+-----------------------+-----+---+---+---------+-------+---+------------+
| #  REVISIONS          |     |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
| DESCRIPTION           |     |   |   |         |       | D |            |
|                       |     |   |   |         |       | A |            |
|                       |     |   |   |         |       | T |            |
|                       |     |   |   |         |       | E |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
| DRAFT                 |     |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
| **DISTRIBUTION        |     |   |   |         |       |   |            |
| STATEMENT A**         |     |   |   |         |       |   |            |
|                       |     |   |   |         |       |   |            |
| **APPROVED FOR PUBLIC |     |   |   |         |       |   |            |
| RELEASE; DISTRIBUTION |     |   |   |         |       |   |            |
| IS UNLIMITED.**       |     |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
| **APPROVALS**         |     |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
| AUTHORIZED SIGNATURES | REP |   |   |         |       | D |            |
|                       | RES |   |   |         |       | A |            |
|                       | ENT |   |   |         |       | T |            |
|                       | ING |   |   |         |       | E |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
|                       | GPS |   |   |         |       |   |            |
|                       | N   |   |   |         |       |   |            |
|                       | AVS |   |   |         |       |   |            |
|                       | TAR |   |   |         |       |   |            |
|                       | JPO |   |   |         |       |   |            |
|                       |     |   |   |         |       |   |            |
|                       | SMC |   |   |         |       |   |            |
|                       | /CZ |   |   |         |       |   |            |
|                       | (AF |   |   |         |       |   |            |
|                       | MC) |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
|                       | Boe |   |   |         |       |   |            |
|                       | ing |   |   |         |       |   |            |
|                       |     |   |   |         |       |   |            |
|                       | SP  |   |   |         |       |   |            |
|                       | ACE |   |   |         |       |   |            |
|                       | S   |   |   |         |       |   |            |
|                       | YST |   |   |         |       |   |            |
|                       | EMS |   |   |         |       |   |            |
|                       | DI  |   |   |         |       |   |            |
|                       | VIS |   |   |         |       |   |            |
|                       | ION |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
|                       |     |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
|                       |     |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
|                       |     |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
| \* An asterisk        |     |   |   |         |       |   |            |
| affixed to the        |     |   |   |         |       |   |            |
| approval signature    |     |   |   |         |       |   |            |
| indicates that the    |     |   |   |         |       |   |            |
| approval is subject   |     |   |   |         |       |   |            |
| to exceptions taken   |     |   |   |         |       |   |            |
| in the \"Letter of    |     |   |   |         |       |   |            |
| Exception\" contained |     |   |   |         |       |   |            |
| in Appendix I of this |     |   |   |         |       |   |            |
| document.             |     |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
| **INTERFACE CONTROL   |     |   |   |         |       |   |            |
| DOCUMENT** 16 April   |     |   |   |         |       |   |            |
| 01                    |     |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
| UNLESS OTHERWISE      | DR  |   |   | ARINC   |       |   |            |
| SPECIFIED:            | BY  |   |   | Incor   |       |   |            |
|                       |     |   |   | porated |       |   |            |
| DIMENSIONS ARE IN     |     |   |   |         |       |   |            |
| INCHES.               |     |   |   | 2250 E. |       |   |            |
|                       |     |   |   | I       |       |   |            |
| TOLERANCES ON:        |     |   |   | mperial |       |   |            |
|                       |     |   |   | H       |       |   |            |
| DECIMALS ANGLES       |     |   |   | ighway, |       |   |            |
|                       |     |   |   | Suite   |       |   |            |
| XX = ±0.03 ±0° 30\'   |     |   |   | 450     |       |   |            |
| XXX = ±0.01           |     |   |   |         |       |   |            |
|                       |     |   |   | El      |       |   |            |
|                       |     |   |   | S       |       |   |            |
|                       |     |   |   | egundo, |       |   |            |
|                       |     |   |   | CA      |       |   |            |
|                       |     |   |   | 902     |       |   |            |
|                       |     |   |   | 45-3509 |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
|                       | CHK |   |   |         |       |   |            |
|                       | BY  |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
|                       | APP |   |   | ICD     |       |   |            |
|                       | ROV |   |   | TITLE   |       |   |            |
|                       | ALS |   |   |         |       |   |            |
|                       |     |   |   | Navstar |       |   |            |
|                       |     |   |   | GPS     |       |   |            |
|                       |     |   |   | Space   |       |   |            |
|                       |     |   |   | Segment |       |   |            |
|                       |     |   |   | / User  |       |   |            |
|                       |     |   |   | Segment |       |   |            |
|                       |     |   |   | L5      |       |   |            |
|                       |     |   |   | Int     |       |   |            |
|                       |     |   |   | erfaces |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
| THIS DOCUMENT         |     |   |   |         |       |   |            |
| SPECIFIES TECHNICAL   |     |   |   |         |       |   |            |
| REQUIREMENTS AND      |     |   |   |         |       |   |            |
| NOTHING HEREIN        |     |   |   |         |       |   |            |
| CONTAINED SHALL BE    |     |   |   |         |       |   |            |
| DEEMED TO ALTER THE   |     |   |   |         |       |   |            |
| TERMS OF ANY CONTRACT |     |   |   |         |       |   |            |
| OR PURCHASE ORDER     |     |   |   |         |       |   |            |
| BETWEEN ALL PARTIES   |     |   |   |         |       |   |            |
| AFFECTED.             |     |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
|                       |     |   |   |         |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+
|                       |     |   |   | SIZE    | CODE  |   | DRAWING    |
|                       |     |   |   |         | IDENT |   | NO.        |
|                       |     |   |   | A       | NO.   |   |            |
|                       |     |   |   |         |       |   | **ICD      |
|                       |     |   |   |         | OVYX1 |   | -GPS-705** |
+-----------------------+-----+---+---+---------+-------+---+------------+
|                       |     |   |   | SCALE:  | REV:  |   | SHEET I    |
|                       |     |   |   | N/A     |       |   |            |
+-----------------------+-----+---+---+---------+-------+---+------------+

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  ------------ ------------------------------------- ------------- ------------
  **REVISION                                                       
  RECORD**                                                         

  LTR          DESCRIPTION                           DATE          APPROVED

  NC           Initial Draft Release                 16 Apr 01     

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   

                                                                   
  ------------ ------------------------------------- ------------- ------------

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[Page Revision Record]{.underline}

[Pages]{.underline} [Revision]{.underline} [Pages]{.underline}
[Revision]{.underline}

(This page intentionally left blank.)

[TABLE OF CONTENTS]{.underline}

**1 SCOPE** 1

1.1 Scope 1

1.2 ICD Approval and Changes 1

**2 APPLICABLE DOCUMENTS** 3

2.1 Government Documents 3

2.2 Non-Government Documents 3

**3 REQUIREMENTS** 5

3.1 Interface Definition 5

3.2 Interface Identification 6

3.2.1 Ranging Codes 6

3.2.1.1 L5-Codes 6

3.2.1.2 Non-Standard Codes 7

3.2.2 NAV Data 7

3.2.3 L5 Signal Structure 10

3.3 Interface Criteria 10

3.3.1 Composite Signal 10

3.3.1.1 Frequency Plan 10

3.3.1.2 Correlation Loss 10

3.3.1.3 Carrier Phase Noise 10

3.3.1.4 Spurious Transmissions 11

3.3.1.5 Phase Quadrature 11

3.3.1.6 Signal Power Levels 11

3.3.1.6.1 User-Received Signal Levels 11

3.3.1.6.2 Satellite-Transmitted Signal Levels 12

3.3.1.7 Equipment Group Delay 13

3.3.1.7.1 Group Delay Uncertainty 13

3.3.1.7.2 Group Delay Differential 14

3.3.1.8 Signal Coherence 14

> 3.3.1.9 Signal Polarization 14

3.3.2 PRN Code Characteristics 14

3.3.2.1 Code Structure 14

3.3.2.2 Code Generation 16

3.3.2.3 Q5 Synchronization Sequence 20

> 3.3.3 Navigation Data 20

3.3.3.1 Navigation Data Modulation 21

3.3.3.1.1 Forward Error Correction 21

3.3.3.1.2 Neuman-Hoffman Code 23

3.3.4 GPS Time and SV Z-Count 23

**4 NOT APPLICABLE** 27

**5 NOT APPLICABLE** 29

**6 NOTES** 31

6.1 Acronyms 31

6.2 Definitions 33

6.2.1 User Range Accuracy 33

6.2.2 SV Block Definitions 33

6.2.2.1 Developmental SVs 33

6.2.2.2 Operational SVs 33

6.2.2.2.1 Block II SVs 34

6.2.2.2.2 Block IIA SVs 34

6.2.2.2.3 Block IIR SVs 34

6.2.2.2.4 Block IIF SVs 34

6.2.2.2.5 Block III SVs 34

6.2.3 Operational Interval Definitions 34

6.2.3.1 Normal Operations 35

6.2.3.2 Short-term Extended Operations 35

6.2.3.3 Long-term Extended Operations 35

6.2.4 GPS Week Number 35

6.3 Supporting Material 35

6.3.1 L5 Received Signals 35

6.3.2 Integrated phase noise characteristics 35

**10 APPENDIX I: LETTERS OF EXCEPTION** 37

10.1 Scope 37

10.2 Applicable Documents 37

10.3 Letters of Exception 37

**20 APPENDIX II: GPS NAVIGATION DATA STRUCTURE FOR DATA ID NO. 3** 39

20.1 Scope 39

20.2 Applicable Documents 39

20.2.1 Government Documents 39

20.2.2 Non-Government Documents 39

20.3 Requirements 40

20.3.1 Data Characteristics 40

20.3.2 Message Structure 40

20.3.3 Message Content 40

20.3.3.1 Message Type 1 Clock, Health and Accuracy Parameters 47

20.3.3.1.1 Message Type 1 Clock, Health and Accuracy Parameter Content
47

20.3.3.1.1.1 Transmission Week Number 47

20.3.3.1.1.2 SV Accuracy (L5) 47

20.3.3.1.1.3 SV Health (L5) 47

20.3.3.1.1.4 Issue of Data, Clock (IODC) 48

20.3.3.1.1.5 Long-term Extended Operations 48

20.3.3.1.1.6 SV Clock Correction 48

> 20.3.3.1.1.7 Estimated Group Delay Differential (L1-L5) 48
>
> 20.3.3.1.1.8 Estimated L2/L5 Group Delay Differential 48
>
> 20.3.3.1.1.9 Estimated L5 Q5/I5 Group Delay Differential 48

20.3.3.1.2 Message Type 1 Clock, Health and Accuracy Parameter
Characteristics 49

20.3.3.1.3 User Algorithms for Message Type 1 Clock Data 49

20.3.3.1.3.1 User Algorithm for SV Clock Correction 49

20.3.3.1.3.2 L1/L5 Group Delay Correction 52

20.3.3.1.3.3 L2/L5 Group Delay Correction 53

20.3.3.1.3.4 L1/L5 Ionospheric Correction 53

20.3.3.1.3.5 L2/L5 Ionospheric Correction 54

20.3.3.1.3.6 L5 Q-code/I-code Correction 54

20.3.3.1.3.7 Example Application of Correction Parameters 54

20.3.3.2 Message Type 1 and 2 Ephemeris Parameters 55

20.3.3.2.1 Message Type 1 and 2 Ephemeris Parameter Content 55

20.3.3.2.2 Message Type 1 and 2 Ephemeris Parameter Characteristics 57

20.3.3.2.3 User Algorithm for Ephemeris Determination 57

20.3.3.2.3.1 Curve Fit Intervals 57

20.3.3.2.3.2 Parameter Sensitivity 57

20.3.3.2.3.3 Coordinate Systems 57

20.3.3.2.3.3.1 ECEF Coordinate System 57

20.3.3.2.3.3.2 Earth-Centered, Inertial (ECI) Coordinate System 57

20.3.3.2.3.4 Geometric Range 57

20.3.3.3 Message Type 3 Parameters 58

20.3.3.3.1 Message Type 3 Parameter Content 58

20.3.3.3.1.1 Coordinated Universal Time (UTC) and GPS Time Parameters 58

20.3.3.3.1.2 Ionospheric Data 58

20.3.3.3.1.3 SV Health (L1/L2) 58

20.3.3.3.1.4 Code(s) on L2 Channel 60

20.3.3.3.1.5 Data ID 60

20.3.3.3.2 Algorithms Related to Message Type 3 Data 60

20.3.3.3.2.1 Coordinated Universal Time (UTC) and GPS Time 60

20.3.3.3.2.2 Ionospheric Model 61

20.3.3.4 Message Type 4 Almanac Parameters 61

20.3.3.4.1 Message Type 4 Almanac Parameter Content 61

20.3.3.4.1.1 Almanac Data 61

20.3.3.4.1.2 SV Health (L1 and L2) 63

20.3.3.4.1.3 SV Health (L5) 63

20.3.3.4.1.4 Anti-Spoof (A-S) Flags and SV Configurations 66

20.3.3.4.1.5 Almanac Reference Week 66

20.3.3.4.2 Algorithms Related to Message Type 4 Data 66

20.3.3.4.2.1 Almanac 66

20.3.3.4.2.2 Almanac Reference Time 66

20.3.3.4.2.3 Almanac Time Parameters 66

20.3.3.5 Message Type 5 66

20.3.4 Timing Relationships 67

20.3.4.1 Paging and Cutovers 67

20.3.4.2 SV Time vs. GPS Time 67

20.3.4.3 Speed of Light 68

20.3.4.4 Data Sets 68

20.3.4.5 Reference Times 68

20.3.5 Data Frame Parity 68

20.3.5.1 Parity Algorithm 69

[LIST OF FIGURES]{.underline}

Figure 3-1 GPS Space Segment to User Segment Interfaces 5

Figure 3-2 Generation of Codes 15

Figure 3-3 Modulation of Signals 16

Figure 3-4 XA Shift Register Generator Configuration 18

Figure 3-5 XB Shift Register Generator Configuration 19

Figure 3-6 Relative Phases Between the XA and XB Sequences 20

Figure 3-7 Convolutional Encoder 22

Figure 3-8 Convolutional transmit/Decoding Timing Relationships 22

Figure 3-9 Time Line Relationship of a Six-Second Message 25

Figure 6-1 User Received Minimum Signal Levels 36

Figure 10-1 Letters of Exception 37

Figure 20-1 Message Type 1 Format 41

Figure 20-2 Message Type 2 Format 42

Figure 20-3 Message Type 3 Format 43

Figure 20-4 Message Type 4 Format 44

Figure 20-5 Message Type 5 Format 45

Figure 20-6 Sample Application of Correction Parameters 56

[LIST OF TABLES]{.underline}

Table 3-I Code Phase Assignments 8

Table 3-II Composite L5 Transmitted Signal Phase 12

Table 3-III Received Minimum L5 RF Signal Strength 13

Table 6-I I5 and Q5 Signal-in-Space Link Budget Summary 36

Table 20-I Message Type 1 Clock, Health and Accuracy Parameters 51

Table 20-II UTC Parameters 59

Table 20-III Almanac Parameters 62

Table 20-IV L5 NAV Data Health Indications 64

Table 20-V Codes for Health of SV L5 Signal Components 65

(This page intentionally left blank.)

