TLAT NON CONCURRENT STATEMENT
AERONAUTICAL MOBILE COMMUNICATIONS PANEL
Working Group C Third Meeting
15-19 October 2001
Anchorage, USA
Agenda Item 6: New Systems to be Considered - UAT
Prepared by Larry Johnsson
TLAT NON CONCURRENT STATEMENT
SUMMARY
This paper presents the non concurrent statement with the TLAT report from the Swedish participants in the TLAT activity.
1. Introduction
The previous meeting pf AMCP WG-C/2 discussed the Universal Access Transceiver (UAT) system under the agenda item “New Systems to be Considered”. The report from the Technical Link Assessment Team (TLAT) was presented to the meeting. In the report from the meeting it is stated in paragraph 6.1.6 “...all the findings had been unanimously agreed upon by the members of the TLAT...”. This statemnt is, according to the Swedish participants in the TLAT activity, not correct.
This paper is presenting a non concurrent statement with the TLAT Report from the Swedish participants in the TLAT activity.
2. Non Concurrent Statement
The participants in the TLAT activity from Sweden, raised during the TLAT activity a number of issues which were disregarded during nthe TLAT process. When the TLAT report became official, the participants from Sweden decided to file a non concurrent statement. It was decided to publish the statement in the avition press. Therefore an article was published in “Air Traffic Technology International 2002”. The text for the article is attached to this paper.
3. Recommendations
The AMCP WG-C/3 meeting is invited to
a) note the information presented in this paper; and
b) to take into account the information in this paper together with the concerns raised in WP/30 to the AMCP WG-C/2 meeting when considering the need for ICAO provisions for the Universal Access Transceiver (UAT) system. .
- END -
2001-09-09
Air Traffic Technology 2002
Need for New Technology
By
Johnny Nilsson,
Swedish Civil Aviation Administration
1. Introduction
Europe and the USA face the same issues regarding saturation of the Air Traffic Control systems. Increasingly crowded skies and mounting delays demand substantial improvements in Air Traffic Control system equipment and performance. International experts agreed more than 10 years ago that new systems and operational procedures are required to reduce delays and enable future growth. The ICAO 10th Air Navigation Conference in September 1991 unanimously endorsed the concept of transitioning from today’s ground-based systems to satellite technology and data link systems. However, the new systems must be designed to maximise user benefits rather than responding to service providers and manufacturers interest.
2. ADS-Broadcast candidate systems
ADS-B is the generic term for automatic broadcast of position (and intent) information from aircraft, vehicles and vessels. There are three candidate systems; two developed in the USA and one in Europe. The two candidates proposed by the US FAA are the 1090 MHz Squitter and the Universal Access Transceiver (UAT). The European candidate, which development started in the early 1980’s, is the VHF Digital Link Mode 4. In March 2001 a two and a half year US FAA initiated technical assessment of the three candidates was completed (Safe Flight 21-TLAT). Dozens of successful European Commission sponsored demonstration projects have been completed with VDL Mode 4 prototype systems during 1996-2000. More than 150,000 flight hours on commercial aircraft have been accumulated. Three large-scale projects focusing on VDL Mode 4 are on going; One in North Europe (NUP) and two in the Mediterranean area (ADS-MEDUP and Mediterranean Free Flight – MFF). This article is discussing the three candidate ADS-B technologies.
1. 1090 MHz Extended Squitter
The 1090 MHz Extended Squitter operates within 3 MHz bandwidth and has been developed as an extension of Mode S technology widely used for aeronautical secondary surveillance radar (SSR) applications. Each extended Squitter message consists of 112 bits, 24 bits of which are used for parity. The data rate used is 1 megabit per second, within a message. Access to the 1090 MHz channel is randomised, and the channel is shared with current Air Traffic Control Remote Beacon System (ATCRBS) and Mode S responses to interrogations from ground-based radars and TCAS. The Squitters proposed for ADS-B are “extended” in the sense that prior Mode S Squitters contained 56-bit messages.
1090 MHz Extended Squitter message formats for ADS-B and transmission rates have been defined by the ICAO Secondary Surveillance Radar Improvement and Collision Avoidance System Panel (SICASP), in conjunction with RTCA Special Committee 186 and EUROCAE Working Group 51. Those bodies approved a joint RTCA/EUROCAE ADS-B MOPS for the 1090 MHz Extended Squitter in September and October 2000, respectively. Changes of the MOPS and ICAO standards (SARPs) are in progress to describe techniques to enhance the range of the Extended Squitter system and to support Traffic Information Services-Broadcast (TIS-B; uplink of radar data). The TLAT evaluated the Extended Squitter system as it was expected to be defined by the augmented MOPS and SARPs. Additional message formats were also proposed by 1090 MHz Extended Squitter experts to support Flight Information Services-Broadcast (FIS-B; uplink of weather data). The system evaluated allowed up to 1 kW of power.
