Atch 15_IS-GPS-153E_RFC394.pdf

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AUTHORITY DESCRIPTION DATE

RFC-00394 Incorporation of IRN-IS-l 53D-003, IRN-IS-l 53D-004 02-MAR-2021

DISTRIBUTION C. Distribution authorized to U.S. Government Agencies and their contractors (Reason: ADMINISTRATIVE OR OPERATIONAL USE). (Date of Determination: 02 March 2021). Other requests for this document shall be referred to the Global Positioning Systems Directorate (ZACS-PNT).

WARNING--This document contains technical data whose export is restricted by the Arms Export Control Act (Title 22, U.S.C..

2751, et seq.) or Executive Order 12470. Violation of these export-control laws is subject to severe criminal penalties.

Dissemination of this document is controlled under DoD Directive 5230.25 and AFI 61-204

DESTRUCTION NOTICE - For classified documents, follow the procedures in DoD 5220.22-M, National Industrial Security Program Operating Manual (NISPOM), chapter 5, section 7; or DoD 5200.1-R, Information Security Program Regulation, chapter IX. For unclassified, Limited Documents, destroy by any method that will prevent disclosure of contents or reconstruction of the document.

REPRODUCTION - LOCAL REPRODUCTION IS AUTHORIZED

This document contains information EXEMPT FROM MANDATORY DISCLOSURE under the FOIA. Exemotion 3 annlies.

APPROVALS

Authored By: RE Vern Slonaker Checked By: RE Adriana Fukuzato

AUTHORIZED SIGNATURES REPRESENTING DATE

DUNN.MICHAEL. Digitally signed by PNT Capability Area Integration, Portfolio Architect

J.1171235045

DUNN.MICHAEL.J.1171235045

Space and Missile Systems Center - LAAFB Date: 2021.05.1713:55:58--07'00'

INTERFACE SPECIFICATION

UNLESS OTHERWISE SPECIFIED: NUMBERS ARE Interface Control Contractor:

REPRESENTED IN DECIMAL FORM. SAIC (GPS SE&I)

200 N. Pacific Coast Highway, Suite 1800

El SeJ;?;undo, CA 90245

TITLE:

THIS DOCUMENT SPECIFIES TECHNICAL

GPS User Equipment Interface Control Document for the GPS Standard Serial Interface Protocol (GSSIP) of DoD Standard GPS

REQUIREMENTS AND NOTHING HEREIN

UE Radio Receivers CONTAINED SHALL BE DEEMED TO ALTER THE

TERMS OF ANY CONTRACT OR PURCHASE ORDER CODE IDENT DOC NO.

BETWEEN ALL PARTIES AFFECTED SIZE A

66RP1

IS-GPS-153

SCALE: NIA REV: E

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IS-GPS-153E

02-MAR-2021

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REVISION RECORD

REVISION DESCRIPTION DOC DATE APPROVED

NC Initial Release 27-Jun-1995 18-Apr-2002

IRN 001 Incorporate corrections to more accurately reflect the interface operation of the AN/PSN-11, as well as editorial/cleanup changes.

IRN 002 Incorporate changes reflecting the new Advanced Navigation capabilities of the AN/PSN-11(v)1, such as 999 waypoints, 26 waypoints per route, Datum Transformation Parameter output, Bullseye Navigation, Navigation Alerts, Extended Setup, Custom Nav Pages, Extended Status, and Man Overboard Navigation.

A Re-Baseline the document: remove change bars and unnecessary blank pages/spaces, and renumber the remaining pages

04-May-1999 03-Jan-2002

B Remove characteristics determined to be specific to the architecture of the AN/PSN-11 and AN/PSN-11(v)1 and add enhancements to additional UE capabilities such as Waypoint Remarks, Extended Routes, SV select/deselect by PRN, 10 PPS UTC, Ionospheric Corrections, Standardized Reprogramming, All-in-View, Integrity, and the Selective Availability/Anti-Spoofing Module (SAASM) security architecture, as well as a comprehensive RCVR Capabilities message.

02-Apr-2002 23-Dec-2002

C Superseded by IS-GPS-153D. Incorporate corrections and resolutions to technical issues outstanding from Revision B.

10-Dec-2002 18-Apr-2002

IRN 003 Add RCVR/HAE serial number output, 10 PPS ready status update. Correct typos, add clarifications (general clean up), and make technical corrections.

D Add new/improved setup (separated for RCVR setup, Units Setup, and COM Port/Interface Setup), Uncorrected Line of Sight data, User Coordinates/Grids, CAS 9-Line Briefs, Azimuth Determination/GLS, WMM coefficients, and Search Grids.

11-Jun-2007 23-Jul-2007

E Incorporates the following IRNs:

Incorporation of IRN-IS-153D-003 (RFC-172) Incorporation of IRN-IS-153D-004 (RFC-405) Incorporated changes needed for Modernized GPS

(RFC-394)

02-Mar-2021 17 -May-2021

02-MAR-2021

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TABLE OF CONTENTS

SECTION PAGE

1. SCOPE ............................................................................................................... 1-1

1.1 Introduction ....................................................................................................... 1-1

1.2 Purpose .............................................................................................................. 1-1

1.3 Item Description ............................................................................................... 1-1

1.3.1 GPS User Equipment ......................................................................................... 1-1

1.3.2 GPS Standard Serial Interface Protocol ............................................................ 1-2

1.3.3 Pulse Per Second (PPS) Interface ...................................................................... 1-2

1.4 Agencies/Contractors ....................................................................................... 1-2

1.4.1 GPS .................................................................................................................... 1-2

