+wildlifewriter Posted January 30, 2006 Share Posted January 30, 2006 It looks like WAAS will finally be usable in areas that is previously was not. I have a couple questions for the experts: (1) does receiving two WAAS signals improve accuracy over receiving only one? I think I was told that Garmin uses only one signal; That's not how it works. Each WAAS-type (SBAS) station transmits the same information for the same service area. The improved performance comes from the fact that your GPSr now has additional or enhanced information with which to calculate a solution. The advantage of having more than one SV covering a service area is availablity: in some local terrain, your GPS might not see one WAAS satellite, but could still receive the same signal from another. -Wlw Quote Link to comment
+Klemmer Posted January 30, 2006 Share Posted January 30, 2006 Just a small contribution that may be perfectly obvious to some, but not others: ALL WAAS satellites are in geostationary orbit, which is over the equator. The pictorials above sometimes show them elsewhere, but that is just "artistic license". They are on the equator or very, very close to it. Quote Link to comment
+GPSlug Posted January 30, 2006 Share Posted January 30, 2006 It looks like WAAS will finally be usable in areas that is previously was not. I have a couple questions for the experts: (1) does receiving two WAAS signals improve accuracy over receiving only one? I think I was told that Garmin uses only one signal; That's not how it works. Each WAAS-type (SBAS) station transmits the same information for the same service area. The improved performance comes from the fact that your GPSr now has additional or enhanced information with which to calculate a solution. The advantage of having more than one SV covering a service area is availablity: in some local terrain, your GPS might not see one WAAS satellite, but could still receive the same signal from another. -Wlw You can also get a little extra geometry by just ranging off the second WAAS satellite. A fairly minor improvement, but an extra measurement is an extra measurement. Theoretically, you could mix the messages from the two satellites to get your 'D's a little faster since the same messages don't come out at the same time. But that's forbidden by the spec because they don't guarantee that it will be compatible, even if it probably is most of the time. 2) does WAAS really improve accuracy? - I've noticed with my 60c that the EPE does not change when receiving "D" corrections. In fact, I've seen EPE of 6 feet without WAAS. I know that EPE is computed and may not reflect reality. Has anyone run tests with two identical units side-by-side, one with WAAS enabled, and one without? I think a test like that would be insightful. The track logs could be downloaded and compared to see the dispersion around a known location. First of all, the EPE will be a better estimate with WAAS than it is without. The errors will be better defined. WAAS tends to be very conservative with it's error bounding because it's more concerned about integrity then accuracy (can I really trust this system to make my final landing approach for me?). Your GPSr probably shaves some of the conservativeness off of the EPE, but it can't necessarily be directly compared to the non-WAAS number. Secondly, I don't know how Garmin handles the case with some 'D' and some without. If you don't mix them in the solution, you might have a solution with 5 'D's which might not be as good as 8 no-'D's was. If you do mix them, you'll probably de-weight the no-'D's and have a similar but muted situation where you don't have as many "good" measurements. Lastly, not all 'D's are created equal. There are three corrections in WAAS, the slow orbit correction, the fast clock correction, and the ionosphere grid. (see the secret screen shots above) The 'D' probably shows up as soon as it gets the fast and slow corrections, because that's enough to get started with. It takes a little longer to get the iono grid which is why you might see the EPE go down further after the point when it first gets all 'D's. Quote Link to comment
GeoBobC Posted January 30, 2006 Author Share Posted January 30, 2006 I guess Joe Mehaffey said it best: "WAAS is not guaranteed to work, it is not guaranteed to increase accuracy, it is not really able to tell you when it will improve accuracy, when it will have no effect, or when it will make for less accuracy than the normal GPS signal." Quote Link to comment
