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From: eugene@pioneer.arc.nasa.gov.arpa (Eugene N. Miya)
Newsgroups: sci.space,sci.space.shuttle
Subject: Re: NASA news - Seasat
Message-ID: <11363@ames.arc.nasa.gov>
Date: 6 Jul 88 19:08:03 GMT
References: <13979@shemp.CS.UCLA.EDU> <1313@daisy.UUCP> <1003@aplcomm.UUCP>
Sender: usenet@ames.arc.nasa.gov
Reply-To: eugene@pioneer.arc.nasa.gov.UUCP (Eugene N. Miya)
Followup-To: sci.space
Organization: NASA Ames Research Center, Moffett Field, Calif.
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A U-2 just took off, must be 1100.  BTW I saw the Nova on Spy Machines.
Thanks for the previous offers.  I go on vacation again and some fool at
NASA HQ has to send a press release out on a skeleton in a closet [well
not that bad].  Let me see if I can address all these notes in
a single article [the last time was grossly misinterpreted].  What's
this doing in space.shuttle?

In article <1003@aplcomm.UUCP> jwm@aplvax.UUCP (Jim Meritt) writes:
>In article <1313@daisy.UUCP> wooding@daisy.UUCP (Mike Wooding) writes:
>} How does the radar altimeter decouple its "height" above ocean
>} surface from the "height" of the ocean's surface? What scales
>} are involved (+-10 meters)? A reference point?
>
>There is an altimeter on the geosat that is a follow-up to the seasat
>one.  I do the real-time processing of the data - you not only get
>"height above ocean", but significant wave height, winds, and a measure
>of roughness off the altimeter.  (probably more, but that is all I lift).
>This can be used to get current "edges", fronts, eddy locations, and all
>kinds of neat stuff.  I asked about satellite oceanography earlier, but
>didn't hear about anyone else using altimeter data for oceanography.

This is basically correct.  Let me sort out some things.  We have
questions of scale and decoupling.  Now, I didn't work on the Altimeter,
(I worked on the SAR) but I had it as a grad school project and talked
to most of the people since they were across the street.

The Altimeter sent out a 1 ns chirp [square wave].  This gave an inherent
resolution of 30 cm (one of GMH's nanoseconds).  This made lots of assumptions:
1) the spacecraft was oriented perfectly vertically, the reality was at
800 KM a slight difference in angle is critical.  The pulse (chirp) hits
the earth in a spherical manner and it radiates it's point of contact.
Now the footprint was designed to be 1 KM (if this seems gross,
please make an other altimeter suggestion [exer. for reader: why can't
use you a laser: answer: won't penetrate clouds].  Anyway, you send out
this perfectly spherical chirp (pulse of 1 ns thinkness against a
topography of unknown surface roughness, and you get a signal back which
is distorted by troughs and peaks of various wave types or land forms.
(Like plane cross-sections).  Yes, you can get undersea features like
canyons and seamounts, but all the instrument does is solve D=cT.

The decoupling isn't done using ground stations in realtime, much too
expensive and the real-time compute and relativistic effect is murder.
Basically I have would have to summarize this book on Accuracy
Assessment of Orbit and Height Measurement for Seasat.  There are models
for satellite orbit which take gravitational anomalies in account, these
are plugged into the T data and the sea height and state are "solved."
Remember this is all done in nano seconds precision.


Also written:
>The position of the satellite can be determined in three-space (X, Y
>and Z co-ordinates, with no reference to the radius of the Earth), by
>the use of radar data from several observers.  Studies of the motion

This was done infrequently as verification.  I have the list of tracking
station, but it was not radar, only radio.

>derived mathematically from the parameters of the reference geoid,
>which was derived from studies of satellite motion in three-space, not
>satellite altitude.  The expected mean sea level can then be compared
>with the actual sea level observed by the radar altimeter, to
>determine the variations caused by tides, waves, and meteorologic
>phenomena.
>
>An aside on SeaSat 1 -- The satellite failed some time before the end
>of its expected service life.  A persistent rumor states that it was
>intentionally disabled, possibly by aiming sensors at the Sun; the
>purported explanation was that it was able to detect the wakes of
>ballistic-missile submarines.
>
>Kevin Kenny			   UUCP: {ihnp4,pur-ee,convex}!uiucdcs!kenny

Also:
>status. I've met one of the engineers in charge of building and
>testing Seasat, and he is still bitter about what happened. It was
>paul cooper

I will be curious, who?

