# Prediction and Analysis of Airport Surface Taxi Time: Classification, Features, and Methodology
## 2 Classification of Research Perspectives
### Type of Surface Taxi Time
#### 2.1.1 Reference Time  
In this section, we introduce the methods for calculating the reference time of aircraft movements, from the Aviation Policy and Planning Office (APO) within the FAA, and the Performance Review Unit (PRU) within EUROCONTROL. (1) FAA APO method  
The FAA defines reference time as the taxi duration of an aircraft under ideal operating conditions without any obstructions. That is, without encountering congestion on the surface, adverse weather conditions, or other factors that might affect the normal taxi time. FAA APO has put forth a method for estimating reference time based on queueing theory [24]. The fundamental concept of this method involves categorizing flight data according to various parameters such as airline, arrival or departure aircraft type, operating season, airport, and actual gate. Subsequently, for each subgroup, a linear regression model is constructed to estimate the reference time, utilizing the departure and arrival queues as independent variables. The constant term in the regression model represents the reference time; i.e., when an aircraft is pushed back from a stand without encountering any queues of aircraft either waiting to take off or taxi in. In other words, the regression model is established under the condition that the aircraft is not queued. Figure 3 shows the steps of the FAA APO method for calculating unimpeded taxi-out time. The number of departure aircraft is usually set to one and the number of arrival aircraft is set to zero [13]. (2) EUROCONTROL PRU method  
EUROCONTROL PRU monitors and reviews the performance of pan-European air navigation services, and utilizes saturation levels to calculate the reference taxi time from each stand to the runway [25]. The saturation level indicates the point at which the taxi time no longer decreases, even if the congestion level decreases. Figure 2: Time mapping of the taxiing phase. The method aims to calculate the reference taxi-out time for a set of similar flights. Initially, each flight is categorized according to the type of stand, the departure runway, and the congestion index, in which the congestion index represents the number of departures of other aircraft between the time a departure aircraft went off-block and its actual takeoff time. Subsequently, for each group of flights (e.g., the same stand-runway combination), the reference time is calculated by taking the truncated average of the flights within the group with lower congestion index values, taking into account the congestion index threshold. This involves setting a congestion index threshold for each group, then trimming flights within the group by applying the threshold on the congestion index. Subsequently, the truncated mean of the remaining flights in the group is calculated, averaging taxi-out times between the 10 th and 90 th percentiles. Then the surface movement delay, which is the difference between the average taxi-out time of all flights within the group and the previously determined reference time, is calculated. Finally, a more accurate result is obtained by performing a weighted average of all surface movement delay groups [13; 26]. In estimating the reference taxi time, this method considers taxi distance; reference taxi time is estimated by applying the level of congestion associated with the surface travel time to the ground travel distance and surface operating conditions of an aircraft as it moves from a particular stand to the runway. In contrast to the FAA APO method, this approach incorporates taxi distance when estimating reference taxi-out time. Figure 4 shows the steps of the method used by the PRU to determine the reference time. Figure 4: EUROCONTROL PRU method for determining unimpeded taxi-out time. Figure 3: FAA APO method for determining unimpeded taxi-out time. Regression modeling method  
Several scholars have proposed heuristic improvements to the FAA APO method, such as incorporating more independent variables to enhance the estimation of reference time. [PERSON] et al.