Appendix F F-3 F.3 Populating Target State and Status Information Register BDS 6216.
This section provides guidance for interfacing the ADS-B system to aircraft systems for complying with the EU ADS-B requirement for Target State and Status BDS 6216.
F.3.1 SILSUPP is based on whether the position source probability of exceeding the reported integrity value is calculated on a per-hour or per-sample basis and should be set based on design data from the position source equipment manufacturer. ADS-B systems interfaced with a GNSS position source compliant with any revision of TSO-C129, TSO-C145, TSO-C146, or TSO-C196 may preset SILSUPP to ZERO, as GNSS position sources use a per hour basis for integrity.
F.3.2 For installations with both an MCP/FCU and an FMS system, the installer should ensure the selected altitude information transmitted is the MCP/FCU selected altitude.
This should be verified in transponder register BDS 4016 and the Target State and Status ADS-B message. If neither the MCP/FCU nor the FMS is providing selected altitude information or the information is invalid, then the installer should ensure the data status is set to all ZEROs. In some legacy aircraft, the selected altitude information may be available but the system does not indicate which source, MCP/FCU or FMS, is in control of the aircraft vertical profile. In this case, the MCP/FCU selected altitude should be encoded when it is valid. If MCP/FCU selected altitude is not valid, and the FMS selected altitude is valid, then the FMS selected altitude should be encoded.
F.3.3 The installer should ensure that when the aircraft vertical intent is to fly the current altitude (e.g., altitude hold) or the pilot is manually flying the aircraft, the MCP/FCU selected altitude is encoded in the Target State and Status ADS-B message.
F.3.4 The installer should ensure that the transponder receives the barometric pressure setting in use. In some installations, it is possible to override the barometric pressure setting and fly the aircraft to the uncorrected pressure altitude. In this case the reported correction should be zero. If the barometric pressure setting is not available or invalid, the installer should verify that the data status is marked invalid.
F.3.5 For installations with both an MCP/FCU and an FMS system, the installer should ensure the selected heading information transmitted by the transponder corresponds to the system in control of the aircraft horizontal profile. If the data is either not available or invalid, verify that it is reported as invalid. In some legacy aircraft, the selected heading information may be available but the system does not indicate which source, MCP/FCU or FMS, is in control of the aircraft horizontal profile. In this case, the MCP/FCU selected heading should be encoded when it is valid. If MCP/FCU selected heading is not valid, and the FMS selected heading is valid, then the FMS selected heading should be encoded.
F.3.6 The accuracy limits for the Navigation Accuracy Category for Position (NACP) allow the surveillance applications to determine whether the reported geometric position has an acceptable level of accuracy for its intended use. The ADS-B equipment must set the NACP based on the real time 95% accuracy metric provided by the position source.
Appendix F F-4 When interfacing GNSS sources, the NACP should be based on a qualified Horizontal Figure of Merit (HFOM).
F.3.7 You should verify the type of altitude source installed in the aircraft and interface the altitude system per the ADS-B equipment manufacturer’s instructions. For aircraft with an approved, non-Gillham altitude source, NICBARO should be preset at installation to ONE. For aircraft with a Gillham altitude source without an automatic cross-check, NICBARO must be preset at installation to ZERO. For aircraft which dynamically cross-check a Gillham altitude source with a second altitude source the NICBARO must be set based on the result of this cross-check. We recommend that ADS-B installations use non-Gillham altitude encoders to reduce the potential for altitude errors.
F.3.8 SIL is typically a static (unchanging) value and may be set at the time of installation if a single type of position source is integrated with the ADS-B system. SIL is based solely on the position source’s probability of exceeding the reported integrity value and should be set based on design data from the position source equipment manufacturer. Installations which derive SIL from GNSS position sources compliant with any revision of TSO-C129, TSO-C145, TSO-C146, or TSO-C196 which output Horizontal Protection Level (HPL) or Horizontal Integrity Level (HIL) should set the SIL = 3 because HPL and HIL are based on a probability of 1x10-7 per hour. Do not base NIC or SIL on Horizontal Uncertainty Level (HUL) information. If integrating with a non-compliant GPS, SIL must be set to 0.
