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Uclock
2007-Feb-19, 03:46 PM
When we try to measure the predicted position of the pioneer probes, are we measuring between two frames?

Tony

Ken G
2007-Feb-19, 06:02 PM
We don't measure the predicted position, we predict that. But note we don't even measure the actual position-- what we measure is a signal that comes from the pioneer spacecraft. So yes, a lot of inferences that may involve separate reference frames occur when we convert from what we measure to a position for the Pioneer. That is natural in science, happens all the time, what of it?

Uclock
2007-Feb-20, 04:30 PM
Hi Ken,

You are right. I should have said ‘calculated the predicted position’.
Is the signal we received being measured between two frames, one being here on the surface of the Earth and the other being the frame of the pioneer probe?
If so is time and space dilation caused by the gravity field of the Earth taken into account?

Tony

Ken G
2007-Feb-20, 05:02 PM
I'd say yes on both counts.

publius
2007-Feb-21, 03:13 AM
The signal data used to note: "hey there's an anamolous acceleration here" is rather complex. It has been gone over and over and analyzed with a fine tooth comb, and the conclusion is there's something there that can't be explained.

Basically, it is Doppler shift that determines the acceleration. The probe broadcasts at a certain frequency. The frequency received by ground stations depend on the relative velocity. Now, take the change in that relative velocity over time, and you have a relative acceleration. Convert that to a barycentric reference frame, and there you go.

I don't think there is any accurate "ranging data" for a tiny little probe that far out. You can know it's angular position in the sky, of course, to some accuracy, but to pin down its exact distance to any precision is beyond current capabilities. They had to wait for the astronauts to put laser reflectors on the moon to even be able to get precise enough distance data for something as big as close as the moon. This lunar ranging data is very precise, and good ol' GR has well, proven its mettle there -- there was even a paper exploring if a gravitomagnetic (frame dragging) signature could be pulled out due to the mass current of the earth-moon system. Believe me, that's some precise measurement.

The probes are too far gone now to receive anymore, but the history is where this is. There was something were they even went back looking for raw data going back even further from they might pull even more information out to analyze this some more.

There are papers galore on this. The pioneers were spin stabilized, and that is what allows some uncertainties in things to be reduced. The voyagers were not spin stabilized and the data is too uncertain to tell if the anomaly shows up there.

One probe showing an anomalous acceleration of ~10^-9 m/s^2 would be nothing but a "funny". However, two probes, heading off in nearly opposite directions showing the same anomaly is something to get interested in. They exhaustively went over just about all systemic effects (is it throwing off mass/radiation in some assymertric manner, is interplanetary dust building up on it, etc, etc, etc), and ruled them all out.

-Richard

Squashed
2007-Feb-21, 03:27 AM
The signal data used to ...

-Richard

I have heard of this anamoly but have not read, or learned, much about it.

So are you saying that we did not send a signal to the spacecraft and then count the seconds until the spacecraft replied ...

... we just receive continuous, or regular, signals from the spacecraft?

publius
2007-Feb-21, 03:50 AM
I have heard of this anamoly but have not read, or learned, much about it.

So are you saying that we did not send a signal to the spacecraft and then count the seconds until the spacecraft replied ...

... we just receive continuous, or regular, signals from the spacecraft?

Squashed,

Seconds? A bit longer than that.

20AU/c (put that in google and it will spit out the answer in hours directly) is 2.7 hours. Round trip would then be on the order of 5 1/2 hours.

Now, imagine something is "out there" in 3 dimensional space. You send a signal, and measure the time it takes to reply. Now, based on that information, and that information alone, how far away is it? Not enough information to solve said problem by far, let alone worrying about the accuracy. :)

The spacecraft was broadcasting some base signal all the time, but now, both are too far out to pick up anymore, if they are still broadcasting. They used to use acquiring the faint Pioneer signals as a training exercise for the deep space network.

-Richard

hhEb09'1
2007-Feb-21, 03:55 AM
Now, imagine something is "out there" in 3 dimensional space. You send a signal, and measure the time it takes to reply. Now, based on that information, and that information alone, how far away is it? Not enough information to solve said problem by far, let alone worrying about the accuracy. :)What accuracy are we looking for in this setup? :)

publius
2007-Feb-21, 04:16 AM
Ok, let's set this up as reasonably as we can. We'll just worry about distance, the 'r' of some spherical coordiante system centered on us. Our spaceprobe is out there moving at some unknown velocity, v. We send a signal and it replies, round trip time = T.

The spacecraft takes some time, delta-t to process the request to reply, and therefore moves some distance v*delta-t during that interval. It may be accelerating, but we'll just absorb that as the average unknown v during the interval.

