Chapter 1. General Information.
1-1. Purpose.
a. This advisory circular (AC) provides guidance material for the installation of ADS-B In technology in aircraft. ADS-B In includes reception of ADS-B, Traffic Information Services-Broadcast (TIS-B) and Automatic Dependent Surveillance – Rebroadcast (ADS-R) messages, but does not include reception of Flight Information Service – Broadcast (FIS-B) messages.
b. The installation of ADS-B In avionics provides the pilot(s) with supplemental information.
No existing responsibility is changed by virtue of installation of this equipment and application(s). The situation awareness applications defined in TSO-C195b supplement, but do not replace, a pilot’s see and-avoid responsibility, as required by Title 14 of the Code of Federal Regulations (14 CFR)
91.113(b).
c. This AC is not mandatory and does not constitute a regulation. This AC describes an acceptable means, but not the only means, to install ADS-B In equipment. However, if the means described in this AC are used, it must be followed it in its entirety. The term “must” is used to indicate mandatory requirements when following the guidance in this AC. The terms “should” and “recommend” are used when following the guidance is recommended but not required to comply with this AC. A list of definitions and acronyms relevant to this AC can be found in Appendix D.
d. This AC provides guidance information intended for new approvals. This AC is not intended to modify, change or cancel existing equipment design or airworthiness approvals. Equipment with existing approvals can continue to be installed within the provisions of their original design and airworthiness certification.
1-2. Audience.
This AC is for installers of ADS-B In equipment, and can assist in obtaining design approval for installation. The installed design can be approved under any means acceptable to the administrator including a type certificate (TC), supplemental type certificate (STC) including approved model list supplemental type certificate (AML-STC), amended type certificate (Amended TC), or amended supplemental type certificate (ASTC).
1-3. Scope.
This AC addresses initial and follow-on installations of ADS-B In systems that comply with TSO-C195b, Avionics Supporting Automatic Dependent Surveillance – Broadcast (ADS-B) Aircraft Surveillance Applications (ASA). Data from a previously approved installation design may be used to fulfill some of the data requirements for a follow-on installation design as appropriate. For example, the latency analysis between a GPS position source and the ADS-B equipment may be reused on a follow-on installation provided that the hardware and software part numbers for both units are identical.
Modifications to previously approved hardware or software must be evaluated to determine data applicability. All installations of ADS -B In should either include ADS-B Out or supplement an existing installed ADS-B Out capability. Full ADS-B In benefits can only be realized when installed in conjunction with a compliant ADS-B Out function. Installation guidance for ADS-B Out can be found in AC 20-165A, Airworthiness Approval of Automatic Dependent Surveillance - Broadcast (ADS-B) 3
Out Systems. Installation guidance for FIS-B applications that make use of the Surveillance and Broadcast Services (SBS) ground system as well as third-party providers can be found in AC 20-149(), Installation Guidance for Domestic Flight Information Services – Broadcast. A list of related documents can be found in Appendix E of this AC.
1-4. Background.
a. The ADS-B In system overview, shown in Figure 1, is a next generation surveillance technology that provides the flight crew with an accurate picture of surrounding traffic. ADS-B Out aircraft provide broadcast messages of their identification, position, altitude, velocity, and other information. Aircraft equipped with ADS-B In capability can “see” these broadcasts from other ADS-B equipped aircraft and display them to improve the pilot’s situation awareness of other traffic, both airborne and on the ground. Suitably equipped surface vehicles may also be visible to ADS-B In capable aircraft. Aircraft that transmit on an alternate link or are transponder equipped are tracked by the FAA ADS-B ground system network and uplinked as ADS-R or TIS-B respectively.
Figure 1. ADS-B In System Overview ATC RADAR Non-ADS-B Target Transponder Reply Track ADS-B 1090 MHz ADS-B on Alternate Link ADS-B on UAT Common Link e.g., 1090MHz ATC ADS-B Ground Station ADS-B Direct Target Sources Traffic Information System Broadcast (TIS-B) ADS-R 1090 MHz TIS-B (1090 and UAT Format) ADS-B Rebroadcast (ADS-R) ADS-B Receiver Airborne Surveillance and Separation Assurance Processor TCAS Position Sensor Barometric Altimeter Other Aircraft Systems Surveillance Tracks Display Data Control Data ADS-B TIS-B ADS-R Reports Antenna Aircraft ADS-B Receiver and Traffic Display Cockpit Display of Traffic InformationSystem Control Data CrewFlight ATC ADS-B Ground Station
b. ADS-B Out refers to an appropriately equipped aircraft broadcasting own-ship information.
