7 ms·
Precisions: * GPS: 2.00—8.76 m * GLONASS: 4.46—7.38 m * GPS+GLONASS: 2.37—4.65 m [citation needed] (source: http://en.wikipedia.org/wiki/GLONASS http://
by dchest 15y ago
Precisions:
* GPS: 2.00—8.76 m
* GLONASS: 4.46—7.38 m
* GPS+GLONASS: 2.37—4.65 m [citation needed]
(source: http://en.wikipedia.org/wiki/GLONASS http://en.wikipedia.org/wiki/GLONASS).
- 0x12 15y agoI don't understand from your table how the combined use of GPS+GLONASS would have a worse 'best case', 2.00 for GPS alone vs 2.37 for GPS+GLONASS.
- jstclair 15y agoI would imagine for the same reason as wearing two watches doesn't improve your knowledge of the current time. You're more likely to be within the largest margin of error (i.e., the latest time on either of the watches is probably no later than the current time), but what can you say about the earliest time? Since it's equally likely to be correct on either watch, the only thing you can do is average it. But in the case that one of the watches is actually correct, you've just introduced error.
- 0x12 15y agoSo, if I understand you correctly, the average error decreases but the best error actually increases?
- Alex3917 15y agoI don't know the explanation, but I know this comes up in biometrics a lot. E.g. using fingerprints + facial geometry to identify someone is much less accurate than using either one or the other. Bruce Schneier has written about this in his book Beyond Fear, and I'm sure there are explanations online somewhere.
- Symmetry 15y agoIf you expect that more information will decrease performance, your algorithms are seriously borked.
- lwhi 15y agohttp://yudkowsky.net/rational/bayes http://yudkowsky.net/rational/bayes
- dfox 15y agoThere probably is some general statistical explanation of why you can get worse accuracy from combining multiple sources than each source by itself, but it seems irrelevant. In actual receiver implementation you really get worse accuracy for simpler reason: most of the hardware resources (ie. receiver channels) are same between GPS and GLONASS, so using GLONASS ties up resources for GPS and vice versa.
- jrockway 15y agoI don't think this is true. Most of the cheap GPS chips I see on sparkfun can track 60 satellites at once, and you're not going to be seeing that many at one time anyway.
- mhw 15y agoFrom which: To improve development of the user segment, on August 11, 2010, Sergei Ivanov announced a plan to introduce a 25% import duty on all GPS-capable devices, including mobile phones, unless they are compatible with GLONASS. I think I can see why the GPS chipsets are starting to adopt this...
- 0x12 15y agoThat's a 25% import duty on devices imported into the Russian federal territory.
- mhw 15y agoYeah, sorry - that's not totally obvious from the bit I snipped. Apple (and presumably the other phone manufacturers) do sell into Russia, and a 25% import duty would probably have a significant impact on their sales.
- listic 15y agoI was surprised when I saw GLONASS support appear in recent Garmin devices. http://garmin.blogs.com/my_weblog/2011/09/garmin-etrex-and-glonass-a-powerful-combination.html http://garmin.blogs.com/my_weblog/2011/09/garmin-etrex-and-g... Now it starts to really make sense.
- MrVitaliy 15y agoThat's hardly a motivation for Apple. They still don't have an official dealer in Russia and afaik don't have any plans. Yet, most of government officials widely use iPads (including the president) and every hip kid around the block has an iPhone. This suggests that Sergei Ivanov is merely looking for more ways to tax their own population, not Apple. Russian GLONASS is an archaic POS that they them selves have trouble working on their own, approved devices and a 25% import duty on GPS-capable devices won't fix it.
- nobody314159 15y agoThe GPS chips include it (and Galileo) anyway since the frequency is close enough that you don't need any extra HW and with ASICs it's easier to just put everything it and fab a single device. Whether the ASIC implementation works and whether the iPhone API actually uses it - is another matter
- deleted 15y ago[deleted]
- singular 15y agothough off-topic and a bit of an aside, at university (I did civ eng) I learnt that it was possible to get cm-levels of accuracy out of GPS through clever arrangement of multiple sensors (for e.g. measuring the movement of buildings due to subsidence, etc.) In fact I just found a wikipedia article all about it - http://en.wikipedia.org/wiki/GPS_augmentation http://en.wikipedia.org/wiki/GPS_augmentation :)
- nodata 15y agoThey use this on automated farms. At known co-ordinates, they stick a GPS receiver in the ground. The difference between the known co-ordinates and what that GPS receiver sees is then continuously transmitted to the vehicles on the farm.
- mikepurvis 15y agoIt's called rtk correction: http://en.wikipedia.org/wiki/Real_Time_Kinematic http://en.wikipedia.org/wiki/Real_Time_Kinematic There's an opensource rtklib which supports a handful of inexpensive receivers: http://gpspp.sakura.ne.jp/rtklib/rtklib.htm http://gpspp.sakura.ne.jp/rtklib/rtklib.htm
- modeless 15y agoWhy don't cell towers do this?
