DME, GPS and ILS

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Instrument

DME
Distance Measuring Equipment
Developed in the 50’s in Australia
Before digital displays, analog gauges were used

DME Specs
Can have an interlock with ILS
When the ILS shuts down DME shuts down
Battery backup of 4 hours
Coverage volumes are the same as for VOR
1000 watts for the H class
100 watts for the L class
However they may reach out up to 199 NM
Line of sight limitations still apply
Accuracy limits are +or- 1200 feet
Better than ½ mile or 3% of the distance
DME can handle 200 interrogators at a time
If there are more than this, the DME automatically desensitizes the receiver to reply to the stronger signals
DME is required at and above FL240 if using VOR for Nav (91.205)

DME’s Are Co-located
DME may be co-located with VOR, LOC, ILS, and NDB’s
The exception may be military where they may not be co-located
If with an NDB, a VOR frequency will be published

Tuning
Tuning is through the VOR or ILS frequency
Each DME frequency is paired to a specific VOR or ILS frequency
The radio is automatically set to the appropriate DME frequency
There are 200 such pairings
UHF frequency range 960-1215 MHz or L band (AIM reference)
Your transponder operates in the UHF band as well
The antenna therefore will be of approximately the same size
Look for the shark fin under the plane

How does it work?
When tuned, the aircraft transmits an interrogation signal to the DME facility
There is a 50 microsecond delay to eliminate uncoordinated operation when close to the station (see pic)
These are in form of a paired pulse RF signal
The ground receiver then sends a transponder signal back
The DME on board measures the time it takes for the signal to return
At a 186,000 fps it doesn’t take very long
In fact it’s about 12.359 microseconds per NM
After that it’s a rate x time = distance

How Does It Work?
So how does it tell your DME from others?
Your DME randomly jitters the spacing of its interrogations
This makes you unique
It looks for its signature in the replies that are being sent out by the ground station
When it finds a match it uses that data to do the calculation

DME Identification
The Morse code for the station in question is the same for the DME
The difference is it occurs every 30 seconds
It is also a higher pitch at 1350Hz
VOR is about every 10-12 seconds and is lower pitch at 1020Hz
So why bother with the ident on a DME if its paired to the VOR?
At high altitudes you may have the DME from a different station on the same freq
It happens
It is possible to have the VOR go down and not the DME

DME Identification
So how do you know if your ILS/LOC has DME?
DME may be used in lieu of the OM if approved

DME Errors
Groundspeed and time readouts are only accurate when going to or from the station
When doing an arc, it only measures your lateral movement on the arc
All distances are slant range
So if you cross the station at 6,000 feet your distance will be 1 NM
Negligible if 1 mile or more for each 1000 feet above the elevation of the station

GPS
First of all there are 3 systems out there
GPS made in the USA (30 sats)
GLONASS the Russians (24 sats)
Galileo the Europeans (30 sats)
The ICAO accepted GLONASS and GPS as the core for international navigation
When this happened some publications changed to refer to the system as GNSS
The DOD runs our system
6 orbital planes
60 degrees apart
11,000 miles out
Not in geosynchronous orbit – which means they move relative to positions on the ground
Each satellite circles the planet twice a day
GPS frequency is 1575.42 MHz

Great Circle Routes
The shortest course between two points on the surface of a sphere.
It lies in a plane that intersects the sphere’s center
To fly a great circle route the heading must change gradually along the course
GPS courses are great circle routes

GPS Components
Space
30 satellite constellation
24 in use plus 7 spares
There are always a minimum of 5 satellites in view
Control
Ground based networked monitoring and control stations
5 monitoring stations Cape Canaveral, Hawaii, Ascension Island, Diego Garcia, Kwajalein Island
3 ground antennas and a master station
Schriever AFB Colorado Springs is the master
User
IFR receivers must meet TSO C-129
They must be operated in accordance with the supplement
The GPS in the B’s do not, they are TSO C-115a
However you may use them to enhance situational awareness during IFR

Accuracy
Basic GPS is good to 25.5 feet 95% of the time
Each satellite carries 4 atomic clocks
These clocks are accurate to a billionth of a sec or nanosecond
A inaccuracy of 1/100th of a second translates into a error of 1860 miles

GPS Scale Sensitivity
There are 3 scale sensitivities
Enroute – +or-5 miles (1 mile per dot)
Defined as 30 NM or more straight line distance from the airport or heliport
GPS must be “armed” and within 30 NM for adjustment to terminal scale to occur
Terminal – +or-1 mile (.2 NM per dot)
From 30 NM to 2 NM outside the FAF
Approach – +or-.3 mile (about 360 feet per dot)
FAF inbound
At 2 NM GPS will shift to “active” indicating RAIM is good and the shift to approach scale
The scales increase sensitivity gradually
At 2 NM the GPS also checks for RAIM availability
If no RAIM is available the message “RAIM not available” will be displayed
At this time level off and fly to the MAWP
If you manually override the armed or active mode, the approach may not be continued
The GPS will not automatically sequence past the MAWP

