal radio
altimeter operation. If the airport does not meet those conditions additional
analysis should be conducted by the responsible aviation authorities. Another
requirement is related to the height at the point where Decision Height DH is
reached. DH, in this case, will be the minimum above the highest obstacle in
the first 3.000 ft of the runway, but the radio altimeter will, at this point be
on the glide path, measuring the height distance immediately below the aircraft.
To determine the required radio altimeter reading when DH is reached, profile
charts are required, which provide elevation information for the first 3.000 ft
of the runway.
Knowing the glide path angle and the glide path intercept point, the required reading on the radio altimeter at DH/alert height can be calculated. Following ICAO Annex 4, Chapter 6 every airport should have a pre-threshold terrain profile chart. Using the data from Sarajevo Airport, Table 1 represents the monthly operations for existing CAT I and corresponding possible CAT II/III. It is obvious, that for arrival operation, with CAT II/III condition all operation would be possible.
3. Characteristics of the Ground-based Augmentation System (GBAS)
GBAS uses the concept of
differential corrections to augment satellite signal enabling precision
approach up to Category CAT III . The GBAS Approach Service Type (GAST)
describes the level of usebility provided by an individual station, in a
similar way of ILS categories. GAST-C is intended to support precision approach
operations in CAT-I minima. GAST-D concept was developed by ICAO Navigation
System Panel (NSP) to allow GBAS to support CAT II/III approach and landing
operations using GPS L1 frequency. Although, ICAO has only issued Standards and
Recommended Practices (SARPs) for GBAS operating over single frequency and
single constellation up to CAT I operations. EUROCONTROL has developed several
documents to support GBAS CAT I and initial GBAS CAT II/III Air Traffic Management
(ATM) requirements based on ICAO standardization work.
GBAS ground station consists of
multiple reference receivers with their antennas installed on previously
surveyed locations in the vicinity of the airport. The information in the
receiver is sent to a processor that computes the corrections for each
navigation satellite in view and broadcasts these differential corrections, via
a VHF Data Broadcast (VDB). The broadcast information is received by the
aircraft together with received information from the navigation satellites.
Using differential correction with received satellite information the position
is accurately calculated. Current certified GBAS airborne receivers and
ground-based installations are limited to CAT-I operations, i.e., precision
approach procedures down to decision height (height above ground at which the
cockpit crew must see either the runway or at least the runway approach lights)
of 200 ft.
4. Open
Issues with GBAS Usage
According to (ICAO, 2018),
several issues must be taken into consideration to use satellite navigation for
aircraft guidance. Faults and space weather have a great impact on the
integrity and accuracy of the position solution, whereas constellation and
frequency interference affects the availability of the service. Due to the
nature of air traffic where operations take place in very short time intervals,
in critical phases, the crew under the high workload does not have time to solve
navigation problems or switch to alternative ones. If the crew notice any
deviations in navigation systems indicators, they will immediately cancel the
approach procedure. A particular limitation is the low power of the GNSS signal
and consequently is prone to jamming and spoofing.
4.1.
System Errors
Errors could occur due to system imperfections both on satellite vehicles and in the aircraft itself. Faults within a Global Navigation Satellite System (GNSS) could be attributed to clock runoff, where the signal broadcast by a given satellite is not properly synchronized to the signals from other satellites in the constellation. Each GNSS has its timing system, and hence, some intersystem clock biases should be considered when dealing with a multi-constellation system. Other faults proposed have been due to upload of faulty navigation data from the GPS control segment to the GPS satellites, or from unannounced maneuvers that render the broadcast satellite position invalid.
4.2.
Space Weather
Space weather can generate severe
ionospheric disturbances. A significant concern for GBAS is the possibility
that very large ionospheric gradients could cause a large spatial error
decorrelation and thus induce differential position errors for arriving
aircraft. The sensitive monitoring for ionospheric gradients within the ground
station is challenging due to strong limitations on the spacing between
reference antennas. For this reason, many airports will have placing problems
due to limited free suitable areas. Another dangerous phenomenon that can
affect the system is solar flares, explosions, which appear on the Sun’s
surface. The frequency of solar flares occurrences coincides with the 11-year
solar cycle. Intense data monitoring during and near the solar cycle peaks is
essential for system analysis to get an insight into the real effect on GNSS
precision.
Shown in Fig. 5, an ionospheric
activity can vary significantly depending on geographic longitude and latitude.
As reported in a study, Brazil is located under the geomagnetic equator, which
implies that Brazilian territory is under harsh ionospheric disturbances. The
amount of error introduced by these anomalies can be much greater than the
correction broadcasted in the GNSS navigation message, which increases the
urgency for assessing the impact of these errors in any satellite-based
aeronautical navigation system.
Sarajevo Airport is located in
the mid-latitude zone, so the ionospheric disturbances are not particularly
pronounced. The aim of the study was to broaden the research to the region of
the Western Balkan to analyze the impact of solar activity on the ionosphere
and on GNSS positioning estimates during two opposing periods of SC (solar
cycle) 24. The study periods were selected to be the second half of October
2014, during pronounced solar activity in the solar maximum, and the first half
of September 2017. Before installing the GBAS system, solar activity in this
area should be studied in detail, and solar patterns should be described in a
larger timeframe and possibly made a local forecast model. Applying a local
ionosphere Total Electron Content (TEC) model of high spatial and time
resolution for the observed area on one-frequency GNSS observations would help
to check the accuracy that could be achieved compared to the usage of global
models, which have limited accuracy and precision. Local TEC model for Bosnia
and Herzegovina is still under development. Using dual-frequency receivers,
ionospheric errors could be minimized, however, many GNSS devices still operate
on single frequency L1. With introducing a second civilian frequency, the
ionosphere error can also be directly corrected onboard the aircraft itself.
