Older GPS receivers had a limited number of channels, 4 or 5 typically. Modern ones have a lot more (at least the decent ones, I suppose). There's a little bit of information on this on Wikipedia.
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I think you should just read up on how GPS works. All the sats near the horizon presents a geometry issue (all within the same plane) that further reduces vertical accuracy. The problem is not that the sats are setting over the horizon, as your posts seem to imply (or refute, rather).
There are also signal propagation issues that get worse as the signal has to travel through more of the atmosphere (i.e. low down). This will be improved as the new systems use more frequencies, allowing receivers to compare how each signal on a wider range of frequencies was affected and correcting errors more accurately than is currently possible.
But yeah, while 7+ sats are possible to see - and I have done many times - often hills or buildings will mean you only see 3 or 4. I've seen receivers that claim to be able to track 21 satellites, which is somewhat pointless.
But yeah, while 7+ sats are possible to see - and I have done many times - often hills or buildings will mean you only see 3 or 4. I've seen receivers that claim to be able to track 21 satellites, which is somewhat pointless.
NigelM
Recovering Member
I'm not that motivated (or that smart? Nobody answer!), but thankfully there are plenty of people on here who know more about this than me. I just wondered if the Roadster elevation display was fixed in 100ft increments.
DZCPA
2.5 2011#1263 "NO GAZZZ"
W.Petefish
Active Member
There is Enhanced GPS as well as DGPS. Using cellular towers along with GPS to determine exact positions within 3 feet is how Enhanced GPS works. DGPS uses dedicated radio towers and a dedicated receiver. DGPS is by far less common now because of it was phased-out by the advent of Enhanced GPS.
In short your best bet for positioning is your iPhone/Android Phone. (at least in an urban setting) Out in the country your best bet is your handheld Garmin or Magellan.
In short your best bet for positioning is your iPhone/Android Phone. (at least in an urban setting) Out in the country your best bet is your handheld Garmin or Magellan.
NigelM
Recovering Member
Although now I have learnt more about GPS than I ever needed to know, I spent this morning driving round with the GPS satellite display on the e-dashboard. My Roadster tracks 11 satellites and always locked on 9 of them. Which 9 from 11 varied but it never changed from 9; all my driving was within a 20 mile radius. The most interesting thing is that my Roadster elevation stayed on 0ft the whole time while the GPS altitude showed anything from 65-113ft over what was pretty flat territory. (From previous explanations, now I know why). BTW: At home now my driveway is 11ft above sea level but the GPS altitude shows it as being 65ft.
I wonder why Tesla included the elevation readout on the VDS? Does it serve a purpose for anyone that I am too dumb to figure out living at sea-level?
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Because it costs energy to climb hills...
For a car without combustion, absolute elevation is not as important as the differential.
strider
Active Member
Hmm. Yeah, I've never seen anything over than "0" on my VDS either. Of course I also live at 8ft above sea level. Should take it up a hill just to make sure it works 
I can confirm: the GPS based navigation system in my phone tells me I live 70 feet below sea level. If that were true, I would have noticed by now!
NigelM
Recovering Member
Hmm. Yeah, I've never seen anything over than "0" on my VDS either. Of course I also live at 8ft above sea level. Should take it up a hill just to make sure it works
Actually, that's one reason why I had my wife watch the VDS as we drove over the Sunshine Skyway....
The reason why altitudes are consistently off by the same amount in many locations is down to the difference between the approximated WGS84 ellipsoid that GPS uses and real local sea levels. Some receivers include a look-up table that corrects this, sounds like the Roadster's doesn't.
dhrivnak
Active Member
For me the vertical has always been within a hundred feet of any posted sign so i am happy with mine.
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