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

---

Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb

-- 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

_______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb

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

---

Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite

program!

...

Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb

-- 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

---

Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb

-- 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

Howard Long wrote:

Hi Ken

...

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.

I may be wrong, but I can't think of a reason why it wouldn't be well under this figure. I was thinking more in terms of 100's if not 10's of nanoseconds.

The SDX (a) takes a batch of samples, (b) does its processing, then (c) outputs the processed batch of samples.

Each of the three parts takes the same amount of time and all three are executed in parallel like a production line using DMA for (a) and (c), and the DSP CPU for (b). This time is dictated by the CPU's crystal oscillator.

I am guessing any variation would be predominantly from the analog circuitry (R's and C's changing value with temperature?), but Bob may have other suggestions I hadn't thought of.

Right on the money. We can learn the variation in these parameters in a joint process with determination of the pseudo-ranges. It is my opinion that doing this joint estimation will yield very good results. I would not be surprised in the least if James and/or Stacey had not already done this code. Stacey has worked on a project he calls Ranger (for obvious reasons) and I have not yet studied all of the details but I know ranging experiments have been done with it.

Bob N4HY

73, Howard G6LVB