+fizzymagic Posted January 30, 2006 Share Posted January 30, 2006 Lastly, not all 'D's are created equal. There are three corrections in WAAS, the slow orbit correction, the fast clock correction, and the ionosphere grid. (see the secret screen shots above) The 'D' probably shows up as soon as it gets the fast and slow corrections, because that's enough to get started with. It takes a little longer to get the iono grid which is why you might see the EPE go down further after the point when it first gets all 'D's. Of course, it is important to note that the ionospheric correction either applies to all satellites or to none. It's not transmitted in a way that allows it to be only applied to one or a few satellites. Since the iono correction is generally the largest, I don't think it makes any sense to show a satellite as 'D' unless the iono corrections have been received. Thus, the only situation in which some satellites might have a 'D' and others would not would be when the iono correction has been received but the other corrections have not for some satellites. But that's only how I would do it. Goodness knows what Garmin actually does; we know they have a somewhat spotty record when it comes to proper use of WAAS. Quote Link to comment
+EraSeek Posted January 31, 2006 Share Posted January 31, 2006 I guess Joe Mehaffey said it best: "WAAS is not guaranteed to work, it is not guaranteed to increase accuracy, it is not really able to tell you when it will improve accuracy, when it will have no effect, or when it will make for less accuracy than the normal GPS signal." This quote was written when WAAS was still in its testing phase. I think you could safely say it is guaranteed to work now. There are times when corrections are needed and times when there is little to correct, thus increase in accuracy will vary. Example, at night there is little or no ionospheric corrections to be made. Quote Link to comment
+EraSeek Posted January 31, 2006 Share Posted January 31, 2006 Whether you are recieving corrections from one WAAS sat (35) or two (35 and 47) makes no difference as they both transmit the same correction data. It is your GPS that applies those corrections in accordance with where your are. The benefit of two WAAS sats over one is the improved ability to "see" a WAAS sat and collect the data. WAAS corrections are only applied when three or four or more corrected sats are collected. It is my understanding from a Garmin tech that corrected sats and uncorrected sats are mixed and used together for the best solution if he understood my question correctly. It was also my understanding from him that, though a postion fix occurs when you recieve 3 or four sats, you still use all the collected sats being recieved to calculate a position. I know some of that may bring up some new questions. As far as the epe, yes it is just a calculated guessimate, and does some funny things when WAAS is first applied. As far as testing it, yes, I have done my own simple test before and found that when a good WAAS signal is in place my accuracy did improve at a known point, from about 12' to 14' out to about 3' to 6' feet within the target. Quote Link to comment
+EraSeek Posted January 31, 2006 Share Posted January 31, 2006 What constitutes a "Differential Position" and a corrected "D" sat? I think Fizzymagic might have it right. Once you get 3 corrected sats "D", that is enough to get you the ionospheric correction to achieve a "Differential Postion" because the ionospheric corrections are based on your GPS's position. The other corrections are errors having to do with the individual sats and their positions. Quote Link to comment
GeoBobC Posted January 31, 2006 Author Share Posted January 31, 2006 Outstanding information. Thank you. Another question: does/can a WAAS satellite serve as one of the three or four standard/non-WAAS satellites to provide position? In other words, if I can lock on two normal satellites plus a WAAS satellite, will my GPS be able to determine its location? I ask because the signal from the new 36 (on my 60c) is often the strongest of all satellite signals. Quote Link to comment
+EraSeek Posted January 31, 2006 Share Posted January 31, 2006 Yes, the WAAS sats also function as a normal GPS sat to provide position. However while they are moving sat 35 (and probably testing 36) they will not perform this function. I guess it is called the ranging function. So yes, but not right now. Sat 47 still will. Quote Link to comment