2.2 Universal Access Transceiver (UAT)
The UAT was developed under an Independent Research and Development project at the Mitre Corporation. UAT operates on a 3 MHz bandwidth and is a “clean sheet” design optimised toward the support of broadcast applications, both air- and ground-based, to support surveillance and situational awareness. The UAT data rate is approximately 1 megabit/second within a message. Access to the UAT medium is time-multiplexed within a 1 second frame between ground-based broadcast services (the first 188 milliseconds of the frame) and an ADS-B segment. While the design presumes time synchronization between ground-based broadcasts to reduce/eliminate message overlap, medium access within the ADS-B segment is randomised. Initial UAT operations have been conducted using the experimental frequency of 966 MHz. Demonstrations in Alaska are using 981 MHz as the UAT frequency. The system evaluated allowed up to 250 W of power.
UAT MOPS development within RTCA was initiated in December 2000.
3. VHF Digital Link (VDL) Mode 4
VDL Mode 4 technology has been under development since the 1980’s, initially in Sweden but since 1996 in a number of States. VDL Mode 4 uses two separate 25 KHz Global Signalling Channels (GSCs), with additional channels used in areas with medium to high traffic density. Access to the VDL Mode 4 medium, within a channel, is time-multiplexed, with a data rate of 19.2 kilobits/second within a message. Various types of prototype single-channel VDL Mode 4 equipment have been fielded since 1991. More recently, prototype dual-GSC equipment has been demonstrated and evaluated in Italy within the European Commission sponsored FARAWAY II-project. FARAWAY II involved installations in the Padua, Rome and Brindisi control centres and demonstration of ADS-B, GNSS Augmentation, TIS-B, FIS-B and Controller-Pilot data link communications (CPDLC).
While VDL Mode 4 technology has been proposed and demonstrated for a wide variety of aviation applications, including two-way aeronautical telecommunications and local area augmentation to GNSS, the TLAT, as directed by the SF21 Steering Committee and the EUROCONTROL ADS Project Steering Group, evaluated all candidate links solely with regard to their ability to support ADS-B, TIS-B, and FIS-B.
VDL Mode 4 Standards and Recommended Practices (SARPS) have been developed by the ICAO Aeronautical Mobile Communications Panel (AMCP) and approved by the ICAO Council in March 2001. Additionally, a EUROCAE MOPS for VDL Mode 4 airborne equipment has been completed and published in July 2001. Also, a European Telecommunications Standardization Institute (ETSI) standard for VDL Mode 4 ground-based radios is being circulated for public comment. The system evaluated allowed up to 25 W of power.
3. Findings by Safe Flight 21 Technical Link Assessment Team (TLAT).
3.1. Traffic scenarios
Safe Flight 21- TLAT agreed on three high-density traffic scenarios to be used in its technical evaluation of the candidate links. The table below summarises the characteristics of these traffic scenarios.
|Scenario |Total Aircraft |Scenario Area |
|LA Basin 2020 (LAX) |2694 |400 nmi radius |
| |(50 percent increase over estimated 1999 | |
| |traffic levels) | |
| |(including 225 on the ground) | |
|Core Europe 2015 (XCE) |2091 aircraft |300 nmi radius |
| |(73 percent increase over estimated 1999 | |
| |traffic levels) | |
| |(including 150 on the ground) | |
|Low Density |360 |400 nmi radius |
| |(all airborne) | |
Table: Selected Traffic Scenarios
3.2. Findings by Safe Flight 21 - TLAT
The following observations concerning the sensitivity of the results of the particular simulation and analytical assumptions were made by TLAT:
• The VDL Mode 4 system is highly configurable and may be optimised in a number of ways in a particular air traffic environment.
• VDL Mode 4 air-ground performance improvements may be achieved through the use of sectorised ground antennas.
• The VDL Mode 4 MOPS requires co-channel interference (CCI) performance (10 dB) at least 2 dB better than the value stated in ICAO SARPs. VDL Mode 4 simulations assumed a 10 dB CCI threshold.
• Trajectory change point transmission rates for UAT and VDL Mode 4 are subject to further optimisation.
• The 1090 MHz Extended Squitter simulations suggest that a breakpoint in 20-40 nm range performance occurs within the fruit (interference) environments examined.