ADS-B In refers to an appropriately equipped aircraft’s ability to receive and display other aircraft’s 4
ADS-B information and ground station broadcast information, such as TIS-B and ADS-R. The TIS-B service provides traffic based on ground surveillance of transponder equipped aircraft. The ADS-R service provides traffic from aircraft equipped with an alternate ADS-B link.
c. There are two ADS-B link options: 1090 extended squitter (1090ES) and universal access transceiver (UAT). The 1090ES equipment operates on 1090 MHz and has performance requirements specified in TSO-C166b, Extended Squitter Automatic Dependent Surveillance - Broadcast (ADS-B) and Traffic Information Service - Broadcast (TIS-B) Equipment Operating on the Radio Frequency of 1090 Megahertz (MHz). The UAT operates on 978 MHz and has performance requirements specified in TSO-C154c, Universal Access Transceiver (UAT) Automatic Dependent Surveillance – Broadcast (ADS-B) Equipment Operating on Frequency of 978 MHz.
Chapter 2. ADS-B In System Installation Guidance.
2-1. S ystem Overview.
ADS-B In refers to an appropriately equipped aircraft’s ability to receive and display other aircraft’s ADS-B information and ground station broadcast information, such as TIS-B and ADS-R. The information can be received by an appropriately equipped aircraft on either or both of two radio frequency (RF) links: 1090 ES or 978 MHz UAT. The received information is processed by onboard avionics and presented to the flight crew on a display. In this AC, guidance is provided for the display of traffic information while on the airport surface and while airborne. This information supports the applications defined in TSO-C195b. This AC will be updated to add appropriate guidance for additional applications as they mature.
2-2. Equipment Classes.
TSO-C195b defines minimum performance standards that provide a basis for installation of ADS-B In equipment in aircraft. The TSO defines four avionics equipment classes: (A) cockpit display of traffic information (CDTI) (surface only); (B) CDTI; (C) airborne surveillance and separation assurance processing (ASSAP); and (D) ADS-B Traffic Advisory System (ATAS) Annunciator Panel. Class A equipment is intended to support the display of ADS-B traffic while own-ship is on the surface and moving slower than 80 knots. Class A equipment must deactivate the CDTI when airborne or at speeds greater than 80 knots. Class B equipment supports the display of ADS-B traffic when airborne as well as on the ground. Class C equipment processes ADS-B messages to generate traffic data for a CDTI. Class D equipment supports an aural only implementation. Currently, this is only available for one application. Table 1 shows which applications are supported by the four equipment classes.
With the exception of the ATAS application, an installation requires both the CDTI and ASSAP functions, which are explained in paragraphs 2-4 and 2-6 of this AC, respectively.
Table 1. ADS-B In Equipment Classes Criticality Equipment Class Application Loss of Function Hazardously Misleading Information CDTI (Surface Only) (A) CDTI (B) ASSAP (C) Annunciator Panel (D)
(1) Enhanced Visual Acquisition (EVAcq) Minor Major Not Permitted B1 C1 Not Applicable
(2) Basic Minor Major (> 80 Knots) A2 B2 C2 Not Surface (SURF) (Runways) Minor (< 80 Knots) Applicable
(3) Basic Minor Major (> 80 Knots) A3 B3 C3 Not Surface (SURF) (Runways + Taxiways) Minor (< 80 Knots) Applicable
(4) Visual Separation on Approach (VSA) Minor Major Not Permitted B4 C4 Not Applicable
(5) Basic Airborne (AIRB) Minor Major Not Permitted B5 C5 Not Applicable
(6) In-Trail Procedures (ITP) Minor Major Not Permitted B6 C6 Not Applicable
(7) ADS-B Traffic Advisory System (ATAS) Minor Major Not Permitted B7 C7 D7
(8) CDTI Assisted Visual Separation (CAVS) Minor Major Not Permitted B8 C8 Not Applicable 7
2-3. ADS-B Applications.
