Navigation: Airborn Equipment Flashcards
Describe the doppler effect.
Doppler effect is the change of frequency between the transmitted and received signals, known as
Doppler shift, due to movement of the transmitter. This effect is present in radio waves and, in
particular, radar.
With a static transmitter and a static receiver, the received frequency is the same as the transmitted
frequency. If the transmitter is moving toward the receiver, more cycles are received every second;
i.e., the frequency increases as one moves toward the receiver. If the transmitter is moving away from
the receiver, fewer cycles are received every second; i.e., the frequency decreases as one moves
away from the receiver. The classic example of the Doppler effect is the change in pitch of a train’s
whistle as it passes a stationary observer.
What is an airborne Doppler system?
Doppler is a self-contained onboard radio/radar navigation system based on the Doppler effect
principle that operates in the 8.8- to 13.2-GHz frequency bands and is used to mathematically
calculate the ground speed of an aircraft
What is an INS/IRS?
An INS is an onboard self-contained inertia navigation system that can provide continuous
information on an aircraft’s position without any external assistance. An IRS (inertia reference
system) is a modern INS that usually has a greater integration into the flight management system
(FMS), and provides the aircraft’s actual magnetic position and heading information with reference to
the FMS required position and heading.
The directional acceleration information provided from the INS’s accelerometers and gyroscopes
is calculated by the position computer that determines the aircraft’s present latitude and longitude
position, provided a correct initial position has been input.
How does an INS/IRS work?
The general principle of all inertia navigation systems (INSs) is that the system measures the
aircraft’s inertia movement from an initial position as a great circle track direction and distance to
continuously determine its up-to-date position.
The components of an INS are
1. Accelerometers
2. Gyroscopes
3. Position computer
The aircraft moves in three dimensions, but the navigation equipment is only interested in
acceleration in the horizontal plane. Therefore, the key to the whole INS arrangement is the
accelerometers
How does an INS/IRS find true north?
An inertia navigation system (INS) is aligned to true north by its gyroscopes
How does an INS/IRS find magnetic north?
INS/IRS systems find magnetic north by applying a stored magnetic variation to the calculated true
north (see preceding question).
What are the advantages of an INS?
The advantages of an inertia navigation system (INS) are as follows:
1. It is a totally global system. It enables an aircraft to fly great circle tracks and to navigate
accurately across vast expanses of open sky where no ground base navigation aids are available,
e.g., the North Atlantic or the Pacific Ocean.
2. It is a completely self-contained system and therefore is free from external navigation aids and
atmospheric errors.
3. It is a very accurate system.
Note: Modern aircraft are fitted with two or more independent INSs, which allows them the
advantage that their positions can be compared for possible system errors.
4. INSs that employ ring laser gyroscopes have the following advantages:
a. Short warmup times
b. No real wander
c. No precession
d. Extreme accuracy
What are the disadvantages of an INS?
The disadvantage of an inertia navigation system (INS) is it errors, which can fall into one of three
categories:
1. Bounded errors, which are errors that do no keep increasing with time or increase and decrease in
a cycle. For example:
a. Schuler loop. Only common to a stable-platform INS that has been programmed to remain
horizontal as the aircraft moves around the surface of the earth. The error exists at the first
accelerometer level, i.e., acceleration, which can be passed up through the integration to affect
velocity and distance, resulting in a distance error during the Schuler loop cycle, but the error
returns to zero at the end of the cycle.
b. North alignment error. This will produce bounded velocity errors.
2. Unbounded errors, which are errors that continue to increase as time goes on. For example:
a. Initial position errors. Incorrect inputs into the system will create unbounded errors in velocity
and position.
b. North alignment error. This will produce serious unbounded errors in position.
3. Inherent system errors. For example:
a. The INS position computer makes no allowance for a distance between two points being greater
at height than on the surface because of the curvature of the earth. Fortunately, these differences
are not large.
b. The INS position computer also makes no allowance for the fact that the earth is not a true
sphere.
As a result, accumulated errors will cause the INS position at the end of the flight to be different
from the ramp position. The magnitude of this accumulated error is known as radial error rate and
should be checked at the end of each flight to determine if the INS is operating within its defined
limits of accuracy.
Describe GPS.
The Global Positioning System (GPS) is the U.S. Department of Defense (DoD) satellite system for
worldwide navigation.
The GPS constellation consists of a minimum of 24 satellites, of which 21 are operational at any
one time. This constellation of satellites is broken down into six orbital planes, each consisting of
three or four satellites. Each circular orbital plane is at 55 degrees to the equator, with the satellites
at a height of 20,200 km (10,900 nm) and covering an orbit of the earth every 12 hours.
