Appendix 1
A1-1 Appendix 1. Latency Analysis
1. Purpose. The purpose of this appendix is to provide guidelines on accomplishing a latency analysis of your ADS-B In system to demonstrate that it complies with the end-to-end budget for ADS-B applications. It is important to minimize latency and the uncertainty of latency (i.e., how the latency differs between updates) at the system integration level. The easiest way to ensure this design goal is met is to provide a direct connection between the position source and the ADS-B equipment. Any other system blocks between them will increase latency and uncertainty. In some cases, an increase in uncertainty can have a more detrimental effect than the latency itself. Refer to RTCA/DO-317A appendix J for additional information on the interfaces described below.
2. Analysis. For ADS-B In installations, the latency analysis consists of two parts; the traffic latency analysis and the own-ship position latency analysis. Together, these analyses must show:
a. That the total latency allowance is not exceeded and,
b. The own-ship position and traffic positions are estimated to a time of applicability within 1 second of the time of display.
Note: Manufacturers should ensure installation instructions adequately address latency to assist the installer.
3. Traffic Latency Analysis. Figure 2 depicts a block diagram of the ADS-B In system and the recommended latency budget allocated to each block. To demonstrate that the system does not exceed the total latency budget, determine the applicable latencies for each component and total all of the individual component latencies. You must include all sources of latency, including, but not limited to: the ADS-B receiver, the ASSAP equipment, the CDTI equipment, and any intermediary devices. The total for your system between interface D and interface G must not exceed 3.5 seconds. It is acceptable for a manufacturer to allocate the total budget among their system components as needed. However, this design choice will limit the flexibility of pairing their equipment with other manufacturers. In calculating worst case latency, the traffic latency analysis must assume the simultaneous processing of the maximum number of traffic symbols the system is designed to support.
4. Traffic Time of Applicability Analysis. Demonstrate by analysis that the traffic displayed to the flight crew has been estimated forward to be within 1 second of the time of display. For instance, if the latency analysis in the previous paragraph comes to 2.6 seconds, the traffic must be estimated forward 2.6 seconds +/- 1 second by the system prior to displaying that traffic.
The actual estimate for each individual piece of traffic will vary as the received ADS-B messages arrive asynchronously. The analysis must demonstrate that this variation is handled appropriately. The analysis must also demonstrate that any variation in the latency due to processes within the equipment chain does not cause the time of applicability to violate the 1 second tolerance. If different vendors’ equipment are paired together to create a complete system, latency performance data for each system component must originate with the
Appendix 1
A1-2 component manufacturer. Reverse engineering another vendor’s latency performance is not an acceptable means of compliance. Manufacturers are encouraged to include their individual component latency performance in an installation manual to facilitate proper ADS-B system integrations.
Figure 2. Traffic Latency Block Diagram
5. Own-Ship Position Latency Analysis. Refer to figures 3 and 4 which depict block diagrams of two potential implementations of ADS-B In system architectures with recommended latency budgets allocated to each block. For either architecture, the ASSAP equipment must receive the own-ship position data with less than 600 ms of compensation error and less than 1 second of total latency. For this portion of the analysis, total latency starts at the time of measurement of the position source (A3) and ends when ASSAP has received the complete position update (B3). Own-ship total latency at the time of display (G) must not exceed 3.5 seconds.
6. Own-Ship Position Time of Applicability. Demonstrate by analysis that the own-ship position displayed to the flight crew has been estimated forward to be within 1 second of the time of display. The 1 second tolerance must include any compensation error present in the system between interfaces A3 and G. Determine the total latency from the position source time of measurement (A3) to the time of display (G). This latency will depend on the path of own-ship position data and vary by system architecture. Refer to Figure 3 and Figure 4 for examples of two potential architectures. The analysis must demonstrate that any variation in the latency due to processes within the equipment chain does not cause the time of applicability to violate the 1 second tolerance. If different vendors’ equipment are paired together to create a complete system, data for each system component must originate with the component manufacturer. Reverse engineering another vendor’s latency performance is not an acceptable means of compliance. Manufacturers are encouraged to include their individual component latency performance in an installation manual to facilitate proper ADS-B system integrations.