a. ADS-B In avionics enable one or more of the following aircraft surveillance applications: enhanced visual acquisition (EVAcq); basic airborne (AIRB); visual separation on approach (VSA), basic surface (SURF) (runways and taxiways, or runways only); In-Trail Procedures (ITP), ADS-B Traffic Advisory System (ATAS), and Cockpit Display of Traffic Information (CDTI) Assisted Visual Separation (CAVS). Refer to Table 1 to see which applications are supported by the four equipment classes.
b. The basic airborne application (AIRB) displays ADS-B traffic on a plan view (bird's eye view) relative to own-ship. This application is the minimum requirement for installations that implement other applications such as VSA or ITP. Each aircraft symbol displayed conveys position, direction, and altitude information. Optionally, additional information, like identity, may be displayed. The traffic information assists the flight crew in visually acquiring traffic out the window while airborne. This application improves both safety and efficiency by providing the flight crew with enhanced traffic awareness. Installations that provide in -flight moving map displays in addition to traffic should comply with TSO-C165a, Electronic Map Display Equipment for Graphical Depiction of Aircraft Position.
c. The enhanced visual acquisition application (EVAcq), also displays ADS-B traffic on a plan view (bird's eye view) relative to own-ship. This application is designed to support only the display and alerting of ADS-B traffic, including ADS-R, TIS-B, and TCAS derived traffic.
Implementations that include application classes other than EVAcq and ATAS must use the AIRB application instead. The traffic information assists the flight crew in visually acquiring traffic out the window while airborne. EVAcq does not relieve the pilot of see and avoid responsibilities under 14 CFR 91.113b. This application is expected to improve both safety and efficiency by providing the flight crew enhanced traffic awareness. Installations that provide in-flight moving map displays in addition to traffic should comply with TSO-C165a.
d. The visual separation on approach (VSA) application builds upon the basic airborne application (AIRB). It allows the pilot to select an aircraft to follow on approach. Additional information about the selected aircraft, including range and ground speed, is displayed to enhance the pilot’s situation awareness. The CDTI display is used to assist the flight crew in acquiring and maintaining visual contact during a visual approach. The application improves both the safety and the performance of visual approach operations. The VSA application should not be confused with creating a new approach operation. No operational responsibility is changed when using the VSA application.
e. The basic surface application (SURF) with runways and taxiways displays ADS-B traffic on a plan view (bird's eye view) relative to own-ship, superimposed on a map of the airport surface. This map consists of all runways at supported airports and includes taxiways when that data is available. Aircraft on -ground and in-air as well as properly equipped surface vehicles are differentiated by symbols to aid the pilot in visual acquisition. The surface application improves flight crew situation awareness during taxi, takeoff, and landing phases of flight. This application reduces the possibility of runway incursion and collision. These installations should also comply with TSO-C165a Electronic Map Display Equipment for Graphical Depiction of Aircraft Position for airport moving map displays. The SURF 8
application with runways only does not display taxiways.
f. The In-Trail Procedure (ITP) application enables aircraft that desire flight level changes in procedural airspace to achieve these changes on a more frequent basis, thus improving flight efficiency and safety. The ITP achieves this objective by permitting a climb-through or descend-through maneuver between properly equipped aircraft, using a new distance-based longitudinal separation minimum during the maneuver. The ITP requires the flight crew to use information derived on the aircraft to determine if the initiation criteria (see Table 2, Section 3-3) required for an ITP are met. The initiation criteria are designed such that the spacing between the estimated positions of ownship and surrounding aircraft exceed the separation minima with acceptable probability throughout the maneuver. ITP requires specific application-unique processing and display parameters. In addition, ITP will require an operations approval by the FAA Flight Standards Service (AFS). Guidance for this operations approval is contained in AC 90-114A, Automatic Dependent Surveillance-Broadcast (ADS-B) Operations.
g. ADS-B Traffic Advisory System (ATAS) is an Automatic Dependent Surveillance-Broadcast (ADS-B) In application intended to reduce the number of mid-air collisions and near mid-air collisions involving general aviation aircraft. Previously known as Traffic Situation Awareness with Alerts (TSAA), the name ATAS has been used in this AC as well TSO-C195b to be more consistent with existing traffic advisory systems. ATAS provides voice annunciations to flight crews to draw attention to alerted traffic and also adds visual cues to the underlying basic traffic situation awareness application (e.g., Enhanced Visual Acquisition [EVAcq] or Basic Airborne Situation Awareness [AIRB]) in installations where a Traffic Display is available. The ATAS application uses ADS-B information, and where available Automatic Dependent Surveillance-Rebroadcast (ADS-R) and Traffic Information Service-Broadcast (TIS-B) information to provide the flight crew with indications of nearby aircraft in support of their see-and-avoid responsibility. ATAS is the only ADS-B application with an aural-only implementation (via an annunciator panel). All other applications require a traffic display as defined by the CDTI requirements.
h. The CDTI Assisted Visual Separation (CAVS) application is designed for use on approach procedures when visual separation is approved by ATC. CAVS requires the flight crew to visually acquire the preceding aircraft out the window during approach procedures.
