IN A large hangar known as End Station 3, sunk half underground at Stanford
University in California, a giant green beast rises, turns slowly and lumbers
off into the cavernous room. A second beast, this one bright red, noses the air
as if it is detecting a scent, and then follows its companion.
These beasts, in reality plastic blimps the size of pick-up trucks, are the
creations of aerospace researcher Jonathan How. As they float through the
hangar, it soon becomes clear that the green blimp is in charge. When it stops,
so does the red one. When the green blimp turns, the red one follows suit, in a
giant game of copycat.
Although they might seem clumsy, the blimps are performing a complex task:
formation flying. This is difficult enough with a human at the controls but
these blimps are flying without any help from How or his colleagues. And what
blimps are doing today, spacecraft will be doing tomorrow. How and a growing
number of aerospace researchers believe that formation flying will revolutionise
space exploration. The technique allows scientists to combine the measurements
from several spacecraft in way that creates telescopes and sensors of
unprecedented size and power, and at an affordable price.
Advertisement
NASA is already designing such a telescope to look for Earth-like planets
around other stars. Its resolution will be hundreds of times that of Hubble but
it will cost only a fraction as much. Meanwhile, the US Air Force is building a
fleet of radar-imaging satellites that will spy on objects on Earth. It鈥檚 all
part of the trend towards smaller, cheaper and more flexible space exploration
vehicles. But before any of this will be possible scientists must perfect the
techniques of flying in formation and this is far from easy.
The basic challenges are well understood. The spacecraft must be able to
determine their location as well as the locations of the other spacecraft in the
fleet. They must be able to calculate their flight paths and decide whether they
are drifting apart. If any adjustments are necessary, they must be able to work
them out without help from the ground. All this must be done using as little
fuel as possible and in a way that minimises the amount of processing and
communications between the craft.
Keeping the spacecraft in formation, however, is tough. Orbiting
spacecraft tend to drift off-course because of such factors as atmospheric drag, the
solar wind and, the biggest problem of all, the nonspherical shape of the Earth.
But in the past few years, scientists have worked on finding a set of individual
orbits that will naturally maintain the spacecraft in the vicinity of each
other. For spacecraft orbiting the Earth, it turns out that there is a special
set of elliptical orbits, called Hill`s orbits, that meets this condition. While
Hill`s orbits have long been used for rendezvous and docking in space, it was
only earlier this year that David Miller at the Massachusetts Institute of
Technology and his colleagues at Texas A&M University realised they could be
used for formation flying. 鈥淚f you want to do formation flying, you have to
accept the constraints that nature puts on you,鈥� says Miller.
The reason Hill`s orbits can be used is that the three-dimensional shape of a
formation is not important when observing the Earth from space. Rather, the
crucial factor is the projection of the formation onto the Earth`s surface, that
is, the way the formation would appear to somebody on the surface looking up
into space. As long as this doesn`t change, the formation can function as a
telescope or radar imaging array. One property of these orbits is that the
formation rotates slowly during each orbit.
Since Miller calculates Hill`s orbits by assuming the Earth is spherical, the
spacecraft still need to correct their orbits to cope with the Earth`s real
shape. But the amount of correction is far less than in other orbits. Indeed,
without these fuel-saving orbits, the satellites would have to carry so much
fuel that formation flying would not be feasible. 鈥淭hat is the breakthrough that
has allowed formation flying to go through,鈥� says Alok Das of the US Air Force`s
Space Vehicle Directorate at Kirtland Air Force Base in New Mexico.
Outside the gravitational influence of the planets, in deep space, orbital
effects are much less of a problem but fuel efficiency is still an issue. 鈥淭he
question is, what are the most efficient ways of rearranging the formation with
the least amount of fuel,鈥� says Paul Wang at the University of California, Los
Angeles. Wang is developing ways of changing formation that are reminiscent of
the way the members of a marching band move to change direction without bumping
into each other.
None of these complex manoeuvres will be possible, however, if spacecraft
cannot measure their position and attitude. 鈥淧eople had proposed formation
flying in the past,鈥� says Frank Bauer, who heads the Guidance, Navigation, and
Control Facility at NASA鈥檚 Goddard Space Flight Center, 鈥渂ut you really can鈥檛 do
it until you have a navigational sensor. That鈥檚 where GPS comes in.鈥�
Constellations in orbit
The Global Positioning System, set up by the US Department of Defense,
consists of 24 NAVSTAR satellites that circle 20 200 kilometres above the Earth.