1.4.2 Interface Control Contractor (ICC) ................................................................... 1-2

2 APPLICABLE DOCUMENTS ................................................................................ 2-1

2.1 Government Documents .................................................................................. 2-1

2.2 Non-Government Documents .......................................................................... 2-2

3 DATA LINK CHARACTERISTICS ........................................................................... 3-1

3.1 GSSIP Definition ................................................................................................ 3-1

3.1.1 Baud Rate ......................................................................................................... 3-1

3.1.1.1 Logic Levels Convention ................................................................................... 3-1

3.1.2 GSSIP Messages ................................................................................................ 3-1

3.1.2.1 Data Messages .................................................................................................. 3-1

3.1.2.2 Command Messages ......................................................................................... 3-2

3.1.3 Message Format ............................................................................................... 3-2

3.1.3.1 Header Description ........................................................................................... 3-5

3.1.3.1.1 Word 1, DLE/SOH ............................................................................................. 3-5

3.1.3.1.2 Word 2, Message ID ......................................................................................... 3-5

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3.1.3.1.3 Word 3, Word Count ........................................................................................ 3-5

3.1.3.1.4 Word 4, Flags .................................................................................................... 3-6

3.1.3.1.5 Word 5, Checksum #1 ....................................................................................... 3-9

3.1.3.2 Data Description ............................................................................................... 3-9

3.1.3.2.1 Words 6 to 5+N, (N Data Words) ...................................................................... 3-9

3.1.3.2.2 Word 6+N, Checksum #2 .................................................................................. 3-9

3.2 GPS Standard Serial Interface Protocol ............................................................ 3-10

3.2.1 Message Protocol ............................................................................................. 3-10

3.2.1.1 Message Timing ................................................................................................ 3-10

3.2.1.2 Message Exchange ........................................................................................... 3-10

3.2.1.2.1 Acknowledge (ACK) .......................................................................................... 3-11

3.2.1.2.2 Negative Acknowledge (NAK) ........................................................................... 3-12

3.2.1.3 Messages Requiring Acceptance ...................................................................... 3-12

3.2.1.4 Default Messages ............................................................................................. 3-13

3.2.1.4.1 STANDARD/CUSTOM Modes ............................................................................ 3-13

3.2.1.4.2 INSTRUM Mode ................................................................................................ 3-14

3.2.1.5 Header Only Messages ..................................................................................... 3-14

3.2.1.5.1 Connect ............................................................................................................. 3-15

3.2.1.5.2 Disconnect ........................................................................................................ 3-16

3.2.1.5.3 Request ............................................................................................................. 3-17

3.2.1.5.4 Resynchronization (RESYNC) ............................................................................ 3-20

3.2.1.6 Message ID, Data Field, and Coded Value Requirements ................................. 3-20

3.2.1.6.1 Development ..................................................................................................... 3-20

3.2.1.6.1.1 Spare Message IDs, Words, and Bits. ................................................................ 3-20

3.2.1.6.1.2 Reserved Message IDs, Words, Bits, and Field Values. ..................................... 3-21

3.2.1.6.1.3 Not Used Field Values. ...................................................................................... 3-21

3.2.1.6.1.4 Legacy Message IDs. .......................................................................................... 3-22

3.2.1.6.1.5 RCVR Specification Functional Tailoring. ........................................................... 3-22

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3.2.1.6.1.6 RCVR Dependent Performance. ......................................................................... 3-22

3.2.1.6.2 Remote Control Input/Output and Data Transfer Messages. ........................... 3-23

4 COMMUNICATIONS MODES ............................................................................. 4-1

4.1 Remote Control Mode ...................................................................................... 4-1

4.2 Data Transfer Mode ......................................................................................... 4-1

4.2.1 RCVR to INTER Data Transfer ............................................................................ 4-2

4.2.2 INTER to RCVR Data Transfer ............................................................................ 4-4

4.2.3 RCVR to RCVR Data Transfer ............................................................................ 4-5

4.2.4 INTER Initiated RCVR to RCVR/INTER Data Transfer ........................................ 4-6

5 PPS INTERFACE CHARACTERISTICS ................................................................... 5-1

5.1 Definition .......................................................................................................... 5-1

5.2 Associated Serial Data Interface ....................................................................... 5-1

5.3 Electrical Characteristics ................................................................................... 5-1

5.4 1PPS In .............................................................................................................. 5-1

5.5 PPS Out ............................................................................................................. 5-1

5.6 Output Switching .............................................................................................. 5-6

5.7 RCVR to INTER Time Transfer ........................................................................... 5-6

5.8 INTER to RCVR Time Transfer ........................................................................... 5-8

5.9 RCVR to RCVR Time Transfer ............................................................................ 5-9

5.10 INTER Initiated RCVR to RCVR/INTER Time Transfer ........................................ 5-11

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LIST OF TABLES

TABLE PAGE

3-1 Inter-Character and Inter-Message Gaps ......................................................... 3-4

3-2 Header Flag Word Logic Diagram ..................................................................... 3-6

3-3 ACK/NAK Time-Outs (milliseconds) .................................................................. 3-11

3-4 Accept/Reject Time-Outs (milliseconds) .......................................................... 3-14

3-5 Requested Message Exceptions ....................................................................... 3-17

4-1 RCVR to INTER Data Transfer ............................................................................ 4-3

4-2 INTER to RCVR Data Transfer ............................................................................ 4-4

4-3 RCVR to RCVR Data Transfer ............................................................................ 4-5

4-4 INTER Initiated RCVR/INTER Data Transfer ...................................................... 4-6

5-1 RCVR to INTER Precise Time Transfer ............................................................... 5-7

