+BletchleyPark Posted August 13, 2002 Share Posted August 13, 2002 This is a follow-up test to the one I performed on July 26th. In this second test I included more samples (10 vs. 4) and recorded more data from each testing sample. Although the test is still far from scientific, it presents some basic data for my fellow geocachers to analyze. I can provide the data in a spreadsheet format to anyone who emails me a request. To recap the parameters of the test. I conducted it on August 13th between 10:26 am and 12:52 pm in the hazy skies on the summit of Mt. Holyoke in Western Massachusetts. My GPSr is a Garmin Vista with software version 2.28; I was using freshly-charged NiMH batteries. For the location of the test I chose a USGS Benchmark (MZ1654) located at the Mt. Holyoke summit. This Benchmark is located at N42°18.028'/W072°35.278', 938 ft in elevation. From this location, there is approxmately a 260° view of the horizon. The Vista was placed on the Benchmark with its patch antenna centerd on the mark and the unit supported in a level position. Ten tests were run, alternating between WAAS disabled/enabled. Care was taken to keep the unit undisturbed during the reconfiguration between each test. The Vista was allowed to "stabilize" for 10 minutes during each test; after which the "Accuracy" in feet, elevation, satellite data and Lat/Lon position were recorded. It took about about 2.5 hours to perform the tests during which the satellite geometry did change, as you will note from the Satellite Geometry data. I have noted the satellite number, satellite elevation (above 45° and 45° or below), signal strength, and, for the WAAS tests, whether correction data was being received. I was careful to stand away from the Benchmark in the "shadow" of the structure. I will leave it to the more experienced folks to arrive at any conclusions this data may indicate. Regards, BP (data posted below) [This message was edited by BletchleyPark on August 13, 2002 at 05:14 PM.] Quote Link to comment
+BletchleyPark Posted August 13, 2002 Author Share Posted August 13, 2002 T We As A Elevt S Beg End Lat Lon Acc Alt GPS1 N 10:26 10:36 42 18.032 72 35.276 22' 928 915 Satellite Geometry >45 <45 Sat# Sig% Data Sat# Sig% Data 17 90 na 6 80 na 23 60 na 9 60 na 26 80 na 18 90 na 29 40 na 21 60 na 24 50 na T We As A Elevt S Beg End Lat Lon Acc Alt GPS2 Y 10:42 10:52 42 18.030 72 35.278 25' 932 954 Satellite Geometry >45 <45 Sat# Sig% Data Sat# Sig% Data 17 80 Y 3 40 Y 23 60 Y 6 80 Y 26 60 Y 9 90 Y 18 100 Y 21 60 Y 35 70 YT We As A Elevt S Beg End Lat Lon Acc Alt GPS3 N 10:55 11:05 42 18.029 72 35.278 18' 942 941 Satellite Geometry >45 <45 Sat# Sig% Data Sat# Sig% Data 17 80 na 3 60 na 23 60 na 6 50 na 26 40 na 9 90 na 29 40 na 18 100 na 21 80 naT We As A Elevt S Beg End Lat Lon Acc Alt GPS4 Y 11:09 11:19 42 18.029 72 35.278 15' 947 961 Satellite Geometry >45 <45 Sat# Sig% Data Sat# Sig% Data 17 100 Y 3 70 Y 23 70 Y 6 40 Y 26 60 Y 9 90 Y 29 60 Y 18 100 Y 21 80 Y 35 60 YT We As A Elevt S Beg End Lat Lon Acc Alt GPS5 N 11:24 11:34 42 18.030 72 35.278 16' 948 944 Satellite Geometry >45 <45 Sat# Sig% Data Sat# Sig% Data 17 90 na 3 80 na 18 100 na 9 90 na 23 60 na 21 90 na 26 40 na 29 30 naT We As A Elevt S Beg End Lat Lon Acc Alt GPS6 Y 11:39 11:49 42 18.029 73 35.277 17' 950 961 Satellite Geometry >45 <45 Sat# Sig% Data Sat# Sig% Data 9 80 Y 3 80 Y 17 100 Y 21 90 Y 18 100 Y 35 60 Y 23 60 Y 26 60 YT We As A Elevt S Beg End Lat Lon Acc Alt GPS7 N 11:55 12:05 42 18.030 73 35.276 21' 951 941 Satellite Geometry >45 <45 Sat# Sig% Data Sat# Sig% Data 9 80 na 3 80 na 17 100 na 14 60 na 18 100 na 21 90 na 26 30 naT We As A Elevt S Beg End Lat Lon Acc Alt GPS8 Y 12:10 12:20 42 18.029 73 35.275 18' 944 938 Satellite Geometry >45 <45 Sat# Sig% Data Sat# Sig% Data 9 80 Y 3 60 Y 17 100 Y 14 50 Y 18 90 Y 21 100 Y 23 30 N 31 40 Y 35 60 YT We As A Elevt S Beg End Lat Lon Acc Alt GPS9 N 12:27 12:37 42 18.030 72 35.277 18' 943 935 Satellite Geometry >45 <45 Sat# Sig% Data Sat# Sig% Data 9 80 na 3 50 na 17 70 na 5 60 na 18 80 na 14 70 na 21 100 na 23 40 na 31 70 naT We As A Elevt S Beg End Lat Lon Acc Alt GPS10Y 12:42 12:52 42 18.029 72 35.278 19' 946 935 Satellite Geometry >45 <45 Sat# Sig% Data Sat# Sig% Data 9 80 Y 3 60 Y 18 60 Y 5 80 Y 21 100 Y 14 80 Y 17 80 Y 23 60 Y 31 40 Y 35 60 Y [This message was edited by BletchleyPark on August 13, 2002 at 05:16 PM.] [This message was edited by BletchleyPark on August 13, 2002 at 05:18 PM.] Quote Link to comment
