5 ms·

GPS is also coupled with WAAS which relies on a network of ground stations at known coordinates to determine GPS error. These errors are caused by bulges in the

GPS is also coupled with WAAS which relies on a network of ground stations at known coordinates to determine GPS error. These errors are caused by bulges in the ionosphere that throw off timing. Error is calculated, broadcast up to satellites and then rebroadcast down to GPS receivers that are WAAS enabled. This is known as a satellite based augmentation system. The other satellite nav systems that compete with GPS are building similar augmentation systems (like GAGAN, EGNOS, MSAS and SDCM). These systems are required for precision approaches like LPV for aircraft on instrument flight plans. Regular GPS precision due to ionosphere changes is too low. They've enabled the FAA to lower the minimums for GPS approaches down to levels that are close to ILS, removing the need for expensive ILS installations at many regional airports. So replacing GPS isn't just about creating a system to locate a point in space. You'd need to replace critical systems like WAAS that enable precision approaches during instrument conditions, for example.

FredPret 4y ago
It’s mindblowing how many wrinkles there are to iron out in a complex system. Who knew that the ionosphere bulges!
- pixl97 4y ago
  SpaceX for one. When solar storms occur they have to release newly launched satellites at a higher altitude to avoid being slowed down by a puffed up atmosphere. Just recently they lost a few starlink sats to this effect.
  - pencilguin 4y ago
    Rather more than a few. Managed to save a few, instead. Expensive lesson.
ajross 4y ago
FWIW, that particular problem isn't likely going to be an issue for Starlink. Starlink satellites orbit at 550km vs. 20000km for GPS. The transmission is much (much) closer to a straight line approximation and ionosphere refraction is negligible. In fact, the line of sight transmission to and from a Starlink satellite is likely to be significantly better than that between an airliner and its local WAAS station, which is confounded by ground reflections. (Edit for clarity: WAAS is just digital data, this isn't an actual error in the signal. The point is more "you have to spend more engineering effort on the transmission environment of the correction signal for GPS than you do on the actual signal for Starlink")
- brandmeyer 4y ago
  Almost all of the ionosphere's total thickness is below 550km, so the lower altitude doesn't matter. Starlink sees less impact from the ionosphere because the carrier frequencies are higher, and dispersion is proportional to 1/f^2.
  - ajross 4y ago
    I don't think that's true? Plasma density goes up with altitude out to quite some distance, though it's nonlinear. And in any case it's a refractive thing. Having a somewhat lower index doesn't make much difference when one lens is 40x thicker than another.
- m463 4y ago
  I would imagine many more satellites to choose from. (I'm uncertain, lower orbit might give you more satellites, but I don't know how many would be in view - many more or fewer)
brandmeyer 4y ago
> Regular GPS precision due to ionosphere changes is too low. ... for single-frequency receivers. Military receivers have been dual-frequency for a long time. GPS L2C's rollout is pretty slow (and L5 is even farther behind), but Galileo and BeiDou are both fully operational with dual-frequency civil signals right now. All dual-frequency receivers can naturally cancel out the effect of the ionosphere. That's part of why some smartphone baseband chips are coming out with L5 capability: GPS L5, Galileo E5A, and BeiDou B2A all operate in the same spectrum.
- Terretta 4y ago
  Apple Watch Ultra has dual frequency: "The Apple Watch series has always offered GPS support (in addition to other global satellite navigation systems), but the Apple Watch Ultra is the first to come with dual-band GPS (L1 + L5). Most smartwatches on the market, including the Galaxy Watch 5, only support single-frequency GPS and can only receive satellite signals on the L1 frequency. Dual-frequency support allows the Apple Watch Ultra to lock onto L1 and L5 bands simultaneously. This greatly improves navigational positional accuracy and reduces multipath errors in urban areas and other challenging environments." https://www.xda-developers.com/apple-watch-ultra-gps-support/ https://www.xda-developers.com/apple-watch-ultra-gps-support...
  - johnwyles 4y ago
    This apparently just came out, was thinking they had this out for some time.
MayeulC 4y ago
And then there's RTK (I like to talk about https://centipede.fr https://centipede.fr ) that measures the offset between GPS signals and litteral ground truth and proadcasts it, enabling centimeter-level precision. The more systems, the merrier, even if they aren't all accurate, you can use them to increase precision (kalman anyone?). Someone mentioned multi band. You can also use signals from multiple systems at different frequencies.
Waterluvian 4y ago
I’ve used VRS RTK for outdoor robotics applications and as a geographer I’ve got to say, 1cm accuracy gives me a partial.

