# ATAC203 Avionics Systems
> # [[C203 Week 3| ◀️ ]] [[C203 Home| Home ]] [[C203 Week 5| ▶️ ]] [[QR C203T SSGW04| 🌐 ]] [📝](https://excalidraw.com/)
> # [[C203 Week 4#ATAC203 Avionics Systems|Week 4]]
>- [[C203 Week 4#Distance Measuring Equipment DME|Distance Measuring Equipment DME]]
>- [[C203 Week 4#Automatic Direction Finding ADF|Automatic Direction Finding ADF]]
>- [[C203 Week 4#Global Positioning System GPS|Global Positioning System GPS]]
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>[!jbPlus|c-blue]- Lesson Intro
>### What
>
In this lesson we continue to look at [[Avionics|avionics systems]], using the knowledge we have already gained in previous lessons. We will look at [[DME|Distance Measuring Equipment]], [[ADF|Automatic Direction Finding]], and [[GPS]].
>
>### Why
>
These systems are critical to safe flight, and your knowledge of them will serve you well in your career.
>
>### Testing
>
You will be tested on this material on Graded Quiz 3 and the Final Test.
>
>### Approach and Objectives
>
By understanding the following topics, you will have achieved the learning outcome for this lesson. Consult your course outline for the learning outcomes and other details of this course.
>
>- CLO 9. Explain principles of [[DME]]
>- CLO 10. Explain principles of [[ADF]]
>- CLO 11. Explain principles of [[GPS]]
## Distance Measuring Equipment (DME)
### DME Purpose and Description
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>[!aside]- Ref
>[[DME|🗺️]]
Distance Measuring Equipment ([[DME]]) provides slant range information to the pilot and other equipment. Range means distance, and slant refers to the fact that this does not mean distance over the ground, but rather on a line between the ground station and the aircraft.
[[Pasted image 20201229225723.png|➡]]![[Pasted image 20201229225723.png|350]]
As shown in the above simple diagram, the ground station is a [[VORTAC]] ground station. DME may also use [[VOR]] or [[Localizer]] stations for reference.
The aircraft sends a signal to the [[transponder]] on the ground, which is immediately processed and returned to the aircraft. The timing of this exchange is used to determine Groundspeed/Time-to-Station (GS/T) information in the form of displays which show [[distance]], [[groundspeed]] and time-to-station.
### DME Components - Ground
#### Antenna%%1%%
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- Often shared with [[VOR]] or [[ILS]]
DME is typically co-located with a VOR or ILS station. In the following illustrations we see a combination DME and VOR array, and in the second, a standalone DME [[transponder]] and [[antenna]].
[[Pasted image 20201229225919.png|➡]]![[Pasted image 20201229225919.png|350]]
[[Pasted image 20201229225939.png|➡]]![[Pasted image 20201229225939.png|200]]
#### Transponder
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The ground station is a [[transponder]]. This means it is a device for receiving a radio signal and automatically transmitting a different signal. You can remember this term by understanding the parts of the word:
- "Trans": transmit, and
- "ponder": responder.
This type of word that is made up of two other words is called a portmanteau. A common example is email, which is a portmanteau of the words electronic and mail. You will encounter several of these in the field of aviation.
### DME Components - Onboard
#### Transceiver
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Onboard the aircraft is a [[transceiver]] called an [[Interrogator]]. The term transceiver derives from these words:
- "Trans": transmitter, and
- "ceiver": receiver.
It is a [[radio]] that can both [[radio transmission|transmit]] and [[receive]], and it is this capability that allows for the interrogation function. The aircraft sends an interrogation signal which is correctly responded to by the transponder on the ground. This response is evaluated and then processed to provide the required information.
##### Small aircraft
Many small aircraft have a combination [[DME]] transceiver/display. Some control heads include the DME distance display, usually indicated as [[NM]] (nautical miles).