F.3.9 For installations that do not have VNAV, LNAV, Autopilot Engaged, Altitude Hold, or Approach Mode information available, the installer should ensure the MCP/FCU Mode bits are reported as invalid. If one or more of the mode parameters are provided to the transponder, the installer should verify the MCP/FCU Mode bits are set in accordance with the horizontal/vertical mode controlling the aircraft. The extent of these modes can vary significantly across different aircraft platforms. A description of each field follows, to clarify the intended use of these parameters by ATC. Refer to the example scenarios for a detailed description on setting the mode field.
F.3.9.1 VNAV indicates when an aircraft is following vertical path guidance using information from the FMS. The pilot may arm this mode manually.
F.3.9.2 LNAV indicates when the aircraft is following lateral path guidance using information from the FMS. The pilot may arm this mode manually.
F.3.9.3 Autopilot Engaged indicates when the pilot has engaged the Autopilot system.
F.3.9.4 Altitude Hold Mode indicates that the aircraft will remain at its current altitude regardless of the current setting or a change to the MCP/FCU selected altitude or FMS selected altitude data.
Note: Altitude Hold Mode may be inhibited when the aircraft has captured the approach (e.g. capturing the glideslope).
Appendix F F-5 F.3.9.5 Approach Mode indicates the pilot has armed the desired approach source (e.g.
VOR/LOC and G/S or LNAV and VNAV).
F.3.10 The installer should verify all reserved fields are unused and set to zero.
F.3.11 The TCAS Installed and Operational field must interface with the TCAS II system if your aircraft has a TCAS II system. This parameter should be preset to ZERO if your aircraft does not have a TCAS II or if your aircraft has a TCAS I. Typically, this parameter will already be provided to the Mode S transponder from the TCAS II.
TCAS II systems compliant with TSO-C119( ) indicate they are operational and able to issue an RA when they transmit Reply Information (RI) = 3 or 4 to the transponder.
F.3.12 MCP/FCU Mode Examples To further clarify how the vertical mode bits should operate, this section presents a set of scenarios below. Figure 3 depicts a series of vertical maneuvers. Table 1 illustrates how the equipment would set the parameters for BDS 4016 during each maneuver.
Similarly, Table 2 illustrates how the equipment would set the parameters for BDS 6216 during each maneuver. The parameters encoded in BDS 4016 and BDS 6216 by the transponder are based on information received from other aircraft equipment or via aircraft configuration strapping. Below the table there is a description of each step along with any assumptions made for the given maneuver. For the purposes of these examples, we assume that the pilot has the ability to manually select the use of MCP/FCU or FMS as the source providing the flight path information to the aircraft’s autoflight guidance system. We also assume that a change by the pilot on the MCP/FCU will override the information provided by the FMS.
Figure F- 1: Aircraft Flight Path Step 1 Step 2 Step 3 Step 4 Step 6 Step 0 Waypoint 1 Waypoint 2 Waypoint 3 Waypoint 4 Waypoint 5 Waypoint 6 Waypoint 7 HDG: 293o Alt: FL250 HDG: 280o Alt: FL220 HDG: 275o Alt: FL220 HDG: 301o Alt: FL210 Step 5 HDG: 275o Alt: FL240 Waypoint 8 HDG: 301o Alt: FL170 Step 7 HDG: 301o Alt: FL200 HDG: 301o Alt: FL190
Appendix F F-6 Table F- 1. BDS 4016 Parameters Step 0 Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 VNAV 1 1 0 0 0 0 1 1 Alt Hold 0 0 1 1 0 0 0 0 Approach 0 0 0 0 0 0 1 1 Target Altitude Source1 11 11 01 01 10 00 11 11 Table F- 2: BDS 6216 Parameters Step 0 Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Autopilot Engage 1 1 1 1 1 0 1 1 VNAV 1 1 0 0 0 0 1 1 LNAV 1 1 1 0 1 0 1 1 Alt Hold 0 0 1 1 0 0 0 0 Approach 0 0 0 0 0 0 1 1 Selected Altitude MCP MCP MCP MCP MCP MCP MCP MCP
Appendix F F-7 F.3.12.1 Step 0 F.3.12.1.1 Assumptions:
F.3.12.1.1.1 Aircraft is flying an en-route segment; the pilot engages the use of the FMS.
