Deep Space Navigation

1
Deep Space Navigation

• Robert Mase
• Jet Propulsion Laboratory
• California Institute of Technology
• Pasadena, California
• Robert.A.Mase_at_jpl.nasa.gov

2
The Navigation Process

• Five tasks need to be performed for successful
  navigation, whether on Earth or in deep space
• Task Student Example Deep Space Example
• 1. Obtain a map Log onto Mapquest Planetary
  ephemerides
• 2. Plot a course Find route to Spring
  Break Select target body, compute
• destination launch/arrival conditions
• 3. Take measurements Speedometer,
  odometer, Radiometric tracking data,
• in-car GPS pictures of target star
  background
• 4. Calculate position Read road signs,
  GPS Numerical least-squares fit of
• tracking data
• 5. Make course Turn the wheel, hit the Perform
  propulsive maneuver
• corrections gas pedal or brakes

3
Navigation Uncertainty - Analogy

• If you know where you are, and where you need to
  get to, why do you need Navigation at all?
• Problem Your friend wants to meet you in Florida
  at your Spring Break destination just as you
  arrive with a cold beverage
• Solution If you start out in Atlanta, heading to
  Spring Break destination, know what time you are
  leaving, and the speed limit, you can calculate
  exactly when you should arrive or can you?
• What if there is an accident, or traffic jam, or
  you stop for gas or food?What if your odometer
  is off by 2 miles/hour?What is the street signs
  are poor and you take a wrong turn?
• Can you predict these things exactly ahead of
  time?
• How can we account for unseen events ahead of
  time to compensate?
• Ill be there at 500 plus or minus an hour
• Friend complains Cold beverage will get warm
  waiting one hour in the hot Florida sun!
• Solution Cellphone! - Call with an update when
  you get close

4
Navigation Uncertainty

• Previous example doesnt really apply to
  deep-space navigation or does it?
• True there are no traffic jams or pit-stops in
  spaceBut what can affect the trajectory?
• Do we know all propulsive events that will
  happen?
• Do we know all forces that will act on the
  spacecraft?
• Do we know all of our measurements are exactly
  right?
• Do we know exactly where our target is?
• Do we know exactly where we are?
• Does any of this matter?

5
Example

• Typical trajectory to Mars is about 250 days
• 250 days x 86,400 sec/day 21,600,000 secs
• Typical propulsive maneuver will impart on the
  order of 1 to 10 meters/sec (2 - 20 mph) of
  velocity change to the spacecraft
• We can measure the velocity change to within 1
  mm/sec (0.002 mph)
• If we are wrong by only 1 mm/sec
• (Simple linear calculation)
• Distance Velocity Time
• 1E-6 km/sec 21,600,000 sec 21.6 km
• We will arrive 21.6 km off from where we should be

6
Example

• Acceleration due to solar radiation pressure
• Typical uncertainty is on the order of 1E-12
  km/sec/sec
• If we are wrong by only 1E-12 km/sec/sec
• Distance 1/2 a t2
• 1/2 (1E-12 km/sec/sec) (21,600,000 sec)2
  230 km
• We will arrive 230 km off from where we should be
• Note The uncertainty will decrease with time to
  go
• The closer we get to the target, the number of
  things that can happen between now and arrival
  decreases
• The effect of any individual perturbation
  decreases with time to go

7
Error Sources

• So what else do we have to contend with?
• Measurement Errors
• Measurement noise
• Earth orientation
• Tracking station locations
• Transmission media
• Instrumental signal delay effects
• Spacecraft oscillator instability
• Force Modeling Errors
• Gravity
• Propulsive maneuver events
• Solar radiation pressure
• Outgassing
• None of these things are known exactly, so we
  must account for our lack of knowledge by
  calculating the associated uncertainty

8
Navigation Uncertainty

• What strategy can we use to account for
  uncertainty or errors?
• We always have the ability to adjust the
  trajectory - fire the thrusters
• In a car we can speed up or slow down, can turn
  left or right
• Small adjustments early in the trajectory have a
  bigger effect
• Can change travel time significantly by driving
  slightly faster all the way
• This will change the planned arrival time, but
  will not decrease the uncertainty in what time
  you will actually arrive
• Adjustments at the last minute more precise, but
  limited in scope
• If you wait until you are almost there, there is
  not much left to deter you
• Cant significantly change arrival time just one
  mile from your destination
• We schedule a series of opportunities to adjust
  the trajectory consistent with our knowledge of
  the trajectory and associated uncertainty
• Make big adjustments early to get in the
  ballpark, make more precise adjustments later
  when there is more certainty in arrival conditions

9
Navigation Measurements

• What measurements do we have to work with?
• Doppler - measures the line-of-site velocity
  between the spacecraft and the tracking station
• Typically accurate to 0.1 mm/sec (0.0002 mph !)
• Range - Measures the line-of-site distance from
  the spacecraft to the tracking station
• Typically accurate to a couple of meters
• Can measure position and velocity quite
  accurately
• But only in one direction
• On Earth we have handheld GPS which uses 4 or
  more satellites to unambiguously triangulate your
  location
• For spacecraft, we can use two tracking stations
  at once to triangulate, referred to as an
  interferometric measurement

10
Mars Odyssey Spacecraft
11
Mars Odyssey Trajectory
Earth at arrival
Launch 07-Apr-2001
Mars Arrival 23-Oct-2001
TCM-4
TCM-1
TCM-3
Mars at Launch
TCM-2
12
Mars Odyssey - Launch

• If the spacecraft was launched and never
  performed any kind of trajectory correction
  maneuver, where would it end up?

13
Mars Odyssey - First Maneuver

• The first propulsive maneuver 3.6 m/s, 46 days
  after launch
• Moved arrival point 60,000 km closer to Mars
• But still large uncertainty (gt 1000 km) in
  expected arrival conditions

14
Mars Odyssey - Final Maneuver

• The final maneuver 0.08 m/s, 12 days prior to
  arrival
• Needed to be accurate to 25 km, ended up lt5 km
  from the target

15
Mars Exploration Rovers

• Two rovers to be landed on the surface of Mars
• Parameter of interest is Entry Flight Path Angle,
  0.10 degree rqmt
• Translates to a position uncertainty at entry of
  a couple of kilometers

16
Mars Exploration Rovers

• Goal Land a Rover on the surface of Mars

17
Cassini - In Orbit Around Saturn

• Goal Deliver Huygens Probe into Titan atmosphere

18
Cassini Probe Delivery Accuracy

• Must wait until Navigation accuracy is sufficient
  to meet entry corridor requirements to release
  the probe
• Parameter of interest is Entry Flight Path Angle,
  3 degree rqmt

19
Genesis

• Mission Collect Particles of Solar Wind
• Navigation Goal Deliver Sample Canister to
  Earth, land in Utah

20
Genesis Trajectory
21
Genesis Landing Zone in Utah

• Navigation to the atmosphere was perfect
• Parachute did not open, canister landed in safe
  zone

22
Conclusion

• Deep Space Missions have unique constraints and
  requirements
• Even all Mars missions are not the sameLanders
  vs Orbiter vs Rovers
• But all require accurate Navigation