[time-nuts] ublox NEO-M8T improved by insulated chamber?
jimlux at earthlink.net
Wed Nov 8 09:33:19 EST 2017
On 11/7/17 8:39 PM, Gary E. Miller wrote:
> Yo Tom!
> On Tue, 7 Nov 2017 20:16:09 -0800
> "Tom Van Baak" <tvb at LeapSecond.com> wrote:
>>> Which is small compared to the published GPS time resolution
>>> (IS_GPS_200H, page 54) of 90 ns.
>> Correct. GPS performs far better than the original spec. Like the
>> Mars rovers...
> Of course, but then you are on a wing and a prayer, not engineering.
Not really - the original spec for GPS was based on being able to track
to a single chip of the PN code at 1 MHz, or about 300m position error,
and 30m for the precise code at 10MHz.
And that was the limit of the technology back in the late 70s early 80s
A fancy multichannel correlator was literally a rack of equipment, and
things like RAKE receivers to deal with multipath were the subject of
papers in IEEE transactions - hardly a "buy it at the hardware store"
What has changed, and what's sort of amazing, given that the GPS signal
hasn't really changed much is that technology has made substantial
advances, both in terms of silicon and in terms of the algorithms.
It is easy now to have a 64 simultaneous channel correlator that
trivially tracks to a fraction of a chip and also recovers the carrier
phase. Codeless receivers allow tracking of the higher rate code, and
today, a L1 only receiver is sort of a legacy oddity - perhaps because
it has some peculiarity for a particular system, or a "drive every tenth
of a penny out of the system cost" item.
Back in "30 m CEP" days who cared about the fact that the receive
antenna phase center wasn't the same for all look angles. Today, folks
go out and extensively calibrate these things, and build antennas where
the phase center is "stable" to 1 mm or better.
We've got much, much better analytical tools to process the data - folks
regularly process long collections of data from inexpensive receivers
where one has to account for things like solid earth tides, not to
mention continental drift. My house in Southern CA is steadily heading
north a few cm/year. That's actually measureable with equipment you
could reasonably buy for under $1000 (I think.. you might be able to do
it for under $100)
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