Implementing Low Cost Airport Systems
with
ICAO's VHF Datalink Mode 4 (VDL Mode 4)
Kim O'Neil
Advanced
Aviation Technology Ltd.
February 2000
ICAO's CNS/ATM concept promises radical improvements
in safety, capacity and efficiency, whilst at the same time reducing overall
costs. As a result of the introduction of new technology - in particular,
ICAO's VDL mode 4, it is now possible to implement many airport services at a
fraction of the cost of traditional solutions. Systems based on VDL mode 4 can
now be installed to replace Landing Systems, Radar Surveillance Systems and a
variety of information services such as ATIS. It is also possible to move far
beyond the capabilities of traditional technologies and implement sophisticated
services such as Fleet Management and Surface Movement Guidance and Control
Systems (SMGCS).
1. Introduction
The cost of implementing Communication, Navigation and
Surveillance (CNS) services can be extremely high using traditional solutions.
Conventional solutions can often be very limited in performance and difficult
to both operate and maintain. New and more effective solutions based on VDL
mode 4 are now available for airports that need to radically improve services.
By exploiting the global navigation satellite system (GNSS) and sophisticated
data-link techniques, many services can now be provided by a common ground
based VHF data-link infrastructure that is both easy to maintain and low cost.
ICAO's VHF Data-link Mode 4 implements many of these functions in a cost
effective, safe and certifiable manner. This includes GNSS based landing
systems. Comprehensive reports of the many tests, evaluations and operational
trials of VDL Mode 4 can be found at:
www.lfv.se/ans/card.
2. VHF Data-Link Mode 4 (VDL Mode 4)
Extensive development, evaluation and standardization
has been carried out in Europe, on a system capable of supporting time-critical
Communication, Navigation and Surveillance (CNS) applications - VHF Data-link
Mode 4 (VDL Mode 4). In addition to this, much effort has been put in to
develop the necessary operational concepts for:
- all elements in the ICAO CNS/ATM concept;
- all phases of flight (gate-to-gate);
- all user groups;
- global implementation.
VDL Mode 4 is a multi-purpose data-link that can
combine several services and applications. It can mix them on a single 25 kHz
channel (frequency) or use several channels for improved redundancy, integrity
and capacity. It is the only known data link suitable for time-critical
applications and is characterized by very high delivery probability from the
unique Self-organized Time Division Multiple Access (STDMA) VHF data-link. Its
FM modulation scheme (GFSK) gives the data-link long range (200 NM) at low
power (e.g. 5-20W) including very good transmission characteristics on the
ground, e.g. at airports. The core function of VDL Mode 4 is Automatic
Dependant Surveillance Broadcast (ADS-B) - where aircraft and vehicles
periodically transmit their GNSS position. This position information can be
used in a variety of essential functions. A key feature of VDL mode 4 is its
cellular nature, which allows ground stations to be networked together to
provide services over a very large geographical area. Europe has implemented
several wide area systems based on VDL mode 4 to demonstrate the potential of
the system.
3. Automatic Dependent Surveillance: ADS-B
ADS-B implemented by VDL mode 4 can improve all
surveillance services, including air-to-air surveillance. For example, ADS-B
data:
- Is broadcast and can be received by any other
aircraft and vehicles.
- Contains more information than radar data and is
transmitted at higher rates.
- Provides end-to-end communications e.g. air-to-air
without ground infrastructure.
- Operates at low power, typically 5-20W, with
long-range coverage e.g. +200 NM.
- Has higher capacity per bandwidth and spectrum
occupancy than other data links.
- Has equal or better surveillance at a significantly
lower cost than radar.