Four satellites will always be in line-of-sight range of an aircraft receiver at any position on the
earth at any one time. The orbiting satellites transmit accurately timed radio signals, and the receiver equipment uses the time delay between transmission and reception to calculate its distance from the satellites. The distance measured from two satellites will establish a latitude and longitude fix. The distance measured from a third satellite will confirm the fix, and the fourth satellite will give altitude information.
Note: GLONASS is the Global Orbiting Navigation Satellite System operated by the
Commonwealth of Independent States (CIS), formerly the Soviet Union, and is similar to the U.S.
GPS.
What are the advantages of the GPS?
The advantages of the Global Positioning System (GPS) are as follows:
1. Truly global.
2. High-capacity use.
3. High-redundant satellite capacity system.
4. Built-in confirmation of the aircraft’s position from the third satellite.
5. Cornerstone of future air navigation systems, including Global Landing Systems (GLS), which
offers the advantage of a curved approach path and missed-approach guidance information at a
lower installation cost compared with a normal instrument landing system (ILS) facility. The first
GLS received Federal Aviation Administration (FAA) approval in 1997.
6. Ability to integrate GPS into other flight management systems.
7. Potential to be very accurate.
8. Ability to fly great circle tracks accurately.
9. Free (although rumors of imposed charges are often floated)
What are the disadvantages of the GPS?
The disadvantages of the Global Positioning System (GPS) for civilian use are twofold:
1. Downgrading of the system’s accuracy, i.e., selective availability
2. System errors
1. Selective availability. The potential accuracy of the system is deliberately downgraded for
civilian users by the U.S. Department of Defense for security reasons. This is known as selective
availability, but a better description is selective unavailability. However, as part of a 1996
presidential directive, President Clinton committed to discontinue selective availability in 2006,
but, in fact, the White House decided to shut it off 6 years ahead of schedule on May 1, 2000. This
move has allowed civilian GPS users to enjoy the same high accuracy as that enjoyed by the U.S.
military. It also will help the FAA in its commitment to adopting a GPS-based sole-means
navigation system for civil aviation in the United States.
2. System errors:
a. Clock bias
b. Satellite ephemeris
c. Ionosphere propagation
d. Instrument/receiver
e. Satellite geometry
f. Signal jamming
Therefore, the normal GPS is unsuitable for precision landing approaches while these poor
accuracy levels are present.
What is differential GPS?
Differential global positioning is a more accurate global positioning system (GPS) than a normal
GPS. It applies a correction factor called dif erential correction to eliminate the two most significant
errors in a normal GPS, namely, (1) selective availability (U.S. DoD downgrading of the normal GPS
system for civil users) and (2) ionospheric errors.
How does differential GPS work?
Global Positioning System (GPS) signals are received at a ground installation, which has been
surveyed accurately. The ground installation then computes the differences between the known
position and the position at which the GPS says it is (known as dif erential position error). It then
sends a differential correction factor to any aircraft within 70 nautical miles using an aircraft
communications and reporting system (ACARS) link that enables the aircraft’s onboard GPS
navigation computer to correct its own normal GPS-derived position into a refined differential GPS
position that is accurate to within 1 to 3 m.
Therefore, differential GPS has the potential accuracy to be suitable for precision landing
approaches, i.e., Global Landing Systems (GLSs).
How is an INS/IRS better than GPS, especially for navigation information?
An inertia navigation system/inertia reference system (INS/IRS) is a better system than the Global
Positioning System (GPS) for providing navigation information mainly because of the following:
1. The downgrading imposed on the normal GPS.
2. An INS/IRS is the only truly onboard self-contained system; therefore, it is not prone to external
influences, i.e., natural effects or transmitter errors such as those which affect the GPS. (See Q:
What are the disadvantages of the GPS? page 95.)
Therefore, the INS/IRS is a better navigation system because it is more self-contained and suffers
fewer errors than the GPS.
What is R Nav?
R Nav is a form of onboard area navigation aircraft equipment that uses either a basic VOR/DME
system or other position sensors, such as DME/DME, INS/IRS, Omega/VLF, or Loran C.
A simple area navigation system allows the operator to input the bearing and distance of a
geographic location with reference to a given station (entered as VOR/DME bearing and range or an
INS/IRS latitude and longitude) to position a waypoint, thus making the waypoint a specified
geographic location. A series of waypoints is used to define an area navigation route or the flight path
of the aircraft.
Note: A waypoint is sometimes likened to a phantom station because it provides the R Nav user
with the same navigation information that a real VOR/DME installation would provide.
Thereafter, the pilot has access to deviation information from the track between waypoints by
means of displayed information on the primary navigation instrument and distance to go from the
DME reading.
The sophistication of the equipment determines the amount of information displayed and the ability
of the system to automatically select and deselect the primary and alternative ground base navigation
aids. The advantage of the system allows for more direct routing, i.e., not restricted to established
airways, which is more efficient.