GBAS
Ground Based Augmentation System
Formally known as LAAS
Ground based station that transmits a differential correction message to the receiver
23 NM radius, so located at or near the airport
Provides enough accuracy for Cat I, II and III
Accurate to less than a meter in horizontal and vertical axis
FAA has delayed purchase
However, you can buy one yourself
Newark and Houston have them
Oh and guess what – Boeing has their own right here in MWH
They have another at Charleston
The 787 can use them
Specially equipped 737s for United have them too

How Does It Work?
Each satellite transmits a continuous stream of data broken into 30 sec blocks
Each of these 30 second blocks include
Status of the transmitting satellite
Clock/time data
Ephemeris data (location of the satellite in the orbit)
Almanac data (describes the orbit)
Data is transmitted at 50 bits a sec
Data for position is in every 30 sec block
Almanac data is spread out over 25 frames taking 12.5 minutes to transmit
How Does It Work?
The receiver calculates the time difference between when the signal is sent and when it is received
This takes very accurate timing thus the need for atomic clocks
The time info is placed in codes broadcasted by the satellite
With the info from 3 satellites it is possible to determine location
With a 4th satellite the receiver avoids the need for an atomic clock
So with 4 satellites in view latitude, longitude, altitude and time are possible
With a 5th satellite error checking is possible which is needed for IFR operations (RAIM)

Trilateration
The receiver then has the distance from 3 satellites
So using 3 signals to calculate position the receiver could be at 1 of 2 points along intersecting planes
The receiver is smart enough to know that your not out in space and chooses the one closer to the surface of the earth

GBAS
Components on the ground
3 or more GPS antennas, computer, VHF data broadcaster
Airborne components
GPS antenna, VHF antenna, the box
The ground station measures the signal from the satellite, compares that to the actual ephemeris data and transmits the correction to the receiver
The signal needs correcting because the earth’s atmospheric density changes which slows down the signal slightly

WAAS
Wide Area Augmentation System
Wide area reference stations transmit data to the master station
The master station generates and transmits error correction to geostationary communications satellites
The satellites broadcast to the WAAS receiver in the plane
The info may also be used for position calculation
Unusable areas, system errors or other hazardous info is broadcast within 6 seconds to the user

WAAS
Can be used as Cat I minimums
Increases accuracy from 100 meters to 7
Designed to replace NDB, VOR, DME and Cat I ILS
Communications satellite with WAAS attached at right

RAIM
Receiver Autonomous Integrity Monitoring
Fault detection
Sometimes corrupt information may be received
5 satellites are needed for this feature
Alternatively 4 satellites and baro-aiding
Fault exclusion
Some receivers may continue with a bad satellite
6 satellites are needed for this feature
Alternatively 5 satellites and baro-aiding
Baro-aiding requires the current altimeter setting to be entered in the receiver
VFR and many hand held units do not have RAIM

Flying IFR With GPS
Must have alternate means of nav
Active monitoring of alternate nav is mandatory if RAIM capability is lost
If you want to do an approach the database must be current
Also the approach must be retrievable from the database
If RAIM Not Available message appears outside the FAF you may not continue the approach
The receiver performs a RAIM prediction prior to 2 miles outside the FAF
If the RAIM Not Available message appears after the FAF go missed immediately
Specs allow delay of annunciation for 5 minutes if RAIM is lost inside the FAF

Flying IFR With GPS
IFR En-route and Terminal ops
1. Determining the aircraft position over a DME fix. This includes en route operations at and above 24,000feet mean sea level (MSL) (FL 240) when using GPS for navigation.
2. Flying a DME arc.
3. Navigating TO/FROM an NDB/compass locator.
4. Determining the aircraft position over an NDB/compass locator.
5. Determining the aircraft position over a fix defined by an NDB/compass locator bearing crossing a VOR/LOC course.
6. Holding over an NDB/compass locator.

Flying IFR With GPS
GPS substitute for ADF or DME pg 9-27 IFH
1. This equipment must be installed in accordance with appropriate airworthiness installation requirements and operated within the provisions of the applicable POH/AFM or supplement.
2. The required integrity for these operations must be provided by at least en route RAIM or equivalent.
3. WPs, fixes, intersections, and facility locations to be used for these operations must be retrieved from the GPS airborne database. The database must be current. If the required positions cannot be retrieved from the airborne database, the substitution of GPS for ADF and/or DME is not authorized
4. Procedures must be established for use when RAIM outages are predicted or occur. This may require the flight to rely on other approved equipment or require the aircraft to be equipped with operational NDB and/or DME receivers. Otherwise, the flight must be rerouted, delayed, canceled, or conducted under VFR.
5. The CDI must be set to terminal sensitivity (1NM) when tracking GPS course guidance in the terminal area.
6. A non-GPS approach procedure must exist at the alternate airport when one is required. If the non-GPS approaches on which the pilot must rely require DME or ADF, the aircraft must be equipped with DME or ADF avionics as appropriate.
7. Charted requirements for ADF and/or DME can be met using the GPS system, except for use as the principal instrument approach navigation source.

