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  • Non ADS-B. Mode S only aircraft

    My own radar is picking up plenty of Mode S only traffic, and sometime these are displayed on the fr24 map originating from an MLAT source which i understand.
    What i don't understand is sometimes this Mode S traffic shows as originating from a T-xxxxxx feeder, which i had assumed is using the same technology as Rpi implementation.

    Can someone be kind enough and explain therefore how t-xxxxxx feeder can show as obtaining a location fix on an aircraft that is only transmitting Mode S ? Is this an Fr24 anomaly or are they getting the GPS co-ordinates from another source ?

    Thanks

  • #2
    Originally posted by awitty View Post
    My own radar is picking up plenty of Mode S only traffic, and sometime these are displayed on the fr24 map originating from an MLAT source which i understand.
    What i don't understand is sometimes this Mode S traffic shows as originating from a T-xxxxxx feeder, which i had assumed is using the same technology as Rpi implementation.

    Can someone be kind enough and explain therefore how t-xxxxxx feeder can show as obtaining a location fix on an aircraft that is only transmitting Mode S ? Is this an Fr24 anomaly or are they getting the GPS co-ordinates from another source ?

    Thanks
    A T-xxxx feeder can be anything other than an official FR24 box, it may well be someone with a radarcape which is basically the same as an official box but privately purchased and running different firmware, these are capable of MLAT but will still be shown as a T-xxxx
    FR24 F-EGLF1, Blitzortung station 878, OGN Aldersht2, PilotAware PWAldersht, PlanePlotter M7.

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    • #3
      Originally posted by F-EGLF1 View Post
      these are capable of MLAT but will still be shown as a T-xxxx
      Thanks, but i don't quite follow your last statement. I had assumed that FR24 servers handled all the multilateration algorithm calculations (and not feeder boxes), and thus displayed the Radar as MLAT when the position was derived from the associated timing data from 3 or more feeders.

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      • #4
        For example Mode S traffic shows as originating from a T-xxxxxx feeder then feeder have 4+ Raspberry Pi receivers with properly working MLAT calculating server backing location data to this receivers.

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        • #5
          Originally posted by DrHyperKALICH View Post
          For example Mode S traffic shows as originating from a T-xxxxxx feeder then feeder have 4+ Raspberry Pi receivers with properly working MLAT calculating server backing location data to this receivers.
          Ok I understand. So it's not uncommon then for someone to have a multi receiver setup and undertake their own Mlat calculations before feeding this data up to fr24.

          Thanks for the help.

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          • #6
            It's more they are likely sending fr24 data flagged as positional, when it's been pre processed but not really (ala mlated somewhere)

            Sent from my XT1092 using Tapatalk
            Posts not to be taken as official support representation - Just a helpful uploader who tinkers

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            • #7
              Originally posted by Oblivian View Post
              It's more they are likely sending fr24 data flagged as positional, when it's been pre processed but not really (ala mlated somewhere)

              Sent from my XT1092 using Tapatalk
              I just did a search for one of the offending radars and found this thread;
              http://forum.flightradar24.com/threa...erry-Pi/page36

              It is suggested that some FR24 feeds are feeding positional data from piaware.

              Thanks for all your help. I understand what's going on now.

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              • #8
                I'm still confused. How would having multiple receivers at the same location make any difference on establishing position?

                I just started feeding to FR from close to KSJC about a week ago and have yet to see my feed in any of the radar boxes.
                There are only 5 or 6 T-XXXX radars that are shown as the source for 95% of the aircraft within 50 miles of the Bay Area and they always have position info when I often have only a ICAO, altitude and speed.

                I've also seen aircraft (PAL airline flight) that I was feeding with full position information and was not showing on FR24 until it was within 30 miles of KSJC.
                Using RPi2 and dump1090. T-KSJC28

                Also, there is no doubt there are at least 20 feeders in the bay area and you rarely see the "MLAT-" radar as the source. I fed my antennas lat and long coordinates to FR24 with 6 digits of accuracy in hopes it would improve the accuracy of flights in the area via MLAT.
                Last edited by Sam26K; 2016-01-23, 07:30.

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                • #9
                  Originally posted by Sam26K View Post
                  I'm still confused. How would having multiple receivers at the same location make any difference on establishing position?

