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Phoenix Final Descent Trajectory
CosmicRocker
post Aug 1 2008, 05:48 AM
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I'm a bit out of my element in this thread, but when I saw the .csv data series files I couldn't help but dive in and see if I could do anything with such easily accessible data. I'm hoping I can take the lat/long/alt data at face value. I started out trying to make a 3D plot of the descent, but settled for this anaglyph rendition of the trajectory.

Does this even appear to be close to what you folks would expect? If it is, I'd like to find a way to plot this above a rendering of the Martian globe. With the white background, it's a bit stark as anaglyphs go. It might appear more clearly to to those of you who can double-up on the density of your red/cyan filters.

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djellison
post Aug 1 2008, 09:36 AM
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The joys of 'plot' - a freeware app for OSX..



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Ant103
post Aug 1 2008, 09:46 AM
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I have made a KMZ file to see what it look in 3D by using alt, long and alt datas.
http://www.db-prods.net/blog/?p=362
(a direct link to the KMZ at the bottom of the GG Maps visualizator).

But, there are data bellow the reference level and in Google Earth, we can't see the last piece of EDL.


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djellison
post Aug 1 2008, 09:50 AM
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What about cheating - just add 2.5k to every alt value.
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Ant103
post Aug 1 2008, 10:01 AM
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Yes, but how? There are a huge quantity of numbers. unsure.gif


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jmknapp
post Aug 1 2008, 12:17 PM
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QUOTE (CosmicRocker @ Aug 1 2008, 12:48 AM) *
I'm hoping I can take the lat/long/alt data at face value.


FYI, the values are planetocentric coordinates of the "near point" on Mars from Phoenix. The SPICE library calls:

subpt_c("near point","MARS",et,"LT+S","PHX",scsubpt,&alt) ; // get Phoenix subpoint, IAU_MARS rectangular coords
reclat_c(scsubpt,&subrad,&sublon,&sublat) ; // convert to rectangular to latitudinal (radius, longtitude, latitude)

For Mars there's not a big difference between planetocentric and planetographic.


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djellison
post Aug 1 2008, 12:18 PM
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Last 55s or so of landing.

Altitude on black line, left axis.

X = blue, Y = red, Z = Green - velocity on right axis in km/sec

Note the brief -Z increase at seperation, then the gradual drop until the constant velocity phase at about 2.3m/sec
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jmknapp
post Aug 1 2008, 12:40 PM
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QUOTE (Ant103 @ Aug 1 2008, 04:46 AM) *
I have made a KMZ file to see what it look in 3D by using alt, long and alt datas.


Love the link to the Google Earth view!

Here's another version of the CSV with the altitudes offset to be zero at touchdown:

PHX EDL CSV altitude corrected (4MB zip file)


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djellison
post Aug 1 2008, 01:41 PM
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FWIW , Excel '08 on my Mac can open the CSV In full - but asking it to plot anything is a death-sentence. So I just use Excel to generate a CSV containing the time I want and the fields I want, and use Plot to then..er..plot it smile.gif

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nprev
post Aug 1 2008, 01:50 PM
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I was kind of curious about that altitude data. Is the offset due to the reference geoid (forgive the term!) used, a delta in the IMU data, a combination of both, or something else entirely? I'd have expected the rad alt data to trump this in any case.


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mcaplinger
post Aug 1 2008, 02:12 PM
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QUOTE (jmknapp @ Aug 1 2008, 04:17 AM) *
For Mars there's not a big difference between planetocentric and planetographic.

Remind me not to let you navigate my spacecraft. smile.gif


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Ant103
post Aug 1 2008, 03:01 PM
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jmknapp : thanks.
Your updated file is great, but, a little big blink.gif
Can you make a version without so many lines? (about the same numbers of the first version, because Google Earth can't view a KML file with so many point)


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jmknapp
post Aug 1 2008, 04:35 PM
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QUOTE (Ant103 @ Aug 1 2008, 10:01 AM) *
jmknapp : thanks.
Your updated file is great, but, a little big blink.gif
Can you make a version without so many lines? (about the same numbers of the first version, because Google Earth can't view a KML file with so many point)


OK, here's one at one second intervals, so only ~500 lines:

PHX EDL CSV, zero alt at touchdown, 1 second steps (26K zip file)


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jmknapp
post Aug 1 2008, 04:50 PM
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QUOTE (nprev @ Aug 1 2008, 08:50 AM) *
I was kind of curious about that altitude data. Is the offset due to the reference geoid (forgive the term!) used, a delta in the IMU data, a combination of both, or something else entirely? I'd have expected the rad alt data to trump this in any case.


Not sure, but I'd bet it's just the difference between the actual altitude and the reference geoid. According to the man page for subpt_c():

QUOTE
method is a short string specifying the computation method
to be used. The choices are:

"Near point" The sub-observer point is
defined as the nearest point on
the target relative to the
observer.

"Intercept" The sub-observer point is
defined as the target surface
intercept of the line
containing the observer and the
target's center.

In both cases, the intercept computation treats the
surface of the target body as a triaxial ellipsoid.
The ellipsoid's radii must be available in the kernel
pool.


In an early press conference someone mentioned that the plains Phoenix is on are quite low, such that liquid water can even exist, so it might make sense that it would be 2.7km below the geoid.


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jmknapp
post Aug 1 2008, 05:05 PM
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QUOTE (mcaplinger @ Aug 1 2008, 09:12 AM) *
Remind me not to let you navigate my spacecraft. smile.gif


Just going by this web page:

QUOTE
Planetocentric coordinates are referred to the equatorial plane of the body concerned and are much used in the calculations of celestial mechanics. Planetocentric longitude is measured around the equator of the body from a prime meridian defined and adopted by international agreement. (The prime meridian may be referred to a visible feature in the case of a solid-surfaced body such as Mars, but in the case of a gaseous planet such as Jupiter it is a purely hypothetical concept.) Planetocentric latitude is measured in an arc above or below the equator of the object in the usual way.

Planetographic coordinates are used for observations of the surface features of those planets whose figures are not truly spherical, but oblate. They are referred to the mean surface of the planet, and are the coordinates actually determined by observation. They can readily be converted to planetocentric coordinates if required. As the oblate planets are symmetrical about their axes of rotation, there is little difference in practice between planetocentric and planetographic longitudes. However, the differences between planetocentric and planetographic latitudes are quite significant for very oblate bodies such as Jupiter and Saturn.



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