5-2 INTER to RCVR Precise Time Transfer ............................................................... 5-9

5-3 RCVR to RCVR Precise Time Transfer ............................................................... 5-11

5-4 INTER Initiated RCVR to RCVR/INTER Time Transfer ........................................ 5-12

LIST OF FIGURES

FIGURE PAGE

3-1 GSSIP Message Format ..................................................................................... 3-2

3-2 STANDARD GSSIP Character Format ................................................................ 3-3

3-3 INSTRUM GSSIP Character Format ................................................................... 3-3

4-1 INTER Initiated RCVR to RCVR/INTER Data Transfer ........................................ 4-7

5-1 PPS IN ................................................................................................................ 5-3

5-2 PPS OUT ............................................................................................................ 5-3

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APPENDICES

APPENDIX PAGE

A DATA FORMATS ................................................................................................ A-1

B MESSAGES ........................................................................................................ B-1

C MESSAGE NUMBER ALLOCATION ..................................................................... C-1

D CRYPTOGRAPHIC STATUS DATA (KDP AND NON-KDP DEFINITIONS) ............... D-1

E ACRONYMS AND ABBREVIATIONS ..................................................................... E-1

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1-1

IS153---1

1. SCOPE

IS153---2

1.1 Introduction

IS153---3

This Interface Specification (IS) identifies and defines the interface protocol and associated data message formats for the Department of Defense (DoD) Standard Global Positioning System (GPS) User Equipment (UE) Radio Receiver (RCVR) (hereafter referred to as GPS UE RCVR or RCVR) Standard Serial Interface Protocol, in support of GPS UE. Electrical characteristics employed by a RCVR implementing GPS Standard Serial Interface Protocol (GSSIP) are not defined in this document. Refer to the RCVR specification for this information.

IS153---4

The use of the word "shall" in this document is intended to convey requirements on the RCVR and an Intelligent Terminal (INTER) in order to be determined compliant with the interface requirements set forth in this document. Whether or not these items are actually requirements on the RCVR or INTER depends on whether compliance with this document is levied as a requirement, either programmatically or contractually.

IS153---5

The use of the term INTER in this document shall specifically refer to INTERs required to be compliant with this revision of this IS, and not necessarily INTERs required to be compliant with earlier revisions of this document. When specifically referring to INTERs required to be compliant with earlier revisions of this document, the term "legacy INTER", "legacy application", or "legacy integration" will be used.

IS153---6

1.2 Purpose

IS153---7

The purpose of the GSSIP is to relay messages (i.e., data and command information) between a RCVR and INTER. A computer or another RCVR may serve as an INTER. This IS is designed to be used in conjunction with a RCVR specification to determine what portions of this IS are applicable to a RCVR. The RCVR specification needs to provide mapping between RCVR requirements and data fields, as well as any tailoring of this IS or additions to this IS required for that RCVR.

IS153---8

1.3 Item Description

IS153---9

1.3.1 GPS User Equipment

IS153---10

GPS is a space-based radio navigation system which provides the user with extremely accurate three-dimensional position, three-dimensional velocity, and time of day. The RCVR receives and processes radio signals from satellites and provides data transfer in accordance with (IAW) the interface protocol and message formats for the GSSIP.

1-2

IS153---11

1.3.2 GPS Standard Serial Interface Protocol

IS153---12

The GSSIP is a bi-directional, full duplex, asynchronous, serial interface. GSSIP data is transmitted in bytes (8-bit fields). The GSSIP is used to transmit and receive data and command messages.

IS153---13

1.3.3 Pulse Per Second (PPS) Interface

IS153---14

The PPS Interface includes a 1PPS input pulse, selectable output pulse, and associated GSSIP data messages.

IS153---15

The 1PPS input pulse is provided by an external source with the leading edge representing the Coordinated Universal Time (UTC) second rollover.

It is input at one pulse per second into the RCVR's input. Refer to Figure 5-1 for input pulse characteristics.

IS153---16

The output pulse is selectable between Off, 1PPS Time Mark (T-Mark), 1PPS UTC, or 10 PPS UTC. When 1PPS T-Mark is selected, the pulse is synchronous with the RCVR's internal tracking sequence. When 1PPS UTC is selected, the pulse is synchronous with the UTC 1 second rollover. When 10 PPS UTC is selected, a pulse is generated at 10 hertz (Hz) that is synchronous with the UTC 0.1 second rollover. Refer to Figure 5-2 for output pulse characteristics.

IS153---17

1.4 Agencies/Contractors

IS153---18

All agencies involved in the development of this IS are identified below.

IS153---19

1.4.1 GPS

IS153---20

The Space and Missile Systems Center (SMC), is responsible for the development, configuration control, and life cycle support of the GPS UE.

The point of contact for the GSSIP Interface Control Working Group (ICWG) is:

SMC/PCU

483 North Aviation Boulevard.

El Segundo, CA 90245-2808

Phone (310) 653-3442

DSN 633-3442

Fax (310) 653-3676

IS153---22

1.4.2 Interface Control Contractor (ICC)

IS153---23

The designated Interface Control Contractor (ICC) is responsible for the basic preparation, approval, distribution and retention of the ICD in accordance with GP-03-001A.

2-1

IS153---24

2. APPLICABLE DOCUMENTS

IS153---25

2.1 Government Documents

IS153---26

The following documents form a part of the GPS UE standards as specified herein. Refer to the document revision that is applicable to the receiver being used.