[[Pasted image 20201229230347.png|➡]]![[Pasted image 20201229230347.png|350]]
[[Pasted image 20201229230355.png|➡]]![[Pasted image 20201229230355.png|350]]
##### Large Aircraft
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Many larger aircraft are equipped with a DME display but with a remote transceiver located elsewhere in the aircraft, often in an area dedicated to [[avionics]]. This display can be quite small, as frequency selection is performed automatically when a [[VOR]] or [[ILS]] [[frequency]] is selected and therefore no separate controls are required in the [[cockpit]].
[[Pasted image 20201229230535.png|➡]]![[Pasted image 20201229230535.png|350]]
#### Control Panel
[[Pasted image 20201229230635.png|➡]]![[Pasted image 20201229230635.png|350]]
#### Antenna
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[[Pasted image 20201229230705.png|➡]]![[Pasted image 20201229230705.png|350]]
Outside of the aircraft, the same [[antenna]], which is approximately 3 inches long, is used for both [[radio transmission|transmit]] (TX) and [[RX]] ([[receiver|receive]] ) functions.
[[Pasted image 20201229230831.png|➡]]![[Pasted image 20201229230831.png|350]]
[[Pasted image 20201229230857.png|➡]]![[Pasted image 20201229230857.png|350]]
[[Pasted image 20201229230904.png|➡]]![[Pasted image 20201229230904.png|350]]
[[Pasted image 20201229230919.png|➡]]![[Pasted image 20201229230919.png|350]]
#### Suppressor Cable
One further component of interest to the technician is the Suppressor Cable to the [[ATC Transponder]]. Because the onboard ATC transponder works in the same frequency band, this suppression cable prevents these two systems transmitting at the same time which could cause interference.
### DME Theory of Operation
In the [[VOR]] section we learned that when VOR is used with DME, we can get positional fixes. We also learned that DME stations are collocated with VOR stations. Before we move on, let's clarify that these are two separate systems. The information provided by them is useful in combination, but technically, VOR does nothing with DME. DME is completely independent piece of equipment that was introduced later than VOR. The only relation is that there is a standard correspondence between VOR and DME channels and the DME receiver automatically tunes to the DME frequency corresponding to the VOR or [[ILS]] frequency tuned on your VOR/ILS receiver.
[[Pasted image 20201229231031.png|➡]]![[Pasted image 20201229231031.png|350]]
#### Jitter
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The aircraft interrogates the ground transponder with a series of pulse pairs whose timing is specific to the aircraft. These unique patterns of pulses are called [[jitter]].
[[ATAC203i070 1.png|➡]]![[ATAC203i070 1.png|350]]
The ground station replies with an identical sequence of reply pulse-pairs so that the aircraft [[transceiver]] can distinguish its interrogation replies from those of other nearby aircraft. These reply pulses will be 63 MHZ higher or lower than interrogation pulses depending on whether the station is designated X or Y as in the table below:
VHF Freq | DME [[radio transmission|TX]] | DME RX | DME MODE
---|---|---|---
108.00|1041|978|X
108.05|1041|1104|Y
"X" channel [[interrogations]] have a pulse repetition frequency ([[PRF]]) of 12 microseconds and "Y" channels PRF is 36 microseconds.
DME measures the time it takes to send a signal to the [[transponder]] and receive a reply. As we know, radio waves travel at the speed of light, that is, 299 792 458 m/s. (This is over a billion kilometres an hour) You can verify this math yourself, and confirm that this means it takes 5.368 microseconds for a radio wave to travel one mile. By dividing the time it takes for the interrogation to reach the station and return by two, we can determine the distance that the RF travelled in that time.
However, there is a variable that must be addressed. The transponder must do some processing to shift the response frequency and generate the exact pulse it received, and the time to do this may vary. For this reason, a standard delay of 50 microseconds is made at the transponder to allow sufficient time for this processing. The aircraft transceiver takes this into account when calculating the distance.
