978 Mhz

[radiodude817]

I have some questions about data being received.

1. I understood to get TIS-B messages you would have to have at least one ADS-B aircraft up to
be able to receive a TIS-B signal. I am 30 miles away from an airport and therefore think it is impossible
to receive any ground based messages. So I assume I am receiving my messages from an aircraft using
ADS-B and repeating the TIS-B info.

2. Initially I always had a ADS-B aircraft in the list as well as TIS-B aircraft. Now I have noticed only TIS-B aircraft
in my list a few times. If I have to "see" a ADS-B aircraft to get TIS-B messages how am I receiving TIB-B messages?

3. Along with receiving "normal" ICAO hex codes "A" & "C" I am getting prefixes of 2B4xxx. I have logged up to 10
hex codes beginning with 2B4 these targets are moving in altitudes of 300' to about 3500'. I thought these may be
the incognito codes but, with only 999 ID's available under 2B4 prefix this doesn't seem likely.

Any comments would be appreciated.

J.

 

[AB5ID]

Do you have a ground station nearby?

ADS-B Towers

towers.stratux.me

 

[radiodude817]

I don't know. I looked on line for locations for UAT sites. Seems to be a work in progress. I did look on my sectional which has a publication date of 12 SEP 2019. I did not see any indicators as shown on some sites. My map is good till 26 MAR 2020 so I assume the info should be on here although it is stated sites at airports are not shown. I do have a RCAG site about 5 miles away but, I assume because of the antenna system construction if there was one there I would be under the radio radiation pattern.

So are you saying I could possibly receiving TIS-B info from the ground? This would explain why I get info without an ADS-B source.

Any idea about the 2B4xxx icao codes?

Thanks,

J

 

[radiodude817]

I don't know. I looked on line for locations for UAT sites. Seems to be a work in progress. I did look on my sectional which has a publication date of 12 SEP 2019. I did not see any indicators as shown on some sites. My map is good till 26 MAR 2020 so I assume the info should be on here although it is stated sites at airports are not shown. I do have a RCAG site about 5 miles away but, I assume because of the antenna system construction if there was one there I would be under the radio radiation pattern.

So are you saying I could possibly receiving TIS-B info from the ground? This would explain why I get info without an ADS-B source.

Any idea about the 2B4xxx icao codes?

Thanks,

J

 

[radiodude817]

I have found a ground station about 11 miles away from my location. It is line of site to me as well.


J

 

[AB5ID]

No idea on the codes. I'm no expert but I thought the ground stations rebroadcast 1090 TISB and mode S targets on 978 UAT

 

[ColoradoRedlands]

Any idea about the 2B4xxx icao codes?

Hi are you within the Cleveland Center area. I show 2B4 as the computer generated TIS-B codes in that area.

 

[radiodude817]

Hi,

Yes I am in the Cleveland Center Area. However, I am very close to Toronto and with in range of the edges of Boston & NY Centers.
Besides having 2B4 prefixes I have received 39F, F85, & 09C units. Is there a publication that identifies the prefix to particular Centers?
Many times I receive 2B4 prefixes along the North shore of Lake Ontario without any rebroadcasting local aircraft.
If the hex id's are by Center shouldn't they change when crossing into another Center?

Would appreciate any information.

Thanks,
J

 

[ColoradoRedlands]

I don't recognize any of those other prefixes you list as TIS-B related. They could always be something else.

After a lot or research several years ago I made a list of the prefixes. I posted it in several groups, but It never really seemed to be interesting to most people so it didn't catch on. I've tried to attach it here, but the system doesn't like it. If I have your email I'll send it to you directly.

As far as 2B4 for Cleveland Center goes, it is actually 2b4 to avoid being confused with a real hex code. Most of our hobby ADS-B boxes don't know the different between the upper and lower case codes so they just mash them all together.

Regarding your question about changing the code, since these are not real hex codes they stay the same until they drop out regardless of boundaries. If they pop up again they will show an entirely different code based on the prefix for whoever displays it. The controllers see these as radar tracks and not as TIS-B, so it doesn't make any difference to them.

The TIS-B tracks don't have any identifying information attached, they just display on a pilot's display with direction, speed, etc so they can see and avoid them. The controllers will still call out traffic, the TIS-B just gives the pilots a heads up that a non ADS-B aircraft is near.

 

[radiodude817]

As 978 was getting closer to becoming operational I watched and there were no "plots" until after January 2020. Maybe now
others will take a greater interest.
I have a TIS-B transmitter about 12 miles West of me.
What is confusing after reading a few articles about the 978 Mhz system is its operation. Initially I understood aircraft
transmitting 978 data was picked up by other aircraft equipped and rebroadcasted it directly to other aircraft.
Now it seems the 978 data is down linked on 1090 or 1030 and then the FAA processes it and rebroadcasts it on the TIS-B transmitters.
Can you shed any light on this.
I am on a hill as is the TIS-B transmitter. My 978 antenna looks across the valley at the TIS-B transmitter. My antenna is up approx 60'
with very good transmission cable. Yet the 978 signals are sometimes erratic. If I am receiving the 978 data from the TIS-B transmitter
I would expect the quality of reception to be better. I understand these aircraft fly a lot lower and terrain may be a factor but, it seems to me if the FAA can see the aircraft then I would think anyone receiving the TIS-B signal should the same thing?

Thank you

J

 

[ColoradoRedlands]

Hi,

It is not downlinked on 1090/1030 per se, but rather the raw radar data of non ADS-B equipped aircraft that is being displayed on the ATC scopes is given a random ID by the ATC computers and then the information is transmitted out via the ground stations for display in the cockpits of suitably equipped aircraft. They are also transmitting location data of ADS-B aircraft to non ADS-B aircraft.

 

[radiodude817]

Ok, that makes more sense.

Thanks,

J