Department of Atmospheric Physics

Overview

An extensive testing of different time-tagging devices (Eventech Ltd., Latvia) was carried out during several dedicated test campaigns that were conducted in various weather conditions at the Warsaw ACTRIS National Facility. The unit responsible for preforming and analysing the tests was the Remote-Sensing Laboratory (RS-Lab) of the Warsaw Observatory Station (WOS) at the Institute of Geophysics, Faculty of Physics, University of Warsaw.

The main goal of the tests campaigns was to obtain the datasets enabling the quality assessment of the existing devices of Eventech Ltd. to provide recommendations for their adaptation to perform lidar-like measurements. Finally, the effectiveness of increasing the vertical range resolution of the measured data in the photon-counting mode was critically assessed.

After the initial testing of the existing Eventech Ltd. device: Stream Time Tagger 7 Series (ESTT 704), the necessary technological changes were identified by Warsaw ACTRIS NF staff and communicated to the company staff. Then the changes were implemented by the company into the new device prototype: Eventech Photon Counter (EPC).

The EPC achieved native spatial resolution as fine as 0.375 m, that is an order of magnitude higher than any classical lidar avaliable within the EARLINET-ACTRIS lidar community. Therefore, this development has a vital role as a complementary detection system to those used in the existing lidars within the EARLINET-ACTRIS.

Here, we introduce the datasets obtained during the EPC testing at the Warsaw ACTRIS National Facility. Below a short description of the EPC device is given, together with description of the Mie-Raman lidar that was used to complete the testing procedures and the critical assessment of the EPC.

Eventech Photon Counter (EPC)

The Eventech Photon Counter (EPC) is a new generation, high-precision, photon-counting signal acquisition system designed for atmospheric, oceanic and environmental lidar applications. It provides an exceptionally fine temporal (and consequently – spatial) resolution reaching 2.5 ns (0.375 m) with sub-nanosecond (10 ps) precision. 

European Space Agency mobile Mie-Raman lidar (EMORAL) 

EMORAL lidar working principle is based on a simultaneous, coaxial, vertical emission of 3 laser pulses at 355, 532, and 1064 nm into the atmosphere with 10 Hz pulse repetition rate. Laser pulses that are scattered by atmospheric constituents in backward direction are detected with a 300 mm Cassegrain reflector telescope with adjustable field of view: 2-3.6 mrad. The data acquisition is done with the Licel GmbH transient recorders with 3.75 m spatial and 60 s temporal resolutions at 9 channels: Mie-scattering (at 355, 532, 1064 nm), Mie-scattering with depolarization (at 355, 532 nm), N₂ Raman scattering (at 387, 607 nm), H₂O Raman scattering (at 408 nm), and a broadband fluorescence channel (420-520 nm). For all channels, analogue and photon-counting mode of detection is used, except for 1064 nm (analog only). In this work we used only photon counting detection type.

Observational coverage:

The dataset covers three observation cases representing different seasonal and time-of-day conditions:

1.      case 1

condition: "Winter daytime"

filename code: “case_id_1”

measurement time: 28 January 2025 11:30 – 11:35 UTC

2.      case 2

condition: "Winter nighttime"

filename code: “case_id_2”

measurement time: 28 January 2025 17:48 – 17:53 UTC

3.      case 3

condition: "Summer evening-to-nighttime"

filename code: “case_id_3”

measurement time:

a)         05 June 2025 20:55 – 21:35 UTC

b)         05 June 2025 22:30 – 22:45 UTC

Data Preparation:

EMORAL dataset

The EMORAL lidar dataset was prepared using the Expandable Module for Rapid Analysis of Lidar Data (EMERALD) software developed by the RS-Lab Team at the Institute of Geophysics, Faculty of Physics, University of Warsaw.

The data preparation workflow consisted of the following steps:

1. Selection of the lidar data files (Licel GmbH) overlapping with EPC detection.
2. Reading the raw lidar data files.
3. Choice of the detection channel of interest.
4. Conversion of range-bins to altitude in meters.
5. Extraction of the lidar signal without applying additional signal processing.
6. Averaging of the selected signal over the chosen measurement period. (for photon-counting data this corresponds to the time-averaged total counts).
7. Exporting of the selected lidar signal as a plain text files (.txt).

NOTE: No additional corrections, filtering, smoothing, or normalization was applied to the lidar signal before the export, unless stated otherwise in the dataset-specific documentation.

EPC dataset

The EPC dataset was prepared for further analyses using a Python-based processing script developed at the RS-Lab Team at the Institute of Geophysics, Faculty of Physics, University of Warsaw.

The data preparation workflow consisted of the following steps:

1. Reading of the raw EPC data file (Eventech Ltd.).
2. Conversion of range-bins to an altitude in meters.
3. Creating the high-vertical-resolution EPC dataset that retains the fine, native altitude sampling (0.375 m).
4. Creating the low-vertical-resolution EPC dataset that is rescaled to match the EMORAL native altitude resolution (3.75 m).
5. Export of the EPC signal as a plain text file (.txt).