[[Pasted image 20201229231739.png|➡]]![[Pasted image 20201229231739.png|350]]
Initially the DME goes into "Search Mode" with a Pulse Repetition Frequency (PRF) of 40-150. The DME ground station receives these pulses, holds them for 50 microseconds, then transmits them back, at a frequency +/-63 MHZ. Once the DME interrogator begins to recognize its own replies it enters "TRACK Mode" (PRF=10-30)
#### Squitter
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The aircraft transceiver will not [[radio transmission|transmit]] interrogation signals until it detects the presence of a DME station. It does this by listening for response pulses from other aircraft. However, if our aircraft was the only one in the vicinity, there would be no responses. To alert the aircraft to its presence, the DME ground transponder will send out random pulses called [[squitter]] when not occupied responding to real interrogations. The use of squitter makes the station visible to DME receivers at all times, and allows for a predictable [[duty cycle]] for the [[transmitter]].
#### Useable Range
The frequencies involved mean that DME radio waves employ line of sight propagation mainly determined by aircraft altitude. In other words, the higher the aircraft is, the farther it can "see".
#### Ident Tone
The DME stations transmits its [[Morse Code]] [[Ident]] Tone at a frequency of 1350Hz. This is done every 30 seconds. Remember that the VOR/LOC tone is 1020Hz.
DME "Memory" mode will hold the last reading or velocity rate for 10 seconds, in order to prevent a return to "Search" after an identification pulse is transmitted.
#### Hold Mode
When DME "Hold" is selected, a new VOR/ILS frequency may be selected, but the DME continues to track the previous DME ground station.
#### Navigating with DME
By measuring the distance (rho) to more than one DME station, a [[position fix]] can be determined
Two DME readings (rho-rho) will often provide a position fix, however 3 readings (rho-rho-rho) are required to truly remove ambiguity.
Don't worry about the term "rho". It is just the technical term. It is just important that you understand that if you can measure the distance to more than one DME station, you can determine where you are on the map.
#### DME/VOR Navigation
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The addition of VOR bearing information provide a Rho-theta (range and bearing) information, and allow the establishment of a position fix.
### DME Indications
[[Pasted image 20201229232157.png|➡]]![[Pasted image 20201229232157.png|350]]
#### Normal
VOR : indication up to 199 NM will be given by the station. Localizer: up to 50 [[NM]].
#### Override
[[Localizer]] indication up to 199NM will be given by the station.
A little explanation about these two modes. In normal mode, the range of the DME is limited so that there is less chance of interference from other, more distant DME stations. This is important in areas with many [[Navigational Aid|nav aids]] , such as western Europe and the United States. At higher altitudes, or in less crowded areas, such as over oceans, using the override mode allows the reception of more distant DME stations.
Modern DME systems are effectively always in override mode, using other technology to reduce interference.
#### Test
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The indicator will display 000.0.
Indication may be on RMI, HSI, or independent DME indicator.
[[Pasted image 20201229232351.png|➡]]![[Pasted image 20201229232351.png|350]]
#### Distance in NM
#### Warning/Fail Flag
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[[Pasted image 20201229232450.png|➡]]![[Pasted image 20201229232450.png|350]]
Note that DME does not measure distance across the ground. It measures the distance from the aircraft to the [[transponder]], and thus is an angular measurement. DME is most accurate when the angle is low, that is, it more closely matches the actual distance on the ground.
DME is least accurate directly over a transponder. In the article that follows this section, you will be asked to consider what the reading of the DME would be in this scenario. Here is the question: If an aircraft is directly overhead its selected DME station, at an altitude of approximately 6000 feet, what would its DME indicate? I would like you to email your prof the answer. Really. I'm not joking. Send me an email with the answer to the question. Why are you not sending me an email? Huh? Is this thing on?