• A 1090 MHz Extended Squitter ADS-B receiver as specified in RTCA DO-260 will exhibit significantly lower performance than that shown for the scenarios considered by the TLAT.
The following observations concerning the capacity (relating to the number of ADS-B system participants) of the candidate links were made:
• In the high-density traffic scenarios considered by the TLAT, there is no excess ADS-B capacity for any of the links as defined in their System Descriptions.
• None of the three links meets all performance requirements in all three traffic scenarios. However, UAT was assessed as meeting all evaluated TLAT range and update rate requirements in the case of the low-density scenario.
• All three links exhibit a graceful degradation of performance in the presence of interference.
4. Additional Findings
For Traffic Information Services-Broadcast (TIS-B), the TLAT considered the capability of the candidate links to support the service, but in terms of simulations it was not taken into account, as the 100% ADS-B equipage scenario is considered more loaded than a mixed ADS-B and TIS-B scenario. All link candidates have the capability to uplink TIS-B information.
Flight Information Services-Broadcast (FIS-B) was evaluated by simulation using the future LA Basin scenario (2020). The following apply to FIS-B capacity for each link relative to the TLAT evaluation rate:
• UAT was the only link shown to have FIS uplink capacity substantially greater than the TLAT evaluation rate. The total capacity of the protected uplink slots had over 80 times the TLAT evaluation rate.
• VDL Mode 4 met the TLAT evaluation rate.
• 1090 MHz Extended Squitter was shown to deliver about one third of the TLAT evaluation rate at the maximum range.
There are several items to consider when assessing the time to implementation—availability of standards, availability of spectrum, and complexity.
Regarding standards the following was noted:
• 1090 MHz Extended Squitter: The system evaluated by TLAT contained features not standardised in the current MOPS (RTCA DO-260/ED-102). RTCA DO-260A currently in progress is expected to include these features. SARPs for Extended Squitter are in place but SARPs harmonised to DO-260A await completion of DO-260A. Complementary AEEC characteristics was expected to be completed by the end of 2001. The TLAT was not aware of any standards activity for 1090 MHz ES ground stations.
• UAT: RTCA MOPS activity has been initiated and was assumed to be completed by February 2002. SARPs and AEEC characteristics have not been initiated. The FAA intends to request initiation of SARPs development. The TLAT was not aware of any standards activity for UAT ground stations.
• VDL Mode 4: SARPs have been approved and will be published by ICAO in November 2001. EUROCAE MOPS (ED-108) was approved and published in July 2001. European Telecommunications Standardisation Institute (ETSI) standards for radio station approval for ground stations are expected by late 2001. Additional ETSI work is ongoing. AEEC activity has not been initiated as yet.
Regarding availability of spectrum:
• 1090 MHz Extended Squitter: International spectrum allocation of the required 3 MHz channel exists. No further action is required.
• UAT: Operating frequencies (supporting the required 3 MHz channel) must be identified. This will be done during the SARPs development process. International co-ordination of the UAT frequency is expected to take until 2006. After identification of a UAT frequency, DME channel(s) will need to be cleared.
• Resolution of interference issues concerning UAT and the military (NATO et al) JTIDS/MIDS, an important military tactical data link, is critical to the deployment of UAT. The UAT evaluation has not taken into account the effects of JTIDS/MIDS systems. The UAT MOPS activity is considering this issue.
• VDL Mode 4: VDL Mode 4 requires seven 25 KHz ground channels (on-board requirement is one transmit and four receive channels) to operate in the high- density scenarios evaluated by TLAT. The seven channels include: two Global Signalling Channels, two Regional Signalling Channels, two Local Signalling Channels, and one ground channel. ICAO working groups are tasked to identify Global Signalling Channels. VDL Mode 4 operation in the VHF navigation band, as desired by the FAA, may require International Telecommunications Union coordination. The international co-ordination of the VDL Mode 4 Global Signalling Channels is expected to be completed by 2003.
The aspect for time to implement relates to risk and complexity.
• Implementation of ADS-B on any of the links—for performance consistent with the System Descriptions—will require new equipment installations.
• A limited 1090 Mhz Extended Squitter capability (supporting Aid to Visual Acquisition and Conflict Detection and Collision Avoidance applications) is available as an option now with new TCAS and transponder installations (installations since 1999), and could offer some near-term benefits.
• Long-range, SARPs- and MOPS-compliant receivers are expected to be available within one year from the completion of DO-260A (receiver availability is estimated by 2003). These estimates apply to applications that require a maximum of 2 Trajectory Change Points (TCPs). The development and certification of avionics to support more than two TCPs applications may take longer.