Appendix 1
A1-3
Figure 3. Own-Ship Latency Block Diagram Simple Architecture
Figure 4. Own-Ship Latency Block Diagram Alternate Architecture
Appendix 2
A2-1 Appendix 2. Symbol Requirements for the CDTI
1. Traffic Symbols and Variations. The “basic” traffic symbol is used to depict airborne traffic. Traffic symbols can be modified from the basic symbol to provide special status information, such as on-ground, selected, designated, and alerted. The symbols depicted are examples. The line width, physical size, and hue of the figures are not requirements. The requirements are stated in the associated text.
a. Basic Directional (see Figure 5).
(1) If directionality is valid, the basic directional traffic symbol must be depicted with an arrowhead shape oriented by the directionality.
(2) The color must be cyan or white.
(3) The color must be the same color as the basic non-directional symbol.
(4) The color should not be the same color as the own-ship symbol.
(5) For displays that do not integrate aircraft surveillance applications system (ASA) with TCAS, the symbol may be filled or unfilled.
(6) For TCAS/ASA-integrated systems, the symbol must be unfilled.
Figure 5. Basic Directional Symbol
b. Basic Non-Directional (see Figure 6).
(1) If directionality is invalid, the basic non-directional traffic symbol must be depicted with a diamond shape.
(2) The color must be cyan or white.
(3) The color must be the same color as the basic directional symbol.
(4) The color should not be the same color as the own-ship symbol.
(5) For displays that do not integrate ASA with TCAS, the symbol may be filled or unfilled.
(6) For TCAS/ASA-integrated systems, the symbol must be unfilled.
Appendix 2
A2-2 Figure 6. Basic Non-Directional Symbol
c. Traffic Directionality. If the traffic symbol indicates directionality, the directionality of the traffic symbol must be displayed relative to the display orientation.
Note: The traffic directionality in air is based on traffic ground track angle, and not necessarily traffic heading. This is important for monitoring traffic such as helicopters that can fly backwards and to account for winds.
d. Traffic Application Capability. The traffic symbol may provide an indication of traffic application capability.
Note 1: Traffic information that does not meet the minimum requirements for enhanced visual acquisition (EVAcq) should not be sent to the CDTI display from ASSAP.
Note 2: ASSAP may provide TCAS-only data that does not support EVAcq. TCAS data will still be displayed.
e. Traffic On-Ground Status (see Figure 7).
(1) If traffic is on-ground, the basic traffic symbol must be modified by changing the color.
(2) The color may be brown/tan.
(3) The size of on-ground traffic symbols may be decreased for additional encoding, and/or to reduce clutter.
(4) The symbol may be filled or unfilled.
Note: Additionally, altitude information is removed from the data tag.
Figure 7. Directional and Non-directional On-ground Traffic Symbols
Appendix 2
A2-3
f. Basic Ground Vehicle
(1) The basic Ground Vehicle symbol must be depicted as a top-down wheeled rectangular shape.
(2) The color should be the same as that used for the basic Traffic On-Ground symbol. The color may be brown/tan.
(3) Ground Vehicle directionality may be indicated by adding a triangular shape to one end of the rectangle, and orienting the entire symbol by directionality. Figure 8 provides an example notional depiction.
(4) The symbol may be filled or unfilled.
Figure 8. Basic Surface Vehicle Symbol
2. Alerts. The following requirements, per TSO-C195a, apply generally to CDTI-displayed alerts based on both ASA and TCAS systems. Additional TCAS-specific alert symbol requirements are provided in Appendix 2 section 3.
a. Traffic that triggers an alert must be indicated on the Traffic Display with a symbol variation. The following requirements only apply to the alerted traffic symbol:
(1) If traffic directionality is valid, directionality information must not be removed during alerts.
(2) The traffic symbol must change to amber/yellow for caution level alerts.
(3) The traffic symbol must change to red for warning level alerts.
(4) For traffic without valid directionality:
(a) If traffic has a caution level alert, the traffic symbol may be modified by changing the shape to a circle.
(b) If traffic has a warning level alert, the traffic symbol may be modified by changing the shape to a square.