1\. SCOPE

1.1 [Scope]{.underline}. This Interface Control Document (ICD) defines
the requirements related to the interface between the Space Segment (SS)
of the Global Positioning System (GPS) and the Navigation Users of the
GPS for the L5 Navigation Signal.

1.2 [ICD Approval and Changes]{.underline}. ARINC Incorporated has been
designated the Interface Control Contractor (ICC), and is responsible
for the basic preparation, approval, distribution, and retention of the
ICD in accordance with YEN 75-13A. The following signatories must
approve this ICD to make it effective:

a\. Space Segment Contractor BLOCK IIF

Boeing,

Space Systems Division

b\. Control Segment Contractor Boeing,

Space Systems Division

c\. Navstar GPS Joint Program Office SMC/CZ (AFMC)

Initial signature approval of this ICD can be contingent upon a letter
of exception delineating those items by paragraph numbers that are not a
part of the approval. Such letters of exception can be prepared by any
of the signatories and must be furnished to the ICC for inclusion in
Appendix I of the approved and officially released version of the ICD.

Changes to the approved version of this ICD can be initiated by any of
the signatories and must be approved by all above signatories. The ICC
is responsible for the preparation of the change paper, change
coordination, and the change approval by all signatories in accordance
with YEN 75-13A. Designated signatories can approve proposed changes to
this ICD without any increase in the scope of a specific contract by so
specifying in a letter of exception. Such letters of exception must be
furnished to the ICC for inclusion in the released version of the
approved change and in Appendix I of the subsequent revised issues of
the ICD.

Whenever all the issues addressed by a letter of exception are resolved,
the respective signatory shall so advise the ICC in writing. When some
(but not all) of the exceptions taken by a signatory are resolved, the
signatory shall provide the ICC with an updated letter of exception.
Based on such notifications \-- without processing a proposed interface
revision notice (PIRN) for approval \-- the ICC will omit the obsolete
letter of exception from the next revision of the ICD and will
substitute the new one (if required).

Review cycles for all Proposed Interface Revisions Notices (PIRNs) is 45
days after receipt by individual addressees unless a written request for
a waiver is submitted to the ICC. Reviewing parties with delinquent
responses will be charged with an automatic letter of exception.

2\. APPLICABLE DOCUMENTS

2.1 [Government Documents]{.underline}. The following documents of the
issue specified contribute to the definition of the interfaces between
the GPS Space Segment and the GPS Navigation User Segment, and form a
part of this ICD to the extent specified herein.

[Specifications]{.underline}

*Federal*

None

*Military*

ICD-GPS-200, Navstar GPS Space Segment / Navigation User Interfaces

*Other Government Activity*

None

[Standards]{.underline}

*Federal*

None

*Military*

None

[Other Publications]{.underline}

+---------------------------+------------------------------------------+
| YEN 75-13A                | Interface Control Working Group          |
|                           |                                          |
| 16 Nov 1979               |                                          |
|                           |                                          |
| with Change Notice 1      |                                          |
+---------------------------+------------------------------------------+

2.2 [Non-Government Documents]{.underline}. The following documents of
the issue specified contribute to the definition of the interfaces
between the GPS Space Segment and the GPS Navigation User Segment and
form a part of this ICD to the extent specified herein.

[Specifications]{.underline}

None

[Other Publications]{.underline}

None

(This page intentionally left blank.)

3\. REQUIREMENTS

1.  ![](media/image1.wmf)[Interface Definition]{.underline}. The signal
    interface between the GPS Space Segment (SS) and the GPS Navigation
    Users consists of three radio frequency (RF) links: L1, L2 and L5.
    Figure 3-1 illustrates the interface between the SS and the users.
    The L5 link is only available on Block IIF and the planned future
    Block III SVs. Utilizing these links, the space vehicles (SVs) of
    the SS shall provide continuous earth coverage for signals that
    provide to the users the ranging codes and the system data needed to
    accomplish the GPS navigation (NAV) mission. These signals shall be
    available to a suitably equipped user with RF visibility to an SV,
    including users in terrestrial and near-terrestrial applications as
    well as users in orbital applications up to geosynchronous altitude.
    The civil functions of the L1 and L2 RF links are specified in
    ICD-GPS-200. Only the L5 link and its relationship with the L1 and
    L2 links will be specified herein.

Figure 3-1. GPS Space Segment to User Segment Interfaces

3.2 [Interface Identification]{.underline}. The carriers of the L5
L-band link are modulated by two bit trains in phase quadrature. One is
a composite bit train generated by the Modulo-2 addition of a
pseudo-random noise (PRN) ranging code, synchronization sequence, and
the downlink system data (referred to as NAV data), and the other is
modulated with only a PRN ranging code and synchronization sequence that
differ from the ranging code used with the NAV data.

3.2.1 [Ranging Codes]{.underline}. Two PRN ranging codes are transmitted
on L5: the in-phase code (denoted as the I5-code); and the quadraphase
code (denoted as the Q5-code). Code-division-multiple-access techniques
allow differentiating between the SVs even though they all transmit at
the same L5 frequency. The SVs will transmit intentionally \"incorrect\"
versions of the I5 and the Q5-codes when needed to protect the users
from receiving and utilizing anomalous NAV signals as a result of a
malfunction in the SV. These two \"incorrect\" codes are termed
non-standard I5 (NSI5) and non-standard Q5 (NSQ5) codes.

3.2.1.1 [L5-Codes]{.underline}. The PRN L5-codes for SV ID number i are
independent, but time synchronized, ranging codes, I5~i~(t) and
Q5~i~(t), of 1 millisecond in length at a chipping rate of 10.23 Mbps.
For each code, the 1-millisecond sequences are the Modulo-2 sum of two
sub-sequences referred to as XA and XB~i~; their lengths are 8,190 chips
and 8,191 chips, respectively, that restart to generate the 10,230 chip
code. The XB~i~ sequence is selectively delayed, thereby allowing the
basic code generation technique to produce a set of 74 (37 I5 and 37 Q5)
mutually exclusive code sequences of 1-millisecond in length. Of these,
32 pairs are designated for use by SVs, while the remaining 5 pairs are
reserved. Assignment of these code phase segments by SV-ID number (or
other use) is given in Table 3-I. SV-ID numbers and PRN are identical to
those for the L1 and L2 signals as specified in ICD-GPS-200.

The 74 codes are a selected subset of over 4000 possible codes that
could be generated using the selective delay. The remaining codes are
reserved for other L5 signal applications such as Satellite Based
Augmentation System (SBAS) satellite signals.

3.2.1.2 [Non-standard Codes]{.underline}. The NSI5 and NSQ5 codes, used
to protect the user from a malfunction in the SV (reference paragraph
3.2.1), are not for utilization by the user and, therefore, are not
defined in this document.

3.2.2 [NAV Data]{.underline}. The system data, D(t), includes SV
ephemerides, system time, SV clock behavior data, status messages and
time information, etc. The 50 bps data is coded in a rate ½
convolutional coder. The resulting 100 symbols per second (sps) symbol
stream is Modulo-2 added to the I5-code only; the resultant bit-trains
are used to modulate the L5 in‑phase (I) carrier. The content and
characteristics of data ID number 3 are given in Appendix II of this
document. Data ID number 2 is used for L1 and L2 C/A-code, and data ID
number 1 is no longer used. In general, the data content is based on
that modulated on the C/A-codes in the L1 and L2 channels of the SV,
with data specific to the L5 signal added, and a different error
detection encoding used.

The L5 quadraphase (Q5) carrier has no data.

+------------+----------+------------+---------------+-----------------+
| Table 3-I. |          |            |               |                 |
| Code Phase |          |            |               |                 |
| A          |          |            |               |                 |
| ssignments |          |            |               |                 |
| (sheet 1   |          |            |               |                 |
| of 2)      |          |            |               |                 |
+------------+----------+------------+---------------+-----------------+
| GPS PRN    | XB Code  |            | Initial XB    |                 |
|            | Advance  |            | Code          |                 |
| Signal     | --       |            | State\*\*\*   |                 |
| No.\*      | C        |            |               |                 |
|            | hips\*\* |            |               |                 |
+------------+----------+------------+---------------+-----------------+
|            | I5       | Q5         | I5            | Q5              |
+------------+----------+------------+---------------+-----------------+
| 1          | 266      | 1701       | 0101011100100 | 1001011001100   |
|            |          |            |               |                 |
| 2          | 365      | 323        | 1100000110101 | 0100011110110   |
|            |          |            |               |                 |
| 3          | 804      | 5292       | 0100000001000 | 1111000100011   |
|            |          |            |               |                 |
| 4          | 1138     | 2020       | 1011000100110 | 0011101101010   |
|            |          |            |               |                 |
| 5          | 1509     | 5429       | 1110111010111 | 0011110110010   |
|            |          |            |               |                 |
| 6          | 1559     | 7136       | 0110011111010 | 0101010101001   |
|            |          |            |               |                 |
| 7          | 1756     | 1041       | 1010010011111 | 1111110000001   |
|            |          |            |               |                 |
| 8          | 2084     | 5947       | 1011110100100 | 0110101101000   |
|            |          |            |               |                 |
| 9          | 2170     | 4315       | 1111100101011 | 1011101000011   |
|            |          |            |               |                 |
| 10         | 2303     | 148        | 0111111011110 | 0010010000110   |
|            |          |            |               |                 |
| 11         | 2527     | 535        | 0000100111010 | 0001000000101   |
|            |          |            |               |                 |
| 12         | 2687     | 1939       | 1110011111001 | 0101011000101   |
|            |          |            |               |                 |
| 13         | 2930     | 5206       | 0001110011100 | 0100110100101   |
|            |          |            |               |                 |
| 14         | 3471     | 5910       | 0100000100111 | 1010000111111   |
|            |          |            |               |                 |
| 15         | 3940     | 3595       | 0110101011010 | 1011110001111   |
|            |          |            |               |                 |
| 16         | 4132     | 5135       | 0001111001001 | 1101001011111   |
|            |          |            |               |                 |
| 17         | 4332     | 6082       | 0100110001111 | 1110011001000   |
|            |          |            |               |                 |
| 18         | 4924     | 6990       | 1111000011110 | 1011011100100   |
|            |          |            |               |                 |
| 19         | 5343     | 3546       | 1100100011111 | 0011001011011   |
+------------+----------+------------+---------------+-----------------+
| \* PRN     |          |            |               |                 |
| sequences  |          |            |               |                 |
| 33 through |          |            |               |                 |
| 37 are     |          |            |               |                 |
| reserved   |          |            |               |                 |
| for other  |          |            |               |                 |
| uses (e.g. |          |            |               |                 |
| ground     |          |            |               |                 |
| tran       |          |            |               |                 |
| smitters). |          |            |               |                 |
|            |          |            |               |                 |
| \*\* XB    |          |            |               |                 |
| Code       |          |            |               |                 |
| Advance is |          |            |               |                 |
| the number |          |            |               |                 |
| of XB      |          |            |               |                 |
| clock      |          |            |               |                 |
| cycles     |          |            |               |                 |
| beyond an  |          |            |               |                 |
| initial    |          |            |               |                 |
| state of   |          |            |               |                 |
| all 1s .   |          |            |               |                 |
|            |          |            |               |                 |
| \*\*\* In  |          |            |               |                 |
| the binary |          |            |               |                 |
| notation   |          |            |               |                 |
| for the    |          |            |               |                 |
| first 13   |          |            |               |                 |
| chips of   |          |            |               |                 |
| the I5 and |          |            |               |                 |
| Q5 XB      |          |            |               |                 |
| codes as   |          |            |               |                 |
| shown in   |          |            |               |                 |
| these      |          |            |               |                 |
| columns.   |          |            |               |                 |
| The        |          |            |               |                 |
| rightmost  |          |            |               |                 |
| bit is the |          |            |               |                 |
| first bit  |          |            |               |                 |
| out. Since |          |            |               |                 |
| the        |          |            |               |                 |
| initial    |          |            |               |                 |
| state of   |          |            |               |                 |
| the XA     |          |            |               |                 |
| Code is    |          |            |               |                 |
| all 1s,    |          |            |               |                 |
| these      |          |            |               |                 |
| first 13   |          |            |               |                 |
| chips are  |          |            |               |                 |
| also the   |          |            |               |                 |
| inverse of |          |            |               |                 |
| the        |          |            |               |                 |
| initial    |          |            |               |                 |
| states of  |          |            |               |                 |
| the I5 or  |          |            |               |                 |
| Q5-codes.  |          |            |               |                 |
+------------+----------+------------+---------------+-----------------+
| NOTE: The  |          |            |               |                 |
| code phase |          |            |               |                 |
| a          |          |            |               |                 |
| ssignments |          |            |               |                 |
| constitute |          |            |               |                 |
| i          |          |            |               |                 |
| nseparable |          |            |               |                 |
| pairs,     |          |            |               |                 |
| each       |          |            |               |                 |
| consisting |          |            |               |                 |
| of a       |          |            |               |                 |
| specific   |          |            |               |                 |
| I5 and a   |          |            |               |                 |
| specific   |          |            |               |                 |
| Q5-code    |          |            |               |                 |
| phase, as  |          |            |               |                 |
| shown      |          |            |               |                 |
| above.     |          |            |               |                 |
+------------+----------+------------+---------------+-----------------+