• Standards-compliant VDL Mode 4 avionics are expected to be available in the near future. The current standards address equipment with a minimum of two receivers. Current production design include one transmit and four receive channels.
• UAT, as currently defined, has the simplest technical concept of the candidates. This simplicity suggests that the necessary validation testing and standards development may be accomplished relatively expeditiously. Presuming that JTIDS/MIDS interference issue is resolved, UAT avionics are expected to be available in 2003.
• The TLAT was aware that Russia has published an order that determines October 1, 2005, as the date to start using ADS-B operationally for air traffic in Russian airspace. Russia has stated to ICAO and widely published that it plans to implement VDL Mode 4-based ADS-B services.
Ability of the candidate links to be integrated with and/or coexist with existing systems:
• Any operational frequency chosen for UAT will require coexistence with the JTIDS/MIDS military tactical data link. The TLAT’s UAT results presume resolution of this important issue in a manner that does not add adverse interference to that used in the TLAT simulations.
The following observation was made concerning the abilities of the candidate links to mitigate potentially catastrophic raised in the FAA’s ADS-B Operational Safety Assessment:
• It is important for the ADS-B system to have a means for independent air-to-air range validation to reduce the risk of spoofing. Both UAT and VDL Mode 4 offer this capability by passive range monitoring. The 1090 MHz Extended Squitter ADS-B system has no provision for air-to-air passive range monitoring. TCAS-equipped aircraft can employ active air-to-air range monitoring; however, the range of this active range monitoring is limited.
Expandability of the candidate links:
• Future applications may require air-to-air two-way data link. The combination of long-range operation and the ability to provide two-way data link may make VDL Mode 4 attractive to support these future applications. UAT as currently defined does not support two-way data link. TCAS-based installations could be modified to provide a two-way air-to-air data link capability for short- to medium-range applications.
• All three links can be upgraded to support the broadcast of additional (to what is specified in RTCA DO-242) information, although VDL Mode 4 and UAT have more flexibility in this respect than does the 1090 MHz Extended Squitter.
• In the high-density scenarios considered, none of the three links appear to have excess air-to- air and air/ground capacity. The UAT System Description provides an uplink mechanism that is independent of the number of aircraft using the channel. In the case of VDL Mode 4, there is also a protected uplink mechanism; however, the capacity is less than that of UAT.
• VDL Mode 4 has the capability to provide Global Navigation Satellite System (GNSS) augmentation services, and the channel loading from this application was considered in the TLAT high-density simulations. Although the ICAO GNSS panel is not currently considering VDL Mode 4 as a means to uplink GNSS augmentation, regional implementation of this capability is planned.
5. Authors observations
In my capacity as a member of the Safe Flight 21-TLAT a number of issues were raised during the many one-week meetings. In summary those were:
• The technical assessments made by TLAT were limited to ADS-B applications only thus preventing a system wide view to be applied. Consequently, the ADS-B system was assumed to be just another add-on equipment. Selecting the right technology for ADS-B could make other systems such as VDL Mode 2, LAAS, WAAS/EGNOS and ultimately radar, etc. redundant. The European Commission ATLAS IIA study made such a system wide study in 1995/96 with the conclusion that VDL Mode 4/STDMA could offer such possibilities and thereby minimise the need for new air and ground equipment required to enhance safety and capacity.
• Using the current version of the RTCA’s ADS-B MASPS (DO-242) is not appropriate since that document mirrors the assumed range performance of the Mode S Squitter technology and TCAS experience. For instance, RTCA MASPS allows four equipment classes (A0-A3) each with range “requirements” of 10, 20, 40 and 90 nm respectively. EUROCAE did not approve the RTCA ADS-B MASPS as a joint document, and the RTCA ADS-B MASPS is currently under review and will be rewritten. Many recent European ADS-B projects have found that the use of intent information and a system providing long-range surveillance capabilities could relax the update rate “requirements” used in the previous RTCA MASPS.
• The fact that Mode S Squitter requires high power for operationally useful ranges (up to 1 kW used for the TLAT evaluation) may prevent its use as an ADS-B system. Dual installations will be required to meet availability requirements for critical applications and available on board power is limited. A single thread system will not be sufficient.
• The GA community has in international resolutions denied the use of the Mode S technology for ADS-B applications.
• Cost consequences of forcing implementation of a technology that may result in large investments by third parties e.g. the military should be at the care of those who is pushing such a system.
What is needed now is to put an end to the countless paper studies completed over the last decade and focus on implementation! Let’s hope that the airlines and the other parties in the user community become more active in this process. If not, politics and vested interests may spoil the opportunity to get the best system fielded at the expense of the users.
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