FlightAware Discussions

Max 978MHz UAT Reception Distance 159.7nm!

Received quite a surprise this morning with the new 978MHz UAT hardware install & testing with the new software release to see reception out to 159.7nm!

A bit of investigation showed the aircraft N228ME is a TBM-700 turboprop cruising at 26,000 feet while apparently running with a 978MHz UAT system above 18,000 feet mentioned upper altitude limit for 978MHz UAT operation. The RSSI reading was -20.6 at 159.7nm, providing an initial indication at what RSSI level long distance reception will drop out. Closer and lower altitude UAT equipped aircraft also appear to drop out in the -19 to -20 RSSI range.

Meanwhile, up to 14 other GA aircraft, largely local flight school aircraft in the Dallas/Fort Worth area, showed early this morning within the more expected reception range of 75nm, with most operating in within 25nm. This all achieved with FlightAware antenna, filter, Pro Stick, all off-the-shelf hardware. Very nice!!

Range Ring Distance Settings

It now appears I need to reset the range rings to a much larger distance scales after just observing a flight coming in from 150 miles out. Perhaps 50,100, and 150nm better suited for the map!

I expect to see longer reception distances achieved as other 978MHz UAT stations are added and more flights are observed. I look forward to seeing performance values from other stations to help determine how well the new station is performing.


Some planes have both systems (ADS-B and UAT) installed.


Updated Range Rings

After seeing 978MHz UAT reception from several aircraft beyond 100nm and 150nm today, I reset the distance ring settings to: 25,50,100,150. This also increases the number of rings to a total of 4 rings now. I find the new settings allow me to get a fast and accurate overall assessment of how far out aircraft are on the map with a quick glance:

Local GA and flight school aircraft appear concentrated within the 25nm and 50nm ranges. It is very easy to quickly estimate distances for the occasional more capable aircraft flying with 978MHz UAT running at higher altitudes using the 100nm to 150nm range rings.

For quick reference, here is the edit info:

Range Ring settings are located in the /usr/share/skyview978/html folder:
$cd /usr/share/skyview978/html folder
$sudo nano config.sys to edit the file with the nano editor

Edit this line within the file (around line 99 from the top):
SiteCirclesDistances = new Array(100,150,200);

Replace the old distance numbers with the new range ring distances to display:
SiteCirclesDistances = new Array(25,50,100,150);

Save changes and close the file:

Restart piaware to load the new range ring settings:
sudo systemctl restart piaware

Go to the Skyview map webpage and hit refresh to display the map with the new range ring settings!

Thanks! I was not expecting to see both UAT and ADS-B simultaneously in use since such use defeats one reason the FAA provided the 978MHz UAT alternative, to relieve 1090MHz ADS-B signal congestion.

Running both UAT and ADS-B simultaneously if you can does provide two obvious benefits. It provides the security of knowing there is a fail-safe feature, second system running, should one system fail. Then it is easier to just run both, not worrying about when each should be used and having to mess with it… That gets my vote!

In my aircraft we operate up to 27,000 feet. The plane is equipped with both Mode S ES 1090 MHz and ADS-B OUT 978 MHz systems.
However the 978 MHz box is deliberately set to inhibit transmission on 978 per the installation manual.
I believe the TBM mentioned has been incorrectly configured. Until that’s corrected though it offers a great long-range UAT source to track.

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Thanks for the insight! I am certainly not going to complain about the UAT being available for a long-range UAT tracking source.I suspect the incorrect configuration will be noticed and corrected soon for this particular aircraft.

I am south of you by 125 miles.

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I see about 200 UAT targets on a great day. If the weather is poor I might track 5. Per day.

Maximum range is over 150 miles but only a tiny fraction of aircraft are seen so far away.

The typical UAT-only aircraft is flying only day VFR and at fairly low altitudes.


The UAT traffic remains variable and strongly affected by local weather. Traffic count on 978 MHz averages about 80 aircraft per day.

On 1090 MHz at the same site a typical day will be 4,500 planes.

UAT is used predominantly on small planes - when the weather is crap, many don’t go up.

Oddly, unless something is configured wrong - I’m getting UAT traffic but my node is reporting the distance as +250mi. Setup is Nooelec 978 rubber duck -> RTL-SDR v3 -> Pi3b+ & FA 3.7.1 install. Configured as UAT only. It’s currently sitting in my window, 20 feet off the ground. This seems a little odd for such a weak gain setup. This is a pretty stock setup, no tweaking.

Noise producing random positions being decoded?
Not sure how that can occur as normally you need 2 position message to decode a position.

Make sure the RPi with dongle is not in the reception plane of the antenna. The dongle itself produces quite a bit of high frequency noise.

Also it seems it is better to reduce the gain somewhat because the demodulation needs a “clearer” signal than dump1090-fa.
(no super-sampling)
Try with a gain of 44 for a bit.

Oh and make yourself a quarter wave antenna.

The Nooelec antennas seem to be completely out of tune.
(Someone looked at them with an antenna analyzer, too lazy to search the thread right now)
You can try removing the plastic cover and see if that helps.
It seems they were designed without the plastic in mind which changes the resonant frequency.

Otherwise i’d just add radials to it and cut the whip to 1/4 wave.

indeed the Nooelec antennas are garbage it seems out of the box, as are the Nano 3 SDRs – but it’s the only 978 antenna I had so I wanted to get something up to play with. My first guess is it is noise as well as the distance didn’t make sense.

I think since I’ve started making my own antennas I’ll probably try to make an antenna next – seems logical with this hobby :b

Regarding changing the gain for the stretch install – where is that done? Similar to how gain is adjusted for the stretch install of dump1090-fa (/etc/default/dump1090-fa)? but in this case /etc/default/skyview978? Now that it’s at least cobbled together it’s time to tinker. Appreciate the feedback & suggestions as always.



--sdr-gain 44

to the receiver or decoder options.

Try getting the plastic sleeve off, it shouldn’t be too hard.
Might improve things quite a bit.

But yeah just building a spider is probably the best thing to do.

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I was digging around - but I couldn’t immediately find instructions for a 978 spider specifically here. My assumption is it would be slightly longer radials? I’ve built my 4th spider for 1090 and each build keeps getting better :smiley:

For 978 MHz you would need a wire with a length of 77 mm or 3.03 in

For 1090 MHz you would need a wire with a length of 69 mm or 2.71 in

Often up to 4 mm shorter works better, i’m not sure why.
The above are the theoretical numbers.

Here are the instructions I sent my brother for DIY 1090 & 978 antennas …

He’s getting great reception with them outside in a IP65 box (SDRs included) on his 2nd floor deck (5% at 200-250 mi range).


So the one question I’ve asked myself while building a few spiders (working on a 978 now, thanks for the input) when the antenna is finally assembled - should the end of the radials be 77mm from the center element - as in, the total length including the SO-239 plate should be 77mm or should each one of the spider elements be 77mm from the edge of the SO-239?

Radial length doesn’t really matter as much.
But if you would be precise, make it total length to the feed point.

Most of the time you need to shorten the whip 2 mm or so anyway for best reception.
Not sure why.

Maybe if you make the radials a bit longer, you don’t need to shorten the whip :stuck_out_tongue:
Without an analyzer it’s really a little bit of guess work.
The design works well in quite a range of length though.

That’s the plus of the design, tolerance to varying dimensions.

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OK awesome - this is somewhat what I assumed re whip/radial. I don’t want to buy an analyzer but may have to otherwise I’ll be trimming/guessing/looking at graphs :slight_smile: Thanks