Interpreting the response from the coverage API endpoint

2 years 7 months ago #1590 by ra3014
Dear all,

I have recently become interested in obtaining coverage data to use it as one of the factors to take into account to reconnect different segments that actually belong to the same flight. Following opensky-network.org/forum/questions/257-coverage-map , I have been able to successfully query and receive coverage data for a day of interest.

However, I had the following questions for which I would very grateful if someone could provide clarifications:
  • It seems that the coverage quality is provided by the "altitude" value, which from my understanding, is the lowest altitude value received for any aircraft in the area. It seems that, the lower this value is, the better coverage in the area. However, I am unsure why the lowest value is used as a measurement for a quality of coverage. Since aircrafts flying higher would be further from the receivers, I thought that intuitively it made sense that a better measurement of quality would be the highest altitude value. Could someone explain why it is actually the lower altitude what brings us information about coverage quality?
  • Is the value of lower altitude obtained from the barometric altitudes reported by all tracked aircrafts, or from the geometric altitudes?
  • I have also seen that the altitude value for some areas is actually negative (for example, see area with latitude 47.4 and longitude -122.1 today, with an altitude of -953 (quite "deep" !). Is this mainly due to faulty sensors in the aircrafts, that reported such a low altitude? If so, would coverage data be sensitive to such faulty sensors? i.e., a single aircraft with a faulty altitude sensors reporting a too low altitude would lead to a low altitude value, which in turn would lead to a misleading high coverage for that area
  • The triples describing coverage for different areas give a single latitude and longitude value, but I guess the coverage (altitude) value would be approximately valid for an area around the specific latitude/longitude pair. How large of an area do these actually cover?
  • Can areas with no coverage at all be identified by the fact that this API endpoint will not return a triple with a nearby latitude/longitude pair?

Thanks a lot in advance for the very helpful information! 

Best wishes,

Rafa

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2 years 7 months ago - 2 years 7 months ago #1591 by strohmeier
Hi, quick reply:

Low altitude coverage is much more difficult to obtain as ADS-B is line of sight. It is intuitively much easier to see an aircraft without obstructions if it is high in the air. There are always exceptions and edge cases but this is generally true. Overall, the process isn't super robust for a very small area/specific location, I would always take it more as an indicator.

I don't know about the exact details of the calculations, it's old code. I'll try and find out but no promises. I suspect it's baroaltitude and if there's no return there should have been no coverage in the bucket.

Btw it is mapped on opensky-network.org/network/facts

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2 years 7 months ago #1592 by ra3014
Hi Martin,

Thanks a lot for the quick reply! That indeed makes sense, and I would indeed be very grateful if you could share any additional information about bucket size / how the actual reported altitude value is chosen, but I completely understand that there is many tasks with higher priority than digging into old code :)

I think it would indeed make sense that it is baroaltitude (if it was geoaltitude, it would be shocking that such negative values would be reported, right? Since my understanding is geoaltitude is basically determined by GPS, so I would expect it to be quite accurate).

I have also noticed that the reported latitude values seem to come at values that are, at the closest, separated by 0.2 degrees, while longitude values come, at the closest, at values separated by 0.3 degrees.

So I guess it would be safe to assume that each reported triple is intended to cover an area of +- 0.1 degrees the reported latitude, and +- 0.15 degrees in longitude?

Best wishes,

Rafa

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