+------------+----------+------------+---------------+-----------------+
| Table 3-I. |          |            |               |                 |
| Code Phase |          |            |               |                 |
| A          |          |            |               |                 |
| ssignments |          |            |               |                 |
| (sheet 2   |          |            |               |                 |
| of 2)      |          |            |               |                 |
+------------+----------+------------+---------------+-----------------+
| GPS PRN    | XB Code  |            | Initial XB    |                 |
|            | Advance  |            | Code          |                 |
| Signal     | --       |            | State\*\*\*   |                 |
| No.\*      | C        |            |               |                 |
|            | hips\*\* |            |               |                 |
+------------+----------+------------+---------------+-----------------+
|            | I5       | Q5         | I5            | Q5              |
+------------+----------+------------+---------------+-----------------+
| 20         | 5443     | 1523       | 0110101101101 | 1100001110001   |
|            |          |            |               |                 |
| 21         | 5641     | 4548       | 0010000001000 | 0110110010000   |
|            |          |            |               |                 |
| 22         | 5816     | 4484       | 1110111101111 | 0010110001110   |
|            |          |            |               |                 |
| 23         | 5898     | 1893       | 1000011111110 | 1000101111101   |
|            |          |            |               |                 |
| 24         | 5918     | 3961       | 1100010110100 | 0110111110011   |
|            |          |            |               |                 |
| 25         | 5955     | 7106       | 1101001101101 | 0100010011011   |
|            |          |            |               |                 |
| 26         | 6243     | 5299       | 1010110010110 | 0101010111100   |
|            |          |            |               |                 |
| 27         | 6345     | 4660       | 0101011011110 | 1000011111010   |
|            |          |            |               |                 |
| 28         | 6477     | 276        | 0111101010110 | 1111101000010   |
|            |          |            |               |                 |
| 29         | 6518     | 4389       | 0101111100001 | 0101000100100   |
|            |          |            |               |                 |
| 30         | 6875     | 3783       | 1000010110111 | 1000001111001   |
|            |          |            |               |                 |
| 31         | 7168     | 1591       | 0001010011110 | 0101111100101   |
|            |          |            |               |                 |
| 32         | 7187     | 6518       | 0000010111001 | 0101111100001   |
|            |          |            |               |                 |
| 33         | 7329     | 749        | 1101010000001 | 1011001000100   |
|            |          |            |               |                 |
| 34         | 7577     | 1138       | 1101111111001 | 1011000100110   |
|            |          |            |               |                 |
| 35         | 7720     | 1661       | 1111011011100 | 0110010110011   |
|            |          |            |               |                 |
| 36         | 7777     | 3210       | 1001011001000 | 0011110101111   |
|            |          |            |               |                 |
| 37         | 8057     | 4332       | 0011010010000 | 0100110001111   |
+------------+----------+------------+---------------+-----------------+
| \* PRN     |          |            |               |                 |
| sequences  |          |            |               |                 |
| 33 through |          |            |               |                 |
| 37 are     |          |            |               |                 |
| reserved   |          |            |               |                 |
| for other  |          |            |               |                 |
| uses (e.g. |          |            |               |                 |
| ground     |          |            |               |                 |
| tran       |          |            |               |                 |
| smitters). |          |            |               |                 |
|            |          |            |               |                 |
| \*\* XB    |          |            |               |                 |
| Code       |          |            |               |                 |
| Advance is |          |            |               |                 |
| the number |          |            |               |                 |
| of XB      |          |            |               |                 |
| clock      |          |            |               |                 |
| cycles     |          |            |               |                 |
| beyond an  |          |            |               |                 |
| initial    |          |            |               |                 |
| state of   |          |            |               |                 |
| all 1s .   |          |            |               |                 |
|            |          |            |               |                 |
| \*\*\* In  |          |            |               |                 |
| the binary |          |            |               |                 |
| notation   |          |            |               |                 |
| for the    |          |            |               |                 |
| first 13   |          |            |               |                 |
| chips of   |          |            |               |                 |
| the I5 and |          |            |               |                 |
| Q5 XB      |          |            |               |                 |
| codes as   |          |            |               |                 |
| shown in   |          |            |               |                 |
| these      |          |            |               |                 |
| columns.   |          |            |               |                 |
| The        |          |            |               |                 |
| rightmost  |          |            |               |                 |
| bit is the |          |            |               |                 |
| first bit  |          |            |               |                 |
| out. Since |          |            |               |                 |
| the        |          |            |               |                 |
| initial    |          |            |               |                 |
| state of   |          |            |               |                 |
| the XA     |          |            |               |                 |
| Code is    |          |            |               |                 |
| all 1s,    |          |            |               |                 |
| these      |          |            |               |                 |
| first 13   |          |            |               |                 |
| chips are  |          |            |               |                 |
| also the   |          |            |               |                 |
| inverse of |          |            |               |                 |
| the        |          |            |               |                 |
| initial    |          |            |               |                 |
| states of  |          |            |               |                 |
| the I5 or  |          |            |               |                 |
| Q5-codes.  |          |            |               |                 |
+------------+----------+------------+---------------+-----------------+
| NOTE: The  |          |            |               |                 |
| code phase |          |            |               |                 |
| a          |          |            |               |                 |
| ssignments |          |            |               |                 |
| constitute |          |            |               |                 |
| i          |          |            |               |                 |
| nseparable |          |            |               |                 |
| pairs,     |          |            |               |                 |
| each       |          |            |               |                 |
| consisting |          |            |               |                 |
| of a       |          |            |               |                 |
| specific   |          |            |               |                 |
| I5 and a   |          |            |               |                 |
| specific   |          |            |               |                 |
| Q5-code    |          |            |               |                 |
| phase, as  |          |            |               |                 |
| shown      |          |            |               |                 |
| above.     |          |            |               |                 |
+------------+----------+------------+---------------+-----------------+

3.2.3 [L5 Signal Structure]{.underline}. The L5 consists of two carrier
components that are in phase quadrature with each other. Each carrier
component is bi-phase shift key (BPSK) modulated by a separate bit
train. One bit train is the Modulo-2 sum of the I5-code, NAV data, and
synchronization sequence while the other is the Q5-code with no NAV
data, but with another synchronization sequence. For a particular SV,
all transmitted signal elements (carriers, codes, synchronization
sequences, and data) are coherently derived from the same on-board
frequency source.

3.3 [Interface Criteria]{.underline}. The criteria specified in the
following define the requisite characteristics of the L5 interface.

3.3.1 [Composite Signal]{.underline}. The following criteria define the
characteristics of the composite L5 signal.

3.3.1.1 [Frequency Plan]{.underline}. The L5 signal is contained within
a 24 MHz band centered about L5. The carrier frequencies for the L1, L2
and L5 signals shall be coherently derived from a common frequency
source within the SV. The nominal frequency of this source \-- as it
appears to an observer on the ground \-- is 10.23 MHz. The SV carrier
frequency and clock rates \-- as they would appear to an observer
located in the SV \-- are offset to compensate for relativistic effects.
The clock rates are offset by
![](media/image2.wmf){width="0.15138888888888888in"
height="0.1638888888888889in"}f/f = -4.4647E-10, equivalent to a change
in the I5 and Q5-code chipping rate of 10.23 MHz offset by a
![](media/image2.wmf){width="0.15138888888888888in"
height="0.1638888888888889in"}f = ‑4.5674E-3 Hz. This is equal to
10.22999999543 MHz. The nominal carrier frequency (f~0~) \-- as it
appears to an observer on the ground -- shall be 1176.45 MHz, or 115
times 10.23 MHz.

3.3.1.2 [Correlation Loss]{.underline}. Correlation loss is defined as
the difference between the SV power received in a 24 MHz bandwidth and
the signal power recovered in an ideal correlation receiver or matched
filter implementation of the same bandwidth. The worst case correlation
loss occurs when the I5 carrier is modulated by the sum of the I5-code
and the NAV data stream. For this case, the correlation loss
apportionment shall be as follows:

1.  SV modulation and filter imperfections: 0.6 dB

2.  Ideal UE receiver waveform distortion (due to 24 MHz filter): 0.4 dB

3.3.1.3 [Carrier Phase Noise]{.underline}. The phase noise spectral
density of the unmodulated carrier shall be such that a phase locked
loop of 10 Hz one-sided noise bandwidth shall be able to track the
carrier to an accuracy of 0.1 radians root mean square (RMS). See
additional supporting material for phase noise characteristics in
section 6.2.1.

3.3.1.4 [Spurious Transmissions]{.underline}. In-band spurious
transmissions shall be at least 40 dB below the unmodulated L5 carrier
over the allocated 24 MHz channel bandwidth.

3.3.1.5 [Phase Quadrature]{.underline}. The two L5 carrier components
modulated by the two separate bit trains (I5-code plus data and Q5-code
with no data) shall be in phase quadrature (within ±100 milliradians)
with the Q5 signal carrier lagging the I5 signal by 90 degrees.
Referring to the phase of the I5 carrier when I5~i~(t) equals zero as
the \"zero phase angle\", the I5- and Q5-code generator output shall
control the respective signal phases in the following manner: when
I5~i~(t) equals one, a 180-degree phase reversal of the I5-carrier
occurs; when Q5~i~(t) equals one, the Q5 carrier advances 90 degrees;
when the Q5~i~(t) equals zero, the Q5 carrier shall be retarded 90
degrees (such that when Q5~i~(t) changes state, a 180-degree phase
reversal of the Q5 carrier occurs). The resultant nominal composite
transmitted signal phases as a function of the binary state of the
modulating signals are as shown in Table 3-II.

3.3.1.6 [Signal Power Levels]{.underline}. The L5 signal power levels
are specified for two service volumes: terrestrial and orbital. The
terrestrial service volume is defined from the earth's surface up to and
including 3000 km altitude. The space service volume is defined from
3000 km above the earth's surface up to and including geosynchronous
orbit. Specified values of radio frequency (RF) minimum signal power are
referenced to the extremities within each service volume. In the
terrestrial service volume case, the minimum case refers to the locus of
points at the intersection formed by the GPS transmit antenna beam and
the earth where any user's elevation angle equals five (5) degrees. In
the space service volume case, the minimum case refers to the locus of
points at the intersection formed by the GPS transmit antenna beam
(measured ±13.9° from boresight) and a sphere with radius equal to 42164
km, corresponding to geosynchronous orbit radius.

3.3.1.6.1 [User-Received Signal Levels]{.underline}. The SV shall
provide L5 navigation for terrestrial users and orbital users, in
accordance with the minimum and maximum levels specified in Table 3-III
into a 0 dB~ic~ circular polarized user receiving antenna (located near
ground for terrestrial users), when the SV is above a 5-degree elevation
angle. For orbital users, the same antenna configuration as terrestrial
users is assumed. Additional related data is provided as supporting
material in paragraph 6.3.1.

3.3.1.6.2 [Satellite-Transmitted Signal Levels]{.underline}. The
specified minimum user-received RF signal power levels are used to
derive electrical power and antenna gain requirements aboard the GPS SV.
Specifically, the *required* Effective Radiated Isotropic Power (EIRP)
is related to *minimum* received isotropic power, when nominal channel
losses are a maximum, by the relationship:

> RIP~MIN~(*f*) = EIRP~REQ~(*f*) - L~FS~(*f*)- L~ATMOS~(*f*) (in dB)

where

> L~ATMOS~(*f*) = Atmospheric Path Loss ≡ 0.5 dB
>
> L~FS~(*f*) is Free Space Path Loss ≡ 10\*log(4πρ~MAX~/λ*~f~*)^2^ (dB)

and

ρ~MAX~ = maximum SV-to-User geometric slant range (in meters)

For a user and a vehicle at a 5 deg elevation, the slant range is
25,251.6 km.

> λ*~f~* = wavelength of L~5~ carrier frequency 1176. 45 Mhz (in meters)
> = 0.2548 m

For the terrestrial service volume the required composite EIRP will be
28.5 dBW and for the orbital service volume it will be TBD dBW.

+-----------------------+-----------------------+-----------------------+
| Table 3-II. Composite |                       |                       |
| L5 Transmitted Signal |                       |                       |
| Phase                 |                       |                       |
+-----------------------+-----------------------+-----------------------+
| Nominal Composite L5  | Code State            |                       |
|                       |                       |                       |
| Signal Phase\*        |                       |                       |
+-----------------------+-----------------------+-----------------------+
|                       | I5                    | Q5                    |
+-----------------------+-----------------------+-----------------------+
| 0°                    | 0                     | 0                     |
|                       |                       |                       |
| -90°                  | 1                     | 0                     |
|                       |                       |                       |
| +90°                  | 0                     | 1                     |
|                       |                       |                       |
| 180°                  | 1                     | 1                     |
+-----------------------+-----------------------+-----------------------+
| \* Relative to 0, 0   |                       |                       |
| code state with       |                       |                       |
| positive angles       |                       |                       |
| leading and negative  |                       |                       |
| angles lagging.       |                       |                       |
+-----------------------+-----------------------+-----------------------+

  -------------- --------- ------------- ------------- ------------- ------------
  Table 3-III.                                                       
  Received                                                           
  Minimum L5 RF                                                      
  Signal                                                             
  Strength \*                                                        

  Production     Channel   User Signal                               
  Version                  Levels (in                                
                           dBW)                                      

                           Terrestrial                 Orbital       

                           Min           Max           Min           Max

  Block IIF      I5        -157          -150          TBD           TBD

                 Q5        -157          -150          TBD           TBD

  Future SVs     I5        TBD           TBD           TBD           TBD

                 Q5        TBD           TBD           TBD           TBD

  \* Minimum                                                         
  power levels                                                       
  based on                                                           
  minimum Space                                                      
  Vehicle EIRP                                                       
  at 13.83                                                           
  degrees off                                                        
  nadir, as                                                          
  averaged over                                                      
  azimuth,                                                           
  nominal                                                            
  orbital                                                            
  temperatures                                                       
  and over its                                                       
  specified                                                          
  design life                                                        
  with an                                                            
  assumed                                                            
  atmospheric                                                        
  loss of 0.5 dB                                                     
  -------------- --------- ------------- ------------- ------------- ------------

3.3.1.7 [Equipment Group Delay]{.underline}. Equipment group delay is
defined as the delay between the radiated output of the L5 ranging
signal from a specific SV (measured at the antenna phase center) and the
output of that SV\'s on-board frequency source; the delay consists of a
bias term and an uncertainty. The bias term is of no concern to the
users of the L1-C/A and L5 Q5-codes since it is included in the clock
correction parameters relayed in the NAV data, and is therefore
accounted for by the user computations of system time (reference
paragraph 20.3.3.1.3.1). However, single frequency L1-C/A, L2-C/A, and
L5 users must correct for the delay between the composite L1/L5 time and
these single-frequency codes. The uncertainty (variation) of these
delays and the relationship of the timing between composite L1/L2
P(Y)-code timing ("GPS Time") and composite L1-C/A and L5-Q codes ("L5
Time") are defined in the following.

