3.1. ETMS source
ETMS data were recorded by the FAA (Federal Aviation Administration) and maintained by the Volpe Centre in Cambridge, Massachusetts. The data were a combination of radar positions and flight plan information. The data were organised into two tables: a flight table listing information on the flight characteristics and a flight chord table containing information on the aircraft positions during the flight (latitude, longitude, flight level, speed and time). The data covered North and Central America, the United Kingdom and many trans-Atlantic and trans-Pacific flights which originate/terminate in the USA.
3.2. ETMS file importing procedure
The importing procedure for ETMS files as developed in the Access prototype tool for the sample data set is described in “AERO2K Import ETMS tool” written by Nicu Stancioi (2002), (Appendix).

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4.1. Back Aviation source
In order to cover as many flights as possible in the world, ATC (Air Traffic Control) data were completed with schedule data extracted from the monthly Back Aviation database. The database contains listings of every scheduled jet and turboprop flights listed by city-pair and airline, and includes departure and arrival times, airplane code, aircraft type and trip frequency.

4.2. Setting constraints
The coverage of Back Aviation database is based on the schedule data submitted by the airlines according to their forecast for the next months. As a result, segment of a flight can be listed more than once. An airline may also code-share with another airline i.e. a single physical flight (same city-pairs and time of departure) will appear under the same code differentiated only by the airline code (ex: X21, Y21). To avoid duplicate counting, some constraints had to be set up when selecting the fields; the constraints are listed in Table 4 and explained below (Back Information Services, 2002).
Table 4: Constraints linked to selected fields.
SELECTED FIELDS  CONSTRAINTS 
Number of stops  EQUAL TO 0 
Flight record type  EQUAL TO U 
Restriction code  EQUAL TO PUB 
Origin  EQUAL TO .CARIB, .SAMER, ETC…. 
Equipment  NOT EQUAL TO .R .A 
Service-fare class  NOT EQUAL TO 2 3 4 

NUMBER OF STOPS
This refers to the number of intermediate stops between an origin and a destination. When origin is set and “Number of stops” is equal to “0”, which specifies a non-stop flight, it avoids that a single flight being counted twice due to multiple stops.
FLIGHT RECORD TYPE
This refers to the existence of unique and duplicate flight records in the OAG (Official Airline Guide) database. Due to the proliferation of commercial agreements between airlines, the OAG database contains duplicate flight records which carriers file with the OAG. In these cases, there may be only one physical flight movement, but the database reports two flight movements because two different carriers each filed a flight record. This results in an inflated number of flight movements. Establishing a constraint “equal to U” for unique flight records avoids duplicate flight records.
RESTRICTION CODE
Unrestricted flights are automatically published in the OAG hard copy publications, while restricted flights may or may not be published. Restricted flights, which were not previously published, are now available in BACK’s OAG database. The addition of these flights presents a more accurate picture of the number of physical operations taking place worldwide. Setting “restriction code” equal to “PUB” allows listing all published flights including restricted and unrestricted flights.
　
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