AMCP WG/WP32
AERONAUTICAL MOBILE COMMUNICATIONS PANEL
Working Group of the Whole
21-24 May 2002
Montreal, Canada
Agenda Item 4(b): Review of developments since the completion of the TLAT Report in March 2001
INTEGRATED ADS-B SYSTEMS INCORPORATING
L-BAND AND VHF TECHNOLOGIES
Presented by: Stephen Heppe / ADSI, Inc.
SUMMARY
This paper describes an airborne architecture using L-band and VHF technologies, which provides a high degree of flexibility for communications and surveillance applications. Preliminary assessment indicates that this architecture can extend the benefits of the underlying systems, reduce channel load in the VHF band, and offer a higher degree of reliability than any single ADS-B system alone.
Members are urged to consider these system alternatives in their consideration of potential new SARPs activities for ADS-B.
1.Introduction
AMCP is considering a new activity to develop SARPs for the Universal Access Transponder (UAT). The UAT operates in a broadcast mode using a wideband random-access channel. Simulations have indicated good performance in both low-density and high-density airspace. However, these simulations have not accounted for the channel load and ground infrastructure management issues that would be needed to satisfy the full set of applications desired in practice (e.g., including inter alia weather and traffic information products). The UAT may also have limitations if an addressed air-to-air data link is determined to be a necessary element for long-range path deconfliction, or other emerging applications.[1]
This paper addresses an integrated ADS-B architecture that includes L-band and VHF elements. The L-band element provides very high update rate tailored for short-range applications, while the VHF element provides addressed air-to-air and air/ground communications for surveillance and other applications. Figure 1 illustrates the conceptual value of an integrated system. Each system supports most or all of the known requirements for ADS-B, but with limited capability to handle new and emerging applications. The integrated system provides greater reliability for known applications (i.e., information is conveyed on multiple links), and allows growth to new applications using the unique strengths of each system.
The suggested architecture uses VDL Mode 4 as the VHF element and either UAT or 1090 MHz extended squitter (1090 ES) as the L-band element. Both UAT and 1090 ES operate at high data rate in a random-access mode on a single wideband channel. The UAT has the advantage (nominally) of a dedicated channel, whereas 1090 ES has the advantage of existing SARPs, integration with ground-based radar and ACAS. There are various ways that these elements could be integrated into a single system offering synergistic benefits; conceivably, if UAT and 1090 ES are supported in a single “L-band cabinet”, all three systems could be integrated to yield a highly robust and capable ADS-B system with straight-forward transition from current systems and high potential for growth to new and emerging applications in the future.
Section 2 of this paper describes a simple integration concept where an L-band system (or combination of systems) is combined with VHF, without any change to the dynamic behavior of either the L-band or VHF system. Section 3 examines a more highly integrated concept where the VHF element is dynamically controlled in real time according to information collected via the two or three systems. Either approach, with or without dynamic control of transmission rate, could serve as the basis of an integrated system. Section 4 identifies areas of future work, which could be included in a possible work program for further development of ADS-B data links.
2. Integration concept with no change in dynamic behavior
Figure 2 illustrates an integrated airborne architecture relying on multi-mode VHF radios, UAT and 1090 MHz extended squitter. The UAT is integrated into a single LRU with ACAS and 1090 MHz extended squitter receivers.[2] This architecture, which is an outgrowth of earlier work relating to integration of VHF and 1090 MHz ES, provides flexible and robust CNS services with no increase in LRUs and no increase in antennas relative to current equipage for transport category users. A recent paper describing this earlier work, presented to the 2nd annual NASA conference on I-CNS Systems, is provided as Annex A.
The architecture supports analog voice (25 kHz and 8.33 kHz), CPDLC, AOC data, ATIS and FIS-B, TIS-B and ADS-B. ADS-B is provided with at least two independent systems – VDL Mode 4 operating in the VHF band and UAT and/or 1090 MHz extended squitter operating in the L-band. Hence, there are a minimum of 4 ADS-B receivers operating, in the aggregate, on two independent systems and two frequency bands. There could be effectively five ADS-B receivers operating if the L-band component integrates UAT and 1090 ES receive/ACAS in a single cabinet with a single receiver, and as many as seven ADS-B receivers if the L-band component is dual-redundant (with dual antennas). Diversity reception yields greater confidence of receiving a message error-free at any given separation distance. Diversity reception in the VHF band (i.e., up to 3 reception opportunities for every message due to the multi-mode, multi-frequency VHF radios connected at baseband) yields equivalent benefits for ATS and AOC communications without any increase in antenna count, and eases airframe integration due to mitigation of cosite interference issues.[3]
Each ADS-B system operates in accordance with its current SARPs and MOPS and provides data to a common data fusion element (which could be integrated with one of the ADS-B systems, or which could alternatively be a dedicated LRU). The dynamic behavior of each system is unaffected by the other(s). The following benefits are achieved by this level of integration:
- Supports all current and emerging comm and surveillance requirements without increasing avionics or antenna count. Supports worldwide interoperability.