F.3.12.1.1.2 The pilot has engaged the Autopilot System F.3.12.1.2 Maneuver: Level Flight to Waypoint 1 F.3.12.1.3 The aircraft autopilot levels off the aircraft at the specified heading and altitude based on the waypoint information provided by the FMS system
(i.e., HDG: 293o, ALT: FL250). The transponder sets the parameters as follows:
F.3.12.1.3.1 BDS 4016 and BDS 6216 Parameters:
F.3.12.1.3.1.1 When the pilot engages the FMS system as the vertical driving source for the aircraft, VNAV is set to “ONE”.
F.3.12.1.3.1.2 Altitude hold is set to “ZERO”.
F.3.12.1.3.1.3 The pilot has not armed the approach. Therefore, approach mode is set to “ZERO”.
F.3.12.1.3.2 BDS 4016 Specific Parameters:
F.3.12.1.3.2.1 Target Altitude Source represents the use of FMS (i.e., “11”) as the source being used for selected altitude information.
F.3.12.1.3.2.2 Reserved F.3.12.1.3.3 BDS 6216 Specific Parameters:
F.3.12.1.3.3.1 Once engaged by the pilot, the Autopilot system sets Autopilot Engage to “ONE”.
F.3.12.1.3.3.2 When the pilot engages the FMS system as the lateral driving source for the aircraft, LNAV is set to “ONE”.
F.3.12.1.3.3.3 Selected Altitude field is populated with MCP/FCU selected altitude and the selected altitude type is set to MCP/FCU.
F.3.12.2 Step 1 F.3.12.2.1 Assumptions:
F.3.12.2.1.1 The pilot has made no changes on the MCP/FCU system.
Appendix F F-8 Note: The altitude in the MCP/FCU must be higher or lower than the achieve altitude desired by the FMS. If not, the MCP/FCU will act as an upper or lower altitude restriction.
F.3.12.2.1.2 Reserved F.3.12.2.2 Maneuver: Descent from Waypoint 1 to Waypoint 2 F.3.12.2.2.1 The FMS determines a descent and heading change as required and transmits the appropriate information to the autopilot system (i.e., HDG:
280o, ALT: FL220). The transponder sets the parameters as follows:
F.3.12.2.2.2 BDS 4016 and BDS 6216 Parameters:
F.3.12.2.2.2.1 VNAV remains set to “ONE” as FMS system is still providing guidance to the aircraft.
F.3.12.2.2.2.2. The pilot has not engaged altitude hold and altitude hold remains set to “ZERO”.
F.3.12.2.2.2.3 The pilot has not armed a method of approach.
Therefore, approach mode remains set to “ZERO”.
F.3.12.2.2.3 BDS 4016 Specific Parameters:
F.3.12.2.2.3.1 Target Altitude Source represents the use of FMS (i.e., “11”) as the source for selected altitude information.
F.3.12.2.2.3.2 Reserved F.3.12.2.2.4 BDS 6216 Specific Parameters:
F.3.12.2.2.4.1 Autopilot Engage remains set to “ONE”.
F.3.12.2.2.4.2 LNAV remains set to “ONE” as the FMS is still providing guidance to the aircraft.
F.3.12.2.2.4.3 Selected Altitude field is populated with MCP/FCU selected altitude and the selected altitude type is set to MCP/FCU.
F.3.12.3 Step 2 F.3.12.3.1 Assumptions:
F.3.12.3.1.1 The pilot has made no heading or altitude changes on the MCP/FCU system.
F.3.12.3.1.2 Upon leveling off, the pilot engages Altitude Hold.
F.3.12.3.2 Maneuver: Level Flight from Waypoint 2 with Altitude Hold engaged
Appendix F F-9 F.3.12.3.2.1 The aircraft achieves the desired heading and altitude provided by the FMS system, and levels off at waypoint 2. The transponder sets the parameters as follows:
F.3.12.3.2.2 BDS 4016 and BDS 6216 Parameters:
F.3.12.3.2.2.1 VNAV is set to “ZERO.”
F.3.12.3.2.2.2 Altitude Hold is set it to “ONE.”
F.3.12.3.2.2.3 The pilot has not armed a method of approach.
Therefore, approach mode remains set to “ZERO.”
F.3.12.3.2.3 BDS 4016 Specific Parameters:
F.3.12.3.2.3.1 Target Altitude Source represents the use of Aircraft Altitude (i.e., “01”) as the source being used for selected altitude information.
