8 October 2001
AMCP WGC3/WP10
AERONAUTICAL MOBILE COMMUNICATIONS PANEL (AMCP)
Working Group C Meeting No. 3
Anchorage, Alaska
15 –19 Oct 2001
ADS-B Candidate Link Developments Since the Issuance of the TLAT
Report
Presented by the U.S. Member
(Prepared by)
George Ligler, PMEI
Lawrence Bachman, Johns Hopkins Applied Physics Laboratory
(The authors acknowledge helpful information provided by Dr. Vincent Orlando, MIT
Lincoln Laboratory, on the 1090 MHz Extended Squitter and Dr. Constantine Tamvaclis,
EUROCONTROL, on ongoing EUROCONTROL Analysis of VDL Mode 4 )
SUMMARY
This paper provides information on further evaluation/development of the three ADS-B
link candidates (1090 MHz Extended Squitter, VDL Mode 4, and UAT) since the
issuance of the TLAT Report in March 2001. In several cases (particularly for UAT),
link design/link equipment assumptions made by the TLAT have been altered by
candidate system experts, with concomitant potential changes in projected link
performance. Additionally, system experts have further evaluated likely future
interference environments in high density air traffic scenarios. For example, for 1090
MHz Extended Squitter, less severe interference environments than those used by the
TLAT are being proposed as being more realistic; for UAT, interference environment
modeling not performed by the TLAT is being addressed in detail.
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Introduction
The issuance of the Technical Link Assessment Team (TLAT) report in March 2001
should be viewed as an indicative, but not “final”, technical evaluation of the three ADSB link candidates, 1090 MHz Extended Squitter, VDL Mode 4, and UAT. While subject
matter experts for all three candidates agreed, within the context of the TLAT charter, to
a set of unanimous technical conclusions, the TLAT Report was in part based upon
agreed assumptions and at several points indicated further technical investigations that
might be undertaken. Furthermore, ADS-B candidate links should not be viewed as
“static”—link design changes have been recently made/suggested and should be expected
to continue (though hopefully at a reduced rate) in the future.
Indeed, at least two of the link candidates have subsequently changed link/link equipment
design assumptions used by the TLAT (alternative design assumptions for the third
candidate have been suggested by particular system experts for that candidate). Also, as
the projection of RF interference environments in future high density air traffic
environments such as the Los Angeles Basin and Core Europe is decidedly not an exact
science, sensitivity analyses involving those interference environments have been
proposed and/or undertaken. Moreover, several analyses which the TLAT did not
perform (e.g., the assessment of UAT in a fully loaded RF interference environment)
have necessarily been investigated. This paper provides a brief summary of post-TLAT
developments/investigations for the ADS-B candidate links.
1090 MHz Extended Squitter
Continuing development of the 1090 MHz Extended Squitter is being undertaken by
RTCA Special Committee 186, Working Group 3, in conjunction with EUROCAE
Working Group 51. A planned revision to RTCA DO-260, the Extended Squitter MOPS
has been under development for almost a year and deals with, inter alia, the codification
of improved Extended Squitter reception techniques. It has recently been determined that
the performance of the Extended Squitter receiver used in TLAT modeling does not
represent the optimum performance that can be achieved with an enhanced Extended
Squitter processor. Sampled data from Extended Squitter measurements taken in
Frankfurt, Germany have been evaluated by Extended Squitter system experts as showing
that enhanced processing techniques provide significantly better reception performance in
a high ATCRBS fruit environment than the processing techniques implemented in the
Extended Squitter receiver used in the TLAT modeling. At present a quantitative
estimate of the performance improvement (over the performance estimates developed by
the TLAT) that could result from using enhanced reply processing techniques does not
exist, but the development of such an estimate is expected in the near future as part of the
DO-260 revision process.
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1090 MHz system experts have also suggested that the fruit rate (interference)
environments predicted in the TLAT Report for the 2020 Los Angeles Basin and 2015
Core Europe are unrealistically severe. A paper developed by the SCRSP Working
Group B in response to AMCP WG-C’s request for comment on the TLAT Report details
the rationale for this suggestion, elements of which involve the effects of ground
geography and recently updated projections of likely future SSR Interrogator
environments in high density airspace. Again, quantitative estimates of performance
improvements that might result from an altered interference environment in future high
density air traffic environments are expected to be provided in the near future.
VDL Mode 4
Several VDL Mode 4 related investigations have been undertaken in order to supplement
the results presented in the TLAT Report. (Responses to AMCP WG-C/2 WP30 are
addressed in another paper.)
First, in the TLAT Report, the receiver diversity for VDL Mode 4 was handled in an
approximate manner, instead of being calculated exactly. This limitation was due to the
nature of the simulation used and the limited time available. However, subsequent to the
issuance of the TLAT Report, the simulation was modified, and the exact diversity
calculation was incorporated, replacing the approximation. The results of this new
simulation did indeed indicate a slight improvement over the results reported by the
TLAT, but the amount of the change was quite small and would not have materially
affected the graphical results depicted in the Report. In addition, none of the TLAT
findings for VDL Mode 4 would have been changed in any appreciable way as a result of
this improvement in the simulation.
