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Are GPS computers accurate?

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Are GPS computers accurate?

Old 11-06-20, 10:06 AM
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Are GPS computers accurate?

I have a cheapo GPS cycle computer with supposedly takes readings from three satellites simultaneously. The issue with this one, and want to know if it is common to all, is when I ride next to cliffs or tall woods (think thick forest) my actual speed reading will drop between 2 to 5 MPH, then pop up again when clear. I would think all would do this when they lose their signal. So other than the mapping function, why would people put up with that?
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Old 11-06-20, 10:23 AM
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Originally Posted by rsbob View Post
I have a cheapo GPS cycle computer with supposedly takes readings from three satellites simultaneously. The issue with this one, and want to know if it is common to all, is when I ride next to cliffs or tall woods (think thick forest) my actual speed reading will drop between 2 to 5 MPH, then pop up again when clear. I would think all would do this when they lose their signal. So other than the mapping function, why would people put up with that?
They use more than 3 satellites (typically).

GPS isn't great for instantaneous speeds at low speeds. It's the wrong tool for that.

If you want more accurate instantaneous speed, get a wheel sensor. Most of the GPS units will support that.

Probably, more people use GPS units for recording their rides than navigation.

(It's possible some of your issues are with being a "cheapo" unit.)
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Old 11-06-20, 10:27 AM
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GPS needs at least three satellites for triangulation. Most GPS units use many more for higher accuracy.

It happens with my Edge Touring when I go through forests with a thickish canopy. It shows a lower mph (well, km/h). The reason is that the GPS units uses the time differences between the reception of the GPS signal to calculate the speed. When the signal is bounced on trees, buildings, rocks/cliffs etc., it will have a hard time doing it properly. My phone isn't as effected by the canopy.

However, with more satellites (including those from GLONASS and what have you), it will be more precise as it can calculate averages between the different sat systems and even prioritise them.
What I'm saying is that if you can find a GPS which can receive signals from more satellites at the same time, it will be more precise even along cliffs and buildings.
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Old 11-06-20, 10:36 AM
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Good to know. Thanks guys. I recently bought a Swiss made Sigma computer which is non-GPS but does provide accurate speed, distance, time which I am happy with. Since I have Strava on my phone I can get all the info I need, even though Strava suffers from the same speed/Ave speed issues.
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Old 11-06-20, 12:22 PM
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Using Strava on my phone is accurate enough for my needs. It's just a bike, I'm not trying to position a spacecraft for reentry.
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Old 11-06-20, 12:34 PM
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A GPS needs a minimum of four satellites, not three. Maybe you could get by with three if the earth was a perfect sphere and GPS units never left the ground, but that does not happen. More is better so it can calculate positions with different combinations of satellites and average their calculated locations.

I get frustrated with a GPS in a major city center when the tall buildings means that I lose GPS location and can't recover it. But that is the only thing that really bothers me. If my GPS says I am doing 2 mph or 5 mph when I am going slow, I already know I am going slow so I really do not care what it says.

I have a separate bike computer with wheel sensor on most of my bikes too, if I was going slowly up a tall hill and did care about my slow speed, I would rely on the data from that instead of from the GPS.

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Old 11-06-20, 12:36 PM
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I have had wire Sigma computers for 20 years also, 16.12 and 14.12. I would never be satisfied with good enough, often enough.
Not for distance or speed. GPS on my phones and Ipad is just for getting unlost. LOL
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Old 11-06-20, 12:53 PM
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Originally Posted by Tourist in MSN View Post
A GPS needs a minimum of four satellites, not three. Maybe you could get by with three if the earth was a perfect sphere and GPS units never left the ground, but that does not happen. More is better so it can calculate positions with different combinations of satellites and average their calculated locations.

I get frustrated with a GPS in a major city center when the tall buildings means that I lose GPS location and can't recover it. But that is the only thing that really bothers me. If my GPS says I am doing 2 mph or 5 mph when I am going slow, I already know I am going slow so I really do not care what it says.

I have a separate bike computer with wheel sensor on most of my bikes too, if I was going slowly up a tall hill and did care about my slow speed, I would rely on the data from that instead of from the GPS.
Three is enough for position. Four is needed for altitude (which is sort of what you say, anyway). And if you don't use the maps, it need not correlate that with the shape of the road you are on. It is your (changing) position in relation to the satellites it calculates, regardless of the shape or height of the road you are on.

