Understanding GPS Links and Codes

This video provides an introduction to the different links and codes used in the Global Positioning System (GPS).
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Timeline:
00:00 Introduction
00:11 About links and codes
01:03 GPS link frequencies
02:04 Why have two (or more) link frequencies
02:49 About L1 and L2
03:30 What do we mean by “code”?
04:06 How codes are used
04:39 Cross-correlation between replica and received code
05:01 Effect of code length and rate
05:49 C/A (“coarse/acquisition”) code
06:52 P (“precision”) code
07:24 Anti-spoofing / P(Y) code
08:00 Direct acquisition of P code
08:27 M code
08:55 L1C (Link 1, Civilian)
09:32 L2C (Link 2, Civilian)
10:37 L5
11:18 L1, L2 … L5? What about L3 and L4?
11:47 Review of GPS links and codes
12:37 Summary

[@jerrymendez9630]

Great fundamentals video on GPS. Once again, nice job Paul.

[@dabdoube92]

I don't know who Paul is, but yeah well done Paul !

[@vestajuno]

Your work is truly awesome. You make it interesting. Thank you very much.

[@pauldenisowski]

Thank you - it is a labor of love :)

[@dadominicanstyl]

You guys make the fundamentals amazing! Awesome video...

[@pauldenisowski]

Thank you!

[@hanskoc3473]

Thanks for the video, I understand now how the gps (and others) system works.

[@pauldenisowski]

Thanks for the feedback! Glad it was helpful

[@sushimirollin8505]

Great video. Very informative.

[@pauldenisowski]

Thank you!

[@mohammadrezarahimi1431]

Thanks for the video. Can you explain that how the phase center measurement error of the GNSS antenna can affected the accuracy of detection the exact position?

[@pauldenisowski]

I can try :) As I'm sure you already know, the phase center of a GNSS antenna is different from the actual physical center of the antenna and this is concern in applications that require extremely high position accuracy. The difference between the phase center and physical center changes based on numerous factors such as the position of the SV (azimuth and elevation) as well as the frequency of the received signal (e.g. L1 vs L2 vs L5 in GPS). In order to minimize this error, calibration of the GNSS receiver antenna can be used. One way of doing this calibration is by sending signals into the antenna at various azimuth and elevation values (in a shielded chamber or at an open air range) and using a vector network analyzer to measure and record the phase differences so that these can be removed in the final navigation solution. Hope that's helpful!

[@Elliot_97]

So if the code is used to determine positioning, why is precise timing via atomic clocks required for PNT? Is it just for accurate timing, (the T in PNT)?

[@pauldenisowski]

The short answer is the GPS is really about timing, not positioning :) Position is determined (by the receiver) based on the time it takes the signal to travel from the well-known positions of multiple satellites to the receiver. Most people tend to think about GPS as providing a "position", but I would say that most (behind-the-scenes) applications of GPS are really more about having a precise time source.

[@ahmednor5806]

🙏🙏🌹🌹

[@TO-jd8rg]

How do you get the "length" of the code in meters?

[@mohandt.6469]

lambda = c/f

[@ve2zzz]

I always tought that L1, L2, L5... referred to L-band 1, L-band 2, etc.
The L-band refers to the 1 - 2 GHz spectrum,

[@pauldenisowski]

If you read the US government ICD's for GPS (e.g. IS-GPS-200), you'll see they are using "L" to abbreviate "link", e.g. "GPS for radio frequency (RF) link 1 (L1) and link 2 (L2)." GPS is in the L-band, but that's not where the abbreviation comes from :)

[@nathanvary1657]

You incorrectly utilized the "cross-correlation" term. Aligning the codes is simply correlation, or perhaps autocorrelation. Cross-correlation is the interference effect between multiple codes. Typically when this happens the codes are mistakenly aligned, causing the receiver to track one satellite, but believe it to be another.

[@pauldenisowski]

Thanks for the feedback. In retrospect, I probably should have just said "(code) correlation," since the distinction between cross-correlation and auto-correlation is not often made (or rigidly adhered to) in the GNSS literature. But I do stand by my use of "cross correlation" since this is the more general description of the process of comparing two time displaced series, regardless of whether they are supposed to be the same or not :)