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Re: SDX transponder timing delay measurements?



Bob -

Perhaps it will not be as accurate as I would like, but there would be the
inevitible (and perfectly fair) question of "...what do you need?..." were I
to simple say that I *need* it to be "rock stable."  The fact is , the more
stable the better and the fewer number of data points I will need to compute
accurate orbital parameters.  Like everything else, it is a trade-off in the
end and I can ultimately live with less than what I want.

The TAPR projects in this area are interesting (as they have always been)
and I will need to go check those out.  Perhaps the work going on there will
define the "knee in the curve" that defines the threshold where higher
stability starts becoming costly in terms of development effort and/or money
invested.

Thank you for your insights.

73, Ken Ernandes N2WWD





-----Original Message-----
From: Robert McGwier [mailto:rwmcgwier@comcast.net]
Sent: Tuesday, September 12, 2006 11:02 AM
To: Ken Ernandes
Cc: Steve Meuse; amsat-bb@amsat.org
Subject: Re: [amsat-bb] Re: SDX transponder timing delay measurements?


The samples will occur at 100 kHz or 200 kHz rate and this will be
stable (very) but it might not be as accurate as you would like.  In
addition,  we have to factor into this the ground station sampling and
LO uncertainties  as well since I think we can safely assume that
sampled systems (almost sure some SDR thing) will be involved.  Since
that will almost surely be the case,  the joint estimation of the
parameters of spacecraft and ground station along with pseudo-ranges
will have to be determined anyway.  I think one of the very nice
potential results coming from the HPSDR group which AMSAT and TAPR are
currently supporting will be sample clocks and local oscillators that
are tamed to a very high accuracy as well as achieving high stability.
The HPSDR Gibraltar board (as in stable as the Rock) will be of great
benefit to AMSAT projects needing stabilized and accurate clocks and
oscillators.

The P3E team would like to include the SDX and we all think we could get
a great Mode B transponder out of this.  The current design for the
telecommand link on P3E is also an SDR and the current plan is to do it
in the IHU.  So this parameter estimation problem will come up again and
again.

Bob
N4HY


Ken Ernandes wrote:
> Just to clarify what Bob said, an extra 35ms delay will not be a big issue
> to the operators.  But as he also indicated, 35 ms is very "noticeable"
when
> making Range measurements -- consider the speed of light (300,000 km/sec)
*
> 0.035 sec = 10,500-km.  Hence his statement that the actual number will be
> important for ranging.
>
> >From a Guidance & Navigation point of view there are two things that are
> important to consider here:
>
> 1.  If the timing delay on Range measurements is accurately estimated (or
> better yet measured), it can be calibrated out for the purposes of Orbit
> Determination (OD).  My OD software has a Transponder Range Bias as an
input
> parameter and has the ability to fine-tune the estimate of that Bias as
part
> of the correction process.  The fine-tuning works well given a good
initial
> estimate -- best done by making measurements with the flight
> hardware/software.
>
> 2.  What is also very important is how *consistent* is the delay.  If it's
a
> rock-solid 35 milli-sec (i.e., rock solid = a very small variation),
that's
> better than an average value of 35 micro-sec with say a +/-5 milli-sec
> (i.e., 1500-km) uncertainty.  The OD process can easily calibrate out a
> constant offset (bias).  But the greater the uncertainty (variability) to
> that bias, the greater will be uncertainty in the OD results (and thus
> poorer quality orbital parameters).  Therefore, if tradeoffs can be made
in
> the design, a bigger delay that's nearly constant is *far* preferable than
a
> smaller average delay with even a modest amount of variance.
>
> As a design goal, it would be helpful to the OD process if the delay could
> be made consistent within about +/-5 micro-sec -- i.e., 1500 meters
(smaller
> is better).  It is also important to quantify the standard deviation to
that
> bias is since that is also factored into the OD process.
>
> 73, Ken Ernandes, N2WWD
>
>
>
>
>
>
> -----Original Message-----
> From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org]On
> Behalf Of Robert McGwier
> Sent: Tuesday, September 12, 2006 8:39 AM
> To: Steve Meuse
> Cc: amsat-bb@amsat.org
> Subject: [amsat-bb] Re: SDX transponder timing delay measurements?
>
>
> I think it is about 35ms.  It is not noticeable and especially with the
> round trip time thrown in the mix,  it will definitely not be
> noticeable.   The number will be important to us when we want to do
ranging.
>
> Bob
>
>
>
> Steve Meuse wrote:
>
>> Douglas Quagliana expunged (dquagliana@aol.com):
>>
>>
>>
>>> I know the design isn't even done yet, but has anyone
>>> ever measured the time delay of an RF signal *through*
>>> the (demo) version of SDX?
>>>
>>>
>> There was a demo of the board at Dayton, I was lucky enough to get a demo
>>
> while we were setting the booth up. The demo setup, if I remember
correctly,
> was using two SDR-1000's. I didn't find the delay noticable at all.
> Propagation delay is much more of an issue for HEO.
>
>> BTW, the SDX sounded FANTASTIC! I had goosbumps when N4HY tuned to the
>>
> beacon :)
>
>> -Steve
>> N1JFU
>>
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>>
>
>
> --
> AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
> NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman
> "You see, wire telegraph is a kind of a very, very long cat.
> You pull his tail in New York and his head is meowing in Los
> Angeles. Do you understand this? And radio operates exactly
> the same way: you send signals here, they receive them there.
> The only difference is that there is no cat." - Einstein
>
> _______________________________________________
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>


--
AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman
"You see, wire telegraph is a kind of a very, very long cat.
You pull his tail in New York and his head is meowing in Los
Angeles. Do you understand this? And radio operates exactly
the same way: you send signals here, they receive them there.
The only difference is that there is no cat." - Einstein

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