About the Skimmer
We thank BATC Qatar-OSCAR 100 Narrowband WebSDR for the usage of their system to build our skimmer.
The system runs on a Windows 10 machine running two instances of WSJT-X v2.6.0. A browser window is open on the BATC QO-100 WebSDR and continualy listens to the FT8/FT4 segment of the band. Audio is fed to the WSJT-X instances via a Virtual Audio Cable. The WebSDR as been configured in the following way:
- Tune frequency is 10489539.000 kHz
- Mode is USB
- Filter bandwidthh is 4.99kHz
- Volume is adjusted such that the signal indicator in WSJT-X is green and hovers around 70dB
One of the WSJT-X instances is configured for the FT8 mode, while the other is for FT4. Each instance produces an ALL.txt file that is sent for processing every 15 seconds approximately.
Using the BATC Qatar-OSCAR 100 Narrowband WebSDR Signal strength plot it has been determined that the noise level runs around -110dB while the lower (CW) beacon tops -80dB.
These values where determined on the afternoon of 2023-02-07 from around 15h00 UT until 17h00 UT.
The WebSDR was tuned for CW with 0.09Hz filter bandwidth and a frequency of 10489502.30kHz. A Signal strength plot was generated with the "very slow" option, returning the following image:
An "eyeball estimation" on the image gives the value of -110dB. The chart spans ~45m time.
Similar to the noise level estimation, the same method was employed for the frequency 10489500.30kHz, covering the higher (in frequency) signal for the lower CW Beacon. The result was the chart:
The same "eyeball estimation" method was employed giving -80dB signal level for the beacon.
Having the values of -110dB and -80dB, the Half Power Signal is midway between them at -95dB, and a tune signal was produced (in WSJT-X) to read this value in the WebSDR level meter. After this calibration was performed a series of test messages were sent with WSJT-X and the reports where recorded at the skimmer. Data was collected while no other station was transmitting on the skimmer bandwidth of 5kHz.
For the Half Power Signal, the skimmer reported +00 for all FT8 messages, and that was chosen as the Reference Signal. For FT4 with the same power level the skimmer reported -01/-02, an effect of the increased signal bandwidth, but for consistency the same value of Reference Signal was chosen +00.
Having a Half Power Signal with a FT8 report of +00, and the beacon at an equivalent +15 implies that any station that is decoded by the skimmer above +15 is operating above the beacon's power level, and risks triggering LEILA.
No tests were performed to determine the true power level needed to trigger LEILA, and we don't recommend such test to be performed.