> Satellites at the GPS altitude travel at the speed of about 2.4 mi/s relative to Earth, which slows the clock down, but they’re also in weaker gravity which causes the clock to run faster. The latter effect is stronger which in total results in a gain of around 4.4647 × 10−10 seconds per second, or around 38 microseconds a day.

> Unfortunately, this is where many sources make a mistake with their interpretation of that result. It’s often erroneously claimed that if GPS didn’t correct for these relativistic effects by slowing down the clocks on satellites, the system would increase its error by around 7.2 mi per day as this is the distance that light travels in those 38 microseconds.

> Those assertions are not true. If relativistic effects weren’t accounted for and we let the clocks on satellites drift, the pseudoranges would indeed increase by that amount every day. However, as we’ve seen, an incorrect clock offset doesn’t prevent us from calculating the correct position.

(Nevertheless there are of course relativistic effects to account for, which Ciechanow proceeds to mention and which are explained in more detail in the other link I shared here: https://news.ycombinator.com/item?id=47861535 )

The author does note that:

> If you want to go much deeper, Bartosz Ciechanowski's interactive explainer on GPS is the gold standard. It covers signal modulation, orbital mechanics, and receiver architecture in far more detail than we do here.

for that I'd recommend this youtube series https://www.youtube.com/watch?v=i7JPjgHa7_A

The time correction is better thought of as the fixed offset between the receiver clock and the GPS clock.

Some considerations:

- They don't use GPS frequencies because there is receiver on the moon that receives GPS L1 signals (LuGRE and potentially more in the future)

- Make it easy to acquire for low complexity hardware

- Use 5G forward error correction code to reuse existing hardware implementation

- Design the signal in a way so that the user can easily find start of a data frame

And those are RF level considerations... there will be more considerations needed for the data transmitted over those navigation signal that the receivers need to use to determine navigation satellite position as lunar orbit is much more complicated than Earth orbit

But yes, it's amazing how small and battery efficient GPS receivers are at this point!

it's more than a couple dozen people.

i'll concede the point that its far, far fewer than those who feel otherwise, and that they like to hate on each other, but the idea that this is tiny and growing smaller is just not true. the surge in homeschooling, for example, would suggest otherwise...

...but at the end of the day the ECEF coordinates we used for everything still require a roughly spherical earth, but I don't think flat earthism is a real thing for most people who talk about it. Most of it is joking/trolling. There are surely some conspiracy-minded people who believe it because they don't give any serious thought to how anything works, but the people that publicly push alternate theories (eg. GPS is balloons, not satellites) have got to just be trolls.