Advanced Navigation

Advanced Navigation

       Advanced navigation equipment being developed by NASA gives pilots more information and freedom to fly in a more efficient manner thus saving fuel and lowering overall flight costs. These controls often combine several indicators into one for a more accurate display. Pilots now use satellite data from global positioning systems for navigation.



Inertial Navigation System (INS)
      An INS is very simple in theory, but complicated in practice. Put simply, it is a totally self-contained dead reckoning system. Given its starting position, INS keeps track of all movements in all directions so it calculates the aircraft's flight position in relation to that point. To detect movement, the INS uses three accelerometers, one north-south, one east-west, and one up-down mounted on a stable platform. An accelerometer is an electronic device that provides information similar to a gyroscope. Part of the accelerometer is in a fixed position and the other part is free to move with the aircraft. A magnetic field is produced by electricity between the two parts. Any change in movement by the free part will disturb the magnetic field. This disturbance will be recorded into the on-board computer which reads the data and calculates the amount of movement. The accelerometers use sliding shuttles and can detect accelerations up to a thousandth of a G force. The platform is stabilized using three gyros, one each for pitch, yaw and roll. This way the aircraft's movement is constantly monitored and helps the pilot keep the aircraft on course.



Newer Inertial Navigation Systems use ring laser gyros that are made up of a series of lasers aligned in the same plane and forming a ring. Interference patterns are generated as the aircraft accelerates indicating changes in the airplane's movement. These changes in movement are measured as nautical miles per hour (nmph) Accuracy is within 1.7 nmph. So after an hour the accuracy is 1.7 nm.



The INS must be initialized on the ramp prior to takeoff. The pilot merely enters the aircraft's coordinates and the system performs the calculations since it has an internal clock calendar. Warm-up time and the time it takes for the INS to "sense" north can take from 2.5 to 45 minutes.
 This system computes for the pilot the following flight data:

• Track
• Drift angle
• Cross track error
• Distance traveled
• Distance remaining
• Flight time remaining

Global Positioning System (GPS)        GPS receivers cost thousands of dollars in 1990, but are available now for under $100 for simple hand held units. Aircraft GPS units designed for IFR flight still cost thousands of dollars each, but many General Aviation (GA) pilots now fly with a low cost hand held GPS receiver. The GPS system uses a constellation of 24 or more satellites, 21 plus spares, at an altitude of 10,900 miles, moving 7,500 nmph. Two UHF frequencies, 1.57542 gHz and 1.22760 gHz are used. Ionospheric distortion is measured by the phase shift between the two frequencies. Two modes are available, the "P", or precise mode, and the "C/A" or Coarse/Acquisition Mode. The P mode used by the military transmits a pseudo-random pattern at a rate of 10,230,000 bits/sec and takes a week to repeat. The C/A code is 10 times slower and repeats every millisecond. The GPS receiver synchronizes itself with the satellite code and measures the elapsed time since transmission by comparing the difference between the satellite code and the receiver code. The greater the difference, the greater the time since transmission. Knowing the time and the speed of light/radio, the distance can be calculated.




The timing comes from four atomic clocks on each satellite. The clocks are accurate to within 0.003 seconds per thousand years. The GPS satellites correct for receiver error, by updating the GPS receiver clock. The GPS satellite also transmits its position, its ephemeris, to the GPS receiver so it knows where it is relative to the satellite. Using information from four or more satellites the GPS receiver calculates latitude, longitude, and altitude. (The math involves matrix algebra and the solution of simultaneous equations with four unknowns. Computers do that sort of computation very well.)

GPS receivers provide all needed navigational information including:

• Bearing
• Range
• Track
• Ground speed
• Estimated time en route (ETE)
• Cross track error
• Track angle error
• Desired track
• Winds & drift angle


Differential GPS or DGPS
 
DGPS uses a ground station to correct the code received from the satellites for 5 meter accuracy. DGPS could be used for Precision approaches to any airport.

Most people think that once they’re on the ground, the flight is over. Believe it or not, many pilots feel this is the most dangerous and stressful part of the route. NASA continues work on a project called the Taxiway Navigation and Situation Awareness System (T-NASA) that will speed up ground operations and aid flight safety.

T-NASA blends Global Positioning Satellite (GPS) abilities with virtual reality technology to create displays that help pilots move around the airport quickly and safely. GPS pinpoints the exact location of the aircraft on the ground, and the T-NASA system displays it on a real-time moving map and a heads-up-display (called a “HUD”) in the cockpit.

The map shows the pilot the aircraft’s exact position, a cleared route, and any other traffic on the taxiways. When looking through the HUD, the pilot sees a virtual representation of the airport surface. The image is projected on a piece of glass (the HUD), but it looks like it’s actually part of the world. The pilot follows virtual cones that are along the edge of the taxiway showing the route that he or she is cleared to follow.

So far, the results with this system are extremely exciting. After a flight test in Atlanta, researchers found that the workload for pilots and controllers goes way down. They also found that the pilots were much more aware of their exact position and the position of other traffic on the surface. Current additions to T-NASA are aimed at adding alerts, so that pilots know when there are other aircraft that need to be avoided. All this translates into significant performance benefits and improved safety.

Whether it’s seeing through fog or booming 1,600 nautical miles an hour, new technologies are giving pilots a clear view of what’s ahead.

The inside of an aircraft using T-NASA technology. The moving map is shown on the bottom right screen, while the HUD display is shown in the top of the image.