FredPret 4y ago

It’s mindblowing how many wrinkles there are to iron out in a complex system. Who knew that the ionosphere bulges!

pixl97 4y ago
SpaceX for one. When solar storms occur they have to release newly launched satellites at a higher altitude to avoid being slowed down by a puffed up atmosphere. Just recently they lost a few starlink sats to this effect.
- pencilguin 4y ago
  Rather more than a few. Managed to save a few, instead. Expensive lesson.

ajross 4y ago

FWIW, that particular problem isn't likely going to be an issue for Starlink. Starlink satellites orbit at 550km vs. 20000km for GPS. The transmission is much (much) closer to a straight line approximation and ionosphere refraction is negligible. In fact, the line of sight transmission to and from a Starlink satellite is likely to be significantly better than that between an airliner and its local WAAS station, which is confounded by ground reflections. (Edit for clarity: WAAS is just digital data, this isn't an actual error in the signal. The point is more "you have to spend more engineering effort on the transmission environment of the correction signal for GPS than you do on the actual signal for Starlink")

brandmeyer 4y ago
Almost all of the ionosphere's total thickness is below 550km, so the lower altitude doesn't matter. Starlink sees less impact from the ionosphere because the carrier frequencies are higher, and dispersion is proportional to 1/f^2.
- ajross 4y ago
  I don't think that's true? Plasma density goes up with altitude out to quite some distance, though it's nonlinear. And in any case it's a refractive thing. Having a somewhat lower index doesn't make much difference when one lens is 40x thicker than another.
m463 4y ago
I would imagine many more satellites to choose from. (I'm uncertain, lower orbit might give you more satellites, but I don't know how many would be in view - many more or fewer)

brandmeyer 4y ago

> Regular GPS precision due to ionosphere changes is too low. ... for single-frequency receivers. Military receivers have been dual-frequency for a long time. GPS L2C's rollout is pretty slow (and L5 is even farther behind), but Galileo and BeiDou are both fully operational with dual-frequency civil signals right now. All dual-frequency receivers can naturally cancel out the effect of the ionosphere. That's part of why some smartphone baseband chips are coming out with L5 capability: GPS L5, Galileo E5A, and BeiDou B2A all operate in the same spectrum.

Terretta 4y ago
Apple Watch Ultra has dual frequency: "The Apple Watch series has always offered GPS support (in addition to other global satellite navigation systems), but the Apple Watch Ultra is the first to come with dual-band GPS (L1 + L5). Most smartwatches on the market, including the Galaxy Watch 5, only support single-frequency GPS and can only receive satellite signals on the L1 frequency. Dual-frequency support allows the Apple Watch Ultra to lock onto L1 and L5 bands simultaneously. This greatly improves navigational positional accuracy and reduces multipath errors in urban areas and other challenging environments." https://www.xda-developers.com/apple-watch-ultra-gps-support/ https://www.xda-developers.com/apple-watch-ultra-gps-support...
- johnwyles 4y ago
  This apparently just came out, was thinking they had this out for some time.

MayeulC 4y ago

And then there's RTK (I like to talk about https://centipede.fr https://centipede.fr ) that measures the offset between GPS signals and litteral ground truth and proadcasts it, enabling centimeter-level precision. The more systems, the merrier, even if they aren't all accurate, you can use them to increase precision (kalman anyone?). Someone mentioned multi band. You can also use signals from multiple systems at different frequencies.

Waterluvian 4y ago

I’ve used VRS RTK for outdoor robotics applications and as a geographer I’ve got to say, 1cm accuracy gives me a partial.