peter Posted January 31, 2006 Share Posted January 31, 2006 I guess it is called the ranging function. So yes, but not right now. Sat 47 still will. #36 has been sending ranging data quite a bit of the time as well. Since it is still in a testing phase I'd expect the data provided by it to vary. Yesterday I was on a bike ride early and never even saw a signal from 36 although there were no obstructions in that direction. I did get corrections and ranging data intermittently from 47, which is quite low in the west from my location outside San Francisco. But later yesterday 36 was back on the air providing both corrections and ranging. On most Garmins you can tell if a given satellite is being used as part of the position calculation by whether the corresponding signal bar is a solid dark color (or black on b&w models) rather than a light shade, gray, or unfilled rectangle. So if you see a solid bar for a WAAS satellite it shows that it must be sending ranging data. OTOH the satellite might still be sending ranging data but not get a solid bar if your receiver just hasn't gotten the full ephemeris data from it yet. Quote Link to comment
+fizzymagic Posted January 31, 2006 Share Posted January 31, 2006 On most Garmins you can tell if a given satellite is being used as part of the position calculation by whether the corresponding signal bar is a solid dark color (or black on b&w models) rather than a light shade, gray, or unfilled rectangle. Rumor has it that the new SiRF-based Garmin units cannot use the ranging info from the WAAS satellites, so the bars never show up as dark, leading people to believe that they don't have a "lock" on the satellite. Something to do with the SiRF chipset. I have seen no confirmation of this, so it could very well be nonsense, but the person who reported it said that he got the information from a Garmin representative. Quote Link to comment
GeoBobC Posted January 31, 2006 Author Share Posted January 31, 2006 I read some comments from new SiRF users about not displaying a solid bar for WAAS. However, I believe those comments were written during the time that no one could get a lock so perhaps it was not related to SiRF, but rather what FAA was doing. If it is a SiRF limitation, that would appear to be a serious problem. Quote Link to comment
+wildlifewriter Posted January 31, 2006 Share Posted January 31, 2006 Rumor has it that the new SiRF-based Garmin units cannot use the ranging info from the WAAS satellites, so the bars never show up as dark, leading people to believe that they don't have a "lock" on the satellite. Something to do with the SiRF chipset. Your rumour is probably a misleading technomyth. There WAS an early issue with WAAS functions on the "old" SirfstarII chipset - but this was sorted out by a firmware mod, ages ago. Garmin's own website and literature state clearly that the new models are "WAAS-capable" - a claim they'd hardly dare to make public if it wasn't true. -Wlw Quote Link to comment
+fizzymagic Posted February 1, 2006 Share Posted February 1, 2006 Garmin's own website and literature state clearly that the new models are "WAAS-capable" - a claim they'd hardly dare to make public if it wasn't true. It's likely a myth, but you seem to misunderstand what I was saying: the rumor is that the Garmins can't use the ranging data from the WAAS satellites; that is, they can't use the WAAS satellites like regular GPS satellites in generating the position solution. There has never been any doubt that the Garmins can make use of the correction data from the WAAS satellites, which is what the Garmin literature claims. Quote Link to comment
+GOT GPS? Posted February 4, 2006 Share Posted February 4, 2006 Bumping thread on the new forums Quote Link to comment
+PDOP's Posted February 4, 2006 Share Posted February 4, 2006 Last week, according to a Garmin tech "the FAA changed the sequence of message that they are using on their test signals to include message MT0 which is the do not use message." I asked Garmin because I noticed that 36 would blip and immediately disappear from my satellite page. If you weren't watching, you might conclude it never saw 36. 36 must be transmitting the do not use message all the time now. I haven't seen it for a while now. Quote Link to comment
+GOT GPS? Posted February 5, 2006 Share Posted February 5, 2006 Last week, according to a Garmin tech "the FAA changed the sequence of message that they are using on their test signals to include message MT0 which is the do not use message." I asked Garmin because I noticed that 36 would blip and immediately disappear from my satellite page. If you weren't watching, you might conclude it never saw 36. 36 must be transmitting the do not use message all the time now. I haven't seen it for a while now. I have not seen number 36 either. Also I have not seen 35 either for some time on the 60Cx. Hope there is SOME coverage here in the east, I like WAASing through the woods with the 60Cx. - Geoff Quote Link to comment
moonpup Posted February 5, 2006 Share Posted February 5, 2006 Hey Timpat, I finally got sat 33 today! Here's a shot from inside my house in MA, sorry it's blurry Quote Link to comment