IS153---299

IS-GPS-800 NAVSTAR GPS Space Segment/ User Segment L1C 13 Aug 2020 Interfaces

IS153---298

IS-GPS-722E NAVSTAR GPS Military Unique User Segment/ 28 Aug 2014 National Security Agency (NSA) Key IRN-IS-722E-001 Utilization and Handling Interface IRN-IS-722E-002 Specification

IRN-IS-722E-003

IRN-IS-722E-004

IRN-IS-722E-005

IRN-IS-722E-006

IRN-IS-722E-007

IS153---297

IS-GPS-705G NAVSTAR GPS Space Segment/User Segment L5 13 Aug 2020 Interfaces

IS153---296

IS-GPS-224E NAVSTAR GPS Selective Availability and Anti- 28 Aug 2014 Spoofing (SA/A-S) Receiver Design IRN-IS-224E-001 Requirements

IRN-IS-224E-002

IRN-IS-224E-003

IS153---268

NGA.STND.0036 Department of Defense World Geodetic System 08 Jul 2014 1984 Its Definition and Relationships with Local Geodetic Systems

IS153---269

NGA.STND.0037 Universal Grids, and Grid Reference Systems 28 Feb 2014

IS153---270

IS-GPS-200L NAVSTAR GPS Space Segment/Navigation User 13 Aug 2020 Interfaces

IS153---271

IS-GPS-154C GPS User Equipment Interface Specification 10 Dec 2002 for AN/PSN-11 Unique Interface IRN-154C-001 Characteristics 06 Apr 04

2-2

IS153---272

IDD-KDP-002 NAVSTAR Global Positioning System (GPS) Version 2.03 Selective Availability Anti-Spoofing Module Nov 24, 1999 (SAASM) Key Data Processor (KDP) Application Microprocessor (AuP) Interface Design Document

IS153---267

ICD-GPS-700H NAVSTAR GPS Military-Unique Space Segment / 16 Mar 2020 User Segment Interfaces

IRN-ICD-700H-001

IRN-ICD-700H-002

IRN-ICD-700H-003

IRN-ICD-700H-006

IS153---43

2.2 Non-Government Documents

IS153---44

The following documents form a part of the GPS UE standards as specified herein.

IS153---301

IS-QZSS-PNT-003 Quasi-Zenith Satellite System Interface 05 Nov 2018 Specification Satellite Positioning, Navigation and Timing Service;

IS153---300

OS-SIS-ICD European GNSS (Galileo) Open Service Signal- DEC 2016 In-Space Interface Control Document

IS153---273

EIA Standard Interface between Data Terminal Equipment RS-232-D and Data Communication Equipment Employing 02 Jan 87 Serial Binary Data Interchange

IS153---274

EIA Standard Electrical Characteristics of Balanced RS-422-A Voltage Digital Interface Circuits Dec 78

IS153---275

RTCM Paper 194-93/ RTCM Recommended Standards for Differential SC104-STD NAVSTAR GPS Service Version 2.1 03 Jan 94

IS153---276

X3.4-1977 American National Standard Code for Information Interchange (ASCII)

IS153---277

NMEA 0183 Standard for Interfacing Marine Version 3.01 Electronic Devices Jan 2002

2-3

IS153---278

NATO STANAG 4392 Standard for Interfacing Marine Edition 1 Electronic Devices 14 Nov 95

3-1

IS153---64

3. DATA LINK CHARACTERISTICS

IS153---65

The STANDARD, INSTRUM and CUSTOM modes of the GSSIP share many of the same characteristics. All configurations are described in this section with the differences noted.

IS153---66

3.1 GSSIP Definition

IS153---67

The GSSIP provides a capability to transfer data and commands between an INTER and a RCVR. An INTER is user-supplied peripheral equipment which operates within the constraints of the GSSIP.

IS153---68

For a definition of the standard implementation of GSSIP using RS-232 and RS-422 electrical characteristics on an HD-15 connector, refer to IS-GPS- 154.

IS153---69

3.1.1 Baud Rate

IS153---70

For the purpose of this document, baud is defined as the serial data transmission rate of signal elements (bits) per second. In the STANDARD mode, the interface transmits and receives data at 9.6 kilobaud. In the INSTRUM mode, the interface transmits data at 76.8 kilobaud and receives data at 19.2 kilobaud. The CUSTOM mode allows independent selection of baud rates for the receive and transmit components of the interface.

IS153---71

The 9600 in/out baud is referred to as the STANDARD configuration. The 19200 baud in/76800 baud out configuration is referred to as the INSTRUM configuration.

IS153---72

3.1.1.1 Logic Levels Convention

IS153---73

The interchange circuits transferring data signals across the interface point will hold "marking" (defined as logic 1) or "spacing" (defined as logic 0) conditions for the total nominal duration of each signal element.

IS153---74

3.1.2 GSSIP Messages

IS153---75

Two types of messages occur on the interface: (1) data messages and (2) command messages.

IS153---76

3.1.2.1 Data Messages

IS153---77

Data messages include header and data words for transfer to or from the RCVR. The data portion of these messages may contain commands to the RCVR. These commands may be distinguished in this IS by their association with a specific message identifier (ID).

3-2

IS153---78

3.1.2.2 Command Messages

IS153---79

Command messages are used to request specific information from the RCVR or are output from the RCVR as acknowledge (ACK)/negative acknowledge (NAK) messages. Header-only-messages are used for the transfer of this type of command. These commands are distinguished by their association with the specific set of commands discussed below (connect, disconnect, request, ACK, NAK, etc.). They are not associated with a specific message ID, but may be used with all message IDs.

IS153---80

3.1.3 Message Format

IS153---81

The data and commands transmitted using GSSIP are formatted into messages.

