National Aeronautics and Space Administration

Glenn Research Center

AwCNS Modeling & Simulation Capability

ACES with Communications, Navigation, and Surveillance (AwCNS) Modeling and Simulation Capability

  • Donald Van Drei, Denise Varga, Mike Zernic – NASA Glenn
  • Greg Kubat – Analex Corp.
  • Norbert Seidel – RS Information Systems, Inc.


In the current NAS, Air Traffic Management and Aircraft rely on specialized Communication, Navigation and Surveillance (CNS) systems to provide and maintain effective control and operations. In previous versions of Advanced Concept Evaluation System (ACES) – a modular, system-level simulation capability, many of these CNS systems are accommodated as ideal functions to provide these services. However, this idealize functionality does not allow for the real-world effects that are inherently imposed with CNS systems such as voice message delays, the impact of voice message collisions and retransmissions, and the accuracy of Surveillance and Navigation system data. ACES with CNS (AwCNS) was developed as a first step to integrate simulated Communication, Navigation and Surveillance systems in ACES.

In selecting which CNS Technologies to model it was decided for the initial implementation to model those technologies in common use in the NAS today. These technologies, VHF Voice Communications, VHF Omni-directional Radar/Distance Measuring Equipment (VOR/DME) Navigation, and Secondary Surveillance Radar (SSR) serve as the baseline for future comparison with emerging CNS technologies that are envisioned.

With the inclusion of these capabilities, not only can individual Communication technology, Navigation technology, Surveillance technology and their corresponding applications be studied for individual system performance, but entire National Airspace System (NAS) simulations can employ these systems for various concept of use and operation evaluations for overall effectiveness and inter-relation performance impacts. Thus, technology performance assessments can be made with respect to a particular Next Generation Air Transportation System (NGATS) utilization concept, or NGATS utilization concept assessments can be made with respect to a particular technology.

Modular Design Approach

With the understanding that a variety of Communication, Navigation and Surveillance systems could potentially be modeled, developing a modular approach was viewed as essential for the integration of the initial (baseline) capabilities. By taking advantage of the ACES Agent paradigm a CNS modular design was developed. This design approach will enable greatly simplified incorporation of future CNS system capabilities, thereby enabling comparative analysis with existing baseline CNS simulations to be performed.

VHF Voice Communications

Technology Description

The VHF band 118-137 MHz provides the primary communications medium for ATS and AOC communications for all areas of the world where ground stations providing radio line-of-sight services can be established in a practical manner. The VHF band is used by civil authorities to provide ATS communications and by the airlines, business aviation, and general aviation to provide AOC communication. (RTCA DO-237)

For Air Traffic Management, Voice Communication message exchanges for directing airport gate departures, ground traffic instructions, take-off clearances, landing sequences, air route change information and aircraft maneuvers as well as air-space, region and sector-to-sector transitions are typical voice sequences which occur via voice communication. AwCNS implements voice communication, simulating voice message exchanges that are typical for gate-to-gate aircraft/flight operations. The simulated voice communication system transmits messages using a VHF Radio communication model to provide voice message delivery and exchange characteristics with representative delay and message collision handling capabilities.

Voice Comm Modeling in ACES

In order to model voice communications in ACES the system uses; 1) a voice message description file, 2) a modification to the ACES Communication Service and 3) a VHF Radio/Propagation model. The operation of these primary functions is shown in the following diagram. In the message delivery process, message information for the particular message to be sent is drawn from the voice message description file first and either carried with the message or used to establish when in the simulation the message will actually occur. For transmission of the message, the message is sent from the sender (via the CNS Forwarder) to the Communication Service where a CNS variation on the Communication Service is applied. In the Communication Service the message is wrapped with the tag for delivery to the Communication Agent in the Generic Master of the receiver Agent and its tag for the Receiver Agent. Once received by the Communication agent, the appropriate Communication Agent activity (model) acts on the message and then publishes the message to the tag of the receiver Agent completing the message delivery process.

Voice Comm Modeling in ACES

Applicability to ATM Systems Analysis

Results of Voice Communication simulations will reflect a representation of voice message load and message timing data that occurs in the NAS for each flight, airport, sector and region. Much of the analysis performed utilizing ACES focuses on efficiency, this information if properly analyzed and reflected into other flight operations analysis is valuable as an additional metric as solutions are presented. With the addition of simulated voice communications, and with planned new technology systems added in the future for operational comparisons, the potential for effective concept studies is enhanced.

VOR/DME Navigation

Technology Description

VHF Omni-directional Radar systems provides azimuth information to aircraft. By transmitting two signals at the same time from a ground station (one signal constant in all directions, while the other is rotated about the station) airborne equipment receives both signals, looks (electronically) at the difference between the two signals, and interprets the result as a radial from the station. VOR is enhanced through use of DME systems. DME equipment onboard the aircraft transmits a stream of interrogations to the ground station. Each interrogation is made up of a pair of RF pulses. When the ground station receives the interrogation, it pauses and then sends a pair of reply pulses back to the aircraft. Airborne DME equipment receives the reply and measures the elapsed time from when it sent the interrogation until it received the reply. The system then calculates the round-trip time. From this, it can figure out its exact distance from the ground station using simple arithmetic.