(5) For traffic with valid directionality:
Appendix 2
A2-4
(a) If traffic has a caution level alert, the traffic symbol may be modified by changing the shape to a circle with a directional inlay.
(b) If traffic has a warning level alert, the traffic symbol may be modified by changing the shape to a square with a directional inlay.
Note: Caution and warning level alerts may use the same traffic symbols as TCAS traffic advisories and resolution advisories, respectively.
(See Appendix 2 section 3)
(6) For airborne applications, alerting traffic that lies outside the configured traffic display range should be positioned at the measured relative bearing, and at the configured display maximum range (i.e., edge of display), and with a symbol shape modification that indicates that the traffic is off-scale.
Note: A half-symbol at the display edge is one acceptable indication method.
b. Proximate Traffic (see Figure 9).
(1) For TCAS/ASA integrated systems, the traffic symbol must indicate airborne TCAS proximate status.
(2) If proximate traffic is displayed, the basic traffic symbol must be displayed as filled. Figure 9 provides example notional depictions.
Note: This requirement is to be consistent with TCAS symbol convention.
Figure 9. Proximate Directional and Non-directional Proximate Traffic Symbols
c. Selected Traffic.
(1) Selected traffic is traffic that is selected by the flight crew. Traffic selection results in display of additional traffic information beyond what is presented in the minimum data tag, and may enable other functions (e.g., designating traffic).
(2) If traffic selection is implemented:
(a) There must be some means of distinguishing the selected traffic from other traffic on the traffic display.
Appendix 2
A2-5
(b) A border must not be used to indicate selected traffic.
Note 1: A border is a discernable line that surrounds an existing symbol. Border types include fixed-shape or conformal.
Note 2: Borders are reserved for depicting designated traffic (see Figure 10).
(3) When traffic is selected, additional information on that traffic must be displayed in a data block or a data tag.
Note: Generally, selecting traffic will bring up the additional information in a data block, but a data tag can also be used for this purpose.
(4) There must be an indication of off-scale selected traffic.
Note: A Selected half-symbol at the display edge and appropriate bearing is one acceptable method of indication.
d. Designated Traffic (see Figure 10).
(1) Designated traffic is traffic upon which a designated application is to be performed. For example, in visual separation on approach (VSA), the traffic to be followed may be displayed as “designated” so that the application and the flight crew both know the specific traffic upon which to act.
(2) If traffic designation is implemented:
(a) There must be some means of distinguishing the designated traffic from other traffic.
(b) If traffic is designated, the basic traffic symbol should be modified by adding a shape-conforming border. Figure 10 provides a notional example.
Figure 10. Directional and Non-directio nal Designated Traffic Symbols
(c) There must be an indication of off-scale designated traffic.
Appendix 2
A2-6 Note: A half-symbol at the display edge and appropriate bearing is one acceptable method of indication.
(d) The loss of “designated” status (e.g., due to signal loss or invalid data) must be indicated to the flight crew.
3. TCAS Alert Symbology for TCAS/ASA-Integrated Systems.
a. If traffic directionality is valid, directionality information must not be removed during a TCAS traffic advisory or resolution advisory.
Note: Directionality information, if available, may assist the flight crew in visual search and identification of the alerted traffic.
b. Traffic Advisories (see Figure 11).
(1) If traffic has a TA, the traffic symbol must be modified by changing the color to amber/yellow, and changing the shape to a circle.
(2) Traffic with valid directionality must include a directional inlay.
(3) The size of TA traffic symbols may be increased to accommodate the shape modification.
(4) Line widths and fill may be changed to improve color interpretation and saliency.
Figure 11. Traffic Advisory Symbols
c. Resolution Advisories (RAs) (see Figure 12).
(1) If traffic has an RA, the traffic symbol must be modified by changing the color to red, and changing the shape to a square.
(2) Traffic with valid directionality must include a directional inlay.
(3) The size of RA traffic symbols may be increased to accommodate the shape modification.
(4) Line widths and fill may be changed to improve color interpretation and saliency.
Appendix 2
A2-7 Note: TCAS aural alerts and resolution guidance are not affected by these requirements or recommendations.
