Police Helicopter

FlightAware ADS-B Flight Tracking
1

Any reason a police helicopter would be sending out messages in such rapid succession? Often, when there is a message, the same message is duplicated several times, e.g.,


MSG,5,111,11111,A820F8,111111,2016/05/06,05:38:04.586,2016/05/06,05:38:04.550,,1375,,,,,,,0,,0,0
MSG,5,111,11111,A820F8,111111,2016/05/06,05:38:04.587,2016/05/06,05:38:04.554,,1375,,,,,,,0,,0,0
MSG,5,111,11111,A820F8,111111,2016/05/06,05:38:04.597,2016/05/06,05:38:04.604,,1375,,,,,,,0,,0,0
MSG,5,111,11111,A820F8,111111,2016/05/06,05:38:04.598,2016/05/06,05:38:04.607,,1375,,,,,,,0,,0,0
MSG,5,111,11111,A820F8,111111,2016/05/06,05:38:04.606,2016/05/06,05:38:04.612,,1375,,,,,,,0,,0,0
MSG,5,111,11111,A820F8,111111,2016/05/06,05:38:04.607,2016/05/06,05:38:04.615,,1375,,,,,,,0,,0,0
MSG,5,111,11111,A820F8,111111,2016/05/06,05:38:04.616,2016/05/06,05:38:04.623,,1375,,,,,,,0,,0,0
MSG,5,111,11111,A820F8,111111,2016/05/06,05:38:04.617,2016/05/06,05:38:04.624,,1375,,,,,,,0,,0,0

EDIT: Well, other aircraft seem to do this as well. I haven’t explored this much. I just happened to finish sorting a file by ICAO when I heard the helicopter outside. I looked at my live data and found the icon for it, not moving around but having a trail that seemed like a helicopter trail. So I looked it up and confirmed it was a police helicopter. Then I thought I’ll look at the sorted file. That’s when I noticed the seemingly large number of duplicated messages (except for time stamp).

2

These are altitude responses to SSR interrogations. Each interrogation generates a response. If the altitude hasn’t changed the response is identical.

3

OK. That just shifts the question to either:

Why so many answers to an interrogation?

or

Why so many interrogations in such fast succession?

Is this the repetition that is being described in this document?

4

The interrogation-and-response is the “radar” bit of SSR, i.e. that’s how it works out where the aircraft is, so it’s fairly normal for it to happen a lot.

5

Yeah, I understand it happens a lot. What I don’t understand is the rate of the repeats. The radar is rotating, and I guess as much of the conversation needs to happen while the main lobe is pointed at the aircraft. So I understand the need for speed. However, 1 1/2 round trips in 1 ms seems really fast.

The document I linked says, “Figure 6 illustrates that an all-call period normally has a Mode S period directly after it and this normally repeats with a frequency of anywhere between 40Hz and 150Hz (the stated ICAO maximum is 250Hz).” If that’s what’s going on, I’d expect to see repeats no more frequent than 4ms.

I’m just trying to understand what is going on.

6

8 responses in about 31 ms does actually seem to be within spec of 4ms.

Is it possible that the SSR signal is bouncing off something causing multiple responses or that 8 radars are operating within the range of the helo?

7

I hadn’t considered multiple radars. That would explain nearly simultaneous reports, I guess. I don’t know what airports within range of a low flying helicopter there are other than KAUS. I can see low flying planes in San Antonio, so surely San Antonio can see helicopters here. Killeen is also probable.

8

There are a few class D airports around KAUS. The class D airports do have ATC services and may have their own radars on premises. Also, military radars will interrogate the civil airplanes as well and with P-49 prohibited area near by I would expect heavy presents of the military radars.

9

Don’t forget about TCAS (SSR). If you are near KAUS than all the big iron landing will will interrogate the traffic in the area.

In the DFW area the replay light on the transpoder is on all the time.

FYI from Wikipedia:

TCAS involves communication between all aircraft equipped with an appropriate transponder (provided the transponder is enabled and set up properly). Each TCAS-equipped aircraft interrogates all other aircraft in a determined range about their position (via the 1.03 GHz radio frequency), and all other aircraft reply to other interrogations (via 1.09 GHz). This interrogation-and-response cycle may occur several times per second.[1][2]

10

I think TCAS doesn’t really care about the squawk (DF5 shown above). It uses Mode-C interrogations for puddle jumpers, and Mode-S DF0 and DF16 for avoidance calculations.

In watching data myself, I see an increase in interrogation rate for low altitude targets from the ATC radar. Both DF5 and DF4 increase. The stuff cruising at 18k and above hardly ever get interrogated for DF5.

Obviously it improves the tracking of maneuvering targets. I used the RTL1030 program on Windows and it is interesting to watch all the interrogations from the radar. Lots of different modes being interrogated every sweep.

11

My mis-understanding… Did not know that when TCAS interrogated, everyone with a transponder mode c or better would not respond.

12

When it interrogates with a Mode-C, the planes that don’t have Mode-C, just reply with two framing pulses. So TCAS is only interested in bearing and range and altitude, and the framing pulses provide everything except altitude.

The theory is, if you see a non-altitude pip on your TCAS screen, that it is probably nearby and you can increase your visual scan in that direction.

The system really comes into use with Mode-S though. I think Mode-3A/C is set to be phased out in another 10 years. By then we will no longer need squawks either. Many airlines in Europe now no longer use squawks. They just set their wheels for 1000 and the ATC uses their Mode-S for everything (so I’ve read).

13

Read the document I linked in my second post. It goes into detail about the interrogation/response processes. I haven’t read through it all yet, but I find it very interesting, so I’ll probably eventually wade through it all.