In any case, most GPS's use way more, but no GPS can be accurate if it uses satellites whose signal is bounced off things. It needs a direct view to the satellite.

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Old 11-06-20, 01:23 PM
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Originally Posted by CargoDane View Post
Four is needed for altitude (which is sort of what you say, anyway).
GPS is poor for elevation.
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Old 11-06-20, 01:31 PM
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Originally Posted by njkayaker View Post
GPS is poor for elevation.
Depends on the GPS and how many satellites it can see. So the higher you are, the farther away the horizon, and the better the altitude position is.
Not all planes use radar altimeters or fly under a height were radar altimeters are useful.


Edited to add:
Although we use "GPS" interchangeably with "Satellite positioning", there are quite a few systems up there right now - either already going, or still running "public trials". And a lot of modern "GPS" navigators makes use of several or all of them. So we should actually refer to these as "GNSS", which stands for Global Navigation Satellite System. So while GPS is a GNNS, not all GNSS are GPS.

There is "GPS" as we know it - the one made by the US military back in the day. In addition to that, there are currently four other global navigation sat systems:

BEIDOU (Chinese), there is Beidou 1, Bidou 2, and I don't know how far they are with the Bidou 3.
GLONASS (Russian)
QZSS (Japanese) (compatible with the US Global Positioning System), and:
GALILEO (European).

A lot of modern "GPS" chips support the actual "GPS" as well as Beidou, Galileo, and Glonass. The QZss is a very Japan-centric setup.

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Old 11-06-20, 01:44 PM
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Speed (avg. and instantaneous) is unimportant anyway, there's too many variables involved that change from ride-to-ride, or hour-to-hour (wind, hill gradient, tire rolling resistance, bike weight, rider weight, clothing aero drag, riding position aero drag, drafting other riders) to make it useful for analysis, except maybe to avoid breaking the speed limit, if that's your thing. Once you come to grips with that, the GPS inaccuracy won't matter to you. If you really care to monitor your performance, I'd recommend getting a torque based power meter, and you'll never look at speed again.
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Old 11-06-20, 02:16 PM
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To be fair, speed matters to a lot of people who actually have to be somewhere and not merely do "trips" for training. A power meter, torque or not, does not matter to when you will be somewhere or how long it took you to get there. I couldn't care less about a power meter.

You don't have to interested in "analyzing the data" to be interested in average speeds and current speed.

Just like I don't care how much power a car produces in a certain gear/and RPM. I couldn't care less. I do care about the speed (or rather the absense of it) I'm going, when I'm going to be somewhere and whether the speed is too much (more often than too little).

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Old 11-06-20, 02:16 PM
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Originally Posted by CargoDane View Post
Three is enough for position. ....
I am not aware of any GPS unit that will give you any answer other than <<searching>> if it only had signal from three satellites.
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Old 11-06-20, 02:26 PM
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Originally Posted by Tourist in MSN View Post
I am not aware of any GPS unit that will give you any answer other than <<searching>> if it only had signal from three satellites.
That's probably because most modern GPS have more set as the minimum, because with only three, it is not that accurate.
It is no different in principle than using a hand bearing compass to calculate where you are. If you have three bearings, you can be pretty sure where you are. Two makes for a whole lot bigger potential area, and four makes for an even more precise position.

The only difference being that the GPS does not use angles, but distance (time) to the various satellite. Hence why with four, you can calculate depth/height rather than your position on a plane (as in something imaginary flat between the satellites).

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Old 11-06-20, 02:48 PM
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Originally Posted by CargoDane View Post
Depends on the GPS and how many satellites it can see. So the higher you are, the farther away the horizon, and the better the altitude position is.
Not all planes use radar altimeters or fly under a height were radar altimeters are useful.
No, it’s not very good for elevation.

a bit old:

Altitude Accuracy

The USGS says cheap units are not accurate for elevation

https://water.usgs.gov/osw/gps/


And we are talking about the very small GPS devices people use on bicycles.

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Old 11-06-20, 02:53 PM
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Originally Posted by njkayaker View Post
No, it’s not very good for elevation.

a bit old:

Altitude Accuracy
Unsafe site. Can't go there as it is blocked as unsafe. But as you said: It is "a bit old".