F.3.12.3.2.3.2 Reserved F.3.12.3.2.4 BDS 6216 Specific Parameters:
F.3.12.3.2.4.1 Autopilot Engage remains set to “ONE.”
F.3.12.3.2.4.2 LNAV remains set to “ONE” as the FMS is still providing guidance to the aircraft.
F.3.12.3.2.4.3 Selected Altitude field is populated with MCP/FCU selected altitude and the selected altitude type is set to MCP/FCU.
F.3.12.4 Step 3 F.3.12.4.1 Assumptions:
F.3.12.4.1.1 The pilot has input a cleared heading assigned by ATC into the MCP/FCU.
F.3.12.4.1.2 Reserved F.3.12.4.2 Maneuver: Altitude Hold with Heading change from Waypoint 3 to Waypoint 4 F.3.12.4.2.1 As the aircraft approaches waypoint 3, the pilot received a new heading from ATC deviating from the heading information being provided by the FMS system (i.e., HDG: 275o). The pilot enters the cleared heading into the MCP/FCU, and disengages the use of the FMS system as the lateral guidance source. The autopilot remains engaged, and begins the maneuver to achieve the selected heading received from the MCP/FCU for waypoint 4. The transponder will set the parameters as follows:
F.3.12.4.2.2 BDS 4016 and BDS 6216 Parameters:
Appendix F F-10 F.3.12.4.2.2.1 VNAV remains set to “ZERO” since the aircraft is still in Altitude Hold mode.
F.3.12.4.2.2.2 Altitude hold remains to “ONE.”
F.3.12.4.2.2.3 The pilot has not armed a method of approach.
Therefore, approach mode remains set to “ZERO.”
F.3.12.4.2.3 BDS 4016 Specific Parameters:
F.3.12.4.2.3.1 Target Altitude Source represents the use of Aircraft Altitude (i.e., “01”) as the source for selected altitude information.
F.3.12.4.2.3.2 Reserved F.3.12.4.2.4 BDS 6216 Specific Parameters:
F.3.12.4.2.4.1 Autopilot Engage remains set to “ONE.”
F.3.12.4.2.4.2 LNAV is set to “ZERO” as the aircraft is now using information from the MCP/FCU.
F.3.12.4.2.4.3 Selected Altitude field is populated with MCP/FCU selected altitude and the selected altitude type is set to MCP/FCU.
F.3.12.5 Step 4 F.3.12.5.1 Assumptions:
F.3.12.5.1.1 The pilot re-engages the use of FMS as the lateral guidance source for the aircraft.
F.3.12.5.1.2 The pilot has input a cleared altitude received from ATC.
F.3.12.5.2 Maneuver: ATC Directed Climb from Waypoint 5 to Waypoint 6 F.3.12.5.2.1 As the aircraft approaches waypoint 5, the pilot received a new altitude from ATC deviating from the altitude information being provided by the FMS system. The pilot enters the cleared altitude into the MCP/FCU
(i.e., FL240), and disengages the altitude hold functionality. The autopilot remains engaged, pilot engages a new vertical mode to initiate a climb (e.g. vertical speed mode) and the aircraft begins the maneuver to achieve the selected altitude received from the MCP/FCU for waypoint 6. The transponder will set the parameters as follows:
F.3.12.5.2.2 BDS 4016 and BDS 6216 Parameters:
F.3.12.5.2.2.1 VNAV remains set to “ZERO.”
F.3.12.5.2.2.2 Altitude hold is set to “ZERO.”
Appendix F F-11 F.3.12.5.2.2.3 The pilot has not armed a method of approach.
Therefore, approach mode remains set to “ZERO.”
F.3.12.5.2.3 BDS 4016 Specific Parameters:
F.3.12.5.2.3.1 Target Altitude Source represents the use of MCP/FCU (i.e., “10”) as the source for selected altitude information.
F.3.12.5.2.3.2 Reserved F.3.12.5.2.4 BDS 6216 Specific Parameters:
F.3.12.5.2.4.1 Autopilot Engage remains set to “ONE.”
F.3.12.5.2.4.2 LNAV is set to “ONE” as the FMS is now providing lateral guidance to the aircraft.