### DME Frequencies
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- 978 MHz to 1213 MHz (UHF)
- Plus/minus 63 MHz
The [[frequency]] range of DME is 978-1213 MHz, which is in the UHF band. DME frequency is automatically selected by association with paired [[Localizer]] frequencies.
Remember that the response from the ground transponder is shifted up or down 63 MHz.
### The Future of DME
DME operation will continue and possibly expand as an alternate [[Navigation|navigation]] source to space-based navigational systems such as GPS or Galileo.
In 2020 one company presented its 'Fifth-Generation DME'. Although compatible with existing equipment, this iteration provides greater accuracy (down to 5 meters using DME/DME [[triangulation]]), with a further reduction to 3 meters using a further refinement. The 3-meter equipment is being considered as part of Europe's SESAR project, with possible deployment by 2023.
In the twenty-first century, aerial navigation has become increasingly reliant on satellite guidance. However, ground-based navigation will continue, for three reasons:
- The satellite signal is extremely weak, can be spoofed, and is not always available;
- A European Union rule requires member states to keep and maintain ground-based navigation aids;
- A feeling of sovereignty, or control over a state's own navigational means. Some states want navigation over their territory to rely on means they control. And not every country has its constellation like the U.S.' GPS or Europe's Galileo.
## Automatic Direction Finding ([[ADF]])
>[!aside]- Ref
>[[ADF|🗺️]]
### ADF Purpose and Description
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- Radio Navigation System
- Needle points to the station
The concept of ADF systems is quite simple. The pilot tunes to a radio station, and a needle on an indicator points to that station. Here is a slightly more technical description: ADF is an [[electronic]] [[navigation aid]] that identifies a [[relative bearing]] of an aircraft from a radio beacon, such as an [[NDB]] or a commercial radio station.
#### Used for station to station navigation
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[[Pasted image 20201229233414.png|➡]]![[Pasted image 20201229233414.png|350]]
So, the ADF is a radio navigation aid, and by using a succession of radio beacons or stations, the pilot can navigate from station to station.
The triangulation of the direction to two stations provides the pilot with a [[positional fix]]. A fix is simply a point in space that establishes the aircraft's current position by referring to external references.
[[Pasted image 20201229233421.png|➡]]![[Pasted image 20201229233421.png|350]]
If a pilot flies directly to the radio beacon or station, that is, he ensures the ADF needle is pointing straight ahead, he is [[homing]] to the station.
### ADF Components - Ground
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ADF requires an [[NDB]] (non directional beacon) or radio station transmitting in the [[MF]] band. NDBs may also broadcast in the [[LF]] band. NDBs are simply a radio [[transmitter]] at a known location that can be used as a navigation aid. It is called non directional because it offers no explicit directional information like [[VOR]].
Remember that AM radio broadcasts in the range of 525KHz to 1705KHz in North America, and the entire [[MF]] band goes from 300KHz to 3 MHz. So, the [[AM band]] is part of the MF range.
### ADF Components - Onboard
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The ADF onboard system is composed of an [[antenna]], a [[receiver]], a [[control head]] and an [[indicator]].
[[Pasted image 20201229233553.png|➡]]![[Pasted image 20201229233553.png|350]]
For some smaller aircraft in the general aviation category, the receiver, control head and indicator may be combined in a panel mounted unit.
In some antenna units, both the loop and sense antenna are combined into one antenna cover. Because of the unique nature of the antenna, it will be covered more thoroughly in the theory of operation section.
[[Pasted image 20201229233611.png|➡]]![[Pasted image 20201229233611.png|350]]
### ADF Theory of Operation
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The technology that makes ADF possible is the properties of an antenna that is formed into a loop. We know that current is induced in an antenna when [[RF waves]] intersect it. When formed into a loop, the antenna picks up the wave differently depending on how it is oriented to the wave.
