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Location accuracy from Ultimate GPS?
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Location accuracy from Ultimate GPS?

by alexitheodore on Mon Sep 02, 2019 2:31 am

Hi all,

I am trying to figure out what kind of accuracy I should be able to expect from my Ultimate GPS + active antenna setup. I've so far been unable to see any documentation that gives me a clear answer to this. I've setup a test that basically evaluates it for myself using the TinyGPSPlus::distanceBetween(...) function and plotting the difference between my stationary position (lat/lon) at start compared with that same position over time. In theory, the distance between should be 0 since the device sits perfectly stationary the entire time. Yet, the readings I get vary from 0m to +10m (the units I am told are in meters) which is a significant variance.

For the record, I am using https://www.adafruit.com/product/960 as my antenna and its running outside and pointing straight up at the sky. Logging tells me that I am getting about 10 sat links and a DGPS fix quality. The code I am running is pretty much the example code out of the box.

I am curious what other people have experienced with regards to accuracy and how they know how to prove it?

alexitheodore
 
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Re: Location accuracy from Ultimate GPS?

by adafruit_support_bill on Mon Sep 02, 2019 7:18 am

Are there any nearby buildings? Accuracy can be affected by multipath reflections from nearby buildings or walls.

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Re: Location accuracy from Ultimate GPS?

by alexitheodore on Mon Sep 02, 2019 1:11 pm

No, thankfully for me. I'm in a pretty open and forested area at the top of a bluff. Like I said - clear view of the sky from one side to the other.

alexitheodore
 
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Re: Location accuracy from Ultimate GPS?

by adafruit_support_bill on Mon Sep 02, 2019 1:43 pm

Can you post a sample of the output you are getting?

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Re: Location accuracy from Ultimate GPS?

by alexitheodore on Mon Sep 02, 2019 1:52 pm

Sure, but could you be more specific what you'd like? I could provide NMEA sentences, but which ones and how many? Keep in mind I am monitoring this over a period of hours to see what the drift looks like over that kind of period.

alexitheodore
 
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Re: Location accuracy from Ultimate GPS?

by adafruit_support_bill on Mon Sep 02, 2019 2:06 pm

Some representative NMEA sentences showing the satellite reception etc. Then some longer term positional data showing the drift that you are seeing.

For the long-term data, use the [code] button and paste your data between the [code] tags.

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Re: Location accuracy from Ultimate GPS?

by alexitheodore on Mon Sep 02, 2019 4:54 pm

Code: Select all | TOGGLE FULL SIZE
$GPGGA,000202.800,,,,,0,00,,,M,,M,,*70
$GPGSA,A,1,,,,,,,,,,,,,,,*1E
$GPRMC,000202.800,V,,,,,0.00,0.00,060180,,,N*4A
$GPVTG,0.00,T,,M,0.00,N,0.00,K,N*32

13:10:48.085 -> $GPGGA,201048.000,4525.8907,N,12238.6667,W,2,09,0.93,70.7,M,-19.4,M,0000,0000*6B
13:10:48.195 -> $GPGSA,A,3,26,15,29,10,21,20,16,13,27,,,,1.28,0.93,0.87*07
13:10:48.228 -> $GPGSV,3,1,10,21,74,322,43,20,53,226,47,29,40,143,49,15,40,107,50*78
13:10:48.333 -> $GPGSV,3,2,10,51,35,158,36,16,33,289,44,26,30,257,45,13,27,064,39*7F
13:10:48.404 -> $GPGSV,3,3,10,10,26,223,46,27,15,314,45*7A
13:10:48.442 -> $GPRMC,201048.000,A,4525.8907,N,12238.6667,W,0.02,47.12,020919,,,D*4C
13:10:48.512 -> $GPVTG,47.12,T,,M,0.02,N,0.04,K,D*0E
13:10:48.623 -> Setpoint ∆: 0.00

13:32:43.506 -> $GPGGA,203243.000,4525.8901,N,12238.6676,W,2,09,0.94,70.7,M,-19.4,M,0000,0000*61
13:32:43.581 -> $GPGSA,A,3,26,15,29,10,21,20,16,13,27,,,,1.93,0.94,1.68*00
13:32:43.656 -> $GPGSV,3,1,10,21,79,359,40,20,63,233,45,15,44,093,49,10,35,228,48*78
13:32:43.731 -> $GPGSV,3,2,10,51,35,158,35,16,31,278,45,29,30,147,41,13,25,054,39*79
13:32:43.806 -> $GPGSV,3,3,10,26,24,248,42,27,23,314,47*73
13:32:43.843 -> $GPRMC,203243.000,A,4525.8901,N,12238.6676,W,0.01,355.81,020919,,,D*78
13:32:43.917 -> $GPVTG,355.81,T,,M,0.01,N,0.02,K,D*31
13:32:44.099 -> Setpoint ∆: 1.78

13:47:33.406 -> $GPGGA,204733.000,4525.8899,N,12238.6685,W,2,09,0.94,70.5,M,-19.4,M,0000,0000*6A
13:47:33.515 -> $GPGSA,A,3,26,15,29,10,21,20,16,13,27,,,,1.75,0.94,1.48*0A
13:47:33.552 -> $GPGSV,3,1,10,21,79,034,43,20,70,240,46,15,44,084,48,10,41,232,46*77
13:47:33.623 -> $GPGSV,3,2,10,51,35,158,36,27,30,313,45,16,29,270,45,29,23,150,38*79
13:47:33.693 -> $GPGSV,3,3,10,13,22,048,39,26,20,242,45*7F
13:47:33.767 -> $GPRMC,204733.000,A,4525.8899,N,12238.6685,W,0.00,210.20,020919,,,D*7B
13:47:33.837 -> $GPVTG,210.20,T,,M,0.00,N,0.01,K,D*38
13:47:34.078 -> Setpoint ∆: 3.19


alexitheodore
 
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Re: Location accuracy from Ultimate GPS?

by dThirteen on Mon Sep 02, 2019 5:02 pm

I am curious as to how many sats are in view as the drift occurs? My understanding is that fewer sats equates to reduced accuracy. Also, does the datasheet for the GPS offer any insights?

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Re: Location accuracy from Ultimate GPS?

by alexitheodore on Mon Sep 02, 2019 7:29 pm

I thought the same. I don’t have an exact answer, but I’ve followed the sat count along side my other measure of distance and found no correlation. Usually there are about 8-10 sats available which I think is normal.

alexitheodore
 
Posts: 5
Joined: Sat Aug 31, 2019 7:51 pm

Re: Location accuracy from Ultimate GPS?

by adafruit_support_bill on Mon Sep 02, 2019 7:51 pm

That is only 3 points over 37 minutes. That doesn't give us any idea of the distribution of the position drift you are seeing.

Please post the code you are using and a complete log of just the position data for the the time period.

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Please be positive and constructive with your questions and comments.