Data files structure:

EMORAL dataset information:

1.      file_name: "<case id number>_EMORAL_<detection_channel>_<detection mode>_<measurement start time>_<measurement stop time>_<EMORAL WSU mode>.txt", where:

1.1. case id number, e.g., “case_id_1” case 1 dataset

1.2. detection_channel: the detection channel contain in the dataset e.g., “532p” for detection channel at 355 nm and parallel polarized state

1.3. detection_mode: mode of the detection acquisition, i.e. “ph” for photon counting

1.4. EMORAL WSU mode: the position of lidar wavelength separation unit (WSU), where "normal" is the WSU position for regular observations, and “rotated+45” refer to WSU in the +45 degree rotation position.

2.      file_type: "Plain text tabular data"

3.      file_format: "Tab-separated values"

4.      delimiter: "Tab character"

5.      header_row: true

6.      number_of_columns: 2

7.      number_of_rows: 16381

8.      row_granularity: "Each row represents of averaged over time of photon count rate value at one altitude value."

9.      record_order: "Rows are ordered by increasing altitude."

10. encoding: "UTF-8 or plain ASCII-compatible text"

11. decimal_separator: "."

12. column_names:

12.1. altitude [m]

12.2. photon_counts [MHz]

13. table_description:  Table contains paired numeric values describing how photon count rate varies with altitude. The first column gives the altitude value for each observation. The second column gives the corresponding averaged photon count rate.

Dataset variables:

1.      name: "altitude [m]"

1.1. position: 1

1.2. data_type: "Floating-point number"

1.3. measurement_level: "Continuous numeric variable"

1.4. role: "Independent or indexing variable"

1.5. description: Altitude value associated with the observation. Values increase at a regular interval across the file.

1.6. unit: "meters"

1.7. allowed_values: "Any valid numeric value within the documented measurement range".

2.      name: " photon_counts [MHz]"

2.1. position: 2

2.2. data_type: "Floating-point number"

2.3. role: "Measured photon count rate by photomultiplier (PMT)"

2.4. description: Numeric averaged over measurement time of photon count rate value corresponding to the altitude value in the same row.

2.5. unit: "MHz"

EPC dataset information:

1.      dataset_versions:

1.1. EPC native resolution

1.1.1. version_name: "epc-resolution"

1.1.2. vertical_resolution: 0.375 meter

1.1.3. description:  High-vertical-resolution version of the dataset, follows the native resolution of EPC.

1.2. EPC rescaled to EMORAL native vertical resolution

1.2.1. version_name: "tr-resolution"

1.2.2. vertical_resolution: 3.75 meter

1.2.3. description: This version follows the same data structure as the EPC native resolution version but uses a coarser altitude spacing. The resolution follows the native EMORAL-TR (standard LICEL resolution) vertical resolution. The coarser resolution derived from integration the counts within coarse resolution sampling from EPC native resolution.

2.      file_name: "<case id number>_EPC_<detection_channel>_<detection mode>_<version_name>_<measurement start time>_<measurement stop time_<EMORAL WSU mode>.txt"

3.      file_type: "Plain text tabular data"

4.      file_format: "Tab-separated values"

5.      delimiter: "Tab character"

6.      header_row: true

7.      number_of_columns: 2

8.      row_granularity: > Each row represents measured number of photon counted at one altitude level.

9.      record_order: > Rows are ordered by increasing altitude.

10. column_names:

10.1. altitude [m]

10.2. photon_count [counts]

 variables:

1.      name: "altitude [m]"

1.1. position: 1

1.2. data_type: "Floating-point number"

1.3. measurement_level: "Continuous numeric variable"

1.4. role: "Vertical coordinate, independent variable, or indexing variable"

1.5. description: Altitude value associated with the observation. Each altitude value defines the vertical level at which the corresponding photon count value is measured.

1.6. unit: "meters"

2.      name: " photon_count [counts]"

2.1. position: 2

2.2. data_type: "Integer"

2.3. description: Number of total photon counts corresponding to the altitude value in the same row.

2.4. unit: " counts"

Recommendation:

1.      “epc-resolution”: Use the high-resolution version when fine-scale vertical variation is important, when detailed profile shape is required.

2.      “tr-resolution”: Use the low-resolution version when a simplified vertical profile is sufficient, when reduced data volume is preferred, or when comparing against lower-resolution products.

ATTENTION:

We offer free access to this dataset. The user is however, encouraged to share the information on the data use with the Remote Sensing Laboratory by sending an e-mail to rslab@fuw.edu.pl.

In the case this dataset is used for a scientific communication (publication, conference contribution, thesis), we would like to kindly ask for considering to acknowledge data provision by adding the following statement in Acknowledgments: "We acknowledge the data originators I.S. Stachlewska, M. Winkowski, A. Hafiz, Ł. Janicka for quality-assurance, evaluation, and provision of data sets of the Remote Sensing Laboratory at the Faculty of Physics of the University of Warsaw, Poland."

Acknowledgements

The experimental work related to this study has been conducted within the Trans-national Access grant "ETTLA" awarded to Eventech Ltd realized at RS-Lab/WOS at the Institute of Geophysics, Faculty of Physics, University of Warsaw within the ATMO-ACCESS project (G.A. no. 101008004) funded by European Commission within Horizon 2020 INFRAIA-03-2020.

The Warsaw ACTRIS National Facility of the Institute of Geophysics, Faculty of Physics, University of Warsaw obtained financial support for operational activities form the Ministry of Science and Higher Education of the Republic of Poland under the programme 'Support for the participation of Polish scientific teams in international research infrastructure projects' as part of ACTRIS-PL project under agreement no. 2024/WK/04.