FlightAware Discussions

Position and Timestamp accuracies


#1

Greetings. We are interested in finding a way to log aircraft positions with a relatively accurate, GPS synchronized timestamp. We are only interested in aircraft flying through a 150 mile radius zone. It is important for us to be able to say “the aircraft was at position Z at time X (+/- Y ms)”. where X are Z are fairly accurate (actual value TDB) and Y is small. I’m thinking +/- 75 ms max. Note that I’m not as interested in the latency of receiving the data (how long it takes the data to travel through the internet), just having a close coupling between the aircraft position and timestamp is what matters.

Originally I started looking down the path of getting an ADS-B receiver, but I discovered that the ADS-B message that the aircrafts transmit does not have a timestamp. From what I understand, the GPS timestamp on most ADS-B receivers are timestamping when the receiver gets the ADS-B message. And since ADS-B messages are sent periodically every second, you can basically only say that the position data is only accurate to within a second.

So that brought me to this site, which looks like I can just receive flight information from the web. How accurate is the information? Could this site be used to meet our needs? If not, does anyone have any recommendations?


#2

You will needs something that uses an FPGA to be that accurate.
The USB bus is too variable to provide decent timing.
You could try using an RPI with PPS timing(from a GPS unit) and just live with the USB variability.
https://store.uputronics.com/index.php?route=product/product&product_id=81

These devices can provide accurate timing, however, they are not cheap.


http://www.kinetic.co.uk/#

I use both the Uputronics GPS unit (to provide PPS NTP timing) and a radarcape(for ABDS-B reception). Both work well.


#3

I am curious… who is “us”?

Space-time localization of a plane with ms accuracy? Are you trying to target it with a non self-guided missile (with a range of 75 miles)?

That’s why the positions of planes on this website are slightly “off”. And the same on FR24.


#4

With regard to running your own receiver, are you at the center point (or near enough to it)?

I think the data is more accurate than that, but ages rapidly.
Assuming you receive one update per second, you’ll always be playing “connect the dots”.


#5

The application is defense contractor in nature, but it’s not related to missiles. That’s about all I can say about it. We have permission from the FCC and other entities, so we’re not doing anything illegal. The receiver will be in the center point of the 150 mile zone. I could subtract out the propagation latency and perform interpolation between the dots, but I can’t find any specification that rigidly defines the latency. It sounds like the aircraft equipment receives pings from GNSS satellites to resolve it’s coordinates, forms a packet of data containing additional information (avionics stuff), then transmit it out over the ADS-B channel. Is there any spec saying “once you have the coordinates, you have n milliseconds to transmit the ADS-B channel?” If the answer is small, I could work with that. If the answer is “just within a second”, then that could vary significantly between each make/model of transmitter.

There’s a few scientific journals out there which use multiple ADS-B receivers and triangulation to get more accurate coordinates. I was hoping that a site like flightaware, which has a cluster of receivers, may be able to do some fancy math and provide the accuracy I am looking for.


#6

I guess there is a spec for uncompensated latency:

https://www.ecfr.gov/cgi-bin/text-idx?node=14:2.0.1.3.10#se14.2.91_1225
§91.227(e)

(1) The aircraft must transmit its geometric position no later than 2.0 seconds from the time of measurement of the position to the time of transmission.

(2) Within the 2.0 total latency allocation, a maximum of 0.6 seconds can be uncompensated latency. The aircraft must compensate for any latency above 0.6 seconds up to the maximum 2.0 seconds total by extrapolating the geometric position to the time of message transmission.