Messages contain a header portion and may or may not contain a data portion. The maximum number of words a message may contain is 2,054 (five header portion words and 2,049 data portion words). The header portion contains four header words plus one header checksum #1 word (words 1 through 5). The data portion may contain a maximum of 2,048 data words (words 6 through 5+N, where 1 ≤ N ≤ 2,048) plus one data checksum #2 word.

See Figure 3-1 for an example message format. The data portion of those message present in the Baseline and Revision A of this document will not exceed 100 words. See Table B-1 for a list of affected messages.

IS153---82

IS153---83

Figure 3-1. GSSIP Message Format

IS153---84

A message word length is 16 bits. However, the GSSIP transmits data in 8-bit groups (bytes). Thus, two bytes are required in order to make up one message word. Only whole 16-bit data words may be transmitted or received. The number of bytes is always even, double the number of data words.

IS153---85

In the STANDARD mode, a byte is transmitted within a 10-bit character.

The 10-bit character is transmitted as follows: start bit first, followed by 8 bits of data, and the stop bit. The character format is presented in Figure 3-2. Refer to APPENDIX A for a description of transmission order.

3-3

IS153---86

IS153---87

Figure 3-2. STANDARD GSSIP Character Format

IS153---88

In the INSTRUM mode, a byte is transmitted within an 11-bit character.

The 11-bit character is transmitted as follows: start bit first, followed by 8 bits of data, the parity bit, and the stop bit. The character format is presented in Figure 3-3. Refer to APPENDIX A for a description of transmission order.

IS153---89

Data can be transmitted or received in the CUSTOM configuration either as an eleven-bit character (odd or even parity) or as a ten-bit character (no parity). The user defines whether parity is even or odd, which results in an eleven-bit character, or no parity, which results in a ten-bit character. The CUSTOM configuration allows the user to define the transmit and receive parities independently. Refer to Message 5331 for details on CUSTOM configuration setup. Special attention should be given to Note 4 of Message 5331.

IS153---90

IS153---91

Figure 3-3. INSTRUM GSSIP Character Format

3-4

IS153---92

The maximum period of time between the interface characters (inter-character gap) in the same message may not exceed the character gaps listed in Table 3-1 measured from the trailing edge of the stop bit of the first character to the leading edge of the start bit of the following character. During this period of time, the interface is held in the "marking" (level "1") state. The minimum period of time between characters that are the ending and beginning characters of two messages (inter-message gap) is listed in Table 3-1, and is measured from the trailing edge of the stop bit of the final checksum of the first message to the leading edge of the start bit of the Data Link Escape (DLE)/Start of Header (SOH) word of the following message. The interface is in the "marking" (level "1") state during this time.

IS153---93

Table 3-1. Inter-Character and Inter-Message Gaps

IS153---94

Maximum Inter-Character Gap

Minimum

Inter-Message Gap

Baud Rate

10 bit Character

(ms)

11 bit Character

(ms)

10 bit Character

(ms)

11 bit Character

(ms)

75 666.67 733.33 1333.33 1466.67 150 333.33 366.67 666.67 733.33

300 166.67 183.33 333.33 366.67

600 83.33 91.67 166.67 183.33

1200 41.67 45.83 83.33 91.67

2400 20.83 22.92 41.67 45.83

4800 10.42 11.46 20.83 22.92

9600 5.21 5.73 10.42 11.46

19200 2.60 2.86 5.21 5.73

38400 1.30 1.43 2.60 2.86

76800 0.65 0.72 1.30 1.43

115200 0.43 0.48 0.87 0.95

230400 0.22 0.24 0.43 0.48

460800 0.11 0.12 0.22 0.24

921600 0.05 0.06 0.11 0.12

IS153---95

Note: For any baud rate, the maximum inter-character gap is 5 character times and the minimum inter-message gap is 10 character times. On input, no action shall be taken to verify the cumulative inter-character gap of messages received. As long as the inter-character gap between any two characters does not exceed the allowed maximum, received messages shall not be rejected on the basis of inter-character gap. These rules apply even when a RCVR implements baud rates other than those identified in the table above.

IS153---96

The sending INTER's protocol will allow for one retransmission of any message that is negative acknowledged or has timed out. This allows the RCVR to control the rate of commands that occur over the interface at any given time.

3-5

IS153---97

3.1.3.1 Header Description

IS153---98

The five message words of the header portion of a message are: DLE/SOH word, message ID word, message word count word, flags word, and checksum #1 word. The transmission order of the header words is described in APPENDIX A. The header words are described in the following paragraphs.

IS153---99

3.1.3.1.1 Word 1, DLE/SOH

IS153---100

Message word 1 of the GSSIP message header contains a 16-bit field composed of the DLE and SOH bytes and is defined below:

DLE - An 8-bit constant with a value of 255 decimal (binary "11111111") forming the first byte (Start Byte) of message word 1 of the header.

In addition, the DLE constant is also used to reestablish interface synchronization.

SOH - An 8-bit constant with a value of 129 decimal (binary "10000001") forming the second byte of message word 1 of the header.

IS153---101

3.1.3.1.2 Word 2, Message ID

IS153---102

Message word 2 forms a 16-bit field that indicates a particular message ID. A message ID of zero indicates the Universal Reset Message. The Universal Reset Message is used for the transmission of flags when no valid message ID is available (i.e., ready/not ready and resynchronize).

The legal range for the message ID field is 0 to 65,535. (Refer to Table B-1 for a list of valid Message IDs contained in this IS and Table C-1 for a list of Message IDs allocated to other ISs and applications).