Figure 12. Resolution Advisory Symbols
Appendix 3
A3-1 Appendix 3. Definitions and Acronyms
1. Definitions. The following are definitions of terms used in this document.
a. 24-bit Address. Unique address assigned to an aircraft during the registration process.
b. ADS-B In- Receipt, processing, and display of other aircraft’s ADS-B transmissions.
ADS-B In is necessary to utilize airborne applications.
c. Advisory. The level or category of alert for conditions that require flight crew awareness and may require subsequent flight crew response.
d. Aircraft Surveillance Applications System (ASAS). An aircraft system based on airborne surveillance that provides assistance to the flight crew in operating their aircraft relative to other aircraft.
e. Alert. A general term that applies to all advisories, cautions, and warning information;
can include visual, aural, tactile, or other attention-getting methods.
f. Application. The function(s) for which the ASA system is used.
g. Aircraft Surveillance Application (ASA). An application that uses aircraft surveillance data to provide benefits to the flight crew.
h. Antenna Offset Parameter. The distance from the nose of the aircraft to the GPS antenna. For large aircraft, this offset is important in accurately placing the aircraft symbol on the airport map.
i. Automatic Dependent Surveillance-Broadcast (ADS-B). A function on an aircraft or surface vehicle operating within the surface movement area that periodically broadcasts its state vector (horizontal and vertical position, horizontal and vertical velocity) and other information.
ADS-B is automatic because no external stimulus is required to elicit a transmission. It is dependent because it relies on on-board navigation sources and on-board broadcast transmission systems to provide surveillance information to other users.
j. Automatic Dependent Surveillance-Rebroadcast (ADS-R). A service of the ground system that rebroadcasts ADS-B messages from one link technology onto another. For example, the SBS ground system provides ADS-R service to rebroadcast UAT messages on 1090 MHz and vice versa.
k. Availability. An indication of the ability of a system or subsystem to provide usable service. Availability is expressed in terms of the probability of the system or subsystem being available at the beginning of an intended operation.
l. Background Application. An application that applies to all traffic of operational interest. One or more background applications may be in use in some or all airspace (or on the
Appendix 3
A3-2 ground), but without flight crew input or automated input to select specific traffic. Background applications include: basic airborne, surface (runways and taxiways OR runways only).
m. Caution. The level or category of alert for conditions that require immediate flight crew awareness and subsequent flight crew response.
n. Coast Interval. The elapsed time since a report from any source has been correlated with the track.
o. Cockpit Display of Traffic Information (CDTI). The pilot interface portion of the Aircraft Surveillance Applications System. This interface includes traffic display(s) and all the controls that interact with such a display. At a minimum, CDTI includes a graphical plan-view (top down) traffic display. Additional graphical and non-graphical display surfaces may also be included. The CDTI receives position information of traffic and Ownship from the airborne surveillance and separation assurance processing (ASSAP) function. The ASSAP receives such information from the surveillance sensors and Ownship position sensors.
p. Compensated Latency. Latency can be compensated by extrapolating position using the last known position measurement, the elapsed time, and the last known velocity measurement.
The elapsed time used to extrapolate is called compensated latency.
q. Conformal. A desirable property of map projections. A map projection (a function that associate points on the surface of an ellipsoid or sphere representing the earth to points on a flat surface such as the CDTI display) is said to be conformal if the angle between any two curves on the first surface is preserved in magnitude and sensed by the angle between the corresponding curves on the other surface.
r. Correlation. The process of determining that a new measurement belongs to an existing track.
s. Controller Pilot Data Link Communication (CPDLC). Provides direct data communication between the pilot and the air traffic controller through a data link.
t. Data Block. A block of information about selected traffic that is displayed somewhere around the edge of the CDTI display, rather than mixed in with the symbols representing traffic in the main part of the display.
u. Data Tag. A block of information that is displayed next to the traffic symbol in the main part of the CDTI display.
v. Designated Application. An application that operates only on specifically-chosen (either by the flight crew or automation) traffic. They generally operate only for a specific flight operation.
w. Designated Traffic. Traffic upon which a designated application is to be conducted.