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Re: SDX transponder timing delay measurements?



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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