Evaluation made by U.S. DOT/Volpe Center:

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Evaluation of Swedish

GP&C System for ASTA Data Link


Stephen Heppe

Woody Phlong

Frederick Seellg


Final Report

October 1994


U.S. DOT/Volpe Center

Cambridge, MA 02142-1093


This report was prepared by Stanford Telecommunications, Inc. (STel), Reston, VA, and its subcontractor, Telenergy, Purcellville, VA, for the John A. Volpe National Transportation Systems Center, Cambridge, MA, and its sponsor, the Federal Aviation Administration (FAA), under Contract DTRS57-94-P-81126.

The authors wish to explicitly thank the GP&C development team, the Swedish Civil Aviation Administration (CAA) and their supporting subcontractors for providing an extremely informative series of briefings and demonstrations. Their depth of knowledge, commitment and responsiveness under questioning materially benefited the quality and completeness of this study.


Authors contributing to this report are:


Stephen B. Heppe (Telenergy)

Woody S. Phlong (STel)

Frederick W. Seelig (STel)


28 October 1994


This report extends the results of an earlier study (Attachment 1) comparing ten alternative data link systems for possible use by ASTA. This current effort was motivated by growing interest in the Swedish Global Positioning and Communications (GP&C) system, which represents an alternative data link technology to support ASTA as well as other applications. The GP&C system combines differential GPS or GNSS functions with data link and automation functions. The GP&C architecture consists of multiple airborne units and one or several groundbased units. The airborne unit, illustrated in Exhibit ES-I, contains a GPS or GNSS receiver (typically differential-capable), a microprocessor controller and a VHF data link processor/transceiver. Optional display devices can be added, along with processor and software augmentations, to support auxiliary or synergistic functions such as moving reap displays, cockpit display of traffic information, and data link input/output. The groundbased unit contains a DGPS reference station, microprocessor controller, VHF data link processor/transceiver, and display input/output terminals (personal or laptop computers) for system management, configuration and control.

The effort reported here included an on-site evaluation of a GP&C demonstration system operated by the Swedish Civil Aviation Administration (CAA) in Nörrkoping, Sweden. Quantitative and qualitative/subjective evaluation data was used to score the system against the following metrics: (1) performance and operations; (2) compatibility with existing NAS and supporting systems; (3) cost; (4) policy impact; (5) risk; and (6) schedule. The individual and aggregate scores of the GP&C system were compared to those of the 10 data link alternatives previously evaluated, and relative rankings were generated. Sensitivity analyses were performed to determine the sensitivity of the overall rankings to the weighting factors of the various metrics.

Technically, the GP&C system is capable of achieving its advertised objectives and those of ASTA data link. It is operationally flexible and synergistic with numerous emerging concepts that rely on aeronautical data link. It is relatively low cost and low risk, and it is free of technical defects that could affect integrity or operational usefulness. It has the philosophical support of a reasonable segment of the aviation community, it is being discussed within multiple ICAO panels, and demonstration programs in Sweden and elsewhere have been successfully completed. Operational systems are in existence, or planned, in Sweden and Malaysia. The U.S. Air Force has announced plans to imbed the GP&C system in larger avionics suites for selected transport aircraft. However, it has been developed outside of the normal standards process followed by the FAA, RTCA and ICAO. Its data link differs substantially from projected Mode

Ref: ICAO AMCP working grow of the whole (October 1994); agenda Item 5 presented by Larry Johnsson

Exhibit ES-1: Architecture of GP&C Airborne Unit

S 1030/1090 MHz operations and future VHF/VDL operations as envisioned by the current draft of the ICAO VDL SARPS. Selected elements and parameters of the GP&C data link protocol are currently driven by test and demonstration objectives, and could be reoptimized for an operational environment. Substantial capacity increases could be achieved with relatively straightforward reoptimization, but additional programmatic delay would ensue as the protocol is adjusted and finalized.

In terms of numerical score, relative to the 10 previously-evaluated systems, the GP&C system is ranked a close third overall behind Mode S squitter and Mode S interrogation. The GP&C system scored well in terms of performance, operational flexibility, cost and synergy with other systems. However, it was penalized for its lack of compatibility with current NAS systems and the relative lack of maturity surrounding the data link standard. When the metric weights for policy impact were reduced 50% in accordance with the sensitivity study protocol, the Mode S and GP&C alternatives were separated by only 0.1 points overall -- essentially a tic given the level of fidelity and confidence associated with the evaluation methodology. The methodology for sensitivity analysis did not formally allow the study of multiple weight shifts; however, GP&C might have emerged as the dominant alternative (by a narrow margin) if there had been a shift of emphasis from policy impact on the one hand, to cost, synergy and acceptability to the General Aviation (GA) community on the other.

The GP&C system has several clear advantages over Mode S. In the far term, it may, actually emerge as the preferred means of supporting automatic dependent surveillance in the air as well as on the surface, while providing a synergistic platform for other applications. It is likely to achieve more rapid penetration of the GA community than equivalent Mode S equipment and systems. On the other hand, it is currently less mature, offers no benefit to ASTA per se relative to the air carrier community already equipped with Mode S, and its use is not likely to be mandated for the foreseeable future. Its current implementation assumes an embedded GPS or GNSS receiver -- this is not strictly necessary, and could increase costs for users already equipped with a GPS or GNSS receiver.

The relative lack of maturity of the system implies that the FAA should exercise caution before relying on GP&C as a key element of ASTA or any other near-term system. On the other hand, given its long-term advantages and natural appeal to the GA community (as well as cost-conscious transport-class operators interested in leveraging VHF technology), there appears to be a national and international benefit associated with the promotion of GP&C standards and systems.


Strongly consider use of GP&C in the near term for ground vehicle support. Perform detailed cost/benefit tradeoff study for GP&C against other surveillance techniques with respect to ground vehicles. Assess potential far-term benefits of VHF data link such as GP&C. Encourage development of true international standards for GP&C data link and operating strategies. Ideally, achieve convergence of GP&C with VHF/VDL SARPS so that a single data link standard emerges for all applications. If further study indicates that GP&C represents a reasonable cost/benefit tradeoff for ground vehicle support, adjust ASTA architecture to incorporate GP&C and use it to "leverage" GA involvement in ASTA as well as general-purpose data link.

28 October 1994


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