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airplanecrazy
Topic Author
Posts: 76
Joined: Thu May 11, 2006 8:09 am

GPS and WAAS

Fri May 08, 2020 9:09 pm

I am looking at some ADS-B data and I have some questions about positional accuracy that I am hoping someone on this forum is knowledgeable enough to answer.. The Garmin website https://www8.garmin.com/aboutGPS/waas.html says that with WAAS the Estimated Position Uncertainty(EPU) is typically less than 3 meters. I am looking at an FAA ADS-B Performance Report for a flight and the maximum NACp was 10, which corresponds to an EPU less than 10 meters. When I look at the actual ADS-B data should I be thinking that the "typical" error is closer to 3 meters or closer to 10 meters? TIA.
 
gloom
Posts: 436
Joined: Thu Jun 30, 2016 4:24 pm

Re: GPS and WAAS

Sat May 09, 2020 6:18 am

There's no simple answer to that, I guess.
All SatNav units are constantly reading information from satellites regarding time, and they calculate transmissioin time and (known) sat location to calculate receiver location. If you need more details, there are plenty of sites explaining the math, I'll stay out of it and consider only error prone elements.
First - SatNav systems were designed to serve army as a primary purpose. They provide two signals, one encoded for own/allies armies, other for civil purpose. The civil channel is "less precise" in time channel, and also adds random tim deviations. That's why army GPS is considered to have gaurantee of about 10cm, while civil would have around 10m (based on 3 sources, which is minimum required to locate receiver).
Second - in specific conditions you could get wrong calculations, due to different physical effects on the receiver antenna/transmitting medium. It's not a problem most of the time, but since you're asking a small difference - yes, experienced designer can take a meter or two for that precision.
There are also methods to improve GPS location precision. Since there are many networks and many SatNavs, you can try to calculate more than one location at a given time to calculate average, or use all location Sats to calculate one position. All of these will improve accuracy.

On the other hand, it's always cost vs use. In a plane running let's say 100m/s (~200kts), do you really need an extra meter of accuracy if it's going to cost you? Or if you started before say Galileo went operational and your receiver is not receiving these decreasing accuracy, are you going to pay for the upgrade to receive Galileo as well?

So, based on my own experience (I've been running fleet position monitoring solution on trains professionally), you can quite easily get about 3m accuracy for 98% of position reports on multi SatNav, and around 10m based on only one network. Back in the days of "start GPS config", when they had 24Sats, it was estimated at a few tens (I think I've read guaranteed difference to real location was 40m). Since the numbers are quite close, it could be explanation to your question.

Cheers,
Adam
 
airplanecrazy
Topic Author
Posts: 76
Joined: Thu May 11, 2006 8:09 am

Re: GPS and WAAS

Sat May 09, 2020 1:10 pm

gloom wrote:
There's no simple answer to that, I guess.
So, based on my own experience (I've been running fleet position monitoring solution on trains professionally), you can quite easily get about 3m accuracy for 98% of position reports on multi SatNav, and around 10m based on only one network. Back in the days of "start GPS config", when they had 24Sats, it was estimated at a few tens (I think I've read guaranteed difference to real location was 40m). Since the numbers are quite close, it could be explanation to your question.


Thanks Adam! The GPS unit that generated the ADS-B data is a Garmin GTX 335. That appears to be a single network device (GPS/WAAS).
 
airplanecrazy
Topic Author
Posts: 76
Joined: Thu May 11, 2006 8:09 am

Re: GPS and WAAS

Sat May 09, 2020 1:50 pm

Adam, I am also seeing some disagreement between some of the altitude reports and some of the vertical rate reports. Do you have any intuition/knowledge about which reports would be more accurate? If I "integrate" (add up) the vertical rate reports I eventually get to the same final altitude as in the altitude reports, but the altitude reports show the aircraft getting to that altitude before the integrated rate reports.
 
mxaxai
Posts: 1679
Joined: Sat Jun 18, 2016 7:29 am

Re: GPS and WAAS

Sat May 09, 2020 5:11 pm

I'm not sure if you're aware of this but WAAS is additional to regular GPS. Your mobile phone will only use GPS signals. A device with WAAS capability uses these additional signals to correct for some error sources. There are also alternative ways to improve accuracy, like using dual-frequency GPS, ground-based augmentation systems or RTK devices.

At the end of the day, the manufacturer of the device will give you some guarantee for the precision (or not, if it's a consumer device like a mobile phone...). Unless there's a specific mention of, for example, "NACp > 10" in the datasheet or handbook, the only thing you know for sure is that the device complies with FAA regulations. For the GTX 335 that appears to be NACp >= 8, i. e. 92.5 m. Anything else is just marketing.

Do note that this is a worst case guarantee. You can achieve NACp = 11 (<3 m accuracy with 95% probability) with some GPS/SBAS systems. Certain high-performance solutions can even give you cm-level accuracy but those are typically only used for geodetic purposes.

Side note: The artificial degradation of civilian signals ("Selective Availability") has been switched off for 20 years (and a week, since 2. May 2000). There is a separate encoding for military signals with certain benefits but the accuracy is virtually identical.
 
airplanecrazy
Topic Author
Posts: 76
Joined: Thu May 11, 2006 8:09 am

Re: GPS and WAAS

Sat May 09, 2020 5:44 pm

mxaxai wrote:
. Unless there's a specific mention of, for example, "NACp > 10" in the datasheet or handbook, the only thing you know for sure is that the device complies with FAA regulations. For the GTX 335 that appears to be NACp >= 8, i. e. 92.5 m. Anything else is just marketing.
.


For the flight I am examining the NACp is a minimum of 8, a maximum of 10, and an average of 9.9 (meaning most of the time it was 10). Do you believe that these values are not really accurate? I was thinking they were accurate, and that the actual uncertainty was between 3 and 10 meters. I was just trying to get a sense if the true value was closer to 10 or closer to 3. Thx.
 
mxaxai
Posts: 1679
Joined: Sat Jun 18, 2016 7:29 am

Re: GPS and WAAS

Sat May 09, 2020 7:00 pm

airplanecrazy wrote:
For the flight I am examining the NACp is a minimum of 8, a maximum of 10, and an average of 9.9 (meaning most of the time it was 10). Do you believe that these values are not really accurate? I was thinking they were accurate, and that the actual uncertainty was between 3 and 10 meters. I was just trying to get a sense if the true value was closer to 10 or closer to 3. Thx.

Oh, if you have those values from the actual flight you can use them. The receiver calculates them while it resolves the position. I was under the impression that you were looking at position data in general (like those found on FR24) so you wouldn't neccessarily have access to the NACp values.

The fact that the NACp was consistently 10 indicates that you were mostly well above the threshold (otherwise you would see regular dips to 9 when a satellite goes out of view and an average well below 10) but also means that you never actually reached 11. Although NACp=11 (< 3m accuracy) is a realistic upper limit for most such systems; you'd only get those values in ideal conditions. Something like 6 - 8 m would be appropriate to assume for most of the flight (you mention that it occassionally did go below 10, so some data points will have significantly higher uncertainty).
Though, if you were using this data in any legal context you'd have to use the maximum uncertainty of 10 m.

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