[[Pasted image 20201229233800.png|➡]]![[Pasted image 20201229233800.png|350]]
In the above diagram, the identical wave hits both sides of the loop at the same time. The identical waves, separated by 180 degrees of loop are out of phase each other, and therefore cancel each other out. This is known as the [[null position]].
As the antenna rotates, the wave hits the two sides of the loop at different times, therefore adjusting the [[phase]] difference between them. The combined waves do not cancel each other out, but reinforce each other according to the phase difference.
So, the strength of the output signal from the loop antenna depends on the angle of the plane of the loop.
When in the null position, the loop plane is [[perpendicular]] to the direction of travel of the waves, and can therefore be used to point to the radio station.
Note however that there are two nulls, that is, two positions of the antenna where its plane is perpendicular to the radio station. In order to resolve this ambiguity, a sense antenna which is [[omnidirectional]] is also used to receive signal from the radio station. When its phase agrees with the phase of the loop antenna null, it's phase is compared and used to drive the needle. If it is out of phase (the other side of the loop), a signal is sent to a motor to turn the antenna 180 degrees to indicate towards the station correctly.
Note that there are two ways to describe this angular representation. The null position is when the plane of the loop is *parallel to the waves* being broadcast, or, *perpendicular to the direction of travel of the waves* being broadcast. You may see this described both ways, but do not be confused.
Look at this image closely and verify the wording used to describe the position of the antenna:
[[Pasted image 20201229234013.png|➡]]![[Pasted image 20201229234013.png|350]]
In ADF mode, the ADF will hunt for the station, and the sense antenna will resolve the ambiguity of the two nulls of the loop antenna to drive the indicator correctly. Don't worry about learning this [[block diagram]], but notice that most of the blocks of circuitry are already familiar to you. You should not be surprised at this point that a radio navigation system has radio circuitry blocks in it. You can also deduce that the receiver uses [[superheterodyne]] circuitry, as evidenced by the LO block.
[[Pasted image 20201229234057.png|➡]]![[Pasted image 20201229234057.png|350]]
In ANT mode, only the sense antenna is used, and the receiver operates as a simple [[AM]] [[receiver]].
[[Pasted image 20201229234140.png|➡]]![[Pasted image 20201229234140.png|350]]
The above block diagram should be familiar to you. This is an example of a [[superheterodyne]] receiver in action, and zooming in to this level should look familiar to you (if you've done the Mandatory Project).
[[Pasted image 20201229234201.png|➡]]![[Pasted image 20201229234201.png|350]]
More modern Fixed Loop ADF's use a [[Goniometer]] in the ADF receiver. The goniometer is synchronized to the pointer in the instrument:
[[Pasted image 20201229234208.png|➡]]![[Pasted image 20201229234208.png|350]]
### ADF Indications
#### ANT Mode
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In ANT mode, the loop antenna is disabled, and the only reception is through the sense antenna. This mode provides the clearest audio reception is and is useful for listening to the identification of a [[beacon]], or listening to the audio broadcast of a radio station.
#### Parking
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In ANT mode, there is no navigational aid happening. It is simply a radio receiver. To indicate to the pilot that the indication for ADF is not valid (because the loop antenna is disabled), many systems will park the needle at the 3 O'Clock position, or at 90 degrees on the indicator.
#### ADF Mode
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In ADF mode, the pointer on the indicator is activated and the ADF points to the station the receiver is tuned to.
[[Pasted image 20201229234435.png|➡]]![[Pasted image 20201229234435.png|350]]
In this example the [[RMI]] display consists of a dial upon which the azimuth is printed, and a needle which rotates around the dial and points to the station to which the receiver is tuned.
On this model, the ADF is indicated by the green pointer, and the VOR/GPS is indicated by the yellow pointer. If both needles are used for ADF, a fix can be determined.
#### BFO
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One other form of identification for ADF is the [[BFO]]. A [[Beat Frequency]] Oscillator will modify the carrier by interrupting it at a frequency that is audible. This technique is call interrupted carrier keying, and is rarely seen in North America except for some marine beacons. However, because of the low costs of this approach, it is often seen elsewhere in the world. Sometimes a separate BFO setting is available to the pilot.