3.3.1.7.1 [Group Delay Uncertainty]{.underline}. The effective
uncertainty of the group delays shall not exceed 3.0 nanosecond (two
sigma).

3.3.1.7.2 [Group Delay Differential]{.underline}. The group delay
differential between the radiated L1-C/A, L2-C/A, and L5-I5 codes are
specified as consisting of random plus bias components. The mean
differential is defined as the bias component and will be either
positive or negative. For a given navigation payload redundancy
configuration, the absolute value of the mean differential delay between
combinations of the L1-C/A, L2-C/A, and L5-I5 shall not exceed 30.0
nanoseconds. The random variations about the mean shall not exceed 3.0
nanoseconds (two-sigma). The group delay differences between the L5-I5
signal and that represented in the SV L5 clock corrections will be
broadcast in the L5 NAV message.

3.3.1.8 [Signal Coherence]{.underline}. All transmitted signals for a
particular SV shall be coherently derived from the same on-board
frequency standard; all digital signals shall be clocked in coincidence
with the PRN transitions for the P-signal and occur at the P-signal
transition speed. On the L5 channel the data transitions of the two
modulating signals (i.e., that containing the I5-code and that
containing the Q5-code) shall be such that the average time difference
between the transitions does not exceed 3.0 nanoseconds (two-sigma). The
signal coherence difference between the L5-I5 signal and the L5-Q5
signal will be broadcast in the L5 NAV message.

3.3.1.9 [Signal Polarization]{.underline} The transmitted signal shall
be right-hand circularly polarized (RHCP). For the angular range of
±14.3 degrees from boresight, L5 ellipticity shall be no worse than TBD
dB.

3.3.2 [PRN Code Characteristics]{.underline}. The characteristics of the
I5- and the Q5-codes are defined below in terms of their structure and
the basic method used for generating them. Figures 3-2 and 3-3 depict
simplified block diagrams of the scheme for generating the 10.23 Mbps
I5~i~(t) and Q5~i~(t) patterns, and for Modulo-2 summing the I5 patterns
with the NAV bit train, D(t), which is rate 1/2 encoded and clocked at
100 sps. In addition, the 100 sps symbols are modulated with a 10-bit
Neuman-Hoffman code that is clocked at 1 kHz. The resultant composite
bit trains are then used to modulate the L5 in-phase carrier. The
Q5-code is modulated with a 20-bit Neuman-Hoffman code that is also
clocked at 1 kHz.

3.3.2.1 [Code Structure]{.underline}. The I5~i~(t) pattern (I5-code) and
the Q5~i~(t) pattern (Q5-code) are both generated by the Modulo-2
summation of two PRN codes, XA(t) and XBI~i~(n~Ii~, t) or XBQ~i~(n~Qi~,
t), where n~Ii~ and n~Qi~ are initial states of XBI~i~ and XBQ~i~ for
satellite i. There are over 4000 unique XB codes generated using
different initial states of which 74 are assigned and identified in
Table 3-I using the same basic code generator.

![](media/image3.wmf)

Figure 3-2. Generation of Codes

Figure 3-3 Modulation of Signals

![](media/image4.wmf)

3.3.2.2 [Code Generation]{.underline}. Each I5~i~(t) pattern (I5-code)
and Q5~i~(t) pattern (Q5-code) are the Modulo-2 sum of two extended
patterns clocked at 10.23 Mbps (XA and XBI~i~ or XBQ~i~). XA is an 8190
length code, with an initial condition of all 1s, that is short cycled
1-chip before its natural conclusion and restarted to run over a period
of 1 millisecond (synchronized with the L1 frequency C/A-code) for a
total of 10230 chips. The XBI~i~ and XBQ~i~, with initial conditions
indicated in Table 3-I, are also 8191 length codes that are not short
cycled. They are restarted at their natural completion and run over a
period of 1 millisecond (synchronized with the XA code) for a total of
10230 chips. The polynomials for XA and XBI~i~ or XBQ~i~ codes, as
referenced to the shift register input, are:

XA: 1+ x^9^ + x^10^ + x^12^ + x^13^, and

XBI~i~ or XBQ~i~: 1 + x + x^3^ + x^4^ + x^6^ + x^7^ + x^8^ + x^12^ +
x^13^.

Samples of the relationship between shift register taps and the
exponents of the corresponding polynomial, referenced to the shift
register input, are as shown in Figures 3-4 (XA code) and 3-5 (XB
codes). In the case of the XB codes, the shift register can either be
initialized with all 1s and delayed n~i~ states as specified in Table
3-I, or initialized with the state indicated in Table 3-I.

The state of each generator can be expressed as a code vector word which
specifies the binary sequence constant of each register as follows: (a)
the vector consists of the binary state of each stage of the register,
(b) the stage 13 value appears at the right followed by the values of
the remaining states in order of descending stage numbers, and (c) the
shift direction is from lower to higher stage number with stage 13
providing the current output. This code vector convention represents the
present output and 13 future outputs in sequence. Using this convention,
at each XA epoch (state 8190), the XA shift register is initialized to
the code vector 1111111111111, while at each XB epoch (state 8191), the
XB shift register is initialized to a code vector peculiar to the PRN
number and phase. The XB code vectors are as indicated in Table 3-I.
Alternatively, the XB shift register is initialized to the code vector
1111111111111 and advanced n~i~ states as indicated in Table 3-I.

The natural 8191 chips of the XA sequence is shortened to 8190 chips to
cause precession of the second XA sequence with respect to the natural
8191 chip XB sequence, as shown in Figure 3-6. Re-initialization of the
XA shift register produces a 10230-chip sequence by omitting the last
6151 chips of the second natural XA sequence, or reinitializing to all
1s at the 1 ms epoch. The XB shift register is simply allowed to run its
natural course until the next 1 ms epoch when it is reinitialized at its
initial state, B0, based upon PRN number and phase. This results in the
phase of the XB sequence leading by one chip during the second XA
sequence in the 1-millisecond period. Depending upon the initial state
of the XB sequence, a third 8191 chip sequence may be started before the
10230-chip sequence is completed. Two different scenarios that may
result are shown in Figure 3-6.

In scenario a, the initial state of the XB sequence, B0, is less than
State 6152. Thus, the second natural XB sequence does not run to
completion prior to the next 1 ms epoch. In scenario b, the initial
state of the XB sequence, B0, is greater than State 6151. Thus, the
second natural XB sequence runs to completion and a third natural
sequence starts (except when B0 is State 6152) prior to the next 1 ms
epoch.

![](media/image5.wmf)

Figure 3-4. XA Shift Register Generator Configuration

![](media/image6.wmf)

Figure 3-5. XB Shift Register Generator Configuration

![](media/image7.wmf)Figure 3-6. Relative Phases Between the XA and XB
Sequences

3.3.2.3 [Q5 Synchronization Sequence]{.underline}. Each of the 1 ms
Q5-code blocks are further encoded with a 20-bit Neuman-Hoffman code.
The 20 bits are Modulo‑2 added to the code chips at the PRN code epoch
rate of 1 kHz. The code, nh20(t), starting coincident with the 20 ms
data epoch on the I5 channel, is as follows:

nh20(t) = 0 0 0 0 0 1 0 0 1 1 0 1 0 1 0 0 1 1 1 0

3.3.3 [Navigation Data]{.underline}. The content and format of the NAV
data for data ID number 3 are given in Appendix II of this document
(reference paragraph 20.3.3.3.1.5). Data ID number 2 is used for L1 and
L2 C/A-code, and data ID number 1 is no longer used.

3.3.3.1 [Navigation Data Modulation]{.underline}. The NAV bit train,
D(t), is rate 1/2 encoded and, thus, clocked at 100 symbols per second
(sps). In addition, the 100 sps symbols are modulated with a 10-bit
Neuman-Hoffman code that is clocked at 1 kHz. The resultant symbol
sequence is then summed with PRN code and used to modulate the L5
in-phase carrier.

3.3.3.1.1 [Forward Error Correction]{.underline}. The NAV bit train,
D(t), will always be rate 1/2 convolutional encoded with a Forward Error
Correction (FEC) code. Therefore, the symbol rate is 100 sps. The
convolutional coding will be constraint length 7, with a convolutional
encoder logic arrangement as illustrated in Figure 3-7. The G1 symbol is
selected on the output as the first half of a 20-millisecond data bit
period coincident with the first bit of the 20-bit Q5 Neuman-Hoffman
code.

Six-second navigation messages broadcast by the SV are synchronized with
every fourth of the SV's P(Y)-code X1 epochs. Although these epochs are
not necessarily accessible to the L5 user, they are used within the SV
to define GPS Time. However, message synchronization does provide the L5
user an access to the time of every 4^th^ P(Y)-code X1 epoch. Thus,
herein, reference will continue to be made to these X1 epochs. See
ICD-GPS-200 for details.

Because the FEC encoding convolves successive messages, it is necessary
to define which transmitted symbol is synchronized to SV time, as
follows. The beginning of the first symbol that contains
[any]{.underline} information about the first bit of a message will be
synchronized to every fourth X1 epoch. The users' convolutional decoders
will introduce a fixed delay that depends on their respective algorithms
(usually 5 constraint lengths, or 35 bits), for which they must
compensate to determine system time from the received signal. This
convolutional decoding delay and the various relationships with the
start of the data block transmission and SV time are illustrated in
Figure 3-8 for the L5 signal.

![](media/image8.wmf)

![](media/image9.wmf)Figure 3-7. Convolution Encoder

Figure 3-8. Convolutional transmit/Decoding Timing Relationships

3.3.3.1.2 [Neuman-Hoffman Code]{.underline}. Each of the 100 sps symbols
are further encoded with a 10-bit Neuman-Hoffman code. The 10 bits are
Modulo-2 added to the symbols at the PRN code epoch rate of 1 kHz. The
code, nh(t), starting at the 100 sps symbol transitions, is as follows:

> Symbol = 1: nh(t) = 1111001010
>
> Symbol = 0: nh(t) = 0000110101

3.3.4 [GPS Time and SV Z-Count]{.underline}. GPS time is established by
the Operational Control Segment (OCS) and is referenced to a UTC (as
maintained by the U.S. Naval Observatory) zero time-point defined as
midnight on the night of January 5, 1980/morning of January 6, 1980. GPS
Time, as defined in ICD‑GPS‑200, is the ensemble of corrected composite
L1/L2 P(Y) SV times, corrected via the clock corrections in the L1 and
L2 NAV data and the relativity correction. L1/L5 Time will be derived
from GPS time to compensate for the offset between L1/L2 P(Y) SV times
and L1‑C/A/L5‑Q5 SV times and may therefore be different than GPS Time.
The largest unit used in stating GPS Time is one week defined as 604,800
seconds, concatenated with the GPS week number. GPS Time may differ from
UTC because GPS Time is a continuous time scale, while UTC is corrected
periodically with an integer number of leap seconds. There also is an
inherent but bounded drift rate between the UTC and GPS time scales. The
OCS controls the GPS time scale to be within one microsecond of UTC
(Modulo one second).

The L5 NAV data contains the requisite data for relating GPS Time to
UTC, as defined in ICD-GPS-200, and L1/L5 Time to GPS Time. The accuracy
of this data during the transmission interval will be such that it
relates L1/L5 Time to GPS Time to within 3.0 nanoseconds (two sigma).
This data is generated by the CS (or provided to the CS); therefore, the
accuracy of these relationships may degrade if for some reason the CS is
unable to upload data to an SV.

In each SV the X1 epochs of the P-code of the L1 and L2 offer a
convenient unit for precisely counting and communicating time. Time
stated in this manner is referred to as Z-count, which is given as a
29-bit binary number consisting of two parts as follows:

a\. The binary number represented by the 19 least significant bits of
the Z-count is referred to as the time of week (TOW) count and is
defined as being equal to the number of X1 epochs that have occurred
since the transition from the previous week. The count is short-cycled
such that the range of the TOW-count is from 0 to 403,199 X1 epochs
(equaling one week) and is reset to zero at the end of each week. The
TOW-count\'s zero state is defined as that X1 epoch which is coincident
with the start of the present week. This epoch occurs at (approximately)
midnight Saturday night-Sunday morning, where midnight is defined as
0000 hours on the Coordinated Universal Time (UTC) scale which is
nominally referenced to the Greenwich Meridian. Over the years the
occurrence of the \"zero state epoch\" may differ by a few seconds from
0000 hours on the UTC scale since UTC is periodically corrected with
leap seconds while the TOW-count is continuous without such correction.
A truncated version of the TOW-count, consisting of its 17 most
significant bits, is contained in each of the six-second messages of the
L5 downlink data stream; the relationship between the actual TOW-count
and its truncated message version is illustrated by Figure 3-9.

b\. The ten most significant bits of the Z-count are a Modulo 1024
binary representation of the sequential number assigned to the current
GPS week (see paragraph 6.2.4). The range of this count is from 0 to
1023 with its zero state being defined as the GPS week number zero and
every integer multiple of 1024 weeks, thereafter (i.e. 0, 1024, 2048,
etc.).