IS153---103

3.1.3.1.3 Word 3, Word Count

IS153---104

Message word 3 forms a 16-bit field that indicates the number of message words contained in the data portion of the message excluding the checksum #2 word. A value of zero in this field indicates that it is a header-only-message. The legal range for the word count field is 0 to 2,048, 16-bit words. On input, this field will be used to determine the number of data words contained in the message and to locate the data checksum.

RCVRs and INTERs shall not restrict processing of message data to the message word counts indicated in this IS or any previous revisions. Words unrecognized by the RCVR or INTER may be ignored. The RCVR and INTER shall accept and process messages received of the length defined in Table B-1.

RCVRs or INTERs designed in a way that limits message growth by restricting or hard coding word count shall be considered non-compliant with this IS.

3-6

IS153---105

Table 3-2. Header Flag Word Logic Diagram

IS153---106

IS153---107

3.1.3.1.4 Word 4, Flags

IS153---108

Message word 4 forms a 16-bit field containing protocol and message-related flags. This field provides flag bits necessary for protocol operation. A logic "1" indicates that the flag is "set". Refer to Table 3-2 for a logic diagram of the GSSIP flags.

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The flags are defined below (Input/Output is in reference to the RCVR):

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BIT NO. FLAG DESCRIPTION

0 Spare:

Input - Ignored by the RCVR.

Output - Always set to "0".

1 Spare:

2 Spare:

3 Spare:

4 Request Message Flag:

Input - Indicates a command to the RCVR from an INTER to output the specified message one time over the interface. Refer to paragraph 3.2.1.5.3.

5 Disconnect Message Flag:

Input - Indicates a command to the RCVR from an INTER to disconnect and discontinue transmitting the specified message. The RCVR will disconnect all messages (Universal Reset) if the Message ID is set to zero while this flag is set. Refer to paragraph 3.2.1.5.2.

6 Connect Message Flag:

Input - Indicates a command to the RCVR from an INTER to connect the specified message and output it repetitively at the scheduled update rate. (Up to 40 messages may be connected at one time.)Refer to paragraph 3.2.1.5.1.

7 Invalid Data Flag:

Input - If this bit is set by the INTER, the message is ignored by the

RCVR.

Output - Indicates that an error has rendered the data contained in the

RCVR transmitted message to be questionable. 0 = data valid, 1 = data invalid.

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BIT NO. FLAG DESCRIPTION (continued)

8 Command Reject Flag:

Input - Indicates that a command to the INTER from a RCVR has been rejected ("1") or accepted ("0"). Ignored by the RCVR if the Handshake Flag is not set. Refer to paragraph 3.2.1.3.

Output - Indicates that a command to the RCVR from an INTER has been rejected ("1") or accepted ("0"). Refer to paragraph 3.2.1.3.

9 Handshake Flag:

Input - Indicates that the INTER has responded to a command from the RCVR. The state of the Command Reject Flag indicates whether the command has been accepted or rejected.

Output - Indicates that the RCVR-transmitted-message is a response to an INTER command. The state of the Command Reject Flag indicates whether the command has been accepted or rejected. Refer to paragraph 3.2.1.3.

10 Negative Acknowledge Flag:

Input - Indicates that a RCVR-transmitted-message, with the Acknowledge Request Flag set, has been received by an INTER with data errors.

The receipt of a Negative Acknowledge Flag by the RCVR will cause the RCVR to retransmit the message in error. Refer to paragraph 3.2.1.2.

Output - Indicates that an INTER-transmitted-message, with the Acknowledge Request Flag set, has been received by the RCVR with data errors.

The receipt of a Negative Acknowledge Flag by an INTER will cause the INTER to retransmit the message in error. Refer to paragraph 3.2.1.2.

11 Acknowledge Flag:

Input - Indicates that a RCVR-transmitted-message, with the Acknowledge Request Flag set, has been received by an INTER correctly. Refer to paragraph 3.2.1.2.

Output - Indicates that an INTER-transmitted-message, with the Acknowledge Request Flag set, has been received by the RCVR correctly. Refer to paragraph 3.2.1.2.

12 Acknowledge Request Flag:

Input - Indicates that the message being transmitted by an INTER requires acknowledgment from the RCVR. Refer to paragraph 3.2.1.2.

Output - Indicates that the message being transmitted by the RCVR requires acknowledgment from the INTER. Refer to paragraph 3.2.1.2.

13 Resynchronization (RESYNC) Flag:

Input - Indicates that the INTER has lost synchronization, and RESYNC of the interface should be initiated by the RCVR. Refer to paragraph 3.2.1.5.4.

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BIT NO. FLAG DESCRIPTION (continued)

14 Test Mode Flag:

Input - Indicates that a test is in progress and that the test INTER will be issuing link commands to the RCVR. No INTER (except the test INTER) can use the interface while the Test Mode Flag is set. This flag is rarely utilized.

Output - Echo of the state of the Test Mode Flag of the last received message.

15 Ready Flag:

Input - The Ready Flag set to "1" indicates that the INTER is ready to receive a message. If set to "0", the RCVR will wait for 100 ms to allow the INTER time to become ready.

Output - The Ready Flag set to "1" indicates that the RCVR is ready to receive a message. The Ready Flag set to "0" indicates that the RCVR is not ready to receive a message, and the INTER will wait 100 ms to allow the RCVR to become ready.

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3.1.3.1.5 Word 5, Checksum #1

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A 16-bit checksum is used to validate the header portion of the message.

The header checksum #1 is computed by summing (modulo 216) the set bits contained in header message words 1 through 4 and then complementing the sum (using 2's complement). In the binary summing of the header message words, the carry bits are ignored and only the least significant 16 bits are used as the checksum. When receiving an input message, if checksum #1 is incorrect, the RCVR will discard the message without any further processing.

