[time-nuts] LEA-M8T

Thu Apr 7 21:55:32 EDT 2016

Tom,

On 04/08/2016 02:06 AM, Tom Van Baak wrote:
> 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.

Your analogy is broken in several ways. More clocks helps, but for 
optimal performance you weight their contribution. If you add 5 crystal 
oscillators against 4 cesiums, a good weigthing mechanism will give them 
weight for the ensemble goal (short-term or long-term).
Additional GNSS signals help, but only if they do not compete out 
tracking other satellites. If you support say 12 GPS satellites and in 
addition can handle 12 GLONASS and 12 Galileo, then with proper weight 
you can get a better performance with more. You can get more assistance 
with tricky geometry, you can get away with some biases and noise.

> 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.

I think one has to be careful, and that goes for both of you. A 
particular platform may perform worse in GNSS mode than pure GPS mode, 
but this is not to say that using the other GNSS systems makes GPS 
position bad in general, it only says that for this platform it doesn't 
give the improvement we thought, in this release.

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

They will, but it's for many reasons. It doesn't even say that GPS will 
give the *right* position, as we know that that is a problematic issue 
on its own. It will be different, and vary somewhat different, but there 
is nothing strange about it, it's a different system, with different 
signals, orbits, constellation size etc. The FDMA properties of GLONASS 
is troublesome.

> 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.

Testing is indeed a good idea. Remember that it will not be saying more 
than what the current release is able to make out of it, for the platform.

Cheers,
Magnus

> /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
>>
>> _______________________________________________
>
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