![](media/image10.wmf)Figure 3-9. Time Line Relationship of a Six-Second
Message

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4\. NOT APPLICABLE

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5\. NOT APPLICABLE

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6\. NOTES

6.1 [Acronyms]{.underline}

> AFMC - Air Force Materiel Command
>
> A-S - Anti-Spoof
>
> Autonav - Autonomous Navigation
>
> bps - bits per second
>
> BPSK - Bi-Phase Shift Key
>
> C/A - Course/Acquisition
>
> CRC - Cyclic Redundancy Check
>
> CS - Control Segment
>
> dB - Decibel
>
> dBW - Decibels with respect to 1 Watt
>
> DN - Day Number
>
> DoD - Department of Defense
>
> ECEF - Earth-Centered, Earth-Fixed
>
> ECI - Earth-Centered Inertial
>
> EIRP Effective Radiated Isotropic Power
>
> FEC - Forward Error Correction
>
> GPS - Global Positioning System
>
> Hz - Hertz
>
> I5 - Inphase L5 Signal
>
> ICC - Interface Control Contractor
>
> ICD - Interface Control Document
>
> ID - Identification
>
> IERS - International Earth Rotation Service
>
> IODC - Issue of Data, Clock
>
> IODE - Issue of Data, Ephemeris
>
> IRM - IERS Reference Meridian
>
> IRP - IERS Reference Pole
>
> LSB - Least Significant Bit
>
> LSF - Leap Seconds Future
>
> MCS - Master Control Station
>
> MSB - Most Significant Bit
>
> NAV - Navigation
>
> NSI5 - Non-Standard I-Code
>
> NSQ5 - Non-Standard Q-Code
>
> OCS - Operational Control Segment
>
> PIRN - Proposed Interface Revision Notice
>
> PRN - Pseudo-Random Noise
>
> P(Y) - Precise (Anti-Spoof) Code
>
> Q5 Quadrature L5 Signal
>
> RF - Radio Frequency
>
> RHCP Right Hand Circular Polarization
>
> RMS - Root Mean Square
>
> SA - Selective Availability
>
> SBAS Space Based Augmentation System
>
> sps Symbols per Second
>
> SS - Space Segment
>
> SV - Space Vehicle
>
> SVN - Space Vehicle Number
>
> TBD - To Be Determined
>
> TBS - To Be Supplied
>
> TOW - Time Of Week
>
> URA - User Range Accuracy
>
> URE - User Range Error
>
> USNO - U.S. Naval Observatory
>
> UTC - Coordinated Universal Time
>
> WGS 84 - World Geodetic System 1984
>
> WN - Week Number

6.2 [Definitions]{.underline}

6.2.1 [User Range Accuracy]{.underline}. User range accuracy (URA) is a
statistical indicator of the ranging accuracy obtainable with a specific
SV and a specific ranging signal or signals. L5 URA (URA~5~) is greater
than or equal to a one-sigma estimate of the user range errors in the L5
navigation data for the transmitting satellite. It includes ephemeris
and clock errors resulting after the corrections specified in Section
20.3 for the L5 or L1/L5 user are applied. It does not include any
errors introduced in the user set or the transmission media. While the
URA~5~ may vary over a given message fit interval, the URA~5~ index (N)
reported in the NAV message corresponds to the maximum value of URA~5~
anticipated over the fit interval.

6.2.2 [SV Block Definitions]{.underline}. The following block
definitions are given to facilitate discussion regarding the capability
of the various blocks of GPS satellites to support the SV-to-user
interface.

6.2.2.1 [Developmental SVs]{.underline}. See paragraph 6.2.2.1 of
ICD-GPS-200.

6.2.2.2 [Operational SVs]{.underline}. The operational satellites are
designated Block II, Block IIA, Block IIR, Block IIF and Block III SVs.
Characteristics of these SVs are provided below. These SVs all transmit
a configuration code of 001 (reference 20.3.3.4.1.4). The navigation
signal provides no direct indication of the type of the transmitting SV.

6.2.2.2.1 [Block II SVs]{.underline}. See paragraph 6.2.2.2.1 of
ICD-GPS-200. These satellites do not broadcast the L5 signal.

6.2.2.2.2 [Block IIA SVs]{.underline}. See paragraph 6.2.2.2.2 of
ICD-GPS-200. These satellites do not broadcast the L5 signal.

6.2.2.2.3 [Block IIR SVs]{.underline}. See paragraph 6.2.2.2.3 of
ICD-GPS-200. These satellites do not broadcast the L5 signal.

6.2.2.2.4 [Block IIF SVs]{.underline}. The block of operational future
SVs developed by Boeing are designated as SVNs 67-TBD and are termed
\"Block IIF\" SVs. The Block IIF operational SVs do broadcast the L5
signal. Paragraph 6.2.3 may not apply.

6.2.2.2.5 [Block III SVs]{.underline}. The block of operational planned
SVs will be termed \"Block III\" SVs. The Block III operational SVs will
broadcast the L5 signal.

6.2.3 [Operational Interval Definitions]{.underline}. The following
three operational intervals have been defined. These labels will be used
to refer to differences in the interface definition as time progresses
from SV acceptance of the last navigation data upload.

6.2.3.1 [Normal Operations]{.underline}. The SV is undergoing normal
operations whenever the fit interval flag (reference paragraph
20.3.3.2.3.1) is zero.

6.2.3.2 [Short-term Extended Operations]{.underline}. The SV is
undergoing short-term extended operations whenever the fit interval flag
is one and the IODE (reference paragraph 20.3.4.4) is in the range
0-239, 256-495, 512-751, or 768-1007.

6.2.3.3 [Long-term Extended Operations]{.underline}. The SV is
undergoing long-term extended operations whenever the fit interval flag
is one and the IODE is in the range 240-255, 496-511, 752-767, or
1008-1023.

Note: the DoD Navigation User Segment and Time Transfer User have no
requirement to operate, and may not operate properly, whenever any SV is
operating in long-term extended operations.

6.2.4 [GPS Week Number]{.underline}. See paragraph 6.2.4 of ICD-GPS-200
and paragraphs 20.3.3.1.1.1, 20.3.3.3.1.1, and 20.3.3.3.2.1.

6.3 [Supporting Material]{.underline}

6.3.1 [L5 Received Signals]{.underline}. The guaranteed minimum
user-received L5 signal levels are defined in paragraph 3.3.1.6.
Additional supporting material is in Figure 6-1 (TBD). The minimum power
of the near-ground user-received L5 signals as a function of SV
elevation angle is calculated using the following assumptions: (a) the
atmospheric path loss is 0.5 dB, (b) the average SV antenna gain with
worst-case polarization loss will be used to determine minimum EOL EIRP
at 14.3 degrees off nadir; and (c) TBD. The specified power levels
identified in §3.3.1.5 are based on the overall link budget given in
Table 6-I.

6.3.2 Integrated phase noise characteristics- As an aid to user
equipment receiver LO design a plot will be provided of a typical GPS
Block IIF carrier phase noise spectral density distribution and
integration interval.

Table 6-I. I5 and Q5 Signal-in-Space Link Budget Summary

+-------------------------------------------------+--------------------+
| #### Link Component                             | I5 or Q5           |
+-------------------------------------------------+--------------------+
| Minimum EIRP\* at 14.3 degrees off nadir angle  | 25.5               |
| (dBW)                                           |                    |
+-------------------------------------------------+--------------------+
| Free Space Loss (dB)                            | -182               |
+-------------------------------------------------+--------------------+
| Atmospheric Loss (dB)                           | -0.5               |
+-------------------------------------------------+--------------------+
| Receiver Antenna Gain (dB)                      | 0                  |
+-------------------------------------------------+--------------------+
| Minimum Received Power (dBW)                    | -157               |
+-------------------------------------------------+--------------------+
| \* Includes polarization loss relative to a     |                    |
| RHCP antenna at end-of-life, nominal orbital    |                    |
| temperatures, and azimuth averaged antenna      |                    |
| gain. Gain variation from 14.3 degrees EOE to   |                    |
| nadir shall not exceed 2 dB decrease. Composite |                    |
| EIRP power is 28.5 dBw , composite received     |                    |
| power is --154 dBw.                             |                    |
+-------------------------------------------------+--------------------+

Figure 6-1. User Received Minimum Signal Levels (TBD)

10\. APPENDIX I. LETTERS OF EXCEPTION

10.1 [Scope]{.underline}. As indicated in paragraph 1.2, initial
signature approval of this document, as well as approval of subsequent
changes to the document, can be contingent upon a \"letter of
exception\". This appendix depicts such \"letters of exception\" when
utilized by any signatory of this document in the initial approval cycle
and/or in the change approval process. The ICC will omit such letters of
exception from subsequent revisions of this document based on written
authorization by the respective signatory (without processing a proposed
interface revision notice (PIRN) for approval). When some (but not all)
of the exceptions taken by a signatory are resolved, the signatory shall
provide the ICC with an updated letter of exception for inclusion in the
next ICD revision (without processing a PIRN for approval).

10.2 [Applicable Documents]{.underline}. The documents listed in Section
2.0 shall be applicable to this appendix.

10.3 [Letters of Exception]{.underline}. If signature approval of this
document \-- as affixed to the cover page \-- is marked by an asterisk,
it indicates that the approval is contingent upon the exceptions taken
by that signatory in a letter of exception. Any letter of exception
which is in force for the revision of the ICD is depicted in Figure
10-1. Signatories for whom no letter of exception is shown have approved
this version of the document without exception.

Figure 10-1 Letters of Exception (TBD)

(This page intentionally left blank.)

20\. APPENDIX II. GPS NAVIGATION DATA STRUCTURE FOR DATA ID NO. 3

20.1 [Scope]{.underline}. This appendix describes the specific GPS
navigation (NAV) data structure denoted by data ID number 3. This data
ID number, when transmitted as part of the NAV data, shall be
represented by the two-bit binary notation as 10. Data ID number 2 is
used for L1 and L2 C/A-code, and data ID number 1 is no longer used.

20.2 [Applicable Documents]{.underline}.

20.2.1 [Government Documents]{.underline}. In addition to the documents
listed in paragraph 2.1, the following documents of the issue specified
contribute to the definition of the NAV data related interfaces and form
a part of this Appendix to the extent specified herein.

[Specifications]{.underline}

None

[Standards]{.underline}

None

[Other Publications]{.underline}

None

20.2.2 [Non-Government Documents]{.underline}. In addition to the
documents listed in paragraph 2.2, the following documents of the issue
specified contribute to the definition of the NAV data related
interfaces and form a part of this Appendix to the extent specified
herein.

[Specifications]{.underline}

None

[Other Publications]{.underline}

none

20.3 [Requirements]{.underline}

20.3.1 [Data Characteristics]{.underline}. The L5 channel data stream
mostly contains the same data as the L1 and L2 channels, but in a
different format. Also, the L5 data stream uses a different parity
algorithm.

20.3.2 [Message Structure]{.underline}. As shown in Figures 20-1 through
20-5, the L5 message structure utilizes a basic format of six-second
300-bit long messages. Each message contains a CRC parity block
consisting of 24 bits covering the entire six-second message (300 bits)
(reference Section 20.3.5). At present, only message types 1-5 are
defined. Message types 6 through 64 are reserved.

20.3.3 [Message Content]{.underline}. Each message starts with an 8-bit
preamble -- 10001011, followed by a 6-bit message type ID and the 17-bit
message time of week (TOW) count. When the value of the message TOW
count is multiplied by 6, it represents SV time in seconds at the start
of the next 6-second message. An "alert" flag, when raised (bit 32 =
"1"), indicates to the user that the SV URA~5~ may be worse than
indicated in Message Type 1, and the SV should be used at the user's own
risk. A 6-bit PRN number, with a range of 1 to 37, starts at bit 33 of
Message Types 1, 3, 4 and 5.

![](media/image11.wmf)

Figure 20-1. Message Type 1 Format

![](media/image12.wmf)

Figure 20-2. Message Type 2 Format

![](media/image13.wmf)

Figure 20-3. Message Type 3 Format

![](media/image14.wmf)

Figure 20-4. Message Type 4 Format

![](media/image15.wmf)

Figure 20-5. Message Type 5 Format

(This page intentionally left blank.)

20.3.3.1 [Message Type 1 Clock, Health and Accuracy
Parameters]{.underline}.

20.3.3.1.1 [Message Type 1 Clock, Health and Accuracy Parameter
Content]{.underline}. The clock parameters in Message Type 1 describe
the SV time scale during the period of validity. The parameters in a
data set shall be valid during the interval of time in which they are
transmitted and shall remain valid for an additional period of time
after transmission of the next data set has started. The timing
information for the messages is described in Section 20.3.4.

20.3.3.1.1.1 [Transmission Week Number]{.underline}. The ten bits
starting at bit 39 shall contain the ten MSBs of the 29-bit Z-count as
qualified in paragraph 20.3.3.3.1.1 of ICD-GPS-200.

20.3.3.1.1.2 [SV Accuracy (L5)]{.underline}. Bits 55 through 58 shall
contain the URA~5~ index of the SV (reference paragraph 6.2.1) for the
unauthorized (non-Precise Positioning Service) user. The URA~5~ index
(N) is an integer in the range of 0 through 15 and follows the same
rules as the URA index, as defined in paragraph 20.3.3.3.1.3 of
ICD-GPS-200.

20.3.3.1.1.3 [SV Health (L5)]{.underline}. The five-bit health
indication in bits 49 through 53 refers to the transmitting SV for the
L5 signal. The MSB shall indicate a summary of the health of the NAV
data, where

0 = all NAV data are OK,

1 = some or all NAV data are bad.

The four LSBs shall indicate the health of the L5 signal components in
accordance with the codes given in paragraph 20.3.3.4.1.3. The health
indication shall be given relative to the "as designed" capabilities of
each SV (as designated by the configuration code - see paragraph
20.3.3.4.1.4). Accordingly, any SV that does not have a certain
capability will be indicated as "healthy" if the lack of this capability
is inherent in its design or if it has been configured into a mode that
is normal from a user standpoint and does not require that capability.

Additional SV health data are given in Message Type 4, the Almanac
message. The data given in Message Type 1 may differ from that shown in
Message Type 4 on other SVs since the latter may be updated at a
different time.

20.3.3.1.1.4 [Issue of Data, Clock (IODC)]{.underline}. Bits 209 through
218 shall contain the IODC. The IODC indicates the issue number of the
data set and thereby provides the user with a convenient means of
detecting any change in the correction parameters. Constraints on the
IODC as well as the relationship between the IODC and the IODE (issue of
data, ephemeris) terms are defined in paragraph 20.3.4.4.

20.3.3.1.1.5 [Long-term Extended Operations]{.underline}. Whenever the
fit interval flag indicates a fit interval greater than 4 hours, the
IODC can be used to determine the actual fit interval of the data set
(reference section 20.3.4.4).