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3.1.3.2 Data Description

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The data portion of the messages defined in this IS can contain up to a maximum of 2,048 data words (words 6 through 5+N, where 1 ≤ N ≤ 2,048) plus one data checksum #2.

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3.1.3.2.1 Words 6 to 5+N, (N Data Words)

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These 16-bit data field message words are completely transparent to the protocol and have no restrictions on bit patterns or character groupings.

The number of data message words "N", is the same number of words specified in the message word count (message word 3 in the header portion). When the message word count is zero, this field does not exist.

The maximum number of data words is 2,048. Data Words are not transmitted in header only messages.

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3.1.3.2.2 Word 6+N, Checksum #2

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This 16-bit checksum field is used to validate the data portion of the message. It is transmitted as the last message word of any message containing data message words. It is computed by summing (modulo 216) the

3-10 set bits contained in the data message words (words 6 to 5+N, where 1 ≤ N ≤ 2,048) and then complementing the sum (using 2's complement). In the binary summing of the data message words, the carry bits are ignored and only the least significant 16 bits are used as the checksum. This word is not transmitted in header-only messages. The RCVR and INTER shall determine the location of Checksum #2 by using word 3 (word count) of the header, and shall not use the word count indicated in the message definitions in APPENDIX B.

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3.2 GPS Standard Serial Interface Protocol

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3.2.1 Message Protocol

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The INTER acts as the interface controller. The RCVR accepts or rejects commands from the INTER based upon the message protocol constraints, and responds appropriately. The message protocol defines a set of constraints concerning: the ability to issue commands, operation in the absence of commands, and limits to the number and amount of commanded outputs. This protocol supports only one active talking INTER on the interface at any one time.

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3.2.1.1 Message Timing

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Refer to section 3.1.3 for a description of inter-character gap and inter-message gap timing.

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3.2.1.2 Message Exchange

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In order to assure interface integrity, the GSSIP supports acknowledging the receipt of a message. For a message to be acknowledged, the ACK request bit of that message must be set. When the ACK request bit is set, the message must be ACK'd, or NAK'd, within the time specified in Table 3-

3. If an ACK/NAK is not received within the time specified in Table 3-3, the message is retransmitted once after the initial transmission. If the acknowledge request bit is not set, the message exchange is considered complete by the transmitting device once the message is sent. Any message transaction requiring an acknowledgment is not considered complete by the transmitting device until a message acknowledgment is received. The transmitting device allows for one retransmission of a message that is not ACK'd, or NAK'd. The transmitting device shall have a maximum of one message awaiting an acknowledgment at any time. The format for ACK’d and NAK’d messages is specified in the following paragraphs. For further detail on how acknowledgement is performed for Remote Control Mode and Data Transfer, see sections 4.1 and 4.2, respectively.

In addition to acknowledgement, messages may require further handshaking as defined in section 3.2.1.3.

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3.2.1.2.1 Acknowledge (ACK)

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If a message is received without parity errors or checksum errors (checksums #1 and #2 produce zero sums) and the ACK request bit is set, the message is acknowledged IAW Table 3-3. If checksum #1 results in a non zero sum, then the header will be assumed invalid (i.e., no flag information is available) and the message will be disregarded without being NAK'd.

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The ACK message is constructed as follows:

1. Set the DLE/SOH word (word 1).

2. Set the message ID (word 2) to the ID of the message requiring acknowledgement.

3. Set word count (word 3) to zero.

4. Set flag bit 11 (ACK Flag) in word 4 to "1".

5. Set flag bit 15 (READY Flag) in word 4 to "1".

6. Compute checksum #1 (word 5).

7. Transmit the new header (No data words are transmitted).

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Table 3-3. ACK/NAK Time-Outs (milliseconds)

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Note: The time period is measured from the trailing edge of the stop bit of the final checksum of the message being ACK'd/NAK'd to the leading edge of the start bit of the DLE/SOH word of the ACK/NAK message.

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3.2.1.2.2 Negative Acknowledge (NAK)

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If a message is received without parity errors in the header and checksum #1 results in a zero sum (i.e., there are no checksum errors in the header), the ACK request bit is set, and there is either a parity error or non zero sum for checksum #2, then the message is negatively acknowledged in the time specified in Table 3-3. An accept/reject message is not output after a NAK.

The NAK message is constructed as follows:

2. Set the message ID (word 2) to the ID of the message requiring negative acknowledgement.

3. Set word count (word 3) to zero.

4. Set flag bit 10 (NAK Flag) in word 4 to "1".

5. Set flag bit 15 (READY Flag) in word 4 to "1".

6. Compute checksum #1 (word 5).

7. Transmit the new header (No data words are transmitted).

If checksum #1 results in a non-zero sum, then the header will be assumed invalid (i.e., no flag information is available) and the message is discarded without being NAK'd. If an ACK/NAK is not received within the time specified in Table 3-3, the message is retransmitted once after the initial transmission. If the retransmitted message is not ACK'd/NAK'd within the time specified in Table 3 3, the RCVR will resume communication by sending the next queued message.

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3.2.1.3 Messages Requiring Acceptance

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After receiving an input message, validity checks will be performed on the input data, regardless of whether the ACK request bit is set in the input message. If all of the data, including the header data, is valid, the message will be accepted and the data processed. If any of the data is invalid (i.e., data is out of the range for specified field), the message will be rejected and no data processed. This is accomplished via the accept and reject messages. An accept/reject message is output in the time specified in Table 3-4. If the ACK request bit is set in the input message, the accept/reject response message is output after the ACK response message. An accept/reject message is not output after a NAK.