### ADF Frequencies
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- 190KHz to 2000KHz
ADF Receivers operate in the range of 190KHz to 2000 KHz.
### ADF Problems and Limitations
#### Coastal Effect
When the ground wave from the ADF [[transmitter]] reaches a coastline, it will change direction slightly. This is similar to the effect of light refracting as it travels from one medium to another. This can cause inaccuracies in the needle position in regard to the position of the station.
#### Night Effect
The ADF uses a ground wave, which follows the contour of the earth. However, a portion of this signal acts as a [[skywave]], and is reflected off the ionosphere, back to earth. An ADF receiver picking up these signals (most pronounced at dusk and dawn) will experience some hunting on the ADF needle.
#### Quadrantal Error
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Electromagnetic fields around the four quadrants (hence the term [[quadrantal error]]) of the airframe can result in errors being introduced into the ADF system. Much like the interference that requires a [[deviation card]] for the compass, ferrous materials and [[electromagnetism]] can cause deviations in the ADF system. These are sometimes referred to as hard iron effects, and can be minimised by use of a Quadrantal Error Corrector.
#### "P" Static
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<!--SR:!2023-02-24,1,230-->
The ADF is particularly prone to noise interference. Poor electrical bonding, inefficient [[static wick]]s, and electrical devices such as [[motor]]s and [[generator]]s will cause noisy reception on the ADF. [[Aircraft Precipitation static]] (Aircraft P-Static) is a term used to describe this interfering noise resulting from the redistribution of charge on an operating aircraft. As an aircraft moves through the air, it acquires charge until sufficient voltage levels are reached to initiate a discharge. This discharge may occur between different parts of the aircraft or between the aircraft itself and the external environment. The noise or "static" resulting from these discharges can cause interference to sensitive aviation equipment resulting in the possible loss of communicate or navigation capabilities for several minutes. This is particularly noticeable on ADF systems.
[[Pasted image 20201229234902.png|➡]]![[Pasted image 20201229234902.png|350]]
[[Pasted image 20201229234907.png|➡]]![[Pasted image 20201229234907.png|350]]
Static wicks and good electrical bonding prevent problems associated with "P" Static
## Global Positioning System GPS
>[!aside]- Ref
>[[GPS|🗺️]]
### GPS Purpose and Description
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- Satellite Based Navigation System
The Global Positioning System (GPS) is a [[satellite]]-based navigation system. Originally developed and intended for military applications, it has been available for civilian use since the 1980s.
#### Triangulation gives exact global position
[[Triangulation]] is used by GPS to calculate a user's exact position on the planet. Once position is determined, other information such as [[speed]], [[bearing]], [[track]], trip [[distance]], distance to destination and more can be calculated and sent to onboard systems and displays in the [[cockpit]].
#### Unparalleled accuracy
GPS is unparalleled for accuracy, currently around 1 ft.
### GPS Components - Ground
- None
### GPS Components - Space
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The GPS system began with 24 satellites orbiting the earth in six orbital paths, each containing four satellites.
[[Pasted image 20201229235652.png|➡]]![[Pasted image 20201229235652.png|350]]
There are more than 24 satellites in current use, with several orbiting as spares.
#### GPS Components - Onboard
Aviation GPS can be divided into these three categories:
#### Handheld
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Handheld GPS units are usually the cheapest, and have the advantage of portability between aircraft. Accuracy is the same as more expensive units, but less information is presented to the pilot.
#### Panel Mounted
Panel Mounted GPS units usually allow for a larger display with more information available to the pilot, often including a moving map.
#### Integrated
A fully integrated GPS system can allow for a large viewing area and enables integration with other systems.