Each satellite transmits a code that can be picked up by portable receivers on
Earth. These receivers use highly accurate clocks to measure the signal鈥檚 travel
time and hence the satellite鈥檚 distance. By comparing the distance to three or
more of the satellites, and with the help of a few tricks, the receivers can
determine their location to within a few centimetres.
This system also works in orbit, and has already been used to help uncrewed
spacecraft rendezvous and dock. 鈥淔or the most part, GPS is there for people on
Earth. We鈥檝e adopted it for space,鈥� says Bauer.
Given accurate position measurements, the next problem is to work out what to
do with them. This is where How鈥檚 work in End Station 3 comes in. Once a
terminal point in a linear particle accelerator, the hangar is now a storage
facility for discarded office furniture and laboratory equipment, as well as the
proving grounds for formation flying. Beneath the yellow lights, How and his
students are testing the computer algorithms necessary to control formation
flying.
Since NAVSTAR signals cannot be received indoors, How鈥檚 colleagues have
mounted their own GPS transmitters high up on the hangar walls. On each blimp
they hung a wooden basket containing a GPS receiver, a circuit board, batteries,
and motors that crank the blimp鈥檚 propellers. Each blimp monitors the signals
and broadcasts its position and attitude to a central computer. The computer
monitors changes in the green blimp鈥檚 position, calculates how the red blimp
should move to maintain the required formation and sends it the required
directions. In future, this processing will be done onboard but the result will
be the same.
鈥淏limps are perfect for testing the control problems in formation flying,鈥�
explains How, who thought of using blimps when he attended a hockey game and saw
a large blimp travelling around inside the arena, dropping T-shirts on the fans.
鈥淭hey are relatively inexpensive and can be rigged fairly easily to carry GPS
谤别肠别颈惫别谤蝉.鈥�
Using GPS indoors, however, is not easy because the signals bounce off walls
and equipment, creating reflections that confuse the receivers. So How plans to
continue his experiments outside with a small fleet of toy trucks. The lead
truck will be controlled via radio remote control, while three other trucks will
use GPS to find the leader and follow it around in formation.
The lessons learnt from the blimps and trucks will soon be put to use in
space. How and his students are taking part in a project to build three
satellites the size of milk crates that will fly in formation sometime in 2001.
The project is part of the University Nanosatellite Program sponsored by the US
Department of Defense, NASA and industry partners.
In the meantime, the first demonstration of semi-autonomous formation flying
in orbit is slated for December this year with the launch of Earth Observing
(EO-1), a land-imaging satellite that will carry a cheap new camera that offers
very high resolution. The idea is to compare the camera鈥檚 images with identical
shots taken by a far more expensive camera already in orbit aboard Landsat 7. To
make an accurate comparison, EO-1 must orbit directly behind Landsat 7, trailing
by only a minute or so.
This task is not straightforward. Although both spacecraft will be in the
same orbit, roughly 700 kilometres overhead, their different shapes and masses
means that EO-1 will have to adjust its orbit from time to time. To accomplish
this, a ground station will measure Landsat 7鈥檚 position and broadcast this
information to EO-1, which will be taking GPS measurements of its own position.
EO-1 will then calculate the future orbits of both spacecraft. If they match,
the spacecraft does nothing. But if its future path deviates from Landsat 7鈥檚,
the onboard computer will work out a manoeuvre to put it back on track, says
David Folta at Goddard, who heads the EO-1 project.
While the EO-1 mission qualifies as formation flying, the fact that the
ground is involved means it is still a long way from the completely autonomous
missions that NASA envisions. Space scientists talk of achieving a 鈥渓ights out鈥�
mode, where ground controllers can supervise the first stages of a mission, then
turn off the lights in the control room and go home, leaving the spacecraft to
perform on their own.
