Author Topic: New Horizons Pluto Flyby Coverage  (Read 253046 times)

Offline JasonAW3

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #20 on: 01/20/2015 03:25 PM »
So, as the communication distance increases, the pointing error becomes more and more relevant on each side, and you cannot beam as the receiver could miss the information.

That doesn't make sense. Any given directional antenna will focus its signal into a specific cone. As distance grows, the cone footprint at Earth increases so the pointing requirements become *less* stringent, not more.

I don't know much about RF comms, but my understanding was that the radio wavelength chosen is what limits how well the signal can be collimated (to reduce "waste" of the footprint) due to diffraction. That's one of the biggest motives for going with optical comms, not that RF is inherently bad. Optical beams can be collimated so tightly that it really does get to a point where you need to have very precise pointing. For radio, not so much.

Basic physics;  As the signal is sent down the cone, the strength of the signal falls off as that same power is spread over a larger width, plus, the signal itself looses energy due to gas, dust, EM fields and general RF interference.  Thus, like all forms of radiation, the signal strength falls of with the square of the distance.

Uh, the inverse square law is exactly what I was talking about in my first post on bandwidth above. I don't see what's that got to do with pointing requirements pagheca talked about. A wider footprint caused by greater distance means that you have more of an angular deadband wiggle room before you need to repoint your antenna at Earth. It is true, though, that you need to keep the Earth withing the angular size of the cone at all times and that doesn't change, but you get progressively more and more of the Earth's orbit inside that cone as distance increases. For Voyager, the "half-power half-width of the antenna beam was 0.32 degrees at X-band and 1.1 degrees at S-band."

You still get the highest strength signal towards the center of the cone. Signal strength bell curve.
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Offline pagheca

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #21 on: 01/20/2015 04:34 PM »
So, as the communication distance increases, the pointing error becomes more and more relevant on each side, and you cannot beam as the receiver could miss the information.

That doesn't make sense. Any given directional antenna will focus its signal into a specific cone. As distance grows, the cone footprint at Earth increases so the pointing requirements become *less* stringent, not more.

If you think to increase the power (reaching the receiver antenna) by using a narrower beam, at some point your beam become so thin it is difficult to track the probe.

« Last Edit: 01/20/2015 04:36 PM by pagheca »

Offline Nomadd

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #22 on: 01/20/2015 05:01 PM »
 It took me a while to figure this argument out. Pointing requirement doesn't change. The cone at the Earth is bigger, but that cone moves more when you slew the antenna. If you have to point withing 1/2 degree at Mars, you still need to maintain that 1/2 degree at Pluto because signal strength doesn't follow a bell curve within the "cone". It stays fairly constant with a sharp dropoff at the edges. Anybody who has ever pointed a dish will tell you that you don't look for a sharp peak at the center. You find a 3db or so dropoff in all directions and center the dish in that bracket. New Horizons probably has the same system, where it acquires the signal, then centers the dish. I have no idea if it has active tracking.
 
 My rough math is getting something like a 1/4 degree correction every week to follow the Earth's orbital path from Pluto. I find it easier to believe that New Horizons is capable of doing once a week what a $20,000 SeaTel does several times a second than to believe reducing the dish tracking from once a week to once a month is any kind of factor. Centering the dish once the signal is acquired can probably be done in 60 seconds or so. (My guess)
« Last Edit: 08/06/2015 07:33 PM by Nomadd »

Offline baldusi

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #23 on: 01/20/2015 05:01 PM »
There is an issue with latency and bandwidth. Part of any communication protocol is the acknowledge of the correct transmission of the data. The requires data to do a round trip (that's roughly 9hrs). If you have a big packet (minimum unit to re transmit), you can send one packet, and then another and so on until you have the acknowledgement of the receiver that the first packet arrived fine. If you receive a retransmit, then you send it again. This is why 1G and 10G ethernet require Jumbo frames. Else, the sped of light (in copper or fibre) determines the maximum bandwidth.
In this case, the low data rate works for you.  But you have to bear in mind that from the last transmitted bit, the spacecraft can't know if the transmission was successful or not for 9hrs. In some cases, like when sending thumbnails of high resolution pictures, this means a lot of wasted bandwidth. And when you thing about the communication sessions, they either have to wait for 9hrs until they know everything went through, or wait for the next communication session for retransmits (which should lengthen that session).

Offline baldusi

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #24 on: 01/20/2015 05:03 PM »
[...] Centering the dish once the signal is acquired can probably be done in 60 seconds or so. (My guess)
You have a 9hs round trip. You can't know your dB drop with anything remotely close to real time. Unless you mean pointing the receptor towards a carrying wave sent from Earth.