#### GPS Antenna
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Since the GPS communicates with satellites, its location is almost always on the top of the [[fuselage]] of an aircraft. Because its size and sometimes shape is similar, the GPS antenna can be mistaken for ADF antennas.
[[Pasted image 20201229235843.png|➡]]![[Pasted image 20201229235843.png|350]]
### GPS Theory of Operation
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- Satellite Transmitters
- [[Atomic Clock]]
#### Onboard receiver calculations
##### Based on time delay
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Each satellite transmits complex radio signals that are received by the GPS receiver on board the aircraft. Each radio signal declares its time based on atomic clocks aboard the satellite, and the receiver makes its calculations based on the time delay.
The arrangement of 24 satellites in 6 orbital planes allows for a number of satellites to be visible to an aircraft receiver at any given time.
Included in the data sent by the satellite by radio is [[ephemeris data]] which is is information specific to each satellite and is used to ensure accuracy. The GPS signal gives the precise "time-of-week" according to the satellite's onboard atomic clock, the GPS week number and a health report for the satellite so that it can be discounted if faulty. Each [[radio transmission|transmission]] lasts 30 seconds and carries 1500 bits of encrypted data. This small amount of data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. GPS receivers know the PRN codes for each satellite and so can not only decode the signal but distinguish between different satellites.
- 3 Satellites = 2D
- 4 Satellites = 3D
#### Altitude information
Only when a GPS receiver is locked onto three satellites is it able to triangulate for latitude and longitude, also known as 2D position. When four satellites are available, altitude information can also be determined, referred to as 3D position.
### GPS Indications
Indications of GPS are many and varied, and may appear on several different instruments. Modern [[EFIS|Electronic Flight Information Systems]] are able to display much useful information to the pilot in real time, including calculations required by the pilot.
[[Pasted image 20201230002141.png|➡]]![[Pasted image 20201230002141.png|350]]
[[Pasted image 20201230002146.png|➡]]![[Pasted image 20201230002146.png|350]]
[[Pasted image 20201230002151.png|➡]]![[Pasted image 20201230002151.png|350]]
[[Pasted image 20201230002205.png|➡]]![[Pasted image 20201230002205.png|350]]
[[Pasted image 20201230002229.png|➡]]![[Pasted image 20201230002229.png|350]]
### GPS Frequencies
- L1 1575.42 MHz
- L2 1227.6 MHz
Each GPS satellite continuously broadcasts a navigation message at 50 bits per second on the microwave carrier frequency of approx 1600 MHz. FM radio, for comparison, is broadcast at between 87.5 and 108.0 MHz and Wi-Fi networks operate at around 5000 MHz and 2400 MHz. More precisely, all satellites broadcast at 1575.42 MHz (this is the L1 signal) and 1227.6 MHz (the L2 signal).
### GPS Maintenance
%%==[[Faculty/Student/Content/C203/Master QB#Q00171|Q]]==%%
- Very little
Highly integrated [[circuitry]] and no moving parts means that GPS equipment on aircraft requires very little maintenance. [[Calibration]] is not required, and there is no [[periodic maintenance]] requirement either. Usually failure is attributed to the receiver or the antenna so troubleshooting is a simple process, usually requiring replacement of the entire component. Wiring may pose maintenance issues in integrated systems.
> [!example]- More
>![[More GPS]]
## Conclusion
Three new avionics systems for your learning pleasure. Notice how the systems are laid out. You can quickly get the details organized in your mind by creating a table with the systems down the left in rows, with columns for the various characteristics. Here's a [head start](https://excalidraw.com/#json=DbVuRsjvs1aIS_8tp77Qv,HWbNqPeNoyUVJgtZWJxyKw). By filling in the blanks, you can create an effective aide-memoire for this material.
Of course, don't forget that the practice quizzes help you to zero in on concepts and facts that you are not fully mastering.
This material will be tested on Assignment 3, and on the Final Test. [[Pasted image 20210326143858.png|😎]]
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