20.3.3.1.1.6 [SV Clock Correction]{.underline}. Message Type 1 contains
the parameters needed by the users for apparent SV L1/L5 clock
correction (t~oc5~, a~25~, a~15~, a~05~). The related algorithm is given
in paragraph 20.3.3.1.3.1.

20.3.3.1.1.7 [Estimated Group Delay Differential (L1-L5)]{.underline}.
The L1-L5 correction term in Message Type 1, T~GD15~, is for the benefit
of "L1-C/A only" or "L5-Q5 only" users; the related user algorithm is
given in paragraph 20.3.3.1.3.2. The value of negative zero or bit
string \[1 0 0 0 0 0 0 0\] shall mean that T~GD15~ is not available.

20.3.3.1.1.8 [Estimated L2/L5 Group Delay Differential]{.underline}. The
L2/L5 group delay correction term, T~GD25~, for the benefit of single
frequency L2-C/A users and dual frequency L2/L5 users is contained in
Message Type 1; the related user algorithms are given in paragraphs
20.3.3.1.3.3 and 20.3.3.1.3.5. The value of negative zero or bit string
\[1 0 0 0 0 0 0 0\] shall mean that T~GD25~ is not available.

20.3.3.1.1.9 [Estimated L5 Q5/I5 Group Delay Differential]{.underline}.
The L5 I5/Q5 group delay correction term, T~IQ5~, for the benefit of L5
users tracking the I5-code is contained in Message Type 1; the related
user algorithms are given in paragraphs 20.3.3.1.3.3 and 20.3.3.1.3.5.
The value of negative zero or bit string \[1 0 0 0\] shall mean that
T~IQ5~ is not available.

20.3.3.1.2 [Message Type 1 Clock, Health and Accuracy Parameter
Characteristics]{.underline}. For those parameters whose characteristics
are not fully defined in Section 20.3.3.1.1, the number of bits, the
scale factor of the LSB (which is the last bit received), the range, and
the units shall be as specified in Table 20-I.

20.3.3.1.3 [User Algorithms for Message Type 1 Clock Data]{.underline}.
The algorithms defined below (a) allow all users to correct the code
phase time received from the SV with respect to both SV code phase
offset and relativistic effects, (b) permit the "single frequency"
(L1-C/A, L2-C/A or L5), "dual L1 and L5 frequency", and "dual L2 and L5
frequency" users to compensate for the effects of SV group delay
differential (the user who utilizes both the L1-C/A and L5-Q5
frequencies does not require this correction, since the clock parameters
account for the induced effects), and (c) allow the user of the L5
I5-code to correct for the timing error between the I5 and Q5-codes.
Those users who utilize the "dual L1 and L2 frequencies" shall use the
corrections provided in the L1 or L2 NAV messages.

20.3.3.1.3.1 [User Algorithm for SV Clock Correction]{.underline}. The
polynomial defined in the following allows the user to determine the
effective SV PRN code phase offset referenced to the phase center of the
antennas (∆t~sv~) with respect to GPS L5 system time (t) at the time of
data transmission. The coefficients transmitted in Message Type 1
describe the offset apparent to the two-frequency (L1 and L5) user for
the interval of time in which the parameters are transmitted. This
estimated correction accounts for the deterministic SV clock error
characteristics of bias, drift and aging, as well as for the SV
implementation characteristics of group delay bias and mean differential
group delay. Since these coefficients do not include corrections for
relativistic effects, the user's equipment must determine the requisite
relativistic correction. Accordingly, the offset given below includes a
term to perform this function.

The user shall correct the time received from the SV with the equation
(in seconds)

> t = t~sv~ - ∆t~sv~ (1)

where

> t = L5 GPS system time (seconds),
>
> t~sv~ = effective SV PRN code phase time at message transmission time
> (seconds),
>
> ∆t~sv~ = SV PRN code phase time offset (seconds).

The SV PRN code phase offset is given by

> ∆t~sv~ = a~f05~ + a~f15~(t - t~oc5~) + a~f25~(t - t~oc5~)^2^ + ∆t~r~
> (2)

where

> a~f05~, a~f15~ and a~f25~ are the polynomial coefficients given in
> Message Type 1, t~oc5~ is the clock data reference time in seconds
> (reference paragraph 20.3.4.5), and ∆t~r~ is the relativistic
> correction term (seconds) which is given in paragraph 20.3.3.3.3.1 of
> ICD-GPS-200.

Note that equations (1) and (2), as written, are coupled. While the
coefficients a~f05~, a~f15~ and a~f25~ are generated by using GPS time
as indicated in equation (2), sensitivity of t~sv~ to t is negligible.
This negligible sensitivity will allow the user to approximate t by
t~SV~ in equation (2). The value of t must account for beginning or end
of week crossovers. That is, if the quantity t - t~oc5~ is greater than
302,400 seconds, subtract 604,800 seconds from t. If the quantity t -
t~oc5~ is less than -302,400 seconds, add 604,800 seconds to t.

The control segment will utilize the alternative but equivalent
expression for the relativistic effect when estimating the NAV
parameters described in paragraph 20.3.3.3.3.1 of ICD-GPS-200.

Since the SV clock corrections of equations (1) and (2) are estimated by
the CS using dual frequency L1 and L5 measurements, the single-frequency
L5 user and the dual frequency L2/L5 user must apply additional terms to
equations (1) and (2). These terms are described in paragraphs
20.3.3.1.3.2 and 20.3.3.1.3.3.

+--------------------+----------+----------+-------------+-------------+
| Table 20-I.        |          |          |             |             |
| Message Type 1     |          |          |             |             |
| Clock, Health and  |          |          |             |             |
| Accuracy           |          |          |             |             |
| Parameters         |          |          |             |             |
+--------------------+----------+----------+-------------+-------------+
| Parameter          | No. of   | Scale    | Effective   | Units       |
|                    |          |          |             |             |
|                    | Bits\*\* | Factor   | Range\*\*\* |             |
|                    |          |          |             |             |
|                    |          | (LSB)    |             |             |
+--------------------+----------+----------+-------------+-------------+
| Week No.           | 10       | 1        | 604,784     | weeks       |
|                    |          |          |             |             |
| SV accuracy        | 4        | 1        |             | (see text)  |
| (URA~5~)           |          |          |             |             |
|                    | 6        | 2^-31^   |             | discrete    |
| SV health (L5)     |          |          |             |             |
|                    | 8\*      | 2^-31^   |             | seconds     |
| T~GD15~            |          |          |             |             |
|                    | 8\*      | 2^-31^   |             | seconds     |
| T~GD25~            |          |          |             |             |
|                    | 4\*      | 2^4^     |             | seconds     |
| T~IQ5~            |          |          |             |             |
|                    | 10       | 2^-55^   |             | (see text)  |
| IODC               |          |          |             |             |
|                    | 16       | 2^-43^   |             | seconds     |
| t~oc5~             |          |          |             |             |
|                    | 8\*      | 2^-31^   |             | sec/sec^2^  |
| a~f25~             |          |          |             |             |
|                    | 16\*     |          |             | sec/sec     |
| a~f15~             |          |          |             |             |
|                    | 22\*     |          |             | seconds     |
| a~f05~             |          |          |             |             |
+--------------------+----------+----------+-------------+-------------+
| \* Parameters so   |          |          |             |             |
| indicated are      |          |          |             |             |
| two's complement,  |          |          |             |             |
| with the sign bit  |          |          |             |             |
| (+ or -) occupying |          |          |             |             |
| the MSB;           |          |          |             |             |
|                    |          |          |             |             |
| \*\* See Figure    |          |          |             |             |
| 20-1 for complete  |          |          |             |             |
| bit allocation in  |          |          |             |             |
| Message Type 1;    |          |          |             |             |
|                    |          |          |             |             |
| \*\*\* Unless      |          |          |             |             |
| otherwise          |          |          |             |             |
| indicated in this  |          |          |             |             |
| column, effective  |          |          |             |             |
| range is the       |          |          |             |             |
| maximum range      |          |          |             |             |
| attainable with    |          |          |             |             |
| indicated bit      |          |          |             |             |
| allocation and     |          |          |             |             |
| scale factor.      |          |          |             |             |
+--------------------+----------+----------+-------------+-------------+

20.3.3.1.3.2 [L1/L5 Group Delay Correction]{.underline}. The L1 and L5
correction term, T~GD15~, is monitored to account for the effect of SV
group delay differential between L1 C/A-code and L5 Q5-code. The value
for T~GD15~ is based on measurements made by the SV contractor during SV
manufacture. The value of T~GD15~ for each SV may be subsequently
updated to reflect the actual on-orbit group delay differential. This
correction term is only for the benefit of "single-frequency" (L1 or L5)
users; it is necessitated by the fact that the SV clock offset estimates
reflected in the a~f05~ clock correction coefficient (see paragraph
20.3.3.1.3.1) are based on the effective PRN code phase as apparent with
two frequency ionospheric corrections. Thus, the user who utilizes the
L1 frequency only must modify the code phase offset in accordance with
paragraph 20.3.3.1.3.1 with the equation

> (∆t~SV~)~L1~ = ∆t~SV~ - T~GD15~

where T~GD15~ is provided to the user as Message Type 1 data. For the
user who utilizes L5 only, the code phase modification is given by

> (∆t~SV~)~L5~ = ∆t~SV~ - γ~15~T~GD15~

where, denoting the nominal center frequencies of L1 and L5 as f~L1~ and
f~L5~ respectively,

> γ~15~ = (f~L1~/f~L5~)^2^ = (1575.42/1176.45)^2^ = (154/115)^2^.

The value of T~GD15~ is not equal to the mean SV group delay
differential, but is equal to the delay differential multiplied by
1/(1-γ~15~). That is,

> $$T_{\text{GD}\text{15}} = \frac{1}{1 - \gamma_{\text{15}}}\left( t_{L5} - t_{L1} \right)$$

where t~Li~ is the GPS time the i^th^ frequency signal is transmitted
from the SV.

20.3.3.1.3.3 [L2/L5 Group Delay Correction]{.underline}. The L2/L5 group
delay correction term, T~GD25~, is calculated by the CS to account for
the effect of SV group delay differential between the composite dual
frequency L1/L5 time and L2 C/A-code time based on measurements made by
the SV contractor during SV manufacture. The value of T~GD25~ for each
SV may be subsequently updated to reflect the actual on-orbit group
delay differential. This correction term is for the benefit of all
L2-only and L2/L5 users; it is necessitated by the fact that the SV
clock offset estimates reflected in the a~f05~ clock correction
coefficient (see paragraph 20.3.3.1.3.1) are based on the effective
composite L1/L5 measurement as apparent with L1/L5 ionospheric
corrections. Thus, the user who utilizes the L2 frequency only must
modify the code phase offset in accordance with paragraph 20.3.3.1.3.1
with the equation

> (∆t~SV~)~L2~ = ∆t~SV~ -- T~GD25~

where T~GD25~ is provided to the user as Message Type 1 data. The value
of T~GD25~ is not equal to the mean SV group delay differential, but is
equal to

> T~GD25~ = t~L1~ -- t~L2~ + T~GD15~

to account for the group delay difference between the L1/L5 and L2
signals.

20.3.3.1.3.4 [L1/L5 Ionospheric Correction]{.underline}. The two
frequency (L1 C/A-code and L5 Q5-code) user shall correct for the group
delay due to ionospheric effects by applying the relationship:

$$\text{PR} = \frac{\text{PR}_{5} - \gamma_{\text{15}}\text{PR}_{1}}{1 - \gamma_{\text{15}}}$$

where

> PR = pseudorange corrected for ionospheric effects,
>
> PR~i~ = pseudorange measured on the L-band channel indicated by the
> subscript.

and

γ~15~ = (f~L1~/f~L5~)^2^ = (1575.42/1176.45)^2^ = (154/115)^2^..

The clock correction coefficients are based on "two frequency"
measurements and therefore account for the effects of mean differential
delay in SV instrumentation.

20.3.3.1.3.5 [L2/L5 Ionospheric Correction]{.underline}. The two
frequency (L2 and L5) user shall correct for the group delay due to
ionospheric effects by applying the relationship:

$$\text{PR} = \frac{\left( \text{PR}_{5} - \text{cT}_{\text{GD15}} \right) - \gamma_{\text{25}}\left( \text{PR}_{2} - \text{cT}_{\text{GD25}} \right)}{1 - \gamma_{\text{25}}}$$

> where
>
> γ~25~ = (f~L2~/f~L5~)^2^ = (1227.6/1176.45)^2^ = (24/23)^2^

This additional correction for the dual frequency L2/L5 users is due to
the fact that the SV clock offset estimates reflected in the clock
correction coefficients (see paragraph 20.3.3.1.3.1) are based on the
effective PRN code phase as apparent with two frequency (L1/L5)
ionospheric corrections, not the two frequency (L2/L5) ionospheric
corrections.

20.3.3.1.3.6 [L5 Q-code/I-code Correction]{.underline}. The L5
Q-code/I-code group delay correction term, ∆T~IQ5~, is calculated by the
CS to account for the effect of SV group delay differential between the
L5 Q-code and L5 I-code based on measurements made by the SV contractor
during SV manufacture. The value of ∆T~IQ5~ for each SV may be
subsequently updated to reflect the actual on-orbit group delay
differential. It is defined such as to be positive when the I-code
transitions lead the Q-code transitions, and negative when the I-code
transitions lag the Q-code transitions.

20.3.3.1.3.7 [Example Application of Correction Parameters]{.underline}.
A typical system application of the correction parameters for a user
receiver is shown in Figure 20-6. The ionospheric model referred to in
Figure 20-6 is discussed in paragraph 20.3.3.3.2.2 in conjunction with
the related data contained in Message Type 3.

20.3.3.2 [Message Type 1 and 2 Ephemeris Parameters]{.underline}

20.3.3.2.1 [Message Type 1 and 2 Ephemeris Parameter
Content]{.underline}. The contents of the ephemeris representation
parameters in Message Types 1 and 2 are defined below, followed by
material pertinent to the use of the data.

The ephemeris parameters describe the orbit of the transmitting SV
during the curve fit intervals described in section 20.3.4. Table 20-II
of ICD-GPS-200 gives the definition of the orbital parameters using
terminology typical of Keplerian orbital parameters; it is noted,
however, that the transmitted parameter values are expressed such that
they provide the best trajectory fit in earth-centered, earth-fixed
(ECEF) coordinates for each specific fit interval. The user shall not
interpret intermediate coordinate values as pertaining to any
conventional coordinate system.