- Offers internal hardware redundancy as well as ability to automatically reconfigure across LRUs to maintain full support of all applications under most failure conditions.
- No substantive change to cockpit procedures.
- Provides better fault-free reception, for VHF data communications, than is feasible with non-integrated VHF systems. No central controller (radios are a peer network).
Technology risk (i.e., what is the right answer for the next 20 years?) is minimized since the radio LRUs can include all emerging standards in the VHF band. The unified approach is compliant with European initiatives for 8.33 kHz analog voice and VDL 4, so international airlines will not face any extra equipage cost.Benefits are summarized in Table 1.
In terms of performance, Table 2 summarizes the advantages of this hybrid approach. A bullet indicates satisfaction of requirements contained in RTCA/DO-242A. It should be noted that operational requirements for ADS-B have not been fully defined within ICAO, and so these data are subject to further validation.
In regard to the performance summary for VDL Mode 4, a diverse set of recent simulations and analyses have collected the following information:
a)The JHU/APL honeycomb analysis, though flawed, can be used as a lower bound on expected performance and demonstrates 100% delivery to a range of 20 nmi. This was based on a nominal update rate of one transmission every 3.75 seconds. Therefore, a more relaxed update rate of once per 5 seconds (consistent with RTCA/DO-242A) will also allow 100% message delivery to a range of at least 20 nmi.
b)If ACAS requirements are satisfied with VDL Mode 4 according to the dynamic method described in RTCA/DO-242A, and currently under consideration in AMCP/WG-W, a small proportion of the user community will transmit at once per 2.5 seconds. Preliminary analysis for the LA Basin 2020 scenario indicates that total channel load in this case will not exceed the nominal load already simulated by JHU/APL. Further validation is desired, including the effects of realistic traffic flow patterns and altitude distributions.
c)Performance beyond a range of 20 nmi cannot be determined based on the JHU/APL results due to errors in diversity modeling and improper extension from single-channel to dual-channel results. However,
d)Simulations for the Core of Europe circa 2015, using corrected versions of SPS, are expected to demonstrate that ADS-B state vector update rate requirements can be satisfied at 40 nmi and 90 nmi using the European regional channel management strategy.[4] The honeycomb strategy, if applied in Europe, is expected to perform at least as well since it deterministically assigns time slots to all users (thereby minimizing the impact of hidden terminals).
e)The performance requirements for transmission of intent information have not been fully defined. Issues include the type of information required as a function of operational domain[5], the update rate during periods when the information is static, and the allowed latency when the information changes. Short-range intent information (e.g., heading and airspeed) may be handled differently, and support different applications, than long-range intent information. Because of the uncertainty surrounding these applications, it is difficult to make statements of requirements satisfaction for any of the data links. However, VDL Mode 4 offers multiple methods to handle these data flows including ground-directed slot assignments on GSCs or local channels, autonomous slot selection on GSCs or local channels, so-called bell-ringer bits to indicate changes, and addressed pair-wise communication for specific cases where active deconfliction, and/or data authentication and validation, is determined to be required.
Table 2: Performance summary of stand-alone and hybrid ADS-B systems
Application (range) / System architecture1090 ES / UAT / VDL/4 / 1090 ES +
VDL/4 / UAT +
VDL/4
Aid to Visual Acquisition (10 nm) / / / / /
Conflict avoidance and collision avoidance
(3-20 nm) / / /
(see note 1) / /
Separation assurance and sequencing (40 nm) / / / / /
Flight deconfliction
planning (90-120 nm)
(see note 5) / TBD / one TCP / multiple TCPs.
Addressed data link if needed. /
(requires validation) /
(requires validation)
Simultaneous approach
(10 nm) / / unlikely
(note 2) / / /
Airport surface
surveillance (5 nm) / / marginal
(note 3) / / /
Airport surface mobile-to-mobile (5 nm (TBV)) / TBD / unlikely
(note 4) / / /
Notes:
- Based on new capability for dynamic rate increase presented at WG-M. Subject to validation.
- Time dithering prevents satisfaction of the 1 second 95% (desired) update interval, and implies that the 1.5 second requirement is only satisfied 93% of the time (assuming uniform dither across 0.8 second subframe). Multipath, shadowing and garble from nearby aircraft in a dense terminal environment, as well as possible TIS-B transmissions from nearby local transmit stations, could suppress the confidence level further. Hence it appears unlikely that these requirements can be satisfied.
- The 1.5 second 95% requirement is marginal based on time dithering alone. RF propagation issues and garble can be mitigated with multiple ground receiving stations located around the airport.
- The 1.5 second 95% requirement is marginal based on time dithering alone. RF propagation issues cannot be mitigated with additional receivers for the mobile-to-mobile application.
- Requirements are not fully defined.
