Download Spacecraft Navigation

Spacecraft Navigation
Kipp Cannon
Ryerson University, March 14th 2015
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Challenges
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Distance: light travel time forces spacecraft to be autonomous,
repair is not possible so robustness and exhaustive pre-flight testing
is essential.
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Distance to space station: 400 km = 1 ms.
Distance to Moon: 384.4 × 103 km = 1.28 s
Distance to Sun: 149.6 × 106 km = 500 s
Distance to Mars upon arrival on Hohmann transfer orbit: ∼ 14 min
Current distance to Voyager 1: 19.4 × 109 km = 18 h
Challenges
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Speed: not possible to carry enough fuel to significantly alter course,
so only small corrections are possible after initial launch
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“Sound barrier” = 0.3 km/s
Low Earth orbit (LEO) requires a speed of 8 km/s = 29 × 103 km/h
Leaving the Earth requires a speed of 11 km/s = 40 × 103 km/h
Leaving Mars requires a speed of 5 km/s = 18 × 103 km/h
Leaving the Solar system (starting at Earth) requires a speed of
42 km/s = 151 × 103 km/h
Challenges
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Precision: enormous distances travelled and inability to make
significant course changes along the way forces guidance system to
be extremely precise.
Challenges
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Absence of local reference: no solid ground or even atmosphere with
which to measure the effects of flight control actions e.g., pressing
gas in car doesn’t always result in forward movement, but by
observing through window it is usually easy to determine if it has or
not.
Challenges
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Relativity: space and time are curved so radio signals do not follow
straight lines in our solar system, clocks at different locations do not
run at the same speeds, and the large speeds of spacecraft also
cause clocks to run at different speeds.
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Not important for people, but important for precision navigation.
Elements of Spacecraft Navigation
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Spacecraft only rarely burn fuel, so flight is almost entirely pure
orbital motion — free-fall under the influence of gravity alone.
Using knowledge of the physical laws governing orbital motion, only
a small number of measurements are needed to determine the
spacecraft’s location and trajectory:
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primary tool is distance from Earth and rate of change of distance.
in some cases position on sky (e.g., using radio interferometers or by
timing occultations)
for attitude control spacecraft generally measures apparent positions
of planets and/or stars from its location (e.g., wrt background stars
or wrt its instruments)
the latter also allows for position determination but not as accurate
as ground-based range/range-rate measurements.
Tools
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On-board star sighting telescopes.
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On-board planetary limb cameras.
Most important: radio range finding
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When things go wrong:
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Coherent link with coded pulses is established with spacecraft and
monitored over a period of time.
Allows distance to be measured to ±1 m.
Loss of attitude leads to misalignment of high-gain antenna with
Earth — no longer possible to communicate, loss of radio ranging.
Spacecraft carries backup omni-directional antenna — can
communicate in all directions but with very bad quality signal.
Compensate with enormous powerful transmitter on Earth and blast
command signal to spacecraft to try to correct attitude.
Goldstone antenna has 500 kW transmitter on 70 m dish.
Take-Away Message
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It’s not easy.
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There aren’t any tricks for making it easier.
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Relies very heavily on understanding of physics: spacecraft motion is
mostly inferred using knowledge of physics.
Interstellar Navigation
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Obviously never attempted.
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Radio ranging from Earth is impractical due to vast distances.
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Might be possible to use pulsars as navigation beacons, use quasars
for attitude reference and cosmic microwave background for velocity
reference.
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Techniques are mostly speculative, but there is research on this
problem.