Model Name: SIMMOD(3/2/96; ARO)
1. Primary Model Category:
Airfield and terminal airspace models. Secondary area: en route and regional airspace models.
2. Summary:SIMMOD can be used to simulate in detail: a full individual airfield (including runways, taxiways and apron areas); an airfield and its associated terminal airspace; a regional system of airports and the associated airspace; or, a regional volume of airspace. Its principal measures of performance (and outputs) are aircraft travel times, flows and throughput capacity per unit of time, delays and fuel consumption.
SIMMOD relies on high-level-of-detail network representations of airfields and airspace. Traffic moves along a network of links and nodes with each link or node (depending on whether airspace or airport surface operations are being modeled) being able to accommodate a single aircraft at a time. Whenever two aircraft converge on the same node or link, the operating strategies programmed into the model determine which of the two candidates will occupy that node or link first and which will incur delay. Aircraft paths on the network are either specified by the user for every origin-destination pair or determined internally by the model according to a shortest-path (Dijkstra) algorithm.
Much of the effort associated with setting up a SIMMOD simulation is, in fact, expended in developing the airspace and/or airfield network on which the traffic will move. For example, if a fan or trombone pattern is to be utilized to increase the efficiency of approach spacing and sequencing, all the possible alternative paths in the fan or trombone must be explicitly "programmed" as part of the network representation.
SIMMOD has several options for simulating probabilistic events and can provide highly detailed output statistics down to the individual aircraft level.
Competitive models are TAAM and, for airspace simulations only, RAMS.
3. Input Requirements:The principal input requirements are the specification of the network structure for the airfield and/or airspace simulated and the description of the traffic that will move on this network, including flight paths and paths between gates and runways. To partially automate the tedious process of developing such networks, one can use a digitizer to trace the network of runways, taxiways and taxilanes from an airport layout map. Such an approach reportedly reduces the amount of time necessary to set up a network representation for a typical major airport to approximately 2 days of effort. It is also possible to use flight plans to generate the route network on which a SIMMOD airspace simulation will be based. SIMMOD includes a database with performance characteristics for 19 types of aircraft. A recently-added SIMMOD capability developed by VPI checks the network specifications of airfields provided by the user to ascertain conformance with FAA standards for separations between runway/taxiway and taxiway/taxiway centerlines, runway exit curvatures, etc.
4. Outputs:SIMMOD provides highly detailed statistics on each aircraft simulated. Outputs can be obtained on: aircraft travel times; traffic flows past specified points; throughput capacity per unit of time; delays by time of day and location on the airfield or in airspace, along with the immediate reason for each delay; and fuel consumption.
5 Major Assumptions:The principal restrictive assumption in SIMMOD is that traffic must move on a pre-specified network of nodes and links according to pre-specified operating strategies or "rules of the road". In terms of conflicts between aircraft paths, SIMMOD is essentially a 1-dimensional model, checking for conflicts along the aircraft’s longitudinal path only, with no possibility of checking for lateral or vertical separation violations.
6. Computational Characteristics:SIMMOD is written in SIMSCRIPT II.5 with a pre-processor and post-processor in C. It can be run on a personal computer, but for large applications a workstation (Sun or HP) is recommended. A 500 MB hard drive is required as well as a tape drive. Operating system: Unix or DOS. The SIMMOD software includes the HOOPS graphics card.
As an indication of speed of execution, a simulation of 24 hours of operation at a major airport takes about 3-5 minutes (single run).
7. Modularity and Flexibility:Ongoing efforts at Eurocontrol and CAA are aiming at developing a data interface (SIMBUS) so that RAMS and SIMMOD can be operated serially, with RAMS bringing aircraft to the final approach and SIMMOD picking them up there to simulate airport operations (and conversely for departing aircraft). Another area of interest at CAA is the development of a capability to specify externally the operating strategy in use at an airport or section of airspace and have SIMMOD execute this strategy in simulating operations. Currently such strategies must be programmed as part of the logic of the model; changing them requires a major effort.