In regard to simultaneous approach and airport surface operations, a distinction is drawn between the surveillance application (situational awareness of the service provider) and the mobile-to-mobile situational awareness application. All systems can support a nominal update rate of 1 Hz, but differ in terms of transmit time dithering and RF propagation characteristics. VDL Mode 4 can command a precise 1 Hz rate with no dither and no garble (i.e., in ground-directed mode), and is expected to exhibit the most reliable RF propagation characteristics on the airport surface. The L-band systems are expected to exhibit less reliable RF propagation characteristics, and of the two, the UAT has a larger random dither compared to 1090 MHz ES.
The performance requirement for simultaneous approach and ADS-B on the airport surface is a 95% confidence of update within 1.5 seconds (with 1 second desired for simultaneous approach). If the UAT employs a random dither across the 0.8 second subframe allocated for aircraft transmissions, the nominal (and desired, for simultaneous approach) 1 second update interval would only be achieved with 50% confidence on any single transmission (i.e., typically half the targets are updated in less than one second, and half are updated in more than 1 second). The 1.5 second requirement would only be achieved with 93% confidence based on consideration of the transmitter time dither alone.[6] Multipath, shadowing and garble from other users would tend to push the confidence level to below 93%.
The RF propagation issue can be mitigated for the surveillance function with additional ground receiving stations, but this is not feasible for the mobile-to-mobile application.
VDL Mode 4 offers a unique capability for air-to-air addressed messaging that is not available with either L-band alternative. This is enabled by source and destination addressing as well as slot reservations which allow long-range communications with high reliability. While destination addresses could be added to UAT with some level of additional complexity, reliable data link at long range would still be infeasible for UAT due to garble from third-party stations. An addressed air-to-air capability may be an emerging requirement.
As may be seen, while the individual links have advantages and disadvantages with respect to certain applications, the hybrid alternatives appear to be the most likely to satisfy all current and emerging requirements. Therefore, a hybrid VHF and L-band system should be considered as a future candidate. It appears that 1090 ES plus VDL Mode 4 can meet all currently-identified needs with systems that are currently available with SARPs. The UAT plus VDL Mode 4 is another alternative. A hybrid consisting of all three systems would meet requirements as well.
- Integration concept with dynamic sensing of ADS-B environment
Current ADS-B systems are designed to operate and provide required levels of performance without the benefit of any auxiliary systems operating in parallel. But in a future integrated architecture, the transmit data rates could be tailored according to real-time knowledge that the ownship system gathers regarding peer stations in the airspace. One method to do this is to:
- allow the L-band systems to operate as currently defined (e.g., 1 Hz transmission rate for UAT; multiple defined rates for 1090 MHz ES), and
- adjust the autonomous transmission rate for the VDL Mode 4 component in a way that does not impair overall ADS-B performance, but maximizes channel resources for long-range applications.
This can be achieved without change to existing SARPs for VDL Mode 4 – the necessary functionality can be embedded in an ADS-B or ASAS entity which operates “above” VDL Mode 4 and merges the data from the multiple data links. However, as noted below, the need for a new “capability code” is subject to further analysis and validation. If needed, such a code could be added to the VDL Mode 4 Implementation Manual without change to underlying SARPs.
3.1ADS-B behavior for aircraft equipped with ADS-B data link in a single frequency band
For aircraft equipped with a single ADS-B data link technology, the aircraft operates in accordance with the current or future SARPs and MOPS for that single data link. This is also true for aircraft equipped with both L-band systems.
3.2ADS-B behavior for aircraft equipped with VHF and L-band ADS-B data links
For aircraft equipped with VDL Mode 4 and at least one L-band ADS-B data link operating in an integrated system with dynamic behavior control, ownship determines the transmit regime for the VDL Mode 4 component according to the following policy:
- If operating in directed mode, ownship operates as directed (i.e., the VDL Mode 4 radio operates as directed by external RF command, and reports that it is operating in directed mode on its baseband output to ADS-B, ASAS or display entities);
- If operating autonomously, the ownship ADS-B or ASAS entity can command a less rapid transmit rate than contained in SARPs for VDL Mode 4 if all VDL Mode 4 peers in the local airspace, to a range of X nmi,[7] are determined to be equipped with the L-band system(s) available on ownship;
Consider an airspace where all users are equipped with e.g. UAT and VDL Mode 4. The VDL Mode 4 component can operate at reduced transmit data rate since the UAT element meets short-range and mid-range requirements. The VDL Mode 4 system acts in a supplemental capacity at short range and offers long-range capability as well as addressed air-to-air data link. The same consideration applies to an airspace where all users are equipped with 1090 ES and VDL Mode 4.
Now consider the same airspace with a single user, U0 , hosting only VDL Mode 4. Other aircraft can determine that this user is limited to VHF ADS-B in two ways: a) no L-band ADS-B transmissions are ever received from this user; or b) a unique capability code transmitted within the VDL Mode 4 system, indicating that U0 can only receive ADS-B information on VHF.[8] Aircraft operating autonomously within X nmi of U0 will operate at the normal default transmit rates. So user U0 will form the center of a “bubble” of high transmit rate activity for VDL Mode 4. If the number of such users is small, overall channel activity in the VHF band may be substantially reduced. Total channel load is never any higher than the case where all users operate according to current SARPs.