F.3.12.5.2.4.3 Selected Altitude field is populated with MCP/FCU selected altitude and the selected altitude type is set to MCP/FCU.
F.3.12.6 Step 5 F.3.12.6.1 Assumptions:
F.3.12.6.1.1 The pilot has not re-engaged the use of FMS as the vertical guidance source for the aircraft.
F.3.12.6.1.2 ATC has cleared the pilot to fly an assigned heading.
F.3.12.6.1.3 ATC has cleared the pilot to fly an assigned altitude.
F.3.12.6.1.4 The pilot has disengaged the autopilot.
F.3.12.6.2 Maneuver: Manual flown descent F.3.12.6.2.1 The aircraft achieves the desired altitude provided by the MCP/FCU, and levels off at waypoint 6. The pilot receives a new altitude and heading from ATC. The pilot disengages the autopilot. The transponder will set the parameters as follows:
F.3.12.6.2.2 BDS 4016 and BDS 6216 Parameters:
F.3.12.6.2.2.1 VNAV remains set to “ZERO.”
F.3.12.6.2.2.2 Altitude Hold remains set to “ZERO.”
F.3.12.6.2.2.3 The pilot has not armed a method of approach.
Therefore, approach mode remains set to “ZERO.”
F.3.12.6.2.3 BDS 4016 Specific Parameters:
Appendix F F-12 F.3.12.6.2.3.1 Target Altitude Source represents Unknown (i.e., “00”) as the source for selected altitude information.
F.3.12.6.2.3.2 Reserved F.3.12.6.2.4 BDS 6216 Specific Parameters:
F.3.12.6.2.4.1 Autopilot Engage is set to “ZERO.”
F.3.12.6.2.4.2 LNAV is set to “ZERO” as the pilot is not using the autopilot to maintain lateral guidance.
F.3.12.6.2.4.3 Selected Altitude field is populated with MCP/FCU selected altitude and the selected altitude type is set to MCP/FCU.
F.3.12.7 Step 6 F.3.12.7.1 Assumptions:
F.3.12.7.1.1 The pilot re-engages LNAV and VNAV.
F.3.12.7.1.2 The pilot re-engages the Autopilot System.
F.3.12.7.1.3 The pilot engages Approach mode.
F.3.12.7.2 Maneuver: Approach Mode from Waypoint 7 F.3.12.7.2.1 The aircraft levels off at the cleared altitude and heading. ATC clears the pilot for approach. The pilot re-engages the use of the FMS as the lateral and vertical guidance source for the aircraft and arms the Approach mode. The transponder will set the parameters as follows:
F.3.12.7.2.2 BDS 4016 and BDS 6216 Parameters:
F.3.12.7.2.2.1 VNAV is set to “ONE” as the aircraft is now using information from the FMS system.
F.3.12.7.2.2.2 Altitude Hold remains set to “ZERO” F.3.12.7.2.2.3 Approach Mode is set to “ONE.”
F.3.12.7.2.3 BDS 4016 Specific Parameters:
F.3.12.7.2.3.1 Target Altitude Source represents the use of the FMS
(i.e., “11”) as the source for selected altitude information.
F.3.12.7.2.3.2 Reserved F.3.12.7.2.4 BDS 6216 Specific Parameters:
F.3.12.7.2.4.1 Autopilot Engage is set to “ONE.”
Appendix F F-13 F.3.12.7.2.4.2 LNAV is set to “ONE” as the FMS is now providing lateral guidance to the aircraft.
F.3.12.7.2.4.3 Selected Altitude field is populated with MCP/FCU selected altitude and the selected altitude type is set to MCP/FCU.
F.3.12.8 Step 7 F.3.12.8.1 Assumptions:
F.3.12.8.1.1 ATC provides the pilot with a final altitude clearance.
F.3.12.8.1.2 The FMS system will fly a step down approach to the cleared altitude.
F.3.12.8.2 Maneuver: Step Down Approach to Waypoint 8 F.3.12.8.2.1 The pilot enters the final altitude clearance into the MCP/FCU. The pilot continues the use of the Autopilot system. The pilot continues the use of the FMS as the vertical and lateral guidance source for the aircraft. The approach mode remains engaged. The transponder will set the parameters as follows:
F.3.12.8.2.2 BDS 4016 and BDS 6216 Parameters:
F.3.12.8.2.2.1 VNAV is set to “ONE” as the aircraft is now using information from the FMS system.
