Position_data4 & Velocity_data4 "mintime"

2 years 8 months ago #1068 by liu1322
Hi!!!! :lol: :lol: :cheer:

I obtained ADS-B raw messages for aircraft icao24='ae0943' on 9/9/2019 between hour>=1568037600 AND hour<=1568066400. After convert epoch to GMT, I sorted Position_data4 & Velocity_data4 together with respect to "mintime". For time period from 6:11:41pm to 6:11:44pm, receivers only received position message and from 6:11:45pm to 6:11:48pm they only received velocity message. How come ADS-B be transmitting only position or only velocity message for such a long period of time? My understanding is that ADS-B should transmit airborne position and velocity messages every 0.4 – 0.6 sec.

To further understand what's going on, may I know how OpenSky keeps track of mintime? and how could I figure out which receiver is receiving that raw message? Thank you in advance for helping me out!

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2 years 8 months ago - 2 years 8 months ago #1069 by strohmeier
You will see this a lot. The reason is simply the loss due to collisions on the 1090MHz channel. We've measured and published about this in 2014 when we started OpenSky [1] and it has been re-measured with the same results many times (e.g. [2]). While the concrete loss depends on type & number receivers, environments, number of aircraft etc. it is very practical to not receive a few position or velocity messages in a row.

You can find the sensors/times that received a message in the 'sensors' structure in the respective tables.

[1] opensky-network.org/files/publications/commag2014.pdf
[2] cri-lab.net/wp-content/uploads/2018/12/S...calepore_SAC2019.pdf
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2 years 8 months ago #1070 by liu1322
Thanks a lot!!! :lol: :lol: One more quick question, I tried to query information from "sensor_visibility_data3" to find the sensors/time information. However it seems to me that there's no information available after 04/27/2016. How may I find information for data during September/2019?

I'm hoping to figure out, for each raw message from position_data4, which OpenSky sensor is receiving and uploading that message.

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2 years 8 months ago - 2 years 8 months ago #1072 by strohmeier
There was a switch to a new table at that time, so everything older is in data_3, everything newer in data_4. However, as I said, you need to query the sensor data structure that is *in* the position/velocity_data4 tables. Or the serials array<int> in the state_vector_data4.
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