                  I just started feeding to FR from close to KSJC about a week ago and have yet to see my feed in any of the radar boxes.
                  There are only 5 or 6 T-XXXX radars that are shown as the source for all the aircraft within 100 miles of the Bay Area and they always have position info when I only receive a ICAO and a few other parameters.

                  I've also seen aircraft (PAL airline flight) that I was feeding with full position information and was not showing on FR24 until it was within 30 miles of KSJC.
                  Using RPi2 and dump1090. T-KSJC28

                  Also, there is no doubt there are at least 20 feeders in the bay area and you rarely see the "MLAT-" radar as the source. I fed my antennas lat and long coordinates to FR24 with 6 digits of accuracy in hopes it would improve the accuracy of flights in the area via MLAT.
                  It comes from this:

                  Some FR24-feeders are also feeding other networks. One of these networks are calculating a position for S-mode aircraft using MLAT (just like FR24) BUT ALSO have the added feature of sending the MLAT position back to the feeder for local display. Some of these feeders then retransmit the MLAT position to FR24 as if it was an ADS-B position received directly form the aircraft and FR24 will then mark it with that feeder name.
                  FR24 are aware of this but have not been able to filter out these 'stolen' positions. This have been going on for a year or so.

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                  • #10
                    Originally posted by Kpin View Post
                    It comes from this:

                    Some FR24-feeders are also feeding other networks. One of these networks are calculating a position for S-mode aircraft using MLAT (just like FR24) BUT ALSO have the added feature of sending the MLAT position back to the feeder for local display. Some of these feeders then retransmit the MLAT position to FR24 as if it was an ADS-B position received directly form the aircraft and FR24 will then mark it with that feeder name.
                    FR24 are aware of this but have not been able to filter out these 'stolen' positions. This have been going on for a year or so.
                    Thanks for explaining that, Kpin. That makes perfect sense why they appear from a T-Feeder instead of an MLAT feeder. I kept thinking that it was a superior receiver or an FR24 fluke in the source reporting.

                    If the data is good, there is no problem with that, but often it's not good when the aircraft are taking off and landing and taking the tight turns they do around KSJC. I was hoping to contribute to improving that situation but so far FR24 does not seem to be tracking my feeder even though it's uploading.

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                    • #11
                      FR24 tracking all feeders, but shows name of receiver with strongerst signal on map. Improve your antenna/position to see your feeder name on map.

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                      • #12
                        Originally posted by DrHyperKALICH View Post
                        For example Mode S traffic shows as originating from a T-xxxxxx feeder then feeder have 4+ Raspberry Pi receivers with properly working MLAT calculating server backing location data to this receivers.
                        I still don't get that theory and don't believe it. More receivers at the same location will not contribute to determining aircraft location.
                        Last edited by Sam26K; 2016-01-23, 08:32.

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                        • #13
                          Originally posted by DrHyperKALICH View Post
                          FR24 tracking all feeders, but shows name of receiver with strongerst signal on map. Improve your antenna/position to see your feeder name on map.
                          Now that makes sense. But it's a poor way for FR24 to pick a feeder based on signal strength and locking into that. I have signal strengths that are very high as I have regular traffic within 10K feet but it's only for less than a minute at a time. Still FR24 never locks on to my feeder so far.
                          Last edited by Sam26K; 2016-01-23, 08:42.

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                          • #14
                            at the end of the day if multiple people are sending in the information you have to pick something, and i imagine that if for example signal strength is sent then it takes very little processing on their behalf (ie take a subset of data go through it in a single pass that feeder was laoudest use that) to do anything more complicated multiple 1000's of feeders would get quite CPU intensive.

                            I doubt there's anything more to it that.

                            also you could do your on own mlat at one location if you had an accurate enough time reference and three receivers equal distances apart.
                            T-EGLF8

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                            • #15
                              Originally posted by SpaxmoidJAm View Post

                              also you could do your on own mlat at one location if you had an accurate enough time reference and three receivers equal distances apart.
                              I agree but the whole idea of MLAT is to increase accuracy by separating the receivers over large distances with freely available feeders and not trying to do it with antennas separated by a few feet with nanometer accuracy.

                              The time thing is a good point, accurate time is important across data sources and is a challenge.
                              Last edited by Sam26K; 2016-01-23, 08:58.

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