UAT reception and Dump978-fa messages

FlightAware ADS-B Flight Tracking
1

I’m trying to optimize my UAT reception.

I have two Pis running only dump978-fa

Both antennas are in my attic

One is using a FA 978 antenna with Radarbox 978 receiver.

The other is using a homemade 978 cantenna with a FA Pro Stick wit the FA 978/1090 filter.

Both have 6ft of coax, LMR400 on the FA antenna and RG58 on the cantenna.

The FA antenna is right at the roof peak, the cantenna is about 4ft below the peak.

For FA antenna I have gain at 20.7 for the cantenna 25.4.

The antennas horizontally are about 18 inches apart (centered on the roof rafters).

Indicated RSSI is roughly the same.

The cantenna outperforms the FA antenna in both message rate and distance. I’ve swapped the receivers between the antennas and that remains the same, the cantenna performs better.

Flightaware UAT position for 11/25
FA antenna 3454
Cantenna 3948

Any thoughts on why the cantenna is performing better than the FA antenna?

I’ve also noticed the cantenna with either SDR show many of these messages on port 30978, they are not present on 30979, and I have never seen them on the FA antenna, I’m not sure what they are

+39d67f962950ba7000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000;rs=6;rssi=-22.9;t=1701007076.148;

2

Hi @kmm0000,

Normally with 978 messages, the ones starting with “-” are downlinks received direct from an aircraft. That is the normal 978 UAT traffic. The “+” messages are usually uplinks. That means your better antenna is likely receiving rebroadcast traffic and other info from a nearby FAA site. The rebroadcast traffic is called TIS-B and ADs-R. Your local displays will show both of those.

The ground stations also broadcast other things like weather, notams, and even graphical images like radar images. FA does not decode these. The towers also frequently broadcast their own coordinates, which makes it easy to identify them. These types of messages are not aircraft, so they do not go out over the 30979 .json port you mentioned. They are likely only on the 30978 raw port. They can be decoded with other utilities that were part of the original dump978 program packages before FA took over development of them.

This is a link to the original dump978 decoder. It does a good job of explaining the format, and also shows how the uat2text program can be used to decode some of those “other” message types.

This is the current version of uat2text and uat2esnt I am using. This site does not have the good examples the mutability one does above.

I decode with dump978-fa here. I have a copy of uat2text which connects to the raw port=30978 and displays text output of everything. I also use a copy of uat2esnt to convert the raw port=30978 to AVR, which I then run through a spare copy of dunp1090-fa to convert to beast binary. I prefer to work with beast. This output is used to feed a completely separate copy of PlanePlotter just for my local 978 UAT, 978 TIS-B, and 978 ADS-R. I do not currently decode the weather graphics, but do see them scroll by.

My local FAA tower broadcasts on both 1090 and 978. The 1090 includes TIS-B and ADS-R. The 978 includes TIS-B, ADS-R, and the weather and notams I mentioned above.

Regards,
-Dan

3

Feeding your exact message above through uat2text does give the tower location as below:

dan@Acer-722:~/uat2esnt$ cat input.txt | ./uat2text
UPLINK:
Site Latitude: +40.6672 (possibly invalid)
Site Longitude: -74.4178 (possibly invalid)
UTC coupled: yes
Slot ID: 26
TIS-B Site ID: 7
RSSI: -22.9 dBFS

4

To clarify, the distinction is more like “downlink has aircraft position data, uplink has services useful to an aircraft via FIS-B” – despite the name, downlink messages can be transmitted by a ground station. TIS-B and ADS-R are downlink messages.

FIS-B carries things like NEXRAD maps, METARs etc.

1 Like
5

Thanks @obj for the correction.

I had forgotten the FIS-B acronym and confused the +/- a bit. I was pleased to see that his posted message did give the exact tower location.

Regards,
-Dan

6

Thanks, that tower is about 20 miles away.

7

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