We are in flat space-time and ignoring any relativistic complications, as we assume the unknown v/c is small. Assume we can put a range on 'v' and the delta-t response time.

What are the error bars on our little cT/2 calculation there? Use something reasonable for the error in T.

Jeff Root
2007-Feb-21, 07:02 AM
I'm using the word "range" in two different ways in this post.
Clumsy but unavoidable without an even clumsier circumlocution.

Pioneer, like other deep Space craft, had a ranging system built in.
Unfortunately, unlike other craft, it didn't work.

The Pioneer receivers were designed to detect a signal from Earth
in a preset range of frequencies. When a transmission was to be
sent to Pioneer, the amount of Doppler shift that the signal would
experience was calculated, and a transmission frequency chosen
that would arrive at Pioneer in the correct range. When Pioneer
detected a carrier, it locked onto it, altered the frequency by a
preset fraction, and retransmitted it. This was done in hardware,
not by a computer. Retransmission was essentially instantaneous.

The ranging technique was to slowly change the carrier frequency
over a period of tens of minutes. Pioneer would remain locked to
the carrier, and the retransmitted carrier would have the same slow
pattern. That pattern would be easily recognizeable in the signal
recieved from Pioneer back on Earth. But Pioneer was not able to
stay locked onto the changing carrier long enough for the pattern
to be recognizeable back on Earth. In the end, they gave up on
attempts to use the ranging feature, and just sent a carrier with
constant frequency.

The received frequency, both at Pioneer and on Earth, still varied
significantly, mostly because of Earth's rotation.

-- Jeff, in Minneapolis

publius
2007-Feb-21, 07:52 AM
Let's see the carrier frequency variation is to get something "obvious" to let you get the phase/time pinned down accurately, right? If you can't do that, your time measurment is shot to heck.

-Richard

RussT
2007-Feb-21, 11:21 AM
The calculations on this stuff has to be far better than what I am seeing in this thread, from what I have seen about different craft that have been sent to different moons and planets.

They inserted a craft 'between' the rings of saturn into orbit! That is unbelievably accurate in time and distances.

And they just inserted the newest Mars orbiter just using gravity in how many elliptical orbits, which took a very precise burn time where even one second off and they would have lost the craft to deep space, which needs extremely precise distance, speed, and time measurements.

publius
2007-Feb-21, 06:21 PM
Russ,

Oh yes, there are far more accurate ways both distant and local to navigate a spacecraft. :) I have no idea of the details much at all. "Long baseline interferometry" is sometimes mentioned by the deep space network. How accurate vs how far out you can get the position of some spacecraft based on telemetry coming back, I don't know, but they certainly have their methods. And then a spacecraft will have some local means of telling it where it is in relation to say Mars.

While it's a long way in the future, the theoretical ground work for a solar system positioning system, like GPS, but on the solar system scale is being worked out. It's possible, but far more complex than GPS. Such a system would allow any spacecraft to know its position and velocity relative to a barycentric coordinate system at all times. Such a thing would be very handy if we become more space-faring with missions to the planets.

-Richard

peteshimmon
2007-Feb-21, 07:16 PM
A few questions if I may. This effect is said
to be present on the ULYSSES spacecraft as well.
This is in orbit while the Pioneers are on
escape velocities. Secondly has the effect
been noted from the start of the Pioneers
flight over 30 years ago or is it improved
instrumentation some time after? It would be
nice if there was a graphic somewhere
illustrating the anomaly. It is interesting to
note that all three spacecraft have been
through the gravitational field of Jupiter.

publius
2007-Feb-22, 06:10 AM
A few questions if I may. This effect is said
to be present on the ULYSSES spacecraft as well.
This is in orbit while the Pioneers are on
escape velocities. Secondly has the effect
been noted from the start of the Pioneers
flight over 30 years ago or is it improved
instrumentation some time after? It would be
nice if there was a graphic somewhere
illustrating the anomaly. It is interesting to
note that all three spacecraft have been
through the gravitational field of Jupiter.

You'll have to do some searching there -- there are papers which go into detail. Basically, someone noticed the anomaly, then they went back through all the data they could get to get as much additional information as they could.

-Richard

Jeff Root
2007-Feb-22, 08:37 AM
The anomaly showed up in the data recorded after the Pioneers
were beyond the orbit of Saturn. Closer to the Sun, the pressure
of sunlight was greater than the anomaly, so it may have hidden
the anomaly in noise. The anomaly wasn't discovered until some
years later, when the data was analyzed in minute detail.

If it wasn't made clear above, the anomaly is a slight difference
between the predicted acceleration toward the Sun due to the
Sun's gravity, and the actual acceleration, which a bit higher
than expected. Lots and lots of possible explanations have been
considered, and none seem to fit.

-- Jeff, in Minneapolis