Navigation Systems (Part Two)

NAVAIDS

NAVAIDS are easily added to a computer system. Very high frequency (VHF) omnidirectional range (VOR) or tactical air navigation system (TACAN) bearings and distance measuring equipment (DME) provide the same information as a radar fix. The computer needs the location and frequency of the transmitter, which can be programmed into the computer before the flight begins. Some corrections must be applied to bearing data. The computer must correct for magnetic variation and slant range from the station to the aircraft.

 

Pressure Altimeter

Pressure altimeter data is an input to the true airspeed computations. Additionally, it can be used with temperature data to compute true altitude.

Radar

When a ground mapping radar is incorporated into the navigation system, present position can be corrected based on the measurements to surveyed radar returns. The operator identifies radar returns on his radar scope and measures the range and bearing to the return. The operator determines the aircraft position relative to the return and updates the aircraft position. Automatic systems allows the operator to pre-load the coordinates of radar returns in a database, place a movable electronic cursor (or crosshairs) on the return, and push a button to update the system. The computer determines the distance and bearing from the aircraft to the set coordinates. The computer then generates the cursor on the radarscope at the calculated range and bearing. If there is any error in the navigation system position, the cursor will not fall on the radar return. The operator adjusts the cursor or crosshairs onto the radar return. The operator pushes a button to automatically update the system.

Temperature Sensors

The air data computer uses the information collected by temperature sensors. Temperature gradients can be used with pressure altimeter data to compute true altitude.

True Airspeed

True airspeed can be calculated from indicated airspeed, temperature, and pressure. True airspeed and winds can be used as a backup for cross-checking groundspeed.

Independent Systems

INS and GPS can also act as sensors for a navigation system. They are discussed in greater detail later in this category.

 

Determining Position

The ever-present problem facing the navigator is determining aircraft position. With a navigation system, this problem is solved because the computer converts input data into a constantly updated present position for the aircraft. Advanced systems provide altitude, attitude, heading, and velocity information.

The mathematics of navigation over the surface of a sphere has been known for several centuries. Starting from an initial position, the computer determines the distance and direction traveled since starting navigation. Aircraft direction, or track, may be supplied by INS, GPS, or the heading reference system in combination with doppler drift. Groundspeed may come from INS, GPS, doppler groundspeed, or may be determined from any NAVAID capable of range and bearing fixes. The computer multiplies speed against time interval to determine distance traveled. Distance is projected along the aircraft track to obtain the new position. Track and speed are sampled and present position is updated many times per second. Waypoint navigation is a simple addition to the navigation computer. A database of coordinates can be added to the system to determine distance to go and estimated time of arrival (ETA). If the aircraft changes speed, the ETA is automatically updated using the new groundspeed.

Decision Algorithm

Simple navigation systems determine position as described above. The operator updates the position for errors that will eventually occur. More complex systems have additional problems. When a system has a variety of sources that provides redundant information, how does the computer decide which source to use? What if the sensors are subject to errors? What if the operator inputs an inaccurate update to the system? How can we get a computer to make simple decisions once left to the navigator? Can we program a computer to analyze and correct for the predictable and unpredictable errors in sensor data? Bias in the accuracy and variability of data are two types of error that navigation systems actually experience and can be solved with the use of statistical software called decision algorithms.

To compensate for these predictable and unpredictable errors in sensor data, we can include statistical measuring software that weigh the accuracy of each data source and the accuracy of the data itself. These programs determine the most likely value for track and velocity in order to compute the most likely present position.

One type of program used to determine the most likely sensor values is called a Kalman filter. Kalman filters are used extensively in computer controlled communications, electronics, and equipment. When used as part of a navigation system, a Kalman filter computes the most likely position of the aircraft and updates the weighing factors with each new position update. The Kalman filter compares the actual sensor data used prior to the update with the data from the update. By comparing the first position with the second position, actual distance and heading can be determined. It then determines the amount of error in the original data and estimates a correction to the data for the next time period. The Kalman filter is an iterative program requiring several updates prior to achieving completely reliable data. If used, the Kalman filter will also be used to evaluate the reliability of operator inputs and weigh how much of each position update to accept. Kalman filtering provides increased reliability in navigation systems so an operator can trust that the information used is valid. Kalman filters protect the operator from inaccurate sensor data and even operator error.