After visual acquisition is established, the flight crew will cross-check the information on the CDTI with what they see out of the window. Once the preceding aircraft is identified on the CDTI, the flight crew is allowed to use the CDTI to continue visual separation even if visual contact out the window is lost, for example, due to glare from rising or setting sun, or against a background of lights at night. CAVS does not aim to modify Visual Meteorological Conditions (VMC) minima as defined by ICAO (see Appendix A.2). Guidance for this operation is contained in AC 90-114A, Automatic Dependent Surveillance-Broadcast (ADS-B) Operations.
The displayed ADS-B information addressed by this AC is not intended for maneuvering based solely on presence or absence of traffic on the display. As future applications are fielded, we expect that certain maneuvers may be found to be safe and acceptable. The analysis and safety studies to justify such procedures are not yet completed. When those activities are concluded and the maneuvers are shown to be safe and acceptable in the national airspace system (NAS), 9
appropriate maneuvers are expected to be allowed based in part on the displayed ADS-B In information. We will revise this guidance accordingly at that time. Operational guidance will be published by AFS.
2-4. CDTI.
a. Displays. The ADS-B In system includes at least one flight deck traffic display (i.e., CDTI) depicting the relative position and related information of ADS-B equipped aircraft in a plan view (bird's eye view), with the exception of audio only ATAS installations. The CDTI display may be presented on a dedicated display or integrated into and presented on an existing display (e.g., electronic flight information system (EFIS), multi-function display (MFD)).
CDTI equipment should be compliant with the Class A or Class B requirements of TSO-C195b.
Class A equipment supports only the Basic Surface application. CDTI equipment should be installed in accordance with manufacturer instructions. Installation in a forward position requiring minimum head and eye movement (14 CFR 23.1321, 25.1321, 27.l321, and 29.1321) will provide the best traffic situation awareness and support subsequent upgrades to other ADS B applications. Side-mounted displays are acceptable for the basic situation awareness applications and ITP, but have limited potential to support more advanced applications. The display must be installed such that the pilots have an unobstructed view of the display when seated in their normal position. For general installation guidance on displays, refer to AC 25-11B, Electronic Flight Displays and AC 23.1311-1C, Installation of Electronic Display in Part 23 Airplanes.
b. ITP Installations. ITP installations must include a traffic display mounted in the forward position or as a side mounted display. The traffic display (plan view) must be visible during the ITP vertical maneuver. It is recommended but not required that a graphical vertical/profile view of the traffic be available for flight crews to aid in assessing initiation criteria. This is particularly helpful in situations where there is a significant angle between the track of the own aircraft and the ITP Traffic aircraft (the angle can be any value less than 45 degrees and still be a valid ITP situation). In these cases, relative geometry (ahead of or behind) may not be intuitive.
c. Traffic Symbols. The FAA worked closely with industry to standardize the ADS-B In symbols and features. The resulting symbols are provided in Appendix B. The traffic display should depict the symbols, features, and information defined in the appendix. However, manufacturers may propose alternate symbols in order to integrate ADS-B with existing flight deck symbology. These alternate symbols will need to be justified by human factors analysis as part of the certification process. Alternate symbol sets are not allowed without additional justification. There is one exception to this guidance. It is acceptable to pair a TSO -C195b Class C ASSAP unit with an existing certified traffic display using legacy symbols (e.g., TCAS, TAS) when either the AIRB or EVAcq application are installed. Minor TSO changes or enhancements may be made to the previously approved traffic display without requiring the equipment to be made fully compliant to TSO-C195b requirements. This exception only applies to previously approved traffic displays. If any other applications beyond EVAcq or AIRB are installed, the display must be fully compliant with TSO-C195b.