hellmaster Posted August 13, 2002 Share Posted August 13, 2002 I dont really know much so tell me if I am right: From what I see WAAS does help. The Altitude was off anywhere from 1 ft to 22 ft. And the position was off anywhere from 15 ft. to 25 ft. If I remember the numbers correctly. Quote Link to comment
+BletchleyPark Posted August 13, 2002 Author Share Posted August 13, 2002 Yup. What was pointed out in the first attempt at testing in July was that WAAS does appear to have a positive effect on the Lat/Lon position. This positive effect is not necessarily shown by looking at the "Accuracy" number the GPSr reports, however. The elevation thing is also interesting. I did not recalibrate my barometric altimeter before the test. I did expect it to be off but I didn't expect the barometric elevation to fluctuate as they did during the course of the test. There was a weather front that moved throught the area this afternoon; perhaps that would explain the numbers changing. As far a the GPSr elevation numbers, they appear to improve during the test. Perhaps acquiring the additional satellites below 45 degrees provided better elevation accuracy? FWIW, BP Quote Link to comment
+apersson850 Posted August 16, 2002 Share Posted August 16, 2002 It seems to me that all the positions measured are quite near the correct point, WAAS or not. Maybe you had the altimeter set for auto-calibration? As far as I can remember, that is the default setting. Anders Quote Link to comment
+BletchleyPark Posted August 16, 2002 Author Share Posted August 16, 2002 Right you are, Anders. Auto Calibration *was* set to on. This may explain some interesting altitude readings I got yesterday during a 1200 ft ascent with the vista in my pocket (altitude jumped down from 862 ft to 509 ft ). Does anyone know how the altimeter auto calibration works? Does it sample the GPS elevation for a reference? The manual seems to overlook the details of this functioni. Regards, BP Quote Link to comment
+BletchleyPark Posted August 16, 2002 Author Share Posted August 16, 2002 Right you are, Anders. Auto Calibration *was* set to on. This may explain some interesting altitude readings I got yesterday during a 1200 ft ascent with the vista in my pocket (altitude jumped down from 862 ft to 509 ft ). Does anyone know how the altimeter auto calibration works? Does it sample the GPS elevation for a reference? The manual seems to overlook the details of this functioni. Regards, BP Quote Link to comment
+apersson850 Posted August 16, 2002 Share Posted August 16, 2002 There is something written at Mehaffey's site about this. To make it short, it seems to apply half of the difference between current elevation (barometric) and the first established GPS elevation. After that, the influence of the GPS elevation decreases exponentially, under the assumption that wheather related changes aren't to fast. The unit will keep up with them as long as it's turned on. Changes that happened when the power was off are compensated for more quickly. Anders Quote Link to comment
+BletchleyPark Posted August 16, 2002 Author Share Posted August 16, 2002 Before I left for the mountain, I calibrated the altimeter to my usual mark. When I arrived at the base, the altimeter read what I expected. I then tried to get a GPS signal and could ony get 3 or 4 satellites. When I did get a GPS elevation figure, it was waaaaaay off. I left the Vista turned on and in my pocket for the ascent. Then, while resting on a ledge at 862 ft (altimeter), I kept the Vista out of my pocket as I took in the views. When I noticed the altimeter again, it read 509 ft. Now I may not be the sharpest tool in the shed but I can guarantee you that I would have felt *that* sudden change in elevation! I should have checked the satellite page to determine what signals I was picking up, but I was unaware of the option and operation of the altimeter's auto calibration. I have now turned auto calibration off. Thanks for making me smarter! BP Quote Link to comment
