[time-nuts] Need info on Trimble 4000S GPS Surveyor

Stewart Cobb stewart.cobb at gmail.com
Mon Mar 11 02:28:39 EDT 2013

The fundamental frequency in the GPS system is 1.023 MHz, chosen so that a
10-bit maximum-length feedback shift register (MLFSR) clocked at that
frequency would complete one cycle of 1023 states in precisely one
millisecond.  The civilian "C/A codes" or "Gold codes" are generated by
combining two such MLFSRs.  This fundamental frequency is often called
"f0", pronounced "f-naught".  It's just far enough away from 1.000 MHz to
get you into trouble if you mistake one for the other.

The military "P/Y-code" signal is transmitted at precisely ten times that
rate, or 10.23 MHz.  Some military-focused documentation calls that
frequency "f0" instead.

The L1 frequency is 1540 * f0 = 1575.42 MHz.  The L2 frequency is 1200 *
f0, or 1227.6 MHz.  Many civilian receivers use an IF of 4 * f0 = 4.092
MHz.  This requires an LO frequency of 1536 * f0.  I'm not familiar with
the 4000S, but some Trimble receivers of similar vintage (late 80's, early
90's) used a LO at 768 * f0 with a sub-harmonic image-reject mixer to
produce the 4* f0 IF frequency.

> The L1 1575.42 MHz chain uses a 16.368 MHz VCXO locked to the 10 MHz
> reference, running an LO of some integer multiple that results in a
> reference around 38.4 MHz labeled "ECL 38.4 F0" on the main board, and
> an unlabeled signal IF called "TTL LIMITER." Internal markings "768" and
> "384" may indicate PLL IFs of 76.8 and 38.4 (76.8/2) MHz.

"38.4 F0"  almost certainly  means 38.4 * f0.  This would produce a digital
clock fast enough to sample P-code data.  (Recall that the P-code wasn't
encrypted to Y-code until about 1990, so this receiver was probably built
to track P-code.)

> The L2 1227.6 MHz chain uses a 28.644 MHz VCXO locked to the 16.368 MHz
> reference, and LO that results in another unknown IF that runs through a
> similar TTL limiter. It appears that the LO is an integer multiple of
> the 28.644 MHz, with a PLL IF possibly around 59.2 MHz, marked "592 FO."
> Only the unknown signal IF from this section goes to the processing
> boards - no PLL IF seems to go beyond these modules. The unknown signal
> IF goes only to one of two apparently identical DSP boards, unlike the
> others that all go to the main board.

If "592 F0" were used in a subharmonic mixer as described above, simulating
an LO of 2 * 592 = 1184 * f0, that would produce an L2 IF signal at 16 * f0
= 16.368 MHz, which is a high enough IF frequency to carry the P/Y
modulation (+/- 10 f0 = 10.23 MHz) without aliasing.  (Not sure that's
what's going on, but it's a plausible explanation for your observations.)

> The L1 downconverter appears to use quadrature mixing, but I can't tell
> what happens after that - the I-Q signals go into a bunch of baseband
> circuitry. The L2 one also has a quadrature mixer, but only one output
> goes into its baseband circuits - the other is just terminated.

The L1 signal carried P-code and C/A code in quadrature.  The L2 signal
only carried P-code.  Sounds like the L1 outputs are split between the
P-code and C/A code processors.  The L2, carrying no C/A code, only goes to
the P-code processor.

> As I
> understand, the L2 is always encrypted, so useless for data, but its
> carrier can be used to enhance overall accuracy - I recall studying that
> a few years ago, but forgot the details. So, maybe the L2 portion is
> only for carrier recovery of some sort.

As above, P-code (on L1 and L2) was not encrypted when this box was

> I'd appreciate any info or ideas on deciphering the rest of the way -
> maybe the modules will be useful for something as a system, rather than
> just parts. I'm especially interested in GPS carrier recovery techniques
> for frequency only - not time.

The GPS signals are spread-spectrum, well below the thermal noise level, so
you can't easily see the carriers without actually tracking the signals.

Hope this helps.


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