NASA Educational Workshop
What Are The Objectives Of Navigation?
- Control the Spacecraft
- Support Scientific Data Processing
What Are The Components Of Navigation?
- Spacecraft Orbit Position and Velocity
- Spacecraft and Sensor Attitude (Orientation)
How Well Do We Need To Know The Locations?
- The SeaWiFS pixels are 1.1 kilometers square. We are required to determine the locations within 1 pixel.
- The spacecraft altitude is 700 kilometers; therefore the viewing direction must be known within 0.09 degree.
Why Is This More Difficult Than Surface Navigation?
- More degrees of freedom; spacecraft position and velocity are not constrained to the Earth's surface; no implicit connection between attitude and direction of motion ("heading") or zenith ("up").
- High velocity (about 5 miles per second).
- Navigation must be performed either autonomously (onboard computer) or remotely (using telemetry).
Is There Anything Which Makes It Easier?
- A spacecraft spends most of its time in free flight, so orbit motion can be predicted using the laws of physics.
- Forces acting on the spacecraft (other than gravity) are small, e.g., atmospheric drag, geomagnetic field.
How Do We Determine the Spacecraft Orbit?
- Our spacecraft has an onboard Global Positioning System (GPS) receiver which computes the spacecraft position and velocity every 10 seconds, with an accuracy of about 50 meters.
- Other spacecraft have used radio-frequency measurements (Doppler shift) to determine the orbit.
- NORAD tracks all spacecraft and orbiting objects using radar.
How Do We Determine the Spacecraft Attitude?
- The spacecraft has onboard sensors to measure the direction of the Sun, the Earth horizon and the geomagnetic field.
- This information is combined with the known Sun direction, nadir (from the orbit) and a model of the magnetic field to determine the spacecraft attitude.
- The attitude is expressed in conventional surface navigation terms: roll, pitch, and yaw.
- Other spacecraft may use different sensors, such as gyros or star trackers.
How Do We Determine the Observed Locations?
- The sensor viewing geometry is combined with the spacecraft attitude in order to compute the "look" direction for each pixel.
- This vector, together with the spacecraft orbit, determines the surface location being observed.
How Do We Check Our Results?
- Features observed in the sensor data (e.g., islands) are compared with reference locations taken from maps.
- For SeaWiFS, a catalog of over 6000 island locations is used to check the accuracy of navigation processing.
SeaWiFS Project Home Page
Frederick S. Patt(firstname.lastname@example.org)