The RCVR may reject a command because the received message ID or data is invalid (i.e., out of range). The accept/reject response is sent with the ACK request bit set. Additional messages should not be sent until the accept/reject message is received or the timeout has occurred. The exception to this is the INTER is not required to provide an accept/reject response to messages output from a RCVR in the Remote Control Mode. The INTER shall provide an accept/reject response to any message output from a

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RCVR in Data Transfer Mode, regardless of whether the ACK request bit is set in the message sent by the RCVR, in the time specified in Table 3-4.

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An accept/reject message is constructed as follows:

1. Set the DLE/SOH word (word 1).

2. Set the message ID (word 2) to the ID of the message to be accepted or rejected.

3. Set word count (word 3) to zero.

4. Set flag bit 8 (COMMAND REJECT Flag) in word 4 to "1" if message was rejected, set to "0" if message was accepted.

5. Set flag bit 9 (HANDSHAKE Flag) in word 4 to "1".

6. Set flag bit 12 (ACK Request Flag) in word 4 to "1" if ACK message response is required, set to “0” if no ACK message response required.

7. Set flag bit 15 (READY Flag) in word 4 to "1".

8. Compute checksum #1 (word #5).

9. Transmit the new header (No data words are transmitted for accept/reject).

Once a connect or disconnect command is responded to with a command accept message, the RCVR shall respond to the command within the time specified in Table 3-4. During this period of time, the RCVR is not required to output any previously connected messages. This condition occurs only after a command has been accepted by the RCVR and the command modifies the current message output.

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3.2.1.4 Default Messages

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3.2.1.4.1 STANDARD/CUSTOM Modes

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The RCVR automatically outputs the Buffer Box Status Request message (ID 253), defined in Section B.2. Message 253 is output once every six seconds with the ACK REQUEST Flag set to "1". This message may be disconnected and reconnected by individual command, but it is automatically reconnected on power reset or clearing of non-volatile memory. This message is not disconnected by the Universal Reset Message (ID 0). When Message 253 is output, and if an INTER is connected, the INTER must acknowledge the message upon receipt. If no acknowledgment is received, the RCVR will retransmit the message once and, if still no acknowledgement is received, determine that there is no interface activity, and disable keyboard lockout, if applicable. Unless disconnected, Message 253 will continue to be output as stated above regardless of whether or not the INTER acknowledges it.

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Table 3-4. Accept/Reject Time-Outs (milliseconds)

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Note: The time period is measured from the trailing edge of the stop bit of the final checksum of the message being accepted/rejected to the leading edge of the start bit of the DLE/SOH word of the acceptance/rejection message.

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3.2.1.4.2 INSTRUM Mode

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When operating in the INSTRUM mode, the Buffer Box Status Request message (ID 253) and the Time Mark Data Message (ID 3) are default output messages. These messages may be disconnected and reconnected by individual command, but they are automatically reconnected on power reset or clearing of non-volatile memory. These messages are not disconnected by the Universal Reset Message (ID 0). Refer to sections 3.2.1.4.1 and B.2 for a description of the Buffer Box Status Request message (ID 253).

Refer to section B.2 for a description of the Time Mark Data Message (ID 3).

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3.2.1.5 Header Only Messages

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Header only command messages command the RCVR or INTER to perform some activity in response to the message. The message consists of only a header; no data words are transmitted. Header only command types, issued by an INTER, and the RCVR response to the commands, are described below.

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3.2.1.5.1 Connect

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A connect command is used to request the repetitive output of a message from the RCVR. The connect command is constructed as follows:

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2. Set message ID (word 2) to ID of desired message.

3. Set data word count (word 3) to zero (no data).

4. Set flag bit 6 (CONNECT Flag) in word 4 to "1".

5. Set flag bit 12 (ACK REQUEST Flag) in word 4 to "1"" if ACK message response is required, set to “0” if no ACK message response required.

6. Set flag bit 15 (READY Flag) in word 4 to "1".

7. Compute checksum #1 (word 5).

8. Transmit the message to the RCVR.

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Upon receipt of the connect command, the RCVR will acknowledge it if the ACK REQUEST Flags is set. If the message ID is valid and if less than 40 messages are currently connected, including Message 253 or default messages, the RCVR shall accept the connect command and then begin transmitting the connected message at the frequency indicated for that message in APPENDIX B. The RCVR may accept commands to connect Message IDs allocated to other ISs in APPENDIX C if implementing the message IAW its respective IS. The RCVR may not implement Message IDs in APPENDIX C for other than their designated purpose. If 40 messages are already connected, or if the message ID is invalid, the RCVR will reject the connect command. The INTER must wait for the accept/reject response before sending any more messages to the RCVR.

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The RCVR stores all connected message IDs in nonvolatile memory so that they are retained during a power down. All previously connected messages will be automatically reconnected at power up. If the total number of bits per second plus the inter-message and inter-character gap times exceed the baud rate, not all connected messages may be transmitted. In addition, the connected message IDs are retained after an emergency zeroization.

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When Message 3 is connected and Messages 4, 8004, and 8005 are not connected, Message 3 will be queued as the first output message after the PPS output when the PPS out mode is 1PPS T-Mark. The output order of all other connected messages is not specified.

When Message 4 is connected and Message 3, 8004, and 8005 are not connected, Message 4 will be queued as the first output message after the PPS output when the PPS out mode is 1PPS T-Mark. The output order of all other connected messages is not specified.

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When Message 8004 is connected and Messages 3, 4, and 8005 are not connected, Message 8004 will be queued as the first output message after the PPS output when the PPS out…