Anyway, just like your position accuracy on a flat plane increases with the number of satellites you receive the signal from, so does the altitude accuracy. So, if you're connected to four you can get the altitude, but your altitude accuracy increases if you can connect to, say, 12, 18, or more.

Do you think your barometer-based altimeter is more accurate over a day going through different systems?
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Old 11-06-20, 02:59 PM
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Originally Posted by CargoDane View Post
That's probably because most modern GPS have more set as the minimum, because with only three, it is not that accurate.
It is no different in principle than using a hand bearing compass to calculate where you are. If you have three bearings, you can be pretty sure where you are. Two makes for a whole lot bigger potential area, and four makes for an even more precise position.

The only difference being that the GPS does not use angles, but distance (time) to the various satellite. Hence why with four, you can calculate depth/height rather than your position on a plane (as in something imaginary flat between the satellites).
Three satellites do not give you a "plane" or flat surface between satellites.
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Old 11-06-20, 03:11 PM
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Originally Posted by njkayaker View Post
The USGS says cheap units are not accurate for elevation

https://water.usgs.gov/osw/gps/


And we are talking about the very small GPS devices people use on bicycles.
You added quite a bit, and I did manage (through another browser) to open your link. It is from 2001. Before D-GPS, and probably also before WAAS (Wide area augmented system).


As to your USCG, link, I can't see the age of the article on that page, but judging from the handheld units shown on the page, I'd guess a ten year old article.
Since then, GPS's (including for bicycles and other handhelds) have improved immensily, even using more sat systems, more individual satellites, and also differential GPS.
So, yeah, the USCG would like to have accuracy to the nearest foot. That is now possible even with handhelds and cycle computers.

From that USCG link, they even talk about "Survey grade GNNS Equipment". Do you think such things would even exist if satellite navigation systems were inherently bad at judging altitude?

Btw, at the very end, the USCG page talks about a "proposed GNSS known as the Galileo", and that "As of June 1, 2009, the Russian government operated GLObal NAvigation Satellite System (GLONASS)".

Maybe modern systems are much better than you think. Especially because those articles and opinions by their very nature (old) do not take the improved accuracy of more modern sat systems into account.

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Old 11-06-20, 03:13 PM
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Originally Posted by Tourist in MSN View Post
Three satellites do not give you a "plane" or flat surface between satellites.
They sort of do. Just like a papermap gives you a "flat" plane between your bearings when you plot them. The map is also just a flat representation of reality. I know they are not eaxct equivalents, but sometimes things have to be explained in simple terms.

Three satellites can position you in two places: Above or below the axis/plane of the three satellites (as they only measure distance - think "spoke length".

It won't know which of the two positions are correct, only that it is one of the two (as it is the distance to the satellites that counts).

If you use a fourth, that can help tell you which side of the plane/axis you are on, and hence get altitude.

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Old 11-06-20, 03:49 PM
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Originally Posted by CargoDane View Post
You added quite a bit, and I did manage (through another browser) to open your link. It is from 2001. Before D-GPS, and probably also before WAAS (Wide area augmented system).


As to your USCG, link, I can't see the age of the article on that page, but judging from the handheld units shown on the page, I'd guess a ten year old article.
Since then, GPS's (including for bicycles and other handhelds) have improved immensily, even using more sat systems, more individual satellites, and also differential GPS.
So, yeah, the USCG would like to have accuracy to the nearest foot. That is now possible even with handhelds and cycle computers.

From that USCG link, they even talk about "Survey grade GNNS Equipment". Do you think such things would even exist if satellite navigation systems were inherently bad at judging altitude?

Btw, at the very end, the USCG page talks about a "proposed GNSS known as the Galileo", and that "As of June 1, 2009, the Russian government operated GLObal NAvigation Satellite System (GLONASS)".

Maybe modern systems are much better than you think. Especially because those articles and opinions by their very nature (old) do not take the improved accuracy of more modern sat systems into account.
The bicycle computers don't use D-GPS. And they may not use WAAS. If these ground-based systems are needed for accurate elevation, it proves the point that satellites are not very good.

No one is using "survey grade" devices on their bicycles.

One way of getting higher accuracy is by keeping the receiver stationary.

Garmin (kind of an expert in this) still rates barometric readings as better than GPS.

Originally Posted by CargoDane View Post
Do you think your barometer-based altimeter is more accurate over a day going through different systems?
Cyclists are not typically interested in absolute elevation.