ILS, SDF & LDA
Instrument Landing System
Simplified Directional Facility
Localizer-type Directional Aid

ILS
There are 4 basic components to the ILS
Localizer
Glideslope
Marker beacons
Approach lights
Optional components include
Compass locator
DME collocated with the glideslope transmitter
Localizer provides horizontal guidance
Glideslope provides vertical guidance
Most ILS provide a 3 degree glideslope
The max is 4 degrees, min is 2.5 degrees
Wenatchee has a 3.6 degree glideslope
Van Nuys has a 3.9 degree glideslope
London City EGLC has a glideslope of 5.5 degrees

ILS Categories
Cat I – 200 feet
Cat II – 100 feet
Cat III – No decision height
In order to Cat II and III special certification is required for pilot, ground equip, and airborne equip

Localizer Antenna
The localizer antenna is on the centerline at the departure end of the runway
Generates the front and back course
Good out to 18 NM and 4,500 AGL
Width of 5 degrees, so full scale at 2.5 degrees
ILS Coverage

Glideslope Antenna
Located 750-1250 feet down the runway from the approach end
400-600 feet laterally off the centerline
The glideslope is a basically a localizer turned on its side
Intersects the MM at about 200 feet and the OM at about 1400 feet nominally
The glideslope only radiates in the direction of the front course
Glidepath is about 1.4 degrees thick
At 10 miles it is 1500 feet thick to just a few feet at touchdown
Glideslope Antenna

ILS Identification
They all start with I
That’s 2 dots Morse code ..
IMWH is the id for here on a freq of 109.50
Some may include a voice feature
Outer Compass Locators are Identified with the first 2 letters e.g. Pelly is MW
Some approaches have inner Compass Locators they are identified with the last 2 lettters

Marker Beacons
There are 4 beacons
Outer marker
Located 4-7 miles out on the front course
Coded with a blue light
Morse code dashes
Middle marker
Located 3,500 feet out on the front course
Right about at where the DH for Cat I occurs
Coded with an amber light
Morse code dash dot
Inner marker
Located at the decision height for Cat II
Coded with a white light
Morse code dots
Back course marker
Located at the FAF for the back course
Most of the time it’s DME fix
They operate on a freq of 75 MHz
Transmit in a vertical cone
They get more “urgent” as you get closer to the threshold

Approach Lights
Used to transition from navaids to outside visual references
The rabbit, 2 trips per second
REIL
VASI
2, 3, 4, 6, 12, or 16
Check the front of your Approach Plate book
Then Google images

How Does It Work?
2 directional signals are sent by the localizer antenna, one modulated at 90 Hz and the other at 150 Hz
The receiver in the plane assigns a left or right deviation to each modulation
90 for left, 150 right
When both signals are equal, the cancel out causing the needle to center
If right of course the 150 Hz will exceed the 90 Hz and the needle will deflect to the left indicating direction for correction
When outside the area full scale deflection occurs and also missed approach

How Does It Work?
The glideslope works on the same basic principle
Basically a localizer turned on its side
The 150 Hz signal lobe is below the 90 Hz on top

How Does It Work?
How Does It Work?

The full picture
Pg 9-36 IFH
Note the yellow on the left and blue on the right

ILS Errors
1. Reflection.
Surface vehicles and even other aircraft flying below 5,000 feet above ground level (AGL) may disturb the signal for aircraft on the approach.
ATC will protect the ILS critical area when ceilings are below 800 and/or vis less than 2 miles
Additionally you may not operate in the critical area when ceilings are less than 200 and RVR less than 2,000 when an aircraft is inside the MM
2. False courses.
Produced at higher slopes
The angle of the lowest of these false courses occurs at approximately 9°– 12°.
You would see gyrations of both the GS needle and GS warning flag as the aircraft passed through the various false courses.
Getting established on one of these false courses results in either confusion (reversed GS needle indications) or in the need for a very high descent rate.
Do the approach at published altitudes
Check you altitude against the published altitude at the FAF

SDF
No glideslope
May not be aligned with the runway
Course may be wider at 6 or 12 degrees
Useable signal limited to 35 degrees off course centerline

LDA
More precise than the SDF
Similar to the localizer portion of an ILS
The LDA is not aligned with the runway
Straight in minimums may be published if within 30 degrees of the runway
Some do have glideslope

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