Grnd-53 Theory of Flight 6 Navigation Systems

GPS systems provide what?

Highly accurate position, velocity, and time information.

GPS Segments

Control segment, space segment, user segment

Full constellation of SV

24

Minimum, maximum, and typical SV in view

4, 12, 7-9

Control segment

Ground based portion of the system that manages and controls the space segment

Monitor stations

Passively track and monitor SVs

Transmits data to MCS for processing

Master Control MCS

Processes SV data

Uploads data

Repair some SV problems

Makes system adjustments

User segment

GPS is used in aviation, marine navigation, surveying, vehicle tracking and exploring

Main types of GPS receivers in aviation

Portable GPS units

Panel mounted

Flight Management Systems (FMS)

Receiver/computer of vehicle using GPS consists of

Antenna

signal processing equipment

computer

oscillator

How does GPS determine position?

Uses ranging and triangulation to determine position.

In aviation, receivers calculate position constantly and, with rapid changes in position, also calculates velocity

Theory of operation

GPS triangulates by obtaining information from a number of SVs. IN order to accomplish this is the receiver must know how long it took the signal to reach receiver and how fast the signal was travelling. This will enable a simple range calculation. Finally, to get a position fix, the receiver must also know the orbital position of the SV transmitting signal

The user’s receiver has the position of each satellite store in its memory and receives precise orbital location in ephemeris data.

By using a common time reference, the receiver measures how long it takes for a coded signal to travel from the satellite to the receiver. Since the speed of propagation is known, the distance can be calculated. By measuring the distance from a variety of satellites, the receiver can accurately determine its position.

Minimum SVs needed for 2D and 3D information

2 SV for 2D, 4 SV for 3D

Satellite geometry

SV position in relation to the receiver can affect accuracy. If all satellites are overhead accuracy may suffer.

Standard positioning service accuracy

Civil users and some military, 60 to 100m error

Precise Positioning Service

Military, 16m accuracy

GPS Characteristics

High accuracy 3D location

Velocity, vector, and time

Global coverage, continuous availability

Passive service, unlimited number of users

Somewhat resistant to interference and jamming

Inertial Navigation Systems

Self-contained systems that operate independently from navigation signals from any external source.

What does INS measure?

Measures acceleration (inertia) in order to produce displacement from a known reference position

5 Basic Components of INS

Accelerometers, stable platform, integrators, computer, control display unit

INS accelerometers

Measure forces in three axes to indicate changers in velocity, vertical and heading accelerations applying Newton’s laws of inertia and opposing force.

INS integrators

Integrates measured accelerations with time to get useful information.

Acceleration/time = velocity

velocity/time = distance

INS stable platform

Direction must be known and maintained. Gyro-stabilised platform maintains accelerometer reference to a known coordinate system (align)

INS computer

Compensates for unwanted accelerations due to gravity, coriolis, and centripetal force.

Processes integrator and platform outputs in order to produce position and attitude data.

INS operation

Power applied and INS activated

Gyros spin up and platform is aligned with the aircraft’s attitude

Alignment position is entered either manually or from another system

Establishes a base heading reference

INS errors

Initial leveling

accelerometers

integration errors

initial azimuth misalignment

leveling gyro drift

total position error

Airborne Doppler Radar

A self contained unit which can provide accurate measurement of an aircraft’s drift and groundspeed

Airborne Doppler Radar Operation

A narrow beam of radar energy is transmitted towards the ground. The relative motion between the aircraft and the ground produces a doppler shift between the transmitted and received frequencies

Doppler Components

Receiver/transmitter

Signal Data Comparator (SDC)

Remote control

Indicator

Antenna Assembley

Doppler Receiver/Transmitter

Transmits, receives and extracts doppler frequency shift and passes it to the SDC

Doppler SDC

Measures doppler shift, supplies antenna position and G/S indicator signals, monitors system

Doppler Remote Control

System operation, includes self test, memory light, and malfunction indicators

Doppler indicator

Drift angle needle, groundspeed, memory lights, off flags

Doppler antenna assembley

Tx/Rx antenna, fixed or electro-mechanically rotated, gyro stabilised, 4 beam janus pattern

Determining drift with 4 beam Janus Pattern

No drift: doppler drift same in beam 1 and 2

When aricraft drifts, one of the beams will detect a higher GS

To indicate drift angle assembley will mechanically rotate to match GS in beams 1&2 and measure angle, or in a fixed assembly beams 1&2 are mathematically compared and DA computed.

Beams 3 and 4 are integrated to reduce inherent errors

Doppler Accuracy and Limitations

Accurate at all altitudes (40 - 50,000’)

Typically ±1/4degree and ±0.5% GS

accurate over land, polar ice cap and rough seas- good returns (land mode)

Smooth seas causes loss of return which requires increased depression angle (sea mode)- results in less accurate information.