ADS-B and VDL Mode 4 can support many CNS
applications, including:
- Pilot situation awareness
- Traffic Information Service Broadcast (TIS-B)
- Flight Information Service Broadcast (FIS-B)
- Airborne Separation Assurance (ASAS)
- Advanced Surface Movement Guidance and Control
(A-SMGCS)
- Enhanced ATC surveillance
- Runway Incursion · Parallel Runway
Monitoring
- Controller-pilot data link communication (CPDLC)
- Pre-departure clearances (PDC)
- GNSS Augmentation
4. GNSS Augmentation
When using GNSS data for navigation or surveillance, a
ground based GNSS Augmentation System can be used to improve the quality and
accuracy of the position data. GNSS Augmentation signals transmitted by
data-link from ground stations provide essential correction and integrity data,
enabling users to fully exploit the opportunities of satellite navigation in
complete safety. There are several possible approaches to augmentation:
Individual ground stations can provide Local Area
Augmentation to support GNSS based landings and also precision navigation on
the airport surface. VDL Mode 4 also supports the GNSS Regional Augmentation
System (GRAS), which is under discussion in the ICAO GNSS Panel (GNSSP). The
GRAS concept proposes a network of ground stations to gather data on GNSS
satellite integrity and calculates augmentation information. VDL mode 4 can
provide this infrastructure.
Using the VDL Mode 4 data link for GNSS Augmentation
can give very high accuracy of position information, typically 1-2 m. This
allows aircraft and ground vehicles to navigate in the air and on the ground
using the augmented position information.
5. Gate to Gate Applications of VDL Mode 4
GNSS Augmentation over VDL Mode 4 is primarily
intended to support area navigation, non-precision approaches and ground
movements in order to achieve the required accuracy and integrity needed for
Advanced Surface Movement Guidance and Control (A-SMGCS) applications.
Gate-to-gate coverage with GNSS Augmentation, will be necessary to support
ADS-B and other CNS applications. DGNSS Approach and Landing applications
employing VDL Mode 4, have been extensively tested at airports, e.g. at
Atlanta/Hartsfield, Amsterdam Schiphol and Stockholm/Arlanda, and is very well
suited for all airport user groups.
For example, at Atlanta/Hartsfield, the probability of
reception over all points of the airport was greater than 99% with a single
ground station. Transmission power is 1 or 5 Watts on ground, facilitating
installations in ground vehicles.
Other applications in Gate-to-Gate trials include:
- Automatic Terminal Information Service Broadcast
(ATIS-B)
- Enhanced surveillance.
- On-ground situation awareness and taxi guidance
- In-flight situation awareness.
- Extended Helicopter Surveillance.
- Runway Incursion.
- GNSS Precision Navigation Capability for En-route
and Approach.
6. Approach and Landing Trials at Angelholm Airport
in Sweden
Trials of GNSS Precision Navigation Capability at
Angelholm airport in Sweden for En-route and Approach. The trials were divided
into DGNSS Instrument Approach with Vertical Guidance and Direct Routes between
Stockholm/Arlanda and Angelholm airport. The Approach tests were started in
late 1997 and ended February 1999.
Earlier accuracy tests were conducted 1994 where over
150 CAT-I to CAT-III landings were conducted using the VDL Mode 4 prototypes.
6.1 Operational context
Angelholm airport has one instrument runway (RWY)
14/32. RWY 14 is equipped with a Category I ILS, whereas RWY 32 only has a
single NDB to support Non-Precision Instrument Approach (NPA). The tested
approach procedure based on DGNSS was overlaid on the respective ILS and NDB
approaches. The Approach was implemented as an Instrument Approach with
Vertical Guidance (IPV). This is equivalent to a Non-Precision Approach with
vertical guidance. Even though the system can support Precision Approach (PA),
IPV was selected for several reasons:
- Non-Precision Approach is a major contributor to
Controlled Flight Into Terrain (CFIT) accidents. Lack of vertical guidance and
poor situation awareness is the main reason.
- Angelholm RWY 32 and its racetrack NDB approach
procedure put undesired operational limitations to SAS operations (similar to
NPA's at many other airports).
- In order to conduct the test in an operational
environment with revenue flights, IPV was more appropriate and straightforward.