They are more interested in elevation gain. Drift over days doesn't really matter.

Garmin rates elevation using barometric data as better than GPS.
​​​​​​​

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Old 11-06-20, 03:54 PM
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Originally Posted by njkayaker View Post
The bicycle computers don't use D-GPS. And they may not use WAAS.

One way of getting higher accuracy is by keeping the receiver stationary.

Garmin (kind of an expert in this) still rates barometric readings as better than GPS.
Again: GPS is the older US military sat system.
Garmin is known to use old tech. But even they have sat (GNSS) computers which use Glonass, Beidou, and, I believe, Galileo, in addition to the old American GPS.

Cyclists are not typically interested in absolute elevation.

They are more interested in elevation gain. Drift over days doesn't really matter.[/quote]


So which is it? If you have absolute elevation, you can easily calculate elevation gain.

I wasn't talking about drift over days. Even during a day where you actually go somewhere, the barometric pressure can change quite a lot. Hell, even the topographical features can change the barometric pressure if wind is present. It is not really as good as you think it is. It is a cheap way to get altitude, but not very precise.

But you do you, relying on data that is decades old.

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Old 11-06-20, 03:59 PM
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Originally Posted by CargoDane View Post
Again: GPS is the older US military sat system.
Garmin is known to use old tech. But even they have sat (GNSS) computers which use Glonass, Beidou, and, I believe, Galileo, in addition to the old American GPS.
Yes. And they still use barometric data over GPS.

https://www.geoawesomeness.com/accur...ter-gps-watch/
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Old 11-06-20, 04:22 PM
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Originally Posted by njkayaker View Post
Yes. And they still use barometric data over GPS.

https://www.geoawesomeness.com/accur...ter-gps-watch/
You first tried to dismiss other sat systems and whatnot because they weren't in cycle computers, but now you post a link to a frigging watch? Yes, a barometer in a watch is a cheap way of getting altitude data - especially as it probably haven't the space to use that many different GNSS systems.
Yes, that watch (in 2017) uses GPS (not Galileo, Glonass, or anything else), so it can be hard for it to receive enough sat signals to get a precise altitude. If you have a watch/cyclecomputer/whatever that uses more than one system, it will have that many more sats to connect to, improving measurements immensely. 2017 is ages ago in the smartwatch "scene".
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Old 11-06-20, 04:40 PM
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Originally Posted by CargoDane View Post
...
From that USCG link, they even talk about "Survey grade GNNS Equipment". Do you think such things would even exist if satellite navigation systems were inherently bad at judging altitude?
....
Survey grade equipment can be very precise, I recall a few years back a land surveyor telling me that the section corner was 0.07 feet (less than an inch) away from where it was supposed to be. I told him that I suspect that the snow plow operator was to blame for that.


Originally Posted by CargoDane View Post
They sort of do. Just like a papermap gives you a "flat" plane between your bearings when you plot them. The map is also just a flat representation of reality. I know they are not eaxct equivalents, but sometimes things have to be explained in simple terms.

Three satellites can position you in two places: Above or below the axis/plane of the three satellites (as they only measure distance - think "spoke length".

It won't know which of the two positions are correct, only that it is one of the two (as it is the distance to the satellites that counts).

If you use a fourth, that can help tell you which side of the plane/axis you are on, and hence get altitude.
You really do not know what you are talking about.

***

I give up, I am not responding any more.
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Old 11-06-20, 04:48 PM
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Originally Posted by Tourist in MSN View Post
Survey grade equipment can be very precise, I recall a few years back a land surveyor telling me that the section corner was 0.07 feet (less than an inch) away from where it was supposed to be. I told him that I suspect that the snow plow operator was to blame for that.
Reading comprehension, please.
They are talking about (a decade ago) just how precise GPS survey equipment is. Meaning even then (and without Glonass, Beidou, or Galileo) they could get VERY accurate results.


You really do not know what you are talking about.

***

I give up, I am not responding any more.
Coming from someone like you who can't grasp what is being said or read links, nor understand how a GPS tells you where you are, nor what triangulation is, I'm perfectly fine with that.

If you still have problems, here's an easy-to-understand link (and why I talked about "spokes" as a way of illustrating it:

https://www.nationalgeographic.org/p...ulation-sized/




https://www.nationalgeographic.org/p...ulation-sized/

Last edited by CargoDane; 11-06-20 at 04:52 PM.
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