Offline Nomadd

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #25 on: 01/20/2015 08:05 PM »
[...] Centering the dish once the signal is acquired can probably be done in 60 seconds or so. (My guess)
You have a 9hs round trip. You can't know your dB drop with anything remotely close to real time. Unless you mean pointing the receptor towards a carrying wave sent from Earth.
I didn't think of the carrier not being up all the time. It would have to be on a schedule. Just pointing using guide stars seems like a hard way to get 1/4 degree accuracy, but maybe that's how they do it.

Online John-H

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #26 on: 01/20/2015 10:06 PM »
[...] Centering the dish once the signal is acquired can probably be done in 60 seconds or so. (My guess)
You have a 9hs round trip. You can't know your dB drop with anything remotely close to real time. Unless you mean pointing the receptor towards a carrying wave sent from Earth.
I didn't think of the carrier not being up all the time. It would have to be on a schedule. Just pointing using guide stars seems like a hard way to get 1/4 degree accuracy, but maybe that's how they do it.

Does the ground station send out a reference beacon  4 1/2 hours before a scheduled transmission to help set up the tracking ? It would help with the pointing, but the station would be on the wrong side of the earth 9 hours before it starts receiving.

Offline LouScheffer

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #27 on: 01/21/2015 01:15 AM »
[...] Centering the dish once the signal is acquired can probably be done in 60 seconds or so. (My guess)
You have a 9hs round trip. You can't know your dB drop with anything remotely close to real time. Unless you mean pointing the receptor towards a carrying wave sent from Earth.
I didn't think of the carrier not being up all the time. It would have to be on a schedule. Just pointing using guide stars seems like a hard way to get 1/4 degree accuracy, but maybe that's how they do it.

Does the ground station send out a reference beacon  4 1/2 hours before a scheduled transmission to help set up the tracking ? It would help with the pointing, but the station would be on the wrong side of the earth 9 hours before it starts receiving.
Yes, they do send a carrier, but not for this purpose.  They do it to measure the spacecraft speed, using doppler shift.  The spacecraft can't carry the super-accurate oscillator needed for this - it stays in a temperature-controlled room on Earth.  So they send the signal, the spacecraft multiplies it by a rational number, then sends it back, and the ground station compares what it sent to what it gets back.  This measures speed very accurately, but not distance.

They also measure distance by sending a ping, which the spacecraft copies and returns.  The round-trip time gives the distance.

Both of these are complicated as New Horizons gets so far away that one facility has to send, but by the time the signal get back, it's out of view and a different station needs to receive.  They needed new and fancy calibration schemes (using mutual observation of quasars) to make this work for New Horizons....

Offline mcgyver

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #28 on: 01/22/2015 04:59 PM »
[/font]
[/font]So, as the communication distance increases, the pointing error becomes more and more relevant on each side, and you cannot beam as the receiver could miss the information.
[/font]That doesn't make sense. Any given directional antenna will focus its signal into a specific cone. As distance grows, the cone footprint at Earth increases so the pointing requirements become *less* stringent, not more.
[/font]
The longer is the distance, the wider is the footprint, the lower is the received power per surface unit.
You can't go below a specific power level, else you won't be able to distinguish signal from noise; so you need a thinner cone; "thinner" and "taller" (=more distant) means a lot more difficult to point. Try turning off light in your room using a broomstick. The longer it is, the more difficult it is.


About who wondered about printing a 3d model of New Horizons, a 3d printer is not needed: just upload the model to shapeways.com , sculpteo.com or others online 3d printing services. But please consider it's very expensive: a 5x5 cm model can cost around 30$!


Offline Comga

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #29 on: 01/22/2015 06:55 PM »
So, as the communication distance increases, the pointing error becomes more and more relevant on each side, and you cannot beam as the receiver could miss the information.
That doesn't make sense. Any given directional antenna will focus its signal into a specific cone. As distance grows, the cone footprint at Earth increases so the pointing requirements become *less* stringent, not more.
The longer is the distance, the wider is the footprint, the lower is the received power per surface unit.
You can't go below a specific power level, else you won't be able to distinguish signal from noise; so you need a thinner cone; "thinner" and "taller" (=more distant) means a lot more difficult to point. Try turning off light in your room using a broomstick. The longer it is, the more difficult it is.
(snip)
But they can't make the beam "thinner".  That is determined by the size of the antenna dish, which is fixed and limited.
The pointing tolerance may get tighter because with longer distance and weaker signals the link can tolerate less roll-off, but that's a pretty slow function.  At some point, halving the pointing error might get one a few percent increase in signal strength or less.  Once they build a system with sufficient control to operate beyond Pluto there is little incentive to be less precise at shorter ranges.

But this discussion is about the spacecraft.  The topic of this thread is observations.  Anyone know when the first LORRI image will be downloaded?
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Offline MarsInMyLifetime

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #30 on: 01/23/2015 03:10 PM »
Thanks, Comga. Now we are talking! According to this article, that first photo of the current science campaign will be taken Sunday, January 25. Ref: http://www.nasa.gov/press/2015/january/nasa-s-new-horizons-spacecraft-begins-first-stages-of-pluto-encounter

Ceres and Pluto unfolding... will I get any work done this spring?
Don

Offline kevin-rf

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #31 on: 01/23/2015 03:14 PM »
Ceres and Pluto unfolding... will I get any work done this spring?