Navigation Modeling in ACES

VOR/DME is the default navigation aid which AwCNS provides to generate simulated latitude/longitude information available to the aircraft. Implementation of the VOR/DME model will be provided as a Navigation activity of the Flight Agent. As shown below, true flight model position is used by the navigation activity to compute the reported position by adding a VOR/DME error to the true position. For this model, an additional input is also provided that identifies the location of the nearest Nav-aid Ground Station for determination of slant distance that adjusts accuracy of the reported position. The new reported position will be logged for post-simulation analysis.

Navigation Modeling in ACES

Applicability to ATM System Analysis

Results of the data collected from simulations with Navigation system models in ACES provide data that indicate uncertainties and inherent inaccuracy that are associated with these systems. In studies where air traffic congestion in and around airports or in-trail separation are being evaluated, analysis can be accomplished that compares aircraft position results.

Secondary Surveillance Radar

Technology Description

Secondary Surveillance Radar provides surveillance information to ATC for appropriately equipped aircraft. SSR operation requires an uplink interrogation, a reception and response by a cooperative airborne transponder’s downlink reply. All SSR systems use 1030 MHz uplink frequency and 1090 MHz downlink frequency. SSR systems are used in conjunction with the primary radar to determine the presence of planes in the airspace allowing controllers to track each plane more precisely and efficiently.

Surveillance Modeling in ACES

SSR is the default Surveillance system which AwCNS provides to generate simulated latitude/longitude information to Air Traffic Controllers. Implementation of SSR simulation is provided by the creation of a Surveillance Agent. As indicated in the figure below, true flight model position is used by the surveillance activity to compute the SSR reported position by adding a SSR error to the true position. The information provided by the SSR model is logged and available for post-simulation analysis.

Surveillance Modeling in ACES

Applicability to ATM system analysis

Results of the data from simulations with Surveillance system models in ACES provide data that indicate uncertainties and inherent inaccuracy associated with the systems that provide ATC with aircraft position information. In studies where air traffic congestion or in-trail separation is being evaluated where direct ATC management of aircraft are being studied. These results can now be used to determine how the uncertainties will affect decisions for directing aircraft in those studies.

Fiscal Year 2006 Additional Models

For the second phase of the AwCNS plan, three new CNS system models and capabilities that were identified as desirable improvements to the baseline system will be added. The new systems are: 1) a CPDLC/VDL-2 Communication Datalink model, 2) an ADS-B Surveillance Model, and 3) a model of both Local/Wide Area Augmentation System (LAAS/WAAS) GPS Navigation. All of these systems will provide simulations capability of new CNS technologies that are under consideration by the FAA for implementation in the NAS. In addition, improvements to the existing baseline CNS models will be made. These are: 1) modifications to the communication system capability that will allow ATC-Pilot message to activate aircraft maneuvers, 2) closed-loop operation for both Navigation and Surveillance systems, 3) enhanced terminal area mapping for communication system frequencies for the ORD and EWR terminal areas, and 4) enhancement to frequency assignments in the en-route airspace that distinguishes short sector duration, aircraft transition times and restricts unnecessary ATC frequency assignments.

Laboratory Technical Description

AwCNS Lab Overview

The AwCNS development lab consists of 6 systems all connected to the External Services Network (ESN). A single processor system is used for Configuration Management, Defect Tracking and serving out software to the other lab systems. It is the only system in the Domain Name Service (DNS) visible externally and is used for tele-conferencing. The 5 dual processors have Windows XP with MySQL database on 1 system as well as ACES software for conducting tests. Two systems have larger disk drives to house the database data and provide backup functions.

Hardware Environment

  • 1 Dual Xeon Processor Intel Server
    • 1GB memory
    • 80GB drive
    • CD-RW
    • Windows XP SP2
  • 5 Dual Xeon Processor Intel Servers
    • Dual 3.6 Xeon XA
    • 2GB, DDR2 – 400 memory
    • 300GB, DVD+-RW (2 systems, 120GB DVD-ROM on 3)
    • Windows XP SP2
  • Switch connected to the ESN
  • A rack containing all the components with a KVM switch and monitor

Software Tools

  • Configuration Management System – Subversion with Tortoise Client Defect Tracking – Bugzilla
  • Collaboration – WebEx, Xythos file sharing
  • Office – MS Office, Open Office
  • Management – MS Project
  • Databases – MySQL
  • Project Software – ACES with CNS


  • Plasma screen with access to WebEx and speaker phone for tele-conferences.