+Timpat Posted February 5, 2006 Share Posted February 5, 2006 (edited) Cool! Glad you got a lock on it at long last. Lately I've been seeing SV 33 instead of 35. I've read where 35 is going thru a move in its position to a new parking spot up in space. Edited February 5, 2006 by Timpat Quote Link to comment
+PDOP's Posted February 6, 2006 Share Posted February 6, 2006 On the newsgroup sci.geo.satellite-nav today. QuickTime animations showing the westward move of Inmarsat 3-F4 (AOR-W)from 54°W to 142°W have been created by the Geodetic Research Lab at UNB. The two animations show the subsatellite point and the 0°, 30°, and 60° elevation angle contours. The animations can be downloaded from here: http://gauss.gge.unb.ca/AOR-W-move.1.mov(cylindrical) http://gauss.gge.unb.ca/AOR-W-move.2.mov(spherical) Richard B. Langley Prof. of Geodesy and Precision Navigation Quote Link to comment
GeoBobC Posted February 7, 2006 Author Share Posted February 7, 2006 Some users are reporting seeing 33; I have not seen it in Seattle. I have not seen 35 recently. I see 36 grab the signal and then shut off - the "do not use" message. Quote Link to comment
moonpup Posted February 8, 2006 Share Posted February 8, 2006 Some users are reporting seeing 33; I have not seen it in Seattle. I have not seen 35 recently. I see 36 grab the signal and then shut off - the "do not use" message. Here in MA I have not seen 33 or 35 for 3 days now Quote Link to comment
GeoBobC Posted February 8, 2006 Author Share Posted February 8, 2006 Here in Seattle, my 60c saw 36 momentarily today and blipped off. Oddly, I experienced the same with 33 today: a quick hit, and then it's gone within seconds. I think the satellite movement and testing are making these WAAS birds unreliable for some period of time. Quote Link to comment
+fizzymagic Posted February 8, 2006 Share Posted February 8, 2006 (edited) Here in Seattle, my 60c saw 36 momentarily today and blipped off. Oddly, I experienced the same with 33 today: a quick hit, and then it's gone within seconds. I think the satellite movement and testing are making these WAAS birds unreliable for some period of time. Huh? This report is bizarre. First, the satellite you see as #36 isn't scheduled to go live until this fall; of course it is unreliable, as it is just being tested. When it goes live, it will not show up as #36, since that PRN is only being used for the testing phase. Second, the satellite you see as #33 is below the horizon in Seattle. I can't understand why your unit would think it saw it, since that is physically impossible. In any case, neither of these satellites is supposed to be usable from your position right now. Edited February 8, 2006 by fizzymagic Quote Link to comment
GeoBobC Posted February 8, 2006 Author Share Posted February 8, 2006 Bizarre or not, that's why I'm experiencing. All other WAAS PRNs are searched for approximately 60 seconds, except for 33 which only appears for a few seconds. Quote Link to comment
+PDOP's Posted February 9, 2006 Share Posted February 9, 2006 36 is transmitting corrections again but no ranging signals (at least I'm not getting a solid bar on my 76C) Quote Link to comment
+GOT GPS? Posted February 13, 2006 Share Posted February 13, 2006 Here is what the Satellites are like today 13 Feb: This is what they were on the 27th of January: Sorry about the bad pictures, I need a new camera Garmin # 33 is almost on the edge of the horizon at 7 deg above Garmin # 35 has moved from Bearing of 140 degrees and 33 deg elevation, to a bearing of 158 degrees, and an elevation of 38 degrees Garmin #36 is showing only on the Map60C, but not the SporTrak Color, but it is close to the edge of the sky view on the Map60C, and I won't have much of anything, until the waas satellite at 107 degree west longitude comes online. Quote Link to comment
GeoBobC Posted February 13, 2006 Author Share Posted February 13, 2006 Very interesting. Do you know if the Garmins have a hidden menu to display almanac data such as this? I've been getting very strong 36 signals on my 60c; no solid bar but it does appear that I'm getting improved accuracy from the "D"s. Quote Link to comment
+PDOP's Posted February 13, 2006 Share Posted February 13, 2006 35 is rising for me It's now at 17° elevation up from an initial 6° Quote Link to comment
+japtkd Posted February 15, 2006 Share Posted February 15, 2006 I've recently gotten my Globalsat 338 setup on a Palm TX and absolutely love it. Has anyone had a problem with the 338 obtaining a WAAS lock? Mine keeps flipping between sat 33 (this one is an EGNOS, correct?) and sat 35. I don't think I've gotten differential data from the unit yet. Any suggestions? Interestingly enough, our Etrex Legend has never spotted sat 33. Differences in sensitivity of the two units? Quote Link to comment
