Comment by pcl | Hacker News Reader

Comment by pcl | Hacker News Reader

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I don’t think that’s true any more. IIRC, the high resolution codes were disclosed to the public in the late 90s / early 00s, and there have been no efforts to rotate them since.

Extra precision can be achieved with fixed-point augmentation signals, which I believe is common at airports and construction sites. I would assume the at militaries similarly augment signals in theaters of war. But that’s different than some separate high-resolution mode.

At this point, so many civilian services depend on the high-resolution data that I’d be pretty surprised to see GPS going back to a two-tier system.

A citation for the above:

“In May 2000, at the direction of President Bill Clinton, the U.S. government ended its use of Selective Availability in order to make GPS more responsive to civil and commercial users worldwide.

“The United States has no intent to ever use Selective Availability again.”

https://www.gps.gov/systems/gps/performance/accuracy/

The way GPS was designed to work originally is two sets of codes. Course Acquisition (CA) code which repeats at 300ms intervals with a deliberate bias was designed to bootstrap the receiver into locating its position in a 1-week Precise (P) long code. This bias was selectively worse in different regions and I think still can be changed in war zones. Happy to be educated further here!

What the US government did was remove the bias from the CA code so it could be used for precise positioning. The military still uses P codes as well. I believe there is a small gain to be had but it’s due to frequencies.

Since then there have been several more advances, mostly to broadcast local augmentation signals. Wide Area (WAAS) and Ground Bases (GBAS) are common in receivers.

L5 is a new band to help solve multipath error in urban areas.

Most receivers also have remote autonomous integrity monitoring, where it can predict its own area of probability (by using groupings of 4 in 5 satellites), and if it’s too large for the intended use case, alert the user. Also with 6 satellites it can calculate combinations of 5 satellite groupings to work out (and exclude) faulty satellites. This is Fault Detection & Exclusion (FDE).

Mobile devices will also download their own separate high resolution almanac and ionospheric data over the internet which is superior to the low data rate GPS almanac. It can also use known cell locations to approximate its position. Combined, this enables rapid (hot) signal lock immediately onto the correct satellite code & Doppler shift frequency, which is why your mobile gets a fix in 3 seconds, versus your car which takes minutes.

> What the US government did was remove the bias from the CA code so it could be used for precise positioning. The military still uses P codes as well. I believe there is a small gain to be had but it’s due to frequencies.

The precision of positioning from the P code is 10 times greater than the C/A code (about 30cm vs 3m). This is due to the wavelength/'chip length' of the code signal which is modulated onto the carrier wave (10.23 Mhz / 29.31 m wavelength for P code, 1.023 Mhz / 293.1 m wavelength for C/A code). Positioning precision is limited to about ~1% of the chip length by signal processing.

This is correct, but I'd like to add that at some point the errors from which frequency and code you use is no longer the dominant factor in the position error. Depending on where you are, either multipath errors (eg due to reflections from buildings or mountains) or athmospheric errors (ie due to the radio signal being distorted in the ionosphere) start to dominate.

Selective Availability being phased out and is no longer a feature on the most recent GPS satellites:

https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_...

There is already a 2 tier system. Clinton turned off "Selective Availability" but kept the P(Y) code for military only use. The P(Y) code now has been supplemented by an M code for military on the newer generation of GPS.

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