The issue of ephemeris data (IODE) term provides the user with a
convenient means for detecting any change in the ephemeris
representation parameters. The IODE is provided in Message Type 2 for
the purpose of comparison with the 10 bits of the IODC term in Message
Type 1. Whenever these two terms do not match, a data set cutover has
occurred and new data must be collected. The timing of the IODE and
constraints on the IODC and IODE are defined in paragraph 20.3.4.4.

Any change in the Message Type 1 and 2 data will be accomplished with a
simultaneous change in both the IODC and IODE words. The CS will assure
that the t~oe~ value, for at least the first data set transmitted by an
SV after an upload, is different from that transmitted prior to the
cutover.

A "fit interval" flag is provided in Message Type 2 to indicate whether
the ephemerides are based on a four-hour fit interval or a fit interval
greater than four hours (reference paragraph 20.3.3.2.3.1).

![](media/image16.wmf)Figure 20-6. Sample Application of Correction
Parameters

20.3.3.2.2 [Message Type 1 and 2 Ephemeris Parameter
Characteristics]{.underline}. For each ephemeris parameter contained in
Message Types 1 and 2, the number of bits, the scale factor of the LSB
(which is the last bit received), the range, and the units are as
specified in Table 20-III of ICD-GPS-200. See Figures 20-1 and 20-2 for
complete bit allocation in Message Types 1 and 2.

20.3.3.2.3 [User Algorithm for Ephemeris Determination]{.underline}. The
user shall compute the ECEF coordinates of position for the phase center
of the SVs' antennas utilizing a variation of the equations shown in
Table 20-IV of ICD-GPS-200. The ephemeris parameters are Keplerian in
appearance; the values of these parameters, however, are produced by the
CS via a least squares curve fit of the predicted ephemeris of the phase
center of the SVs' antennas (time-position quadruples; t, x, y, z
expressed in ECEF coordinates). Particulars concerning the periods of
the curve fit, the resultant accuracy, and the applicable coordinate
system are given in the following subparagraphs.

20.3.3.2.3.1 [Curve Fit Intervals]{.underline}. Bit 275 of Message Type
2 is a "fit interval" flag which indicates the curve-fit interval used
by the CS in determining the ephemeris parameters, as follows:

0 = 4 hours,

1 = greater than 4 hours.

The relationship of the curve-fit interval to transmission time and the
timing of the curve-fit intervals is covered in section 20.3.4.

20.3.3.2.3.2 [Parameter Sensitivity]{.underline}. See paragraph
20.3.3.4.3.2 of ICD-GPS-200.

20.3.3.2.3.3 [Coordinate Systems]{.underline}

20.3.3.2.3.3.1 [ECEF Coordinate System]{.underline}. See paragraph
20.3.3.4.3.3.1 of ICD-GPS-200.

20.3.3.2.3.3.2 [Earth-Centered Inertial (ECI) Coordinate
System]{.underline}. See paragraph 20.3.3.4.3.3.2 of ICD-GPS-200.

20.3.3.2.3.4 [Geometric Range]{.underline}. See paragraph 20.3.3.4.3.4
of ICD-GPS-200.

20.3.3.3 [Message Type 3 Parameters]{.underline}. The contents of
Message Type 3 are defined below, followed by material pertinent to the
use of the data.

20.3.3.3.1 [Message Type 3 Parameter Content]{.underline}. Message Type
3 contains UTC and ionospheric parameters and other data.

20.3.3.3.1.1 [Coordinated Universal Time (UTC) and GPS Time
Parameters]{.underline}. Message Type 3 shall contain the parameters
related to correlating UTC time with GPS Time and parameters to relate
L5 GPS Time to GPS Time. The bit length, scale factors, ranges, and
units of these parameters are given in Table 20-II. The related
algorithms are described in paragraph 20.3.3.3.2.1.

The UTC and GPS Time parameters shall be updated by the CS at least once
every six days while the CS is able to upload the SVs. If the CS is
unable to upload the SVs, the accuracy of the UTC parameters transmitted
by the SVs will degrade over time.

20.3.3.3.1.2 [Ionospheric Data]{.underline}. The ionospheric parameters
which allow the "L1 only", "L2 only" or "L5 only" user to utilize the
ionospheric model (reference paragraph 20.3.3.3.2.2) for computation of
the ionospheric delay are contained in Message Type 3. The bit lengths,
scale factors, ranges, and units of these parameters are given in Table
20-X of ICD-GPS-200. See Figure 20-3 for complete bit allocation in
Message Type 3.

The ionospheric data shall be updated by the CS at least once every six
days while the CS is able to upload the SVs. If the CS is unable to
upload the SVs, the ionospheric data transmitted by the SVs may not be
accurate.

20.3.3.3.1.3 [SV Health (L1/L2)]{.underline}. The six-bit health
indication in bits 43 through 48 refers to the transmitting SV for the
L1 and L2 signals. The health indication is described in paragraph
20.3.3.3.1.4 of ICD-GPS-200.

Additional SV health data are given in Message Type 4, the Almanac
message. The data given in Message Type 3 may differ from that shown in
Message Type 4 on other SVs since the latter may be updated at a
different time.

+-------------+----------+----------+-------------+--------------------+
| Table       |          |          |             |                    |
| 20-II. UTC  |          |          |             |                    |
| Parameters  |          |          |             |                    |
+-------------+----------+----------+-------------+--------------------+
| Parameter   | No. of   | Scale    | Effective   | Units              |
|             |          |          |             |                    |
|             | Bits\*\* | Factor   | Range\*\*\* |                    |
|             |          |          |             |                    |
|             |          | (LSB)    |             |                    |
+-------------+----------+----------+-------------+--------------------+
| A~0~        | 32\*     | 2^-30^   | 602,112     | seconds            |
|             |          |          |             |                    |
| δA~05~      | 8\*      | 2^-30^   | 7           | seconds            |
|             |          |          |             |                    |
| A~1~        | 24\*     | 2^-50^   |             | sec/sec            |
|             |          |          |             |                    |
| δA~15~      | 8\*      | 2^-50^   |             | sec/sec            |
|             |          |          |             |                    |
| ∆t~LS~      | 8\*      | 1        |             | seconds            |
|             |          |          |             |                    |
| t~ot~       | 8        | 2^12^    |             | seconds            |
|             |          |          |             |                    |
| WN~t~       | 8        | 1        |             | weeks              |
|             |          |          |             |                    |
| WN~LSF~     | 8        | 1        |             | weeks              |
|             |          |          |             |                    |
| DN          | 8        | 1        |             | days               |
|             | \*\*\*\* |          |             |                    |
| ∆t~LSF~     |          | 1        |             | seconds            |
|             | 8\*      |          |             |                    |
+-------------+----------+----------+-------------+--------------------+
| \*          |          |          |             |                    |
| Parameters  |          |          |             |                    |
| so          |          |          |             |                    |
| indicated   |          |          |             |                    |
| are two's   |          |          |             |                    |
| complement  |          |          |             |                    |
| with the    |          |          |             |                    |
| sign bit (+ |          |          |             |                    |
| or -)       |          |          |             |                    |
| occupying   |          |          |             |                    |
| the MSB;    |          |          |             |                    |
|             |          |          |             |                    |
| \*\* See    |          |          |             |                    |
| Figure 20-3 |          |          |             |                    |
| for         |          |          |             |                    |
| complete    |          |          |             |                    |
| bit         |          |          |             |                    |
| allocation  |          |          |             |                    |
| in Message  |          |          |             |                    |
| Type 3;     |          |          |             |                    |
|             |          |          |             |                    |
| \*\*\*      |          |          |             |                    |
| Unless      |          |          |             |                    |
| otherwise   |          |          |             |                    |
| indicated   |          |          |             |                    |
| in this     |          |          |             |                    |
| column,     |          |          |             |                    |
| effective   |          |          |             |                    |
| range is    |          |          |             |                    |
| the maximum |          |          |             |                    |
| range       |          |          |             |                    |
| attainable  |          |          |             |                    |
| with        |          |          |             |                    |
| indicated   |          |          |             |                    |
| bit         |          |          |             |                    |
| allocation  |          |          |             |                    |
| and scale   |          |          |             |                    |
| factor;     |          |          |             |                    |
|             |          |          |             |                    |
| \*\*\*\*    |          |          |             |                    |
| Right       |          |          |             |                    |
| justified.  |          |          |             |                    |
+-------------+----------+----------+-------------+--------------------+

20.3.3.3.1.4 [Code(s) on L2 Channel]{.underline}. Bits 41 and 42 of
Message Type 3 shall indicate which code(s) is (are) commanded ON for
the L2 channel. The codes are described in paragraph 20.3.3.3.1.2 of
ICD-GPS-200.

20.3.3.3.1.5 [Data ID]{.underline}. Bits 39 and 40 of Message Type 3
(and the same bits in Message Types 4 and 5) shall contain the data ID
which defines the applicable GPS NAV data structure. Data ID one
(denoted by binary code 00) was utilized during Phase I of the GPS
program and is no longer in use; data ID two (denoted by binary code 01)
is used on the L1 and L2 C/A and P(Y)-code signals. Data ID three
(denoted by binary 10) is used on this L5 signal. Future data IDs shall
be defined as necessary.

20.3.3.3.2 [Algorithms Related to Message Type 3 Data]{.underline}. The
following algorithms shall apply when interpreting Coordinated Universal
Time, L5 GPS Time and Ionospheric Model data in the NAV message.

20.3.3.3.2.1 [Coordinated Universal Time (UTC) and GPS
Time]{.underline}. Message Type 3 includes: (1) the parameters needed to
relate GPS Time to UTC and L5 GPS Time to GPS Time, and (2) notice to
the user regarding the scheduled future or recent past (relative to NAV
message upload) value of the delta time due to leap seconds (∆t~LSF~),
together with the week number (WN~LSF~) and the day number (DN) at the
end of which the leap second becomes effective. Information required to
use these parameters to calculate t~UTC~ is in paragraph 20.3.3.5.2.4 of
ICD-GPS-200.

The GPS Time/L5 GPS Time relationship is given by

t~E~ = t~E,L5~ + δA~05~ + δA~15~ (t~E,L5~ -- t~ot~)

where t~E,L5~ is the estimated GPS L5 Time in seconds relative to
end/start of week.

20.3.3.3.2.2 [Ionospheric Model]{.underline}. The "two frequency" (L1
C/A and L5 Q5) user shall correct the time received from the SV for
ionospheric effect by utilizing the time delay differential between L1
and L5 (reference paragraph 20.3.3.1.3.2). The "one frequency" user,
however, may use the model given in Figure 20-4 of ICD-GPS-200 to make
this correction. The calculated value of T~iono~ is referred to the L1
frequency; if the user is operating on the L5 frequency, the correction
term must be multiplied by γ~15~ (reference paragraph 20.3.3.1.3.2). It
is estimated that the use of this model will provide at least a 50
percent reduction in the single - frequency user's RMS error due to
ionospheric propagation effects. During extended operations, if the CS
is unable to upload the SVs, the use of this model will yield
unpredictable results.

20.3.3.4 [Message Type 4 Almanac Parameters]{.underline}. The contents
of Message Type 4 are defined below, followed by material pertinent to
the use of the data.

20.3.3.4.1 [Message Type 4 Almanac Parameter Content]{.underline}.
Message Type 4 contains almanac data.

20.3.3.4.1.1 [Almanac Data]{.underline}. Message Type 4 contains the
almanac data and SV health words for up to 32 SVs (the health words are
discussed in paragraphs 20.3.3.1.1.3 and 20.3.3.3.1.3). The almanac data
are a reduced-precision subset of the clock and ephemeris parameters.
The number of bits, the scale factor (LSB), the range, and the units of
the almanac parameters are given in Table 20-III. The algorithms and
other material related to the use of the almanac data are given in
paragraph 20.3.3.4.2.

The almanac parameters shall be updated by the CS at least once every 6
days while the CS is able to upload the SVs. If the CS is unable to
upload the SVs, the accuracy of the almanac parameters transmitted by
the SVs will degrade over time.