- VDL Mode 4 is a gate-to-gate system focusing on
several CNS Applications covering all phases of flight and applications "up to"
IPV and A-SMGCS.
- ICAO standards are being developed for CAT-I/II/III
using other techniques.
Note that the VDL Mode 4 accuracy was proven to meet
CAT-I already in 1994 and that the participating SCAT-I (Special CAT I) ground
station is already developed with the required accuracy as well as integrity,
continuity and availability for SCAT-I.
6.2 Aircraft Installations
Tests were conducted using two SAS Fokker 28s on
scheduled service between Stockholm-Arlanda and Angelholm. A Swedish CAA Beech
200 was used for flight inspection purposes prior to the Swedish CAA Flight
Safety Department approving the final test program. The approach into Angelholm
was made as an Instrument Approach with Vertical guidance (IPV). Two separate
Track Deviation Indicators (TDI), with lateral (VOR/LOC) and vertical (GS)
guidance were installed to aid the pilots during final approach. The TDI was
used together with the Cockpit Display of Traffic Information (CDTI), which
provided situation overview throughout en-route navigation and approach phases
of flight.
Figure 1. The CDTI during Approach to RWY 14
Initially, only a limited number of SAS Fokker 28
pilots were allowed to fly the approach but by the end of 1998, a decision was
made to allow all pilots to do it. Prior flight experience with the CDTI was
required but the education was straightforward.
6.3 Ground Equipment Installations
A ground station was developed and installed on the
airport. This station incorporated as one component a commercial Navigation and
Landing System SCAT-I ground station for generation of differential GNSS
corrections and integrity data. The data was output to Saab Dynamics prototype
VDL Mode 4 transponder system, which provided the two-way data link capability
for DGNSS broadcast and reception of ADS-B reports. The SCAT-I ground station
was designed to meet RTCA/DO-217 standard (undergoing FAA certification) and
the VDL mode 4 transponder to Joint Aviation Authorities (JAA) Form 1
non-hazardous level.
Figure 2: Ground Station at Angelholm Airport
New display equipment was installed in the control
tower (TWR), approach control displays were updated and new phraseology was
used. The TWR display included also the vertical component, enabling the
controller to graphically monitor the approach in four dimensions. Data from
the ground station and extended VDL mode 4 infrastructure allowed the
controller to monitor, in a seamless fashion, aircraft positions from the
departure gate at Stockholm/Arlanda through the en-route, approach, landing and
taxiing phases into the parking position at Angelholm.
6.4 Results of IPV Landing Trials
No unexpected behavior was detected in the
differential corrections received by the aircraft, all visible errors were
detected by the system and reported to the pilot and the analytical discussion
indicated that the system architecture will met the integrity requirements in a
full production system. All application-specific objectives were met, and the
assumptions on expected benefits and system characteristics were accepted. The
evaluated services provided operational benefits in terms of improved situation
awareness for pilots and controllers. The system provides support for approach
and landing and can support seamless gate-to-gate operations.
The combination of ADS-B and GNSS Augmentation using a
single data link provides a cost-effective solution for all phases of flight. A
number of ATC benefits were enabled by ADS-B, including:
- ground surveillance capability
- potential enhancements to capacity and flexibility
- gate-to-gate surveillance.
The majority of the controllers were positive to
pilots having a detailed picture of the traffic situation through the
ADS-B/CDTI and improved situation awareness.
7. Conclusions
VDL Mode 4 is a key technology enabling many of the
functions outlined in ICAO's CNS/ATM Concept. The integration of CNS services
has been proven feasible in projects mentioned in this article. The IPV
Approach Trials highlighted several possible benefits by integrating DGNSS and
ADS-B, both for ATC and aircrew. Both aircrew and controllers were very
positive to the system and its' potentials. Extensive trials have successfully
demonstrated that seamless gate-to-gate services can be implemented with VDL
Mode 4 and associated CNS Applications. Greater operational benefits can be
achieved with VDL mode 4 in a way that is safe, certifiable and at much lower
cost than traditional techniques. |