(1) Traffic Symbol and Ownship Symbol Reference Point. The airplane or rotorcraft flight manual or pilot’s guide for the ADS-B equipment must specify the location of the horizontal position reference point on the traffic symbols and the ownship symbol. For example, this position may be the center of the symbol or the tip of the traffic directional arrow.
The ownship horizontal position reference point should be consistent with the existing flight deck philosophy. The traffic and ownship symbols are an abstract representation and are not required to reflect the physical extent of the aircraft. This becomes more evident when implementing the surface application with the underlying airport map.
d. CAVS Installations. For installations that include the CAVS application, Differential Ground Speed (when aircraft are in-trail) and Digital Traffic Range must be displayed in the primary field of view to facilitate pilot monitoring. Traffic Identification, Ownship Ground Speed, and Traffic Ground Speed should be displayed in the primary field of view, but may be displayed in a forward position requiring minimum head and eye movement.
Refer to AC 25-11B. CAVS installations must include a means to designate traffic for CAVS.
e. Required Controls. The CDTI control panel may be a dedicated control panel or it may be incorporated into another control, such as a multifunction control display unit (MCDU) or Flight Management Computer (FMC) control display unit (CDU). CDTI controls must be readily accessible from the normal seated position. Pilot controls for the ADS-B In equipment must be provided as follows:
values.
(1) A means to adjust the display range between the minimum and maximum values.
(2) A means to adjust the altitude band between the minimum and maximum
(3) A means to adjust the brightness of the display.
f. Optional Controls. The following optional controls may be provided:
(1) A means to select between display of relative and actual altitude.
(2) A means to select at least one traffic element.
(3) A means to select alternate display criteria (e.g., filters and vertical views).
(4) A means to declutter, which removes optional traffic information when display of the information is not desired. If decluttering is implemented,
(a) A means must be provided for the flight crew to control the decluttering.
(b) The flight crew must be able to perform the declutter operation by a simple action.
(c) The flight crew should be able to return to the previous state by a simple action.
(d) If automatic decluttering is implemented, a means should be provided for the flight crew to control the automated decluttering function.
(e) An indication that decluttering is active must be provided.
(5) A means to pan the view. If panning is implemented:
(a) There must be a means to control panning.
(b) There must be a means to return to the original view with a simple action.
(6) A means to designate traffic for an application (e.g. CAVS).
2-5. Alerts.
a. Alerts should be consistent with the flight deck alerting system and philosophy, and should not interfere with higher priority alerts. Advisory Circular 25.1322-1, Flightcrew Alerting, contains guidance for flight crew alerting functions.
b. Each ATAS alert must include a visual caution level alert either on the Traffic Display or on an annunciator panel in an aural-only implementation.
c. Each ATAS aural alert should be annunciated by a dedicated voice message consistent with the requirements in TSO-C195b. The voice message will be annunciated over a cockpit speaker and through a headset/helmet at a volume adequate for clear understanding at high cockpit noise levels, but not excessively loud at low noise levels.
The evaluation of the message annunciated through a cockpit speaker includes the case where a flight crew member is wearing a headset covering the outboard ear, when appropriate. ATAS aural alerts provide bearing, range, relative altitude, and optionally, a vertical sense of the alerted traffic. Vertical sense of the alerted traffic can be of three types: climbing, descending, or level.
d. ATAS installations must include a means to acknowledge an alert in progress that causes the aural to be discontinued.
e. ATAS installations must include a means to request an update of the aural alert to announce the new traffic state (bearing, range, relative altitude).
f. Aural alerts must be automatically inhibited by all higher priority alerts including alerts issued by TCAS II, if so equipped. Inhibit priorities for TCAS are established by AC 20-151(), Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II), Versions 7.0 & 7.1 and Associated Mode S Transponders.
2-6. Airborne Surveillance and Separation Assurance Processing (ASSAP).
The ASSAP subsystem accepts data from one or more sources including ADS-B reports, TIS-B reports, ADS-R reports, and TCAS tracks (if installed). ASSAP correlates data from these sources, generates tracks, and performs application-specific processing. Surveillance tracks and application-specific alerts or guidance are output by ASSAP to the CDTI function. The ASSAP equipment must be compliant with the Class C requirements of TSO-C195b and should be installed in accordance with manufacturer instructions. TCAS processors track transponderequipped aircraft. Therefore, TSO-C195b equipment requires installations with TCAS to provide these tracks to the ASSAP equipment to complete the traffic picture. TCAS in this AC is meant to apply to all versions of certified traffic advisory system (TAS) or TCAS compliant with TSO-C147(), TSO-C118(), or TSO-C119(). Hybrid surveillance TCAS are included. For aircraft installations without TCAS, the TIS-B service provides tracks of transponder-equipped aircraft.
