Followup post – this issue has now been fixed.
The problem was caused by the “Up in the Air” feature on the HWT site not using the refraction value, effectively using a value of 0 when displaying the terrain rings. The rest of the site, and the API used to generate the JSON data, was using the refraction value. This meant that the rings displayed on HWT were the wrong size, and the rings displayed in Skyview from the JSON data were correct.
Thanks to Michael from HeyWhatsThat for diagnosing and fixing the problem. In order to ensure that the correct terrain rings for 1090MHz are displayed on the HWT site when using the “Up in the Air” feature you must use the URL
and enter 0.25 into the Refraction box in the Optional parameters section
If you don’t use the full URL you won’t see a box for Refraction and in that case the default value of 0.14 will be used which will result in rings of the wrong size for our use.
The API continues to work as it did before. Obtain the panorama ID and specify a refraction of 0.25 and the altitudes in metres in the API call, eg:
wget -O upintheair.json "http://www.heywhatsthat.com/api/upintheair.json?id=XXXXXXXX&refraction=0.25&alts=6096,12192"
and move this file into
/usr/share/dump1090-fa/html/
With the HWT site now correctly using the specified refraction value in its terrain ring calculations both maps now show exactly the same result.