GeoBobC Posted February 15, 2006 Author Share Posted February 15, 2006 My guess on the Legend is that it only searches for WAAS satellites it has previously observed. My Map76 works that way: it only searches for 35 and 47. It will not find 36, 33, or any other WAAS bird as far as I can tell. I suppose I could reset the almanac but that's painful. Quote Link to comment
+weakfish Posted February 16, 2006 Share Posted February 16, 2006 Riddle me this.... Why is my Legend C seeing 42 and showing corrections when my 60Cx isn't getting any WAAS birds at all? Quote Link to comment
+GOT GPS? Posted February 16, 2006 Share Posted February 16, 2006 Riddle me this.... Why is my Legend C seeing 42 and showing corrections when my 60Cx isn't getting any WAAS birds at all? It has something to do with how the SiRF chip works, in the new units. - Geoff Quote Link to comment
+vds Posted February 17, 2006 Share Posted February 17, 2006 In the heart of Kent WA today, got twelve birds including two WAAS and the 'ol B+W Garmin Legend was showing an accuracy of 7 feet. Amazing. Never got a WAAS hit before at all out here. Quote Link to comment
+fizzymagic Posted February 18, 2006 Share Posted February 18, 2006 In the heart of Kent WA today, got twelve birds including two WAAS and the 'ol B+W Garmin Legend was showing an accuracy of 7 feet. Amazing. Never got a WAAS hit before at all out here. I've really been noticing the difference here in the Bay Area as AOR-W is rising in the sky. I am getting a WAAS lock on my Magellan pretty much instantly. Today I turned on a GPS that I hadn't used in a few months, to see how long it would take to update the almanac and show AOR-W in its current position. Took about 15 minutes or so; but it was receiving the WAAS corrections within less than 1 minute. Quote Link to comment
Delaypat Posted February 18, 2006 Share Posted February 18, 2006 turned on my 60cs today and got a lock, inside my house within 30 secs..I was flippin happy..No where near a window and getting 7+/- WOW, I have never gotten waas inside my house before..Very cool!! very cool! Quote Link to comment
packerfan1964 Posted February 18, 2006 Share Posted February 18, 2006 been getting strong waas signals here in wis too Quote Link to comment
+EScout Posted February 20, 2006 Share Posted February 20, 2006 (edited) The eastern WAAS sat (35) has moved west. It is showing on my eXplorist, Meridian and Legend in its new position. I can now receive it in the house, and get WAAS corrections on the eXplorist and Meridian in my house which I never could previously, here in So Calif. I should add that because of a hill, I can never get the western WAAS (47) here. Edited February 20, 2006 by EScout Quote Link to comment
+GOT GPS? Posted February 21, 2006 Share Posted February 21, 2006 My SporTrak Color is showing WAAS 122(35) at bearing 183 deg and elevation of 41 deg, so I missed it yesterday. I tried to get the SporTrak Color to update the waas satellite position yesterday, and it wouldn't so I missed seeing the satellite being at a bearing of 180 deg(due south) and an elevation of 42 deg. OH WELL. Quote Link to comment
+hardwire Posted February 22, 2006 Share Posted February 22, 2006 (edited) Kinda cool watching 35 move on the satellite screen. Signal strength of 35 now matches 36 at full. ------------------------------ Hardwire http://geo.clearpowerzone.com Edited February 22, 2006 by hardwire Quote Link to comment
GeoBobC Posted February 28, 2006 Author Share Posted February 28, 2006 Yesterday I had a strong signal on 38 on the satellite page of my 60c. Today I had a strong signal on 48. They appear to be in the same location as 36 was previously. Quote Link to comment
+PDOP's Posted March 2, 2006 Share Posted March 2, 2006 Yesterday I had a strong signal on 38 on the satellite page of my 60c. Today I had a strong signal on 48. They appear to be in the same location as 36 was previously. I saw the same thing this morning. Quote Link to comment
+fizzymagic Posted March 2, 2006 Share Posted March 2, 2006 Yesterday I had a strong signal on 38 on the satellite page of my 60c. Today I had a strong signal on 48. They appear to be in the same location as 36 was previously. Maybe that's because it's the same satellite. As previously noted in this thread, the PRN numbers (from which Garmin gets its satellite designations) for testing are not the same as those that the satellite will use when it is operational. Quote Link to comment
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