●Stories
●Firehose
●All
●Popular
●Polls
●Software
●Thought Leadership
Submit
●
Login
●or
●
Sign up
●Topics:
●Devices
●Build
●Entertainment
●Technology
●Open Source
●Science
●YRO
●Follow us:
●RSS
●Facebook
●LinkedIn
●Twitter
●
Youtube
●
Mastodon
●Bluesky
Follow Slashdot stories on Twitter
Forgot your password?
Close
This discussion has been archived.
No new comments can be posted.
Load All Comments
Full
Abbreviated
Hidden
/Sea
Score:
5
4
3
2
1
0
-1
More
Login
Forgot your password?
Close
Close
Log In/Create an Account
●
All
●
Insightful
●
Informative
●
Interesting
●
Funny
The Fine Print: The following comments are owned by whoever posted them. We are not responsible for them in any way.
byHadlock ( 143607 ) writes:
Accurate to what? 1km? 5km? You might be able to triangulate with similar accuracy simply based on signal strength of terrestrial AM and FM radio signals.It's another story if they can reliably resolve position within 100 meters. That will put you within a month's crawling distance of your science objective
byEntrope ( 68843 ) writes:
https://www.asi.it/en/2025/02/... [www.asi.it] says:
Despite the significant distance and high speed, the position was calculated with very high accuracy, with an error margin of about 1.5 kilometers for position and about 2 meters per second for velocity.
The distance from Earth means there is not much angular separation between signals, which increases the "dilution of precision" and thus leads to higher uncertainty/error in the solution (colloquially called "bad geometry").
byangel'o'sphere ( 80593 ) writes:
No, there is no bad geometry.
Because that is not how it works.
The satellite tells you *exactly* where it is above the earth surface.
And with that you know *exactly* where the satellite is in 3D space.
And that means, you only need three to calculate *exactly* where you are.
You had a point if you compared it with Astro navigation on the earth/water surface, looking at a star using a Sextant and measure its angle above the horizon. Where you would need two or three measurements in short timely succession and w
byEntrope ( 68843 ) writes:
There absolutely is dilution of precision and it absolutely is affected by geometry.
https://en.wikipedia.org/wiki/... [wikipedia.org]
Also, if you have only three satellites, you will not get a solution in this kind of application. You could get a solution if you have some other constraint on the variables, such as being on the surface of the Earth.
byangel'o'sphere ( 80593 ) writes:
You only need two satellites and the surface of the earth.
As we are talking about moon: you know how far away you are from the moon. So: same way to solve the problem.
So, two satellites, for two possible solutions, which in general on earth would mean: you know you are on the ocean, and not in the middle of Africa. So, you can rule out one.
Or you have three satellites: then you know exactly where you are.
More than three is interesting/relevant if you are in a city with sky scrapers and have reflections, aka
bypsmears ( 629712 ) writes:
You only need two satellites and the surface of the earth.
No, that's not true. I'll try to explain why.
Each satellite transmits (very roughly) "My precise position is (here), and the time by my clock is (timestamp)". (In fact the satellites send a bit more - including each other's positions - but that needn't concern us here.)
With one satellite, this tells you nothing about your position: you know the timestamp the satellite thought it was when it sent its transmission, but you have no idea how long that transmission took to arrive. (Unless you already happen to have a pre-synchronised atomic clock, which most GPS receivers do not!)
With two satellites, you can compare the timestamps you receive - the difference in the times tells you that one of the satellites is closer to you than the other, and by how much. That's enough to narrow down your position to anywhere on a specific curved surface (a hyperboloid, as it happens),
Another, third, satellite then narrows the position down further - this time to anywhere on a specific curve. If you know you're on the surface of the Earth, that's usually enough to give your position - job done; if not, a fourth satellite will do the job.
But that's not the end of the story: while you know the positions of the satellites to rather high precision, the measurements of when you received the timestamps are approximate - for a variety of reasons, but including the fact that atmospheric conditions may change the propagation speed of the signal (and you have no way of knowing whether this has happened), and the fact that (since your receiver doesn't have an atomic clock) its local clock will drift slightly over time.
If all of the satellites you're tracking are close together, then the differences between the timestamps you measure from the different satellites will be small. But the absolute error in your measurement (due to local clock drift, atmospheric conditions etc) remains roughly the same - so the percentage imprecision in the differences becomes much larger - and so the uncertainty in your position increases accordingly. This is what is referred to as "bad geometry".
Parent
twitter
facebook
●rrent threshold.
There may be more comments in this discussion. Without JavaScript enabled, you might want to turn on Classic Discussion System in your preferences instead.
Slashdot
●
●
Submit Story
It is much harder to find a job than to keep one.
●FAQ
●Story Archive
●Hall of Fame
●Advertising
●Terms
●Privacy Statement
●About
●Feedback
●Mobile View
●Blog
Do Not Sell or Share My Personal Information
Copyright © 2026 Slashdot Media. All Rights Reserved.
×
Close
Working...
Working...