[time-nuts] Trimble Mini-T Timing Glitches

Tom Van Baak tvb at LeapSecond.com
Thu Nov 30 17:42:13 EST 2017


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
> 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


----- Original Message ----- 
From: "Bob kb8tq" <kb8tq at n1k.org>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Thursday, November 30, 2017 2:14 PM
Subject: Re: [time-nuts] Trimble Mini-T Timing Glitches


I’d freely admit it is *very* much the “Cliff notes” version of what is 


> On Nov 30, 2017, at 4:31 PM, Leo Bodnar <leo at 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 <kb8tq at 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.

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