# Contrails and Their Dependence on Meteorological Situations
## 2 Materials and Methods
### Contrail Observation and Pairing  
The current system of contrail research at the CTU uses three fixed cameras located in Decin to collect video records. The advantage of this placement and routing of the cameras was to record curved aircraft trajectories near the boundaries of adjacent FIRs, where aircraft above the navigation point of the original upper airspace flight paths changed direction, and the curved trajectory made the resulting long-lived contrail more easily distinguishable from natural cloud cover. The output is a set of discovered contrails with data on the time of contrail formation, camera number, and contrail lifetime. Next, data are collected from secondary radar receivers of Mode S messages, which, after decoding, contain the ICAO aircraft address, date and time, heading, latitude, longitude, altitude, GS, TAS, and Mach number. In the last step, the recorded contrails are matched with the decoded information from the aircraft that flew through the monitored airspace and caused the contrail [15]. Additional data were obtained by tracking contrails near Prague, which are created as close as possible to the active aerological probe to maximize the temporal and spatial accuracy of the matched aerological data. For this reason, an alternative system for collecting information into the database was proposed and is shown in Figure 1. The left side shows the receiving and processing of the signal from the active position of the RS41 aerological probe, which is used to precisely adjust the camera that records the contrail. The middle shows the process of collecting combined camera records with flight and meteorological data, resulting in a database of contrails. The right side shows the process of receiving, processing, and filtering Mode S messages from the aircraft transponders, which leads to data about the aircraft that generated the contrail. The alternative system focuses on finding out how to record the contrails formed as close as possible to the position of the aerological probe at the appropriate altitude and obtaining available additional information about the environment and aircraft in the investigated area. By receiving and processing the signal of the active probe and decoding the received message, the current position of the probe at a specific time can be obtained, and this position with the time stamp is stored in a database. At the same time, from the relative position of the current decoded probe position and the position of the camera site, the elevation and horizontal angle (from the defined base camera position) for setting the camera direction can be determined, and from the calculated distance of the probe from the camera site, the optimal focal length of the camera lens can be determined. These calculated values of the angles and focal lengths belonging to a specific probe position are used for continuous changes in the camera settings during the probe flight, stored in the database, and at the same time used to calculate the coordinates of the plan projection  
Figure 1: Flight data processing diagram of an alternative system for collecting information. of the area currently occupied by the camera, needed for filtering the received Mode S messages from the aircraft transponders. In parallel with the described processes, the receiving and processing of the signal from the Mode S transponders and the decoding and filtering of the individual messages are performed. Each decoded message must first be tested for the value of the DF field; only messages with DF 4, 17, 20, and 21 are processed. If a DF 17 message containing a position is found to be within the current calculated plan pattern and altitude range, the selected decoded information shall be stored in the database together with the timestamp and ICAO address, and information from other received DF 4, 20, and 21 messages with the same ICAO address shall also be stored, as long as the position in the DF 17 messages with this ICAO address is within the current pattern; otherwise, the messages shall be ignored.