This is the kind of autonomy demanded by a NASA project called Space
Technology 3 being developed at the Jet Propulsion Laboratory in Pasadena,
California. Due for launch in 2003, ST3 will combine the images from two small
telescopes flying in formation to produce images almost as good as those of a
very expensive giant telescope. Two collector spacecraft will reflect light into
a third instrument where the two beams are combined. The interference pattern
created will then be processed to produce an image of the object that generated
the light.
A space-based interferometer with a baseline distance of 200 metres between
collectors would have a resolution 800 times better than that of the Hubble
Space Telescope. With this increased capability, scientists will be able to look
for new stars, planets around other stars and black holes.
The problem is to maintain formation while flying several million kilometres
from Earth. Since the spacecraft will not orbit the Earth but will fly in deep
space, orbiting the Sun, they cannot rely on the NAVSTAR GPS signals. Instead,
the spacecraft will carry GPS transmitters, creating their own mini-GPS
constellation. 鈥淓ach spacecraft is like a NAVSTAR satellite to the other
spacecraft in the group,鈥� says Kenneth Lau, the key architect of JPL鈥檚
Autonomous Formation Flying system. This will be accurate to within a few
centimetres. A more specialised system will then fine-tune the spacecrafts鈥�
positions to within a billionth of a metre鈥攁 feat that can be achieved
using laser measurements and fine positional corrections.
All this has to be done without any help from Earth. When devising autonomous
control algorithms, a key design question is which spacecraft gets to be the
boss. There are two basic types of control architectures. One is a centralised
master-slave arrangement, where the lead spacecraft detects the locations of all
follower spacecraft and sends them instructions on how to manoeuvre.
The second is a decentralised system, in which the lead spacecraft sends out
instructions but leaves it to the followers to calculate their own positions and
determine how to move to stay in formation. In this scenario, each spacecraft
could take turns being the leader, similar to the alternation of leaders in a
flock of flying birds. Each spacecraft carries the necessary equipment to lead,
making the system far more versatile, but also more complex and more expensive.
鈥淭he best approach might be to switch from centralised to decentralised control
depending on the application,鈥� says Lau.
With either of these control mechanisms spacecraft can change position and
damaged craft can be taken out of service and replaced. 鈥淏efore, we had to build
huge platforms for sensors and other equipment. Now, we will be able to update
specific pieces of equipment with new technology鈥攚e don鈥檛 have to do it
all at once,鈥� says Fred Hadaegh, director of JPL鈥檚 formation flying programme.
鈥淔ormation flying is an incredibly powerful tool for doing distributed
communications, sensing, and telemetry.鈥� He envisions an armada of spacecraft
each carrying a different scientific instrument that can be swapped in and out
depending on the mission.
Flying spies
Autonomous formation flying will also be used for spying. The US Air Force is
developing a GPS-controlled fleet of spacecraft called TechSat 21 that will use
synthetic aperture radar to spot moving objects on the ground. Fleets of
spacecraft provide much greater flexibility than a single craft, says Das. 鈥淵ou
can actually do several jobs with the same equipment,鈥� he says.
When looking for moving objects, for example, the ideal spacing between the
spacecraft is about 500 metres. But when pinpointing signals on the ground, the
ideal baseline is several kilometres. 鈥淪o you command just one of them to drift
back. When the job is done, you move the satellite back to the cluster,鈥� Das
says.
The Techsat 21 system will be able to decide for itself what formation to
adopt based on the tasks it is asked to carry out. The first few satellites are
due to be launched late in 2003.
The list of missions involving formation flying continues to grow. The
European Space Agency plans to start its own formation flying programme this
year. It has long wanted to build a giant interferometer using spacecraft
separated by distances many times larger than that between the Earth and the
Moon. The mission, called LISA, will look for ripples in space called gravity
waves that are produced when black holes collapse and neutrons stars collide
(快猫短视频, 10 August 1996, p 36). Gravity waves have never been
detected directly, and doing so could open up a whole new branch of astronomy.
And earlier this year, NASA announced plans to study interactions between the
Earth鈥檚 magnetosphere and the solar wind by making simultaneous measurements
over huge volumes of space.
Once the technical challenges are worked out, NASA researchers are confident
that autonomous formation flying will revolutionise the way we explore space.
And for this, they will have the lumbering ballet of a few giant blimps to
thank.