Second, analysis was continued on the “honeycomb” channel management scheme
proposed as an alternative to the primary VDL Mode 4 channel management plan
recommended for evaluation by the TLAT’s VDL Mode 4 subject matter experts. For
reasons including non-availability of important information needed to conduct the study
until a few days prior to the final meeting of the TLAT, only preliminary analysis of the
honeycomb proposal was included in the TLAT Report. Analysis by Johns Hopkins APL
continued thereafter, however, using the metrics described in the TLAT Report. This
analysis substantiated the preliminary conclusion stated in the Report, namely that the
“honeycomb” channel management scheme did not appear to provide better results than
the primary VDL Mode 4 channel management plan evaluated by the TLAT. Indeed, the
“honeycomb” channel management scheme might well not perform as well as the
primary channel management scheme proposed by the TLAT’s VDL Mode 4 subject
matter experts.
Third, since the publication of the TLAT Report, EUROCONTROL has continued the
analysis of the three main candidate links (1090 MHz Extended Squitter, VDL Mode 4,
and UAT) focusing on the validation/refinement of the TLAT results and the possible use
of multi-link combinations. For VDL Mode 4, additional simulation runs have been made
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on the Core Europe 2015 scenario. Runs for scaled-down traffic densities corresponding
to 2013, 2011, 2009 and 2007 have been done. A number of changes have been
incorporated on the VDL Mode 4 model to remove some of the limitations of the
original. The results obtained so far indicate that the six VHF channel solution [two
global channels, two regional channels, and two local channels] considered by the TLAT
would reach saturation around 2013. Additional simulations are also being done
assuming a four VHF channel configuration [two global channels, two local channels]
starting from the year 2005. The results obtained so far suggest that the system might not
be able to support full equipage beyond 2007, but it is not yet clear what would happen
under more realistic conditions [partial equipage and use of TIS-B]. The simulation effort
continues as the variance of the results remains significant.
UAT
Subsequent to the issuance of the TLAT report in March, 2001, there have been a number
of developments pertinent to the process of estimating the performance of the UAT
system in future high density air traffic environments. The three main factors which have
affected the TLAT-reported system performance are:
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

Discovery of a “bug” in the receiver performance model used by the TLAT,
which penalized performance in a self-interference environment; i.e., the
reception of a UAT message under conditions of interference from other UAT
transmitters (the vast majority of cases) was degraded by around 9 dB from
the actual expected performance. Correcting this error produced significantly
improved system performance to that presented in the TLAT Report
(mitigating the impact of the inclusion of JTIDS/MIDS and DME interference
discussed below).
Modifications to the transmit powers used on the various classes of aircraft
during the development by RTCA Special Committee 186, Working Group 5,
of a UAT MOPS. The TLAT Report used a specific set of transmit power
requirements for the various classes of aircraft provided by the TLAT’s UAT
subject matter expert. Work on the transmit power requirements for the UAT
MOPS involves establishing a reasonable compromise between performance
and cost, and the final transmit power requirements have not yet been
finalized. A nominal set of transmit powers is being used for analysis.
Inclusion of severe interference scenarios from JTIDS/MIDS (Link 16)
airborne and DME ground transmissions. The TLAT evaluation of UAT did
not, as stated several times in the TLAT Report, include consideration of RF
interference from systems other than UAT itself. Given the importance of
establishing UAT compatibility with existing systems in its band, the
scenarios used for these sources of interference, provided by Link 16 and
DME experts, are necessarily harsh. Clearly, the addition of these sources of
external interference degrades system performance. As a consequence of this,
the UAT waveform has been modified from that assessed by the TLAT to
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provide greater resistance to these sources of pulsed interference.
Additionally, a narrower bandwidth receive filter has been evaluated during
the UAT MOPS development. These modifications have been incorporated in
the UAT simulation.
As part of the UAT MOPS effort, the cases corresponding to the TLAT scenarios have
been re-analyzed, corresponding to the inclusion of the three effects mentioned above.
In the 2020 Los Angeles Basin scenario, a severe Link 16 environment was included but
no DME interference was inserted, as there are no DMEs operating or planned to operate
in the United States at either the target UAT operating frequency of 978 MHz or at 979
MHz. Even in the presence of severe Link 16 interference, the UAT system is projected
by UAT system experts in the 2020 LA Basin scenario to achieve all air-to-air ADS-B
state vector update TLAT requirements, including the support of air-to-air deconfliction
at ranges of 150 nmi as proposed by EUROCONTROL. Trajectory Change Point (TCP)
update requirements are projected to be met to a range of 150 nmi for two TCPs (as
presently required by the RTCA ADS-B MASPS, DO-242). This result is an
improvement over that reported by the TLAT for the 2020 LA Basin scenario. Air-toground performance and support of the timely delivery of more than two Trajectory
Change Points (EUROCONTROL has indicated a potential requirement for four) are
presently being evaluated.
The majority of UAT MOPS performance modeling efforts, however, have focused on
the Core Europe 2015 scenario. In this scenario, the same severe Link 16 interference
environment was used as in the 2020 Los Angeles Basin scenario, and two adjacent
channel (979 MHz) DME ground transmitters were included, both within around 30
nautical miles of the victim receiver aircraft. In this combined severe interference
environment, the TLAT state vector and TCP (for two TCPs) update requirements are
projected by UAT system experts to be met at more than 100 nautical miles, while in the
TLAT Report, this distance was reported to be around 70 miles. Additional sensitivity
analyses involving DME interference levels and additional TCPs are being conducted.
Further efforts in the UAT arena will be undertaken as part of the UAT MOPS effort in
order to complete characterization of UAT performance in areas including receiver
performance at a number of different altitudes, performance of different receiver
implementations, and provision of FIS-B and TIS-B.
Recommendation
The working group is invited to note the benefits of UAT, and recommend the initiation
of ICAO SARPs development for that system.