Nytro Posted February 1, 2014 Report Posted February 1, 2014 [h=3]Mystery signal from a helicopter[/h] Last night, YouTube suggested for me. It was a raw clip from a news helicopter filming a police chase in Kansas City, Missouri. I quickly noticed a weird interference in the audio, especially the left channel, and thought it must be caused by the chopper's engine. I turned up the volume and realized it's not interference at all, but a mysterious digital signal! And off we went again. The signal sits alone on the left audio channel, so I can completely isolate it. Judging from the spectrogram, the modulation scheme seems to be BFSK, switching the carrier between 1200 and 2200 Hz. I demodulated it by filtering it with a lowpass and highpass sinc in SoX and comparing outputs. Now I had a bitstream at 1200 bps. The bitstream consists of packets of 47 bytes each, synchronized by start and stop bits and separated by repetitions of the byte 0x80. Most bits stay constant during the video, but three distinct groups of bytes contain varying data, marked blue below: What could it be? Location telemetry from the helicopter? Information about the camera direction? Video timestamps? The first guess seems to be correct. It is supported by the relationship of two of the three byte groups. If the 4 first bits of each byte are ignored, the data forms a smooth gradient of three-digit numbers in base-10. When plotted parametrically, they form an intriguing winding curve. It is very similar to this plot of the car's position (blue, yellow) along with viewing angles from the helicopter (green), derived from the video by magical image analysis (only the first few minutes shown): When the received curve is overlaid with the car's location trace, we see that 100 steps on the curve scale corresponds to exactly 1 minute of arc on the map! Using this relative information, and the fact that the helicopter circled around the police station in the end, we can plot all the received data points in Google Earth to see the location trace of the helicopter: Update: Apparently the video downlink to ground was transmitted using a transmitter similar to Nucomm Skymaster TX that is able to send live GPS coordinates. And this is how they seem to do it. Posted by Oona Räisänen Sursa: absorptions: Mystery signal from a helicopter 1 Quote
takko Posted February 1, 2014 Report Posted February 1, 2014 genius! nu ma gandeam niciodata la asa ceva Quote
djwap Posted February 9, 2014 Report Posted February 9, 2014 Este vorba despre ADS-B Automatic dependent surveillance-broadcast - Wikipedia, the free encyclopedia Quote
odumy Posted June 9, 2014 Report Posted June 9, 2014 si ce posibilitatea ar fi de dispersat acel semnal ? desi nu prea am mari cunostinte cu undele electromagnetice , ce posibilitate ar fi de a le da peste cap ?? Quote
