Leo, About GPS and 1 ms...
1) Bob's version: Bob's succinct description is fine. There is often a 1 ms loop in GPS receiver firmware (you can see this in the spec for some timing receivers). It is not impossible that off-by-1 errors would occur at this level. 2) Book version: > Fundamentals of Global Positioning System Receivers: A Software Approach > > CHAPTER FIVE > GPS C/A Code Signal Structure > > The C/A code is a bi-phase modulated signal with a chip rate of 1.023 MHz. > Therefore, the null-to-null bandwidth of the main lobe of the spectrum is > 2.046 > MHz. Each chip is about 977.5 ns (1/1.023 MHz) long. The transmitting > bandwidth > of the GPS satellite in the L1 frequency is approximately 20 MHz to > accommodate the P code signal; therefore, the C/A code transmitted contains > the main lobe and several sidelobes. The total code period contains 1,023 > chips. > With a chip rate of 1.023 MHz, 1,023 chips last 1 ms; therefore, the C/A code > is 1 ms long. This code repeats itself every millisecond. The spectrum of a > C/A > code is shown in Figure 5.2. > > In order to find the beginning of a C/A code in the received signal only a > very limited data record is needed such as 1 ms. If there is no Doppler effect > on the received signal, then one millisecond of data contains all the 1,023 > chips. > Different C/A codes are used for different satellites. The C/A code belongs to > the family of Gold codes,(5) which will be discussed in the next section. > > Figure 5.3 shows the GPS data format. The first row shows a C/A code with > 1,023 chips; the total length is 1 ms. The second row shows a navigation data > bit that has a data rate of 50 Hz; thus, a data bit is 20 ms long and contains > 20 C/A codes. Thirty data bits make a word that is 600 ms long as shown in > the third row. Ten words make a subframe that is 6 seconds long as shown in > row four. The fifth row shows a page that is 30 seconds long and contains 5 > subframes. Twenty-five pages make a complete data set that is 12.5 minutes > long as shown in the sixth row. The 25 pages of data can be referred to as a > superframe. 3) Tom's haiku version: atomic clocks fly coded signals drop from space position is time /tvb ----- Original Message ----- From: "Bob kb8tq" <kb...@n1k.org> To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Sent: Thursday, November 30, 2017 2:14 PM Subject: Re: [time-nuts] Trimble Mini-T Timing Glitches Hi I’d freely admit it is *very* much the “Cliff notes” version of what is happening. Bob > On Nov 30, 2017, at 4:31 PM, Leo Bodnar <l...@leobodnar.com> wrote: > > Bob, this is quite an unorthodox description of how GPS works. > You probably want to rephrase that before it gets ripped to shreds. > Leo > > From: Bob kb8tq <kb...@n1k.org> >> GPS extracts time and location by locking on to various codes in the >> transmissions. > One of them happens to run at about a 1 KHz clock rate. A slip on that part > of the > process gives you a (modulo) 1 ms clock jump. Certain types of interference > may > “help” the receiver make these sorts of mistakes. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.