2-7. ADS-B In Receiver and Antenna.
The installation must include a UAT (per TSO-C154c) or a 1090 ES (per TSO-C166b) receiver.
Ideally, installation of a dual-band receiver would allow for dual-link interoperability where ADS-R coverage is not provided. The ASSAP equipment may interface with the ADS-B receiver equipment or it may be integrated. If TCAS is installed, the ADS -B In equipment must contain or interface with the TCAS equipment so that the TCAS tracks may be used. Guidance material concerning the installation of the UAT or 1090ES equipment, and associated antenna(s), is provided in AC 20-165A.
2-8. Integration Considerations.
a. System Definition. ADS-B In installations include the ADS-B In receiver, antennas, traffic processor, control panels, audio system, and display components. All of these component part numbers must be identified as part of the integrated system. Any change to any of the components’ hardware or software requires evaluation of the potential impact to the ADS-B In function.
b. Equipment Compatibility Requirements. A critical component of the ADS-B In system is the position sensor. Compatibility between the sensor and the surveillance processor must be established by the equipment manufacturer(s) and detailed in an installation manual or supplement. Position source compatibility should consider the position source requirements in AC 20-165A. Compatibility between all other system components should be documented in an installation manual or supplement.
c. Aircraft Integration with ADS-B In System.
(1) Provide electrical power and grounding in accordance with the manufacturer’s installation manual. Conduct an electrical load analysis to verify that there is adequate power capacity for the ADS-B In equipment.
(2) The total latency to receive, process, and display traffic data must be less than
3.5 seconds. The traffic time of applicability must be within 1 second of the time of display.
The total latency of own-ship position at the display must be less than 3.5 seconds. The ownship time of applicability must be within 1 second of the time of display. Perform a latency analysis in accordance with Appendix A to demonstrate compliance. The total latency figures here are to be interpreted to mean when an ADS-B message is received. They do not address data age issues while the system is waiting to receive the next position report for an existing track. Data age and timeout requirements are handled separately for each application in TSO-C195b compliant equipment.
(3) The same position source used to provide ownship data for transmission on ADS-B Out should be used to provide position to the ASSAP equipment. Position sources interfaced to the ASSAP equipment must meet the quality metric requirements in DO-317B, section 2.2.4. Further guidance on integration with ADS-B position sources can be found in AC 20-165A. Future applications may require that ASSAP and the ADS-B Out equipment use the same position source. The FAA encourages applicants to plan accordingly to prevent extensive future redesign. An alternate position source may be used to provide ownship position to the CDTI display, but the accuracy, latency, and display time of applicability requirements still apply (refer to Appendix A). Provide connections in accordance with the manufacturer’s installation manual.
(4) The manufacturer’s instructions for strapping and/or programming of configurable aircraft parameters should be followed. Manufacturers are highly encouraged to provide instructions to installers for setting the global navigation satellite system (GNSS) antenna offset parameter during installation. The GNSS antenna offset information can be extremely valuable for ADS-B In surface situation awareness and future surface collision alerting applications on large aircraft with GNSS antenna far from the nose.
(5) Verify the equipment’s environmental qualifications (e.g., environmental categories of RTCA/DO-160( ) Environmental Conditions and Test Procedures for Airborne Equipment) are suitable for the aircraft type and equipment location.
(6) Record any limitations associated with use of the ADS-B In equipment in the Aircraft Flight Manual.
(7) ITP requests and clearances can only be granted using Direct Controller Pilot Communication (DCPC). Although it may be possible to perform ITP requests and clearances via voice communications, in most non-radar regions this means requests and clearances will be accomplished using Controller Pilot Datalink Communications (CPDLC). ITP requests can be lengthy and prone to typographical errors. It is recommended that ITP designs integrate the CDTI and data link systems in order to populate ITP requests automatically. An alternative is to provide the request text on the CDTI so that the flight crew can reference the text while entering the information manually. This reduces the possibility of human error while entering the ITP request. Examples of standardized free text CPDLC message formats for an ITP request (downlink message) can be found in Change 1 to RTCA/DO-306.