F.3.12.8.2.2.2 Altitude Hold remains set to “ZERO” F.3.12.8.2.2.3 Approach Mode is set to “ONE.”
F.3.12.8.2.3 BDS 4016 Specific Parameters:
F.3.12.8.2.3.1 Target Altitude Source represents the use of the FMS
(i.e., “11”) as the source for selected altitude information.
F.3.12.8.2.3.2 Reserved F.3.12.8.2.4 BDS 6216 Specific Parameters:
F.3.12.8.2.4.1 Autopilot Engage is set to “ONE.”
F.3.12.8.2.4.2 LNAV is set to “ONE” as the FMS is now providing lateral guidance to the aircraft.
F.3.12.8.2.4.3 Selected Altitude field is populated with MCP/FCU selected altitude and the selected altitude type is set to MCP/FCU.
Appendix F F-14 Figure F- 2. Format of BDS register 4016, Selected Vertical Intent Information 1 STATUS PURPOSE: To provide ready access to information about the aircraft’s current vertical intentions, in order to improve the effectiveness of conflict probes and to provide additional tactical information to controllers.
1) Target altitude shall be the short-term intent value, at which the aircraft will level off (or has leveled off) at the end of the current maneuver. The data source that the aircraft is currently using to determine the target altitude shall be indicated in the altitude source bits (54 to 56) as detailed below.
Note. This information which represents the real “aircraft intent,” when available, represented by the altitude control panel selected altitude, the flight management system selected altitude, or the current aircraft altitude according to the aircraft’s mode of flight (the intent may not be available at all when the pilot is flying the aircraft).
2) The data entered into bits 1 to 13 shall be derived from the mode control panel/flight control unit or equivalent equipment. Alerting devices may be used to provide data if it is not available from “control” equipment. The associated mode bits for this field (48 to 51) shall be as detailed below.
3) The data entered into bits 14 to 26 shall be derived from the flight management system or equivalent equipment managing the vertical profile of the aircraft.
4) The current barometric pressure setting shall be calculat ed from the value contained in the field (bits 28 to 39) plus 800 mb.
When the barometric pressure setting is less than 800 mb or greater than 1 209.5 mb, the status bit for this field (bit 27) shall be set to indicate invalid data.
5) Reserved bits 40 to 47 shall be set to ZERO (0).
6) Bits 48 to 56 shall indicate the status of the values provided in bits 1 to 26 as follows:
Bit 48 shall indicate whether the mode bits (49, 50 and 51) are already being populated:
1.
2. 0 = No mode information provided
3. 1 = Mode information deliberately provided
4.
Bits 49, 50 and 51:
5.
6. 0 = Not active
7. 1 = Active
8.
9. Reserved bits 52 and 53 shall be set to ZERO (0).
10.
Bit 54 shall indicate whether the target altitude source bits (55 and 56) are actively being populated:
11.
12. 0 = No source information provided
13. 1 = Source information deliberately provided
14.
Bits 55 and 56 shall indicate target altitude source:
15.
16. 00 = Unknown
17. 01 = Aircraft altitude
18. 10 = FCU/MCP selected altitude
19. 11 = FMS selected altitude
20.
21. .
2 MSB = 32 768 feet 3 4 5 MCP/FCU SELECTED ALTITUDE 6 7 Range = [0, 65 520] feet 8 9 10 11 12 13 LSB = 16 feet 14 STATUS 15 MSB = 32 768 feet 16 17 18 FMS SELECTED ALTITUDE 19 20 Range = [0, 65 520] feet 21 22 23 24 25 26 LSB = 16 feet 27 STATUS 28 MSB = 204.8 mb 29 30 31 32 BAROMETRIC PRESSURE SETTING 33 MINUS 800 mb 34 35 Range = [0, 410] mb 36 37 38 39 LSB = 0.1 mb 40 41 42 43 44 RESERVED 45 46 47 48 STATUS OF MCP/FCU MODE BITS 49 VNAV MODE 50 ALT HOLD MODE MCP/FCU Mode bits 51 APPROACH MODE 52 RESERVED 53 54 STATUS OF TARGET ALT SOURCE BITS 55 MSB TARGET ALT SOURCE 56 LSB