SIMMOD can be linked to the Integrated Noise Model (INM) so that the noise impacts of airport operations can be estimated.
8. Status of Model:SIMMOD is a mature model, having undergone many revisions and improvements over a period of more than 15 years. Funding does not currently exist in the FAA for additional development of SIMMOD. The FAA recognizes the value of SIMMOD and is considering new funding methods to ensure SIMMOD's future growth and development. One proposed funding scenario involves bringing SIMMOD under the umbrella of the FAA's proposed Aviation Operations Research Center of Excellence (COE).
9. Extent of Model Validation:The ATAC Corporation conducted a validation study of SIMMOD in 1988 (see Bobick, J. C., "Validation of the SIMMOD Model," ATAC Corporation, Mount View CA, Contract No. DTFA03-85-C- 00043). The ability of the model to provide realistic results under quite complex operating conditions has been confirmed repeatedly in a number of airport and airspace simulation studies.
10. Principal Applications:SIMMOD is possibly the most widely utilized airport and airspace model in the world today, with about 300 registered users worldwide, 50-100 of whom are believed to be currently active. The model has also been the beneficiary of significant support and promotion by the FAA over the past decade.
The great majority of applications to date have dealt with the capacity and delay impacts of a variety of operational alternatives at airports. More recently, several studies dealing with reconfiguring regional or terminal airspace to reduce delays, reduce fuel consumption or improve operational efficiency have also utilized SIMMOD.
11. Model Availability:SIMMOD is available at a nominal cost from the FAA (about $1,500 for the workstation version, about $400 for the PC version). Several companies in the United States (ATAC, CACI, SDT) offer training courses, typically one-week long, and/or provide software support for SIMMOD.
Contact persons in the FAA are Tony Vanchieri ((202) 358-5198, fax (202) 358-5543, firstname.lastname@example.org ) and Steve Bradford ((202) 358-5234, email@example.com).
12. Information Base for Model Evaluation:Interview with Steve Bradford on 12/18/95.
Informal discussions with several users in the United States, Europe and Australia.
Review of a draft report (October 1995) by DLR on an extensive series of simulation experiments at Frankfurt airport and terminal airspace.
Review of the following documents: (i) Federal Aviation Administration, SIMMOD Data Input Manual, 1989; (ii) Federal Aviation Administration, SIMMOD Information Brief, 1989; (iii) Federal Aviation Administration, SIMMOD Reference Manual, Office of Operations Research (AOR-200) 1989.
13. Summary Evaluation:In the hands of a skilled user, SIMMOD is possibly the most powerful existing tool for "fine granularity" simulation of airport surface operations, allowing for arbitrarily high levels of detail (e.g., simulation of push-back operations, gate occupancies, de-icing procedures, etc.). Several airport studies conducted with SIMMOD to date illustrate this point.
The principal perceived weakness of SIMMOD is that it is a "labor intensive" model whose users must undergo a significant amount of training. Moreover, to avoid several potential pitfalls, SIMMOD users must have a very good understanding of ATM and airport operations. For example, because SIMMOD is essentially a one-dimensional model (i.e., it can check for conflicts between aircraft only along the paths traced by the elements of a network) care must be taken so that the network structure on which the traffic moves is based on sets of nodes and links with sufficient lateral and vertical separations to avoid the presence of undetected conflicts during the simulation.
Another difficulty in SIMMOD is the modeling of dynamic rerouting of aircraft to simulate the ATM system's responses to local congestion problems.
In summary, especially when its low acquisition cost is considered, SIMMOD may be the model of choice for high-level-of-detail airport simulations, with TAAM the principal competitor. The model's steep "learning curve" should, however, be recognized. For airspace studies, both RAMS and TAAM may be better alternatives at this point. For the specific case of evaluating the Free Flight concept in en route, transitional and terminal area airspace, an important limitation is the pre-specified underlying network structure on which traffic is restricted to move in the SIMMOD model.
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