4.3.
Constellation Weakness Effects
Dilution of precision (DOP) or geometric dilution of precision is the term that describes the positional errors related to the geometric positions of Space Vehicles (SVs) in the sky relative to the receiver. The better the geometry is, the lower the DOP, and, hence, the better the position solution (Karaim et al., 2018). For high precision navigation, multiple constellations of satellites are required especially due to the aircraft high speed where there is a rapid change in the tracking of the SV. Constellation strength means that the GNSS constellation is adequately replenished and that all key aircraft operations are adequately supported all of the time. Generally, GPS users only need four satellites to estimate their position. However, aircraft on approach typically need seven or more satellites to guarantee the performance needed to ensure safe operation.
4.4.
Radio-Frequency Interference (RFI)
RFI can be naturally or
artificially induced. Artificial can be intentional and unintentional. Signals
are received below the user background thermal noise level, therefore, these
signals are weak and readily overwhelmed by any of the multitude of signals
emanating from terrestrial sources. Intentional interference is, in many cases,
a significant source of GNSS sig na l deg radat ion. Intentional interference,
known as signal jamming, is caused by the broadcast of malicious radio
frequency (RF) signals to prevent GNSS receivers from tracking GNSS signals in
a specific area. The creation of fallback systems is crucial, as well as the
ability of the system to quickly return in operation if an inevitable
interruption occurs. Spoofing represents the intentional signal transmission
that is providing the receiver with misleading signals. The receiver uses
counterfeit signals in space and computes erroneous position calculation.
Signal spoofing is more harmful than jamming because it is not readily
detected. Of all the errors listed above, prediction of intentional human-made
errors is most demanding. Protection against such interference is under
consideration for the next generation of avionics standard. Cybersecurity will
have a special place in the aviation research community during the
implementation of future GNSS technology.
5.
Results and Discussion
Current civil aviation GNSS use
is predominantly based on a satellite single frequency constellation, mainly L1
frequency . ICAO standards for L1 f frequency have already been developed for
GPS and GLONASS systems and augmentations already exist. SARPs (Standards and
Recommended Practices) for dual-frequency GPS and GLONASS systems are still in
development. Introducing the L5 frequency in combination with existing
frequencies will significantly increase the integrity parameters of the
precision guidance system.
To increase system reliability,
it is necessary to improve the receiver’s ability to receive signals from
multiple GNSS constellations. Dual-frequency receivers could eliminate the
ionospheric delay in the monitor metrics using the linear combination of the pseudo
range and carrier phase measurements. GNSS constellations offering
dual-frequency signals will be introduced into service during the 2020s by the
United States (GPS), the Russian Federation (GLONASS), Europe (Galileo), and
China (BeiDou).
Standards for the CAT-II/III
capable service type (GAST D) were agreed and developed by the International
Civil Aviation Organization (ICAO) at the end of 2016 and will be in effect
from 2018 on (Felux, 2018). Contributors for the Single European Sky ATM Research
(SESAR) projects were the main manufacturers of GBAS airborne and ground
equipment (namely Thales, IndraNavia, Honey well, etc.). Table 3 shows the cost
calculation or revenue loss as an example based on referenced aircraft of 77
000 kg, with 113 passengers, on-route of 1000 km, for the three most important
subject (Cost calculation is made on the basis for the scenario that flight
operate in Bosnia and Herzegovina, and Sarajevo Airport):
• Air carrier lost revenue for
113 passengers with an average ticket price of 200 €
• Air Navigation Service
Provider, lost revenue for navigation fees on EUROCONTROL basis
• The airport at destination, lost revenue for basic aeronautical charges (landing, handling, passenger tax).
For a GBAS GAST D price of
1.194.000 €, not considering equipping the aircraft, we just need 48 operations
to pay off the initial investment. Table 4 compares the ILS system with the
GBAS system by several criteria. According to multiple comparisons, we conclude
that GBAS has 15 advantages of total of 24 criteria points. Multiple research
has been done on this topic, however, below are presented all criteria that
future users of this system will be able to evaluate and make a cost-benefit
analysis. We cannot say that GBAS is a better or worse solution, though each
operator will decide by its local conditions which system will meet their
needs.
6.
Conclusion
Considering the implementation of
GBAS means an analysis of factors that affects airport capacity decline. In the
first stages, upgraded navigational procedures should be combined with existing
conventional radio navigation aids, which should be used as backup systems.
Geographic and weather conditions are the primary limiting factors at the
Sarajevo Airport for the implementation of higher ILS approach categories. Two
major factors affecting Sarajevo Airport operations are weather with low
visibility in the winter months and the proximity of high terrain, which limits
the freedom of aircraft movement at lower altitudes. GBAS implementation would
be one of the possible solutions for lowering approach minimums since there are
no conditions for a higher ILS category implementation yet. Higher approach
category would increase overall airport capacity, and especially could reduce a
large number of cancelled flights during the winter season. This paper promotes
GBAS benefits and encourages air navigation service providers, safety
regulators and other users to implement such a system at Sarajevo International
Airport
Source: Flying Bosnian
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