[time-nuts] Unique TBolt GPS characteristics

[Stewart Cobb]

Every GPS receiver calculates its clock offset (phase error, in time-nut
terms) as part of its four-dimensional position fix. It can apply almost
the same math to calculate its clock rate (frequency error) as part of a
four-dimensional velocity fix.

In "position hold" mode, the Thunderbolt calculates its timing errors (both
phase and frequency) using similar one-dimensional algorithms. The accuracy
of these measurements is determined by all the factors that affect GPS, but
the precision (resolution) of these measurements is effectively infinite,
limited only by IEEE double-precision floating-point math.

Almost every other GPSDO uses a hardware time-to-digital converter (TDC,
interpolator, etc) to compare the OCXO timebase to the PPS output of a
separate GPS receiver.

The PPS/TDC scheme has four disadvantages relative to the Trimble scheme:

A) The resolution of the phase error measurement is limited by the TDC
hardware.  For example, the HP Z38xx units appear to have 10ns resolution.

B) The accuracy of the phase error measurement may be degraded by analog
effects in the PPS connection.

C) The phase error measurement must be compensated by a software "sawtooth
correction" for best accuracy, because the PPS output is quantized by the
receiver clock.

D) Frequency error cannot be measured directly, but must be derived from
successive phase measurements. The derivative process introduces noise, so
the derived frequency error must be heavily filtered.

Unfortunately, the Trimble scheme is only available to GPSDO builders who
have access to the internal architecture of their GPS receiver.
Historically, among the major players, only Trimble and perhaps Zyfer did.
Even mighty HP did not.

Fortunately, SwiftNav is now selling a (mostly) open-source GPS receiver.
Only the FPGA correlator chip is closed-source, and that can be ignored for
timing purposes. One would need to build a VCXO-based synthesizer to create
the SwiftNav receiver clock frequency from a good OCXO, add a DAC to
control the OCXO, and do some software work to add timing functions to the
receiver firmware. Adding WAAS corrections to this hypothetical open-source
GPSDO could make it noticeably more accurate than a Thunderbolt.

Cheers!
--Stu