FlightAware Discussions

Dual mode aircraft

Interesting to see Dual mode aircraft and how close their plots are.

This aircraft running at 978 MHz.

Same aircraft running at 1090 MHz.

Interesting, but why is the line in the table for that aircraft in the 1090 plot shaded grey indicating other rather than the pale blue that indicates ADS-B?


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That I have no clue about but however after looking closer both the 1090 and the 978 side are using ADSB.

Grey just means that line is selected.

If it really is from a transponder why should the lines be any different, they have the same GPS source probably.

Good illustration of how the 978 demodulator is not as good as the 1090 one yet!


I don’t disagree, but isn’t the range difference exaggerated by the fact that 978 UAT typically transmits at a lower power than 1090 ES?

Aircraft must transmit signals at a certain level of power to ensure ground stations and ADS-B In-equipped aircraft and vehicles can receive the transmitted signals. As proposed, the final rule requires UAT systems to broadcast at a 16-watt minimum-transmit power, and 1090 MHz ES systems to broadcast at a 125-watt minimum-transmit power.

16W on a frequency not spammed by Mode A/C and TCAS (I get thousands of Mode A/C messages per second on my receiver) would probably be pretty useful and be as effected as 125W on 1090Mhz.

The AM radios on small aircraft are only 10 or 16W. Their range is not bad for such a wide signal. A digital signal with error correction and repetition should be better.

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Many people don’t have filtering for 978 MHz at all, the mobile phone frequencies are a whopping 100 MHz closer.
Also the demodulator for dump1090-fa uses oversampling which improves reception quite a bit.

You said you had no filter correct, which gain are you using?

With the yellow dongle and UAT i’ve read in another thread going down to a gain of 26 or something provided the best results.

Without filtering the noise is just too much it seems.
If you have a dongle without an LNA i would try that as well for UAT.

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my 978 side (FA yellow pro stick) no filter and 25 set for the gain.

This is what I did. I plugged my pro stick plus into my windows computer and ran SDRConsole and set the frequency to 978 and watched the spectrum for noise. I found that anything above 28 the dongle becomes saturated with noise and interference. I found at Max gain no decodes. The blue pro stick plus ran on SDR console at 1090 MHz interference is low but the signals are so saturated the display is all lit up bright yellow.

I do have a selective band pass filter in front of the LNA. It is attenuating the 800 MHz cellular data band and “smart meters” in the 902-928 MHz band by more than 40 dB.

My SDR gain adjustment consisted of adjusting until the near-by ground station at BWI is coming through at about -6 dBFS. It may not be gain optimized. I have seen a few UAT planes flying at 7,000’ nearly 90 miles away.

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I have one RTL-SDR with the hab/nevis amp/filter. I can’t remember if I put a cavity filter in the mast box with the amp.
The other is the FA Pro dongle with a cavity filter.
I can’t really compare them as one connects to a vertical antenna and the other to a directional antenna(also lower than the vertical antenna).

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I can’t remember if I put a cavity filter in the mast box with the amp.

How does that work? Antenna --> cavity filter --> LNA --> Airspy / Bias-tee --> RPI ?
or is the cavity filter at another spot? Does it even matter?

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Yes. Cavity filter is closest to the antenna. Otherwise I can’t power the amp by bias-t.
Lots of noise around my location. Cavity filter improve my reception considerably.

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I see these all the time also, probably half of the 978 MHz hits i get also show up on 1090 MHz. At first I thought I was having some sort of leakage or intermodulation issue that was causing ghosts. Finally convinced myself they were real by detaching the antennas one at a time.

One think I’ve noticed is that the 1090 signal is often MLAT. I think many of these aircraft may have had older 1090 transponders and then added 978 to meet the FAA mandate but didn’t decommission the older transponder. I wonder is this is allowed, I honestly don’t know.


In the USA light aircraft are permitted to retain their 1090 MHz Mode A/C transponder and add a 978 UAT device for ADS-B out.
So yes, we might well see both 978 and 1090 MHz A/C signals from a single airplane.
After 1 January 2020 in most US airspace ADS-B Out is mandated.


In 2020 in the US ADS-B out will be mandated in airspace that currently requires a Mode C transponder. It will not be required in all airspace.

Light aircraft have the choice of 978 or 1090 transponders, or both.
I am not aware of a restriction on any aircraft from having both.

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It does give us a method of comparing 1090 MHz MLAT-derived positions with true GPS positions transmitted via 978 MHz UAT. I’ve been looking at this and the results are pretty impressive in both accuracy and how quickly the MLAT positions are returned (minimal lag to the 978 Mhz reported positions).

I use both FA and ADSB-exchange MLAT. FA looks to have more filtering as the point to point positions are smoother than ADSB exchange. On the other hand, ADSB exchange often reports MLAT aircraft positions sooner than FA.

In order to be approved for operation in the air traffic system an aircraft’s ADS-B system must operate with its horizontal (position) accuracy at Level 8 or better.


978/UAT and 1090/ES systems may use the same or independent GPS sources to provide the aircraft’s position. Assuming independent GPS sources for a dual-equipped aircraft, the theoretical worst-case reported position should not be greater than 0.05 + 0.05 nautical miles (about 185 meters (~600 feet).
Obviously, if they use the same source the position reported should be identical.

However, the systems we use to receive the 978 and 1090 signals have differing characteristics (sensitivity, processing power, processing algorithms, etc.) . In addition, there are known and unknown differences in the transmitting power of the aircraft’s 978 and 1090 units, as well as the location of the antennas on the aircraft (shielding). Adding it all up, there is a chance that the 978 signal may be received intermittently while the aircraft is at the edge of our 978 ground equipment’s capability. Therefore, a delay of a few seconds between a “good” 978 signal and the constant reception of “good” 1090 signals over the same time period might allow the display of an aircraft at two different locations, especially on separate displays. This would also be dependent on the update rate of our displays and any designed target-retention-before-drop.

Once Flightaware provides a PiAware capability to show both ADS-B system types on a single display the dual location problem should less apparent or gone.

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