- HeyItsDiogenes 15y agoBecause cellular technology is actually designed to handle long distance with obstructions?
- fleitz 15y agoCell towers kind of do by providing an accurate time and other features in AGPS.
- 15y ago
- flipbrad 15y agoI'm wondering whether it actually matters to people whether their GPS device has 2, 4 or 7m accuracy. Anything with lower resolution than about 5 cm is going to be largely useless for robotic driving, etc, particularly anything worse than 2 m res - so really (and not meaning to be rude to you and your research efforts) - isn't the different resolution irrelevant, and isn't the only reason GPS users should be happy about GLONASS integration is redundancy (especially in the rare military event that the US decides to switch public GPS off?)
- aw3c2 15y agoThink of augmented reality apps, 2 vs 7 meters can be a big difference here if you are close to the "to be augmented" target. Also for OpenStreetMap this would be great. This would get the accuracy down to "error less wide than a street" levels on customer devices.
- jws 15y agoComputer driving is more about vision systems than GPS. Maps certainly aren't accurate into the centimeter range in any event. Sailors find that their charts are some times off by kilometers. I've personally seen a large, erroneous discontinuity in the western coast of St. Lucia while using a GPS chart plotter for navigation. Chart data is copied forward from older charts, many of which were made in the 1800s, those guys were good with a sextant, chronometer, and a pen, but when near land you need your eyeballs.
- FrojoS 15y agoWow. Shouldn't it be strate forward to correct such large errors by using public available satellite images?
- ChuckMcM 15y agoIf your maps were wrong you could start a war :-) [1] [1] http://abcnews.go.com/blogs/politics/2010/11/google-nearly-starts-a-war-seriously/ http://abcnews.go.com/blogs/politics/2010/11/google-nearly-s...
- Tloewald 15y ago
- lucaspiller 15y agoAlso from the same page: Some modern receivers are able to use both GLONASS and GPS satellites together, providing greatly improved coverage in urban canyons and giving a very fast time to fix due to over 50 satellites being available. In indoor, urban canyon or mountainous areas, accuracy can be greatly improved over using GPS alone. Is there any reason, other than the higher number of sources, why GPS+GLONASS is better than just GPS in these situation? I.e. Is GLONASS better in these sorts of environments?
- omgtehlion 15y agoIt's all about (visible) satellite count
- CWuestefeld 15y agoIn situations like the "urban canyon", where the problem is that only a small slice of sky is visible, I would think that the advantages of using both systems would only be marginal. The most accurate GPS fix is had when the satellites you're seeing are widely separated in the sky. In the urban canyon, the problem isn't a lack of satellites, but the fact that you can only see those that happen to be in a narrow slice. And narrow slices are bad for constraining the solution to the equations. The way I think about it is each satellite being used in the solution is like a rope tied to the satellite (where stretchiness of the rope is analogous to measurement error). Your GPS receiver can calculate your location to within the accuracy of how far you can move while keeping all ropes taut. So if you had a fix from two satellites that are smack next to each other, you really don't have a fix any better than a single sat. If you've got two ropes at 180-degrees opposite each other (i.e., at either end of the street in the urban canyon), you've still got ambiguity due to your potential for lateral movement. For a good fix, you've got to bring in another rope at a wide angle from those -- but the tall buildings interfere with getting that rope, even if there are lots of satellites up there that could be offering it.
- sjwright 15y agoYeah, but if there are more useful satellites in the sky, there's a greater chance of the visible satellites having a wider separation. Even in the 'urban canyon', you still frequently have partial sky visibility in some non-vertical directions. And there will be occasions when a GLONASS satellite will be better placed than a GPS satellite. To use a reductio ad absurdum argument, imagine a satellite positioning network with a thousand nodes. Wouldn't it have a greater chance of providing a useful fix in this environment: http://g.co/maps/sa8zk http://g.co/maps/sa8zk ?
- jdietrich 15y agoRaw precision is only part of the story. GPS + GLONASS provides a number of significant practical advantages over GPS alone, mainly due to the increased number of satellites. Performance in urban canyons or heavily wooded areas is greatly improved, as there's a much better chance of the receiver seeing enough satellites. Likewise, startup times can be much quicker. An intelligent receiver using GLONASS only when needed could have dramatically better real-world performance with only a minor increase in battery consumption. We're inevitably going to see GLONASS everywhere due to the Russian import duty, but it's a real boon to most ordinary GNSS users, for whom the main limit on practical precision is being able to see enough satellites. This is likely to become increasingly important as the existing GPS satellites reach end-of-life.
- 7952 15y agoGPS is already better than 2m with the use of Wide Areas Augmentation Systems such as WAAS or EGNOS. This is already widely supported by chipsets.