| A decade ago when I came
to the Geostationary Operational Environmental Satellite
(GOES) program, we had one limping spacecraft, plus a
satellite rented from the Europeans. I had to start by
assuming, essentially, that we had no resources in orbit.
GOES is by no means an inconspicuous program. Every
night when you watch the weather on the evening news,
you see GOES satellite pictures. My customer, the National
Oceanic and Atmospheric Administration (NOAA), requires
two operating satellites, with a spare ready to be put
into operation when an existing satellite goes out of
service. Clearly, we needed to build our first two satellites
and get them launched as fast as we could. There was
money available, and a contractor lined up to do the
work. Easy so far, from a scheduling point of view:
Build the spacecraft and launch it.
But what do you do when events beyond your control
dictate when you launch a spacecraft?
Back in those days the people who built launch vehicles
were doing a lot of launches. Thus, we expected long
launch queues. The idea of launching a spacecraft the
moment it was needed didn't seem very realistic. In
addition, storing a backup for extended periods of time
seemed too risky. There were certain detectors that
we couldn't check at room temperature; we would have
to go back in the thermal vacuum chamber. How long could
we have a spacecraft out of thermal vac and still have
confidence that it would work when launched? We didn't
know, and it made us nervous to think about putting
things in storage for two or three years, then trying
to get hold of a thermal vac chamber, then hoping to
fit into a launch queue.
So, I sold my customer on the idea of having an on-orbit
spare. That meant I could build the third spacecraft
and launch it as soon as it was ready. We built the
first two as fast as we could, and then tailored the
third one to when we wanted it to pop out and get ourselves
in the launch queue. Thus far, we are still talking
about a fairly easy scheduling scenario.
We assumed one failure out of every five spacecraft;
one of the five satellites budgeted was for insurance.
In the end, all five succeeded. We never had that launch
or spacecraft failure. The second spacecraft had trouble
with a momentum wheel and we took it out of service
after three years—two years short of its expected
operational lifetime. On the other hand, the one we
launched in 1994 still operates.
Things began to get complicated as money became less
available. Isn't that how it always is? To save $4 to
$5 million dollars, we launched a spare earlier than
planned, so that we could reduce the number of contractors.
It left us with two on-orbit spares. How many spacecraft
are you going to have on-orbit before you get criticized
for having too many? But we also worried about experienced
people being available for the launch, and we were right
to be concerned in this regard; thousands and thousands
of people have been laid off in the aerospace industry
in the past 18 months.
What else did we have to figure into our scheduling?
To put it simply: fuel. Eventually, a working satellite
runs low on fuel and its usefulness as an operational
spacecraft diminishes quickly. We have to retire the
satellite or use it for some other function where it
is not mainline operational. How long will these satellites
continue to perform? Will they go all the way to fuel
depletion? I don't know. But you look pretty funny trying
to take one out of service that is working well, and
you would look even funnier if you put too many of them
up and used up their lifetimes orbiting as hot spares.
All this comes into play in the way you schedule the effort
to build a spacecraft, to store it on the ground, and
then to put it in orbit so that you get it up there before
you need it—not knowing when you're going to need
it. It's a guessing game and the best you can do is to
try to balance all the resources. Here's the average timetable
we work with: five years ground storage, two years on-orbit
storage, five year operational lifetime. But what lifetime
do you use for a planning schedule? Is it the five years?
Or is it an estimate of fuel depletion?
of launching a spacecraft the moment it was needed
didn't seem very realistic.
Sometimes you make a schedule that you use for budget
purposes to get the money you need, assuming the five-year
lifetime, and then anything you get beyond that is gravy.
But do you get accused of lying to Congress or Office
of Management and Budget when you do that? That's something
we face as we do schedules for an ongoing program like
this. NOAA can no longer go back and say, "This is what
we need," and get all the money they need for satellites
because Congress says, "Look, they're working fine.
You've solved your problem." Congress isn't planning
as far ahead as we need to. If you want to look at a
long-term program, this is it. We have launch dates
slated through 2021.
What I want to get across here is that when you get
a multiple-unit situation like we have in satellites,
and you have something like on-orbit performance to
evaluate, the scheduling becomes complicated and it
requires ongoing attention in order to make adjustments
for changing situations.
Periodically we evaluate the health of the on-orbit
assets and revise our schedule as necessary. When we
make revisions, does it appear to an outsider that we
don't know what we're doing? Yes, is the answer. I call
this "scheduling in the real world."
- Balance best — and worst — case scenarios
when scheduling. This may make scheduling more complicated,
but it will yield a more realistic, sustainable project
- If established approaches aren't likely to achieve
desired results, challenge the status quo and be willing
to take calculated risks.
How have you planned for uncertainty on a project?
| Watching the Weather
Flash floods, hail storms, tornadoes, and hurricanes
— all severe weather conditions worth keeping
an eye on. Since 1975, NASA has produced that eye
for the National Oceanic and Atmospheric Administration
(NOAA). NASA's latest series of geostationary operational
environmental satellites (GOES) provide high spatial
and temporal resolution images from a vantage point
of 22,300 miles above the earth, as well as full-time
temperature and moisture profiles of the atmosphere.
Together, two satellites produce a full-face picture
of the earth, 24 hours/day. For more information
about the GOES project, visit http://goespoes.gsfc.nasa.gov.
Marty Davis is the Program Manager of the
Geostationary Operational Environmental Satellite
(GOES) at the NASA Goddard Space Flight Center (GSFC)
in Greenbelt, Maryland. The recipient of many honors,
he received NASA's highest award, the Distinguished
Service Medal, in 1995. He has also received the
NASA Outstanding Leadership Medal (1991) and the
NASA Exceptional Service Medal (1979). He has worked
at NASA since 1962.