+BletchleyPark Posted August 16, 2002 Author Share Posted August 16, 2002 Before I left for the mountain, I calibrated the altimeter to my usual mark. When I arrived at the base, the altimeter read what I expected. I then tried to get a GPS signal and could ony get 3 or 4 satellites. When I did get a GPS elevation figure, it was waaaaaay off. I left the Vista turned on and in my pocket for the ascent. Then, while resting on a ledge at 862 ft (altimeter), I kept the Vista out of my pocket as I took in the views. When I noticed the altimeter again, it read 509 ft. Now I may not be the sharpest tool in the shed but I can guarantee you that I would have felt *that* sudden change in elevation! I should have checked the satellite page to determine what signals I was picking up, but I was unaware of the option and operation of the altimeter's auto calibration. I have now turned auto calibration off. Thanks for making me smarter! BP Quote Link to comment
+EraSeek Posted August 17, 2002 Share Posted August 17, 2002 quote:Originally posted by Anders:It seems to me that all the positions measured are quite near the correct point, WAAS or not. Anders WAAS only corrects when there is something that needs correcting. Sometimes, like at night, the ionoshere is pretty settled. Signal delay is at a minimum. But so far when I have done tests there has also been little difference. A matter of a few meters. Quote Link to comment
+EraSeek Posted August 18, 2002 Share Posted August 18, 2002 I did another quickie test today. Walked up to the NGS marker with WAAS in and out, but ''D's'' up at the time. When I first stepped on the marker I had an accuracy reading of a whopping 68' but the unit said I was 5.6' to the target. After standing there for a few minutes that accuracy reading dropped to 24' and I had 0.0' to the target. Accuracy readings do not necessarily reflect reality (but hey, it was WITHIN that range!) Quote Link to comment
+BletchleyPark Posted August 18, 2002 Author Share Posted August 18, 2002 Originally posted by EraSeek: ... After standing there for a few minutes that accuracy reading dropped to 24' and I had 0.0' to the target. At 0.0', was your lat/lon position spot on? BP Quote Link to comment
+EraSeek Posted August 18, 2002 Share Posted August 18, 2002 quote:Originally posted by BletchleyPark: At 0.0', was your lat/lon position spot on? BP Yes. It was. Quote Link to comment
Kerry. Posted August 18, 2002 Share Posted August 18, 2002 that's as spot on as the reciever precision will allow . Cheers, Kerry. I never get lost everybody keeps telling me where to go Quote Link to comment
Kerry. Posted August 18, 2002 Share Posted August 18, 2002 that's as spot on as the reciever precision will allow . Cheers, Kerry. I never get lost everybody keeps telling me where to go Quote Link to comment
+Krenath Posted August 19, 2002 Share Posted August 19, 2002 I've just recently been seeing the D's show up in the satellite signal strenght bars on my GPS, but I haven't read anywhere in any documentation what exactly they mean (come to think of it, I can't find anything explaining what gray bars as opposed to black mean) Sorry if I'm interrupting your conversation with a rather menial question, but it soulds like several of you may know the answer... Quote Link to comment
+EraSeek Posted August 19, 2002 Share Posted August 19, 2002 D's above signal bars mean you are recieving WAAS corrections for those satellites. Gray bars mean your are recieving a signal and are in the process of downloading the data from those sats. Black bars mean you are now using the satellite and it's data for navigation. Quote Link to comment
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