is it absolutely true that NO aircrafts is removed from adsb exchange ? I find it strange that none of the norwegian police helicopters is visible on adsb exchange. They should be within mlat coverage now and then.
Assuming these helicopters are transmitting mode S, how many receiving stations are there in Norway? As i understand it for mlat to work an aircraft has to be in range of 4 or 5 stations at one time, if an aircraft is at low level this is not likely to be the case. I'm not sure if this map is accurate, if it is then there only 4 receiving stations in Norway https://www.adsbexchange.com/coverage-4/ Paul
No, this map is not accurate, and it is not covering the area either (you need to go to eastern europe). But mode S is not a requirement to show up on flight radars ? It only needs mode A or C ? (but, I know the police helicopters does have mode S anyway). Norway has ordered 3 new police helicopters, expected to be in service within a few months, and I expect them to have ADS-B with positioning signal.
So do these aircraft show up on your system when they are flying in your area? - If they have full mode S they should do. So is this the correct map? if so it still only shows 4 stations in Norway https://www.adsbexchange.com/coverage-5/ - if it's not the correct map please post a link
Yup, they show up on my system with registration, HEX code, altitude, etc. Yup, this is the correct map. And, since Norway is divided in two, there are 8 VISIBLE receivers in Norway. But there are about twice as many according to the adsb exchange staff.
Ok, well can't really be any more help, perhaps James will reply, but to the best of my knowledge aircraft are not censored or deliberately not shown - certainly here in the UK Police helicopters show up usually as mlat aircraft not full mode S
ADSBx does not filter. MODE A and MODE C transponders do not transmit ICAO hex, only altitude. That means you can't differentiate between multiple mode a or c signals. dump1090 pretty much ignores mode a/c unless you turn on the option. You cannot MLAT mode a/c without a complex system, more complex than ADSBx has funding for. Only MODE S will show up. MODE S transmits mode s code and altitude. MLAT of non-ADSB mode s requires 4 feeds picking up the same aircraft. Helicopters typically fly very low altitude. Thus feeds need to be close versus picking up a jet at 40,000 feet. https://en.wikipedia.org/wiki/Aviation_transponder_interrogation_modes Mode A/C Transponder - needs interrogation from radar, a multilat System, or TCAS. Special processing is used to manage matching of individual replies when received at multiple ground stations. Mode S transponder - transmits DF11 download format automatically without radar interrogation and allows unique matching of messages received at all ground stations. Mode S provides error free mode C data. ADS-B transmitter - transmits DF17/18 download formats automatically without radar interrogation and allows unique matching of messages received at all ground stations. Mode S provides error free mode C data https://www.icao.int/APAC/Documents/edocs/mlat_concept.pdf
I created these simple images to try to get some people on another forum to understand how this works. Helo is within the footprints of receivers in the top image. However when viewed laterally you can see it's actually below the footprints, therefore not received and plotted.