Lee @ JPL wrote:
>The investigation into the failure of Seasat assigned the probable
>cause to a short across the slip rings that transfer the power
>generated by the solar cells to the power buss where the solar panels
>rotate.  They attributed this to the poor design of the slip rings
>that had the various voltages (48 rings) alternating plus and minus,
>creating the maximun potential to catastrophic electical short.  It
>was shown  that there was a galling problem (in the ring bearings I
>believe) that created metal slivers.  These slivers more than likely
>shorted the main power buss at the rings. The telemetery showed large
>voltage and current excursions in the milliseconds prior to loss of
>signal from the spacecraft.
>
>I too have heard unconfirmed rumors of the possibility that SSBM wakes
>could be seen in the radar imaging data.  I was clear on the images
>that I saw that surface wakes were very visible.
>
>The spacecraft lasted only 90 days in a planned life of one year.  I
>have had people tell me that they were not unhappy that the spacecraft
>had shut down because of the enourmous quantity of data that was
>pouring in.

We called Seasat-A then -1 after launch (B's and C's were planned as
exercises).  The "slip ring" on the Agena bus was the cited case of
failure by the Congressional Investigative Service.  LMSC [Lockheed
Sunnyvale] had the burden slapped on them.  They had "gotten too lax in
the quality control on Agena boosters.  You have to understand this
satellite was slapped together with parts of an existing booster, not
designed from scratch.  JPL's scientists were too lax in overseeing
LMSC Corp.  So said Congress.  So how would you lose $90M of the
People's money?

Regarding FMB subs: shortly before I came on board the project, the
Navy Department came by and the SAR group had discussions about
resolution, visibility, etc.  They didn't want this thing flying at all.
Fortunately, other parts of the Navy like the Numerical Weather Central
people did want it.  Compromises were made.  This is the SAR now, not
the altimeter.  [Oh, SAR== Synthetic Aperature Radar, aka Side-looking
Radar, SLAR].  These fears were partially unfounded because the digital
processing time for one image was 2 weeks.  Partially because it could
show where they had been rather than where they were.

Optically processed images came out in about a week.

Anyway, the project is over, reports are made, a few images were made,
SIR [Shuttle Imaging Radar] is off the ground [having a few problems].
Most of the data sits unused.  Time for other projects (in this case
Magellan).

Some reflections, the other day someone stopped me at PARC and noticed
an old and faded Project sticker and expressed the conspiracy theory
yet again [tired of this].  Launching money into orbit is a sticky
thing.  One senior engineer (who will go unnamed) was hoping the Atlas
was blow up on the pad at VAFB because there were no indication his
antenna would unfold in 0-G.  I know others who felt the same way about
the kludges they had installed and got flight certified.  It's like
saying, "Your next school project will make or break your future."
No second chances.

Anyway, we launched at 6:01 PDT. Into the fog, gone in 3 seconds.
We went to Solvang for dinner (about 30 miles from where I went to
college).  Quite a birthday present, three of us (Vickie, myself, and
Dave Drake [now at DEC]) -1,0,+2 days).

BTW: Dave and I tried getting an early v6 Unix system running on a
PDP-11/34 [without MMU].  We learned of Joe Ossanna's satellite tracker
[azel] which was not distributed, too bad, we were the space program.
Years later, I got "track" from Phil Karn [thanks Phil].  And I
occasionally run track with seasat-1 and watch the numbers tick by.
It reminds me a bit of the old film Robinson Crusoe on Mars knowing
something up there is orbiting because of thing you did.

Another gross generalization from

--eugene miya, NASA Ames Research Center, eugene@aurora.arc.nasa.gov
  resident cynic at the Rock of Ages Home for Retired Hackers:
  "Mailers?! HA!", "If my mail does not reach you, please accept my apology."
  {uunet,hplabs,ncar,decwrl,allegra,tektronix}!ames!aurora!eugene
  "Send mail, avoid follow-ups.  If enough, I'll summarize."

Lee, if you want track, I don't think Phil would object.