Simple, hide under a rock on Mars ;)
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Offline MarsInMyLifetime

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #32 on: 01/23/2015 03:35 PM »
Fun simulation of how the approach will change, and how dynamic the last month will be... Bring up the NASA/JPL Solar System Simulator:
http://space.jpl.nasa.gov/

Then select Pluto as seen from New Horizons, set the field of view to 0.5 degree (approximating LORRI--it is actually 0.3 degree) and check out the view. In the URL for that image, you can change the month=1 parameter to 2, 3, 4... and watch the view change right up to fly-by in July. By early May, Pluto's scale on the LORRI imager will start to improve on the best images from Hubble, according to here: https://en.wikipedia.org/wiki/New_Horizons#Key_mission_dates
Don

Online Calphor

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #33 on: 02/04/2015 09:51 PM »
New images of Pluto from New Horizons.

http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150204

A Long Distance Look from LORRI:

Pluto and Charon, the largest of Pluto's five known moons, seen Jan. 25 and 27, 2015, through the telescopic Long-Range Reconnaissance Imager (LORRI) on NASA's New Horizons spacecraft. New Horizons was about 126 million miles (203 million kilometers) from Pluto when the frames to make the first image were taken; about 1.5 million miles (2.5 million kilometers) closer for the second set. These images are the first acquired during the spacecraft's 2015 approach to the Pluto system, which culminates with a close flyby of Pluto and its moons on July 14.

Pluto and Charon subtended 2 pixels and 1 pixel, respectively, in LORRI's field of view. The image was magnified four times to make Pluto and Charon more visible, though during the next several months, the apparent sizes of Pluto and Charon, as well as the separation between them, will continue to expand in the LORRI images.

The image exposure time was only a tenth of a second, which is too short to detect Pluto's smaller moons. LORRI will also be taking images with longer exposure times (10 seconds) that should reveal both Nix and Hydra.


« Last Edit: 02/04/2015 09:52 PM by Calphor »

Offline kevin-rf

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #34 on: 02/04/2015 10:39 PM »
Can I feel old, remembering the first dead tree pre HST publication of a picture of Pluto and Charon resolved as two separate objects in an image?
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Offline Blackstar

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #35 on: 02/04/2015 11:46 PM »
Can I feel old, remembering the first dead tree pre HST publication of a picture of Pluto and Charon resolved as two separate objects in an image?

Why would that make you feel old? Don't all these new discoveries make you feel young and lucky to be alive at a time when we learn such new and fascinating things every day?

Online redliox

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #36 on: 02/05/2015 12:24 AM »
Can I feel old, remembering the first dead tree pre HST publication of a picture of Pluto and Charon resolved as two separate objects in an image?

Why would that make you feel old? Don't all these new discoveries make you feel young and lucky to be alive at a time when we learn such new and fascinating things every day?

Seconded  :)

The only time I ever feel old is when I realize I played the original Nintendo in this era of Playstation-blahs and online-cell phone-aps.  :P  But ever since I was about 7 I was an avid space enthusiasm, and I certainly would have thought a Pluto-bound-probe would be a great idea. 

Barring an insisted-upon expedition to Eris, things now become a matter of revisiting and thoroughly investigating.  Neptune and Uranus need this the most, followed by neglected Venus and then a long list of moons.
« Last Edit: 02/05/2015 12:24 AM by redliox »
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Offline kevin-rf

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #37 on: 02/05/2015 01:32 PM »
I had ET for the 2600 and lifted him up hill each way ;) It just amazes me what a couple decades makes in our understanding of Pluto. 

For kicks, the 1978 discovery image of Charon (it is the bulge in the image, it wasn't until the early 80's when an image was obtained that showed Pluto and Charon as separate objects). Yes, I've been an avid space fan since I was a kid.
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Offline Sesquipedalian

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #38 on: 02/05/2015 02:21 PM »
Barring an insisted-upon expedition to Eris

We have one of those launching in November, actually. ;)



Sadly this isn't a real-life mission, but it is an extremely cool concept.  I had no idea until watching that how truly far away Eris is.

Online redliox

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Re: New Horizons begins Pluto observations ahead of July flyby
« Reply #39 on: 02/05/2015 07:10 PM »
Sadly this isn't a real-life mission, but it is an extremely cool concept.  I had no idea until watching that how truly far away Eris is.

I doubt Eris will get any immediate visits for a long while.  Regarding the video, a very nice fly-by vehicle.  I certainly would like to see something like that sent to either Uranus or Neptune, more specifically the later since there's a vague chance Uranus might get a full fledged orbiter in the next two decades.
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