[time-nuts] LEA-M8T

[Tom Van Baak]

Hi Bjorn,

> For navigation more measurements have always been prefered - that
> is use as many GNSS systems as all your receicers support.

I would like to believe this. There's a common myth with clocks that more is always better. For example, if you have 4 cesium then adding 5 more gives you an improvement of sqrt(4) to sqrt(9) or 50% better. And similarly, if you go from receiving 4 SV to 9 SV your position and timing fixes will get 50% better.

But it seems to me that "more is better" only works when each unit has similar accuracy and stability to begin with. If you have 4 cesium and add 5 quartz you do not get better performance. Instead the quartz will strongly degrade your net result.

Someday when GLONASS and Galileo and BeiDou match GPS in accuracy at the cm and sub-ns level your claim that more is always better will be true. >From what I've read we're not at that point yet. If you can find some papers to the contrary please let me know. We're in a very exciting decade or two of GNSS evolution and coordination.

Meanwhile it would not surprise me if each GNSS system gives a slightly different position and a slightly different time than GPS does.

If someone has a couple of LES-M8T to spare, would you configure each one to a single GNSS and then post a one day or one month record of 3D position and timing amongst all 4 systems? That would replace all our mutual conjecture with actual hard numbers.

/tvb

----- Original Message ----- 
From: "Björn" <bg at lysator.liu.se>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Thursday, April 07, 2016 7:43 AM
Subject: Re: [time-nuts] LEA-M8T


For navigation more measurements have always been prefered - that is use as many GNSS systems as all your receicers support.

That should be true also for common view timing.

--
Björn

<div>-------- Originalmeddelande --------</div><div>Från: Bob Camp <kb8tq at n1k.org> </div><div>Datum:2016-04-07  18:41  (GMT+07:00) </div><div>Till: Bob Stewart <bob at evoria.net>, Discussion of precise time and frequency measurement <time-nuts at febo.com> </div><div>Rubrik: Re: [time-nuts] LEA-M8T </div><div>
</div>Hi

Indeed, if you have not turned off the other systems for timing, you will have issues. Even for 
precision navigation, you need to turn them off. Until the European system goes up, there will 
not ba a coordinated approach between any two of the systems. Right now they each make their
own assumptions and their own definitions. If you are driving a car down the road, that’s not a 
big deal. If you are trying to do TimeNuts stuff … A one meter delta is a big deal for timing. A “few”
nanoseconds (say >10) is also a big deal. 

Bob


> On Apr 6, 2016, at 9:39 PM, Bob Stewart <bob at evoria.net> wrote:
> 
> Hi Logan,
> 
> I seem to remember Bob Camp mentioning that you can't have multiple satellite sources in the mix, because the other satellites are inferior to the GPS sats in timing.  Maybe Bob or someone could address this.  I would love to discover that I've set something wrong in all the many, many data structures.
> 
> Bob
> 
> 
> --------------------------------------------
> On Wed, 4/6/16, Logan Cummings <logan.cummings at gmail.com> wrote:
> 
> Subject: Re: [time-nuts] LEA-M8T
> To: "Bob Stewart" <bob at evoria.net>, "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Date: Wednesday, April 6, 2016, 8:18 PM
> 
> Hi
> Bob, 
>     Can't speak to
> jitter accuracy but the M8 series is definitely not the same
> receiver in the 6 series. As you probably know, M8
> introduced multi-GNSS support so in addition to GPS you have
> Beidou and Glonass satellites.    
>       At work we've had some gnashing of
> teeth about the wider filter passband requirements for
> multi-GNSS support since we're operating in a noisy
> environment, but I have nothing further on degraded
> performance when using only GPS.
>      Would be interesting to let
> it have all the constellations and see what
> happens.
> -Logan
> On Wed, Apr 6, 2016 at
> 10:04 AM, Bob Stewart <bob at evoria.net>
> wrote:
> I
> recently bought a number of LEA-M8T receivers and I have to
> say that I am unimpressed, so far.  They don't survey
> to the same reported accuracy as the LEA-6T in the same
> amount of time.  They certainly aren't better in the
> jitter after sawtooth correction.  So, have I managed to
> overlook some new field, or are they just not the same
> receiver as the 6T?  I did shut all sats off except GPS
> sats.
> 
> 
> 
> Bob - AE6RV
> 
> _______________________________________________