+-------------+----------+----------+-------------+--------------------+
| Table       |          |          |             |                    |
| 20-III.     |          |          |             |                    |
| Almanac     |          |          |             |                    |
| Parameters  |          |          |             |                    |
+-------------+----------+----------+-------------+--------------------+
| Parameter   | No. of   | Scale    | Effective   | Units              |
|             |          |          |             |                    |
|             | Bits\*\* | Factor   | Range\*\*\* |                    |
|             |          |          |             |                    |
|             |          | (LSB)    |             |                    |
+-------------+----------+----------+-------------+--------------------+
| e           | 16       | 2^-21^   | 602,112     | dimensionless      |
|             |          |          |             |                    |
| t~oa~       | 8        | 2^12^    |             | seconds            |
|             |          |          |             |                    |
| ![](med     | 16\*     | 2^-19^   |             | semi-circles       |
| ia/image17. |          |          |             |                    |
| wmf){width= | 16\*     | 2^-38^   |             | semi-circles/sec   |
| "0.13888888 |          |          |             |                    |
| 88888889in" | 24       | 2^-11^   |             | meters^1/2^        |
| height="    |          |          |             |                    |
| 0.19375in"} | 24\*     | 2^-23^   |             | semi-circles       |
| ~i~\*\*\*\* |          |          |             |                    |
|             | 24\*     | 2^-23^   |             | semi-circles       |
| OMEGADOT    |          |          |             |                    |
|             | 24\*     | 2^-23^   |             | semi-circles       |
| (A)^1/2^    |          |          |             |                    |
|             | 11\*     | 2^-20^   |             | seconds            |
| (OMEGA)~0~  |          |          |             |                    |
|             | 11\*     | 2^-38^   |             | sec/sec            |
| ![](medi    |          |          |             |                    |
| a/image18.w |          |          |             |                    |
| mf){width=" |          |          |             |                    |
| 0.166666666 |          |          |             |                    |
| 66666666in" |          |          |             |                    |
| height="    |          |          |             |                    |
| 0.152777777 |          |          |             |                    |
| 7777778in"} |          |          |             |                    |
|             |          |          |             |                    |
| M~0~        |          |          |             |                    |
|             |          |          |             |                    |
| a~f05~      |          |          |             |                    |
|             |          |          |             |                    |
| a~f15~      |          |          |             |                    |
+-------------+----------+----------+-------------+--------------------+
| \*          |          |          |             |                    |
| Parameters  |          |          |             |                    |
| so          |          |          |             |                    |
| indicated   |          |          |             |                    |
| are two's   |          |          |             |                    |
| complement  |          |          |             |                    |
| with the    |          |          |             |                    |
| sign bit (+ |          |          |             |                    |
| or -)       |          |          |             |                    |
| occupying   |          |          |             |                    |
| the MSB;    |          |          |             |                    |
|             |          |          |             |                    |
| \*\* See    |          |          |             |                    |
| Figure 20-4 |          |          |             |                    |
| for         |          |          |             |                    |
| complete    |          |          |             |                    |
| bit         |          |          |             |                    |
| allocation  |          |          |             |                    |
| in Message  |          |          |             |                    |
| Type 4      |          |          |             |                    |
|             |          |          |             |                    |
| \*\*\*      |          |          |             |                    |
| Unless      |          |          |             |                    |
| otherwise   |          |          |             |                    |
| indicated   |          |          |             |                    |
| in this     |          |          |             |                    |
| column,     |          |          |             |                    |
| effective   |          |          |             |                    |
| range is    |          |          |             |                    |
| the maximum |          |          |             |                    |
| range       |          |          |             |                    |
| attainable  |          |          |             |                    |
| with        |          |          |             |                    |
| indicated   |          |          |             |                    |
| bit         |          |          |             |                    |
| allocation  |          |          |             |                    |
| and scale   |          |          |             |                    |
| factor;     |          |          |             |                    |
|             |          |          |             |                    |
| \*\*\*\*    |          |          |             |                    |
| Relative to |          |          |             |                    |
| i~0~ = 0.30 |          |          |             |                    |
| se          |          |          |             |                    |
| mi-circles. |          |          |             |                    |
+-------------+----------+----------+-------------+--------------------+

20.3.3.4.1.2 [SV Health (L1 and L2)]{.underline}. Message Type 4
contains two types of SV health data for the SV indicated with by the
PRN~a~ parameter: (a) L1 and L2 health, and (b) L5 health. The L5 health
data is described in paragraph 20.3.3.4.1.3. The eight-bit L1/L2 health
words are defined in paragraph 20.3.3.5.1.3 and Table 20-VII of
ICD-GPS-200.

The data given in Message Type 3 of the other SVs may differ from that
shown in Message Type 4 since the latter may be updated at a different
time.

The predicted health data will be updated at the time of upload when a
new almanac has been built by the CS. The transmitted health data may
not correspond to the actual health of the transmitting SV or other SVs
in the constellation.

20.3.3.4.1.3 [SV Health (L5)]{.underline}. The three MSBs of the
seven-bit L5 health words in Message Type 4 shall indicate health of the
NAV data in accordance with the code given in Table 20-IV. The five-bit
L5 health words in Message Type 3 provide a one-bit summary of the NAV
data's health status in the MSB position in accordance with paragraph
20.3.3.3.1.3. The four LSBs of both the seven-bit and the five-bit words
provide the health status of the SV's signal components in accordance
with the code given in Table 20-V.

The data given in Message Type 3 of the other SVs may differ from that
shown in Message Type 4 since the latter may be updated at a different
time.

The predicted health data will be updated at the time of upload when a
new almanac has been built by the CS. The transmitted health data may
not correspond to the actual health of the transmitting SV or other SVs
in the constellation.

+------+------+-----+--------------------------------------------------+
| T    |      |     |                                                  |
| able |      |     |                                                  |
| 20   |      |     |                                                  |
| -IV. |      |     |                                                  |
| L5   |      |     |                                                  |
| NAV  |      |     |                                                  |
| Data |      |     |                                                  |
| He   |      |     |                                                  |
| alth |      |     |                                                  |
| Ind  |      |     |                                                  |
| icat |      |     |                                                  |
| ions |      |     |                                                  |
+------+------+-----+--------------------------------------------------+
| Bit  |      |     | Indication                                       |
| Posi |      |     |                                                  |
| tion |      |     |                                                  |
| in   |      |     |                                                  |
| 7    |      |     |                                                  |
| -Bit |      |     |                                                  |
| L5   |      |     |                                                  |
| He   |      |     |                                                  |
| alth |      |     |                                                  |
| Word |      |     |                                                  |
+------+------+-----+--------------------------------------------------+
| 1    | 2    | 3   |                                                  |
+------+------+-----+--------------------------------------------------+
| 0    | 0    | 0   | ALL DATA OK                                      |
|      |      |     |                                                  |
| 0    | 0    | 1   | PARITY FAILURE \-- some or all parity bad        |
|      |      |     |                                                  |
| 0    | 1    | 0   | PREAMBLE FORMAT PROBLEM \-- any departure from   |
|      |      |     | standard format (e.g., preamble misplaced and/or |
| 0    | 1    | 1   | incorrect, etc.), except for incorrect Z-count,  |
|      |      |     | as reported in TOW                               |
| 1    | 0    | 0   |                                                  |
|      |      |     | Z-COUNT BAD \-- any problem with Z-count value   |
| 1    | 0    | 1   | not reflecting actual code phase                 |
|      |      |     |                                                  |
| 1    | 1    | 0   | Message Types 1 and 2 \-- one or more elements   |
|      |      |     | in one or more of the messages are bad           |
| 1    | 1    | 1   |                                                  |
|      |      |     | Message Types 3 and 4 \-- one or more elements   |
|      |      |     | in one or more of the messages are bad           |
|      |      |     |                                                  |
|      |      |     | ALL UPLOADED DATA BAD \-- one or more of the     |
|      |      |     | uploaded elements in any one (or more) of the    |
|      |      |     | messages are bad                                 |
|      |      |     |                                                  |
|      |      |     | ALL DATA BAD \-- one or more elements in any one |
|      |      |     | (or more) of the messages are bad                |
+------+------+-----+--------------------------------------------------+

+-------------+--------------------------------------------------------+
| Table 20-V. |                                                        |
| Codes for   |                                                        |
| Health of   |                                                        |
| SV L5       |                                                        |
| Signal      |                                                        |
| Components  |                                                        |
+-------------+--------------------------------------------------------+
| MSB LSB     | Definition                                             |
+-------------+--------------------------------------------------------+
| 0 0 0 0     | All Signals (I & Q) OK                                 |
|             |                                                        |
| 0 0 0 1     | All Signals (I & Q) Weak\*                             |
|             |                                                        |
| 0 0 1 0     | All Signals (I & Q) Dead                               |
|             |                                                        |
| 0 0 1 1     | I Signal Has No Data Modulation                        |
|             |                                                        |
| 0 1 0 0     | I Signal Is Weak                                       |
|             |                                                        |
| 0 1 0 1     | I Signal Is Dead                                       |
|             |                                                        |
| 0 1 1 0     | Q Signal Is Weak                                       |
|             |                                                        |
| 0 1 1 1     | Q Signal Is Dead                                       |
|             |                                                        |
| 1 0 0 0     | L5 Specific Data Is Bad                                |
|             |                                                        |
| 1 0 0 1     | L5 Parity Is Bad                                       |
|             |                                                        |
| 1 0 1 0     | Spare                                                  |
|             |                                                        |
| 1 0 1 1     | Spare                                                  |
|             |                                                        |
| 1 1 0 0     | Spare                                                  |
|             |                                                        |
| 1 1 0 1     | Spare                                                  |
|             |                                                        |
| 1 1 1 0     | Spare                                                  |
|             |                                                        |
| 1 1 1 1     | More Than One Combination Would Be Required To         |
|             | Describe Anomalies                                     |
+-------------+--------------------------------------------------------+
| \* 3 to 6   |                                                        |
| dB below    |                                                        |
| specified   |                                                        |
| power level |                                                        |
| due to      |                                                        |
| reduced     |                                                        |
| power       |                                                        |
| output,     |                                                        |
| excess      |                                                        |
| phase       |                                                        |
| noise, SV   |                                                        |
| attitude,   |                                                        |
| etc.        |                                                        |
+-------------+--------------------------------------------------------+

20.3.3.4.1.4 [Anti-Spoof (A-S) Flags and SV Configurations]{.underline}.
Bits 225 to 228 of Message Type 4 shall contain a four-bit-long term for
each of up to 32 SVs to indicate the A-S status (of the L1 and L2
signals) and the configuration code of each SV. The terms are defined in
paragraph 20.3.3.5.1.6 of ICD-GPS-200.

20.3.3.4.1.5 [Almanac Reference Week]{.underline}. Bits 47 through 56 of
Message Type 4 shall indicate the number of the week (WN~a~) to which
the almanac reference time (t~oa~) is referenced (see paragraphs
20.3.3.4.1.1 and 20.3.3.4.2.2). The WN~a~ term consists of eight bits
which shall be a Modulo 256 binary representation of the GPS week number
(see paragraph 4.2.4) to which the t~oa~ is referenced. Bits 73 through
80 of Message Type 4 shall contain the value of t~oa~, which is
referenced to this WN~a~.

20.3.3.4.2 [Algorithms Related to Message Type 4 Data]{.underline}. The
following algorithms shall apply when interpreting Almanac data in the
NAV message.

20.3.3.4.2.1 [Almanac]{.underline}. The almanac is a subset of the clock
and ephemeris data, with reduced precision. The user algorithm is
essentially the same as the user algorithm used for computing the
precise ephemeris from the Message Type 1 and 2 parameters (see
paragraph 20.3.3.2.3). The almanac content for one SV is given in Table
20-III. Important information about the almanac calculation is contained
in paragraph 20.3.3.5.2.1 of ICD-GPS-200.

20.3.3.4.2.2 [Almanac Reference Time]{.underline}. See the two
subparagraphs labeled [Normal and Short-term Extended
Operations]{.underline} and [Long-term Extended Operations]{.underline}
of paragraph 20.3.3.5.2.2 of ICD-GPS-200.

20.3.3.4.2.3 [Almanac Time Parameters]{.underline}. See paragraph
20.3.3.5.2.3 of ICD-GPS-200.

20.3.3.5 [Message Type 5]{.underline}. Message Type 5 is reserved for
special messages with the specific contents at the discretion of the
Operating Command. It can accommodate the transmission of 29 eight-bit
ASCII characters. The requisite 232 bits occupy bits 41 through 272 of
Message Type 5. The eight-bit ASCII characters shall be limited to the
set described in paragraph 20.3.3.5.1.10 of ICD-GPS-200.

20.3.4 [Timing Relationships]{.underline}. The following conventions
shall apply.

20.3.4.1 [Paging and Cutovers]{.underline}. Paging of messages is
completely arbitrary, but sequenced to provide optimum user performance.
Message types 1 and 2 shall be broadcast at least once every 24 seconds.
All other messages shall be broadcast in-between, not exceeding the
maximum update interval in Table 30-III. Message type 5 will be
broadcast as needed, but will not reduce the maximum update interval of
the other messages. Type 5 messages that are longer than one page shall
not necessarily be broadcast consecutively.

+----------------------+------------------------+----------------------+
| Table 20-III.        |                        |                      |
| Message Content      |                        |                      |
| Broadcast Intervals  |                        |                      |
+----------------------+------------------------+----------------------+
| Message Data         | Message Type           | Maximum Update       |
|                      |                        | Interval (seconds)   |
+----------------------+------------------------+----------------------+
| Clock/Ephemeris      | 1                      | 24                   |
|                      |                        |                      |
| Ephemeris            | 2                      | 24                   |
|                      |                        |                      |
| Ionosphere/UTC       | 3                      | 96                   |
|                      |                        |                      |
| Almanac              | 4                      | 48\*                 |
|                      |                        |                      |
| Text message         | 5                      | as needed            |
+----------------------+------------------------+----------------------+
| \* At least one SV   |                        |                      |
| almanac will come    |                        |                      |
| within 48 sec. All   |                        |                      |
| almanacs for the     |                        |                      |
| entire constellation |                        |                      |
| will be broadcast at |                        |                      |
| least every 576 sec. |                        |                      |
+----------------------+------------------------+----------------------+

20.3.4.2 [SV Time vs. GPS Time]{.underline}. In controlling the SVs and
uploading of data, the CS shall allow for the following timing
relationships:

a\. Each SV operates on its own SV time;

b\. All time-related data (TOW) in the messages shall be in SV-time;

c\. All other data in the NAV message shall be relative to GPS time;

d\. The acts of transmitting the NAV messages shall be executed by the
SV on SV time.

20.3.4.3 [Speed of Light]{.underline}. See paragraph 20.3.4.3 of
ICD-GPS-200.

20.3.4.4 [Data Sets]{.underline}. The IODE is a 10 bit number equal to
the 10 bit IODC of the same data set. The transmission of IODC and IODE
values in different data sets shall be such that the transmitted IODC
and IODE will be different from any value transmitted by the SV during
the preceding seven days. The range of IODC and IODE will be as given in
Table 20-XII of ICD-GPS-200.

Cutovers to new data sets follow the rules in the second subparagraph of
paragraph 20.3.4.4 of ICD-GPS-200.

The start of the transmission interval for each data set corresponds to
the beginning of the curve fit interval for the data set. Each data set
remains valid for the duration of its curve fit interval.

[Normal Operations]{.underline}. Message Type 1 and 2 data sets are
transmitted by the SV for periods of one hour. The corresponding curve
fit interval is four hours. The corresponding curve-fit interval will be
four hours.

[Short-term and Long-term Extended Operations]{.underline}. The
transmission intervals and curve fit intervals with the applicable
IODC/IODE ranges are given in Table 20-XII of ICD-GPS-200.

20.3.4.5 [Reference Times]{.underline}. See paragraph 20.3.4.5 of
ICD-GPS-200.

20.3.5 [Data Frame Parity]{.underline}. The data signal contains parity
coding according to the following conventions.

20.3.5.1 [Parity Algorithm]{.underline}. Twenty-four bits of CRC parity
will provide protection against burst as well as random errors with a
probability of undetected error ≤ 2-24 = 5.96×10-8 for all channel bit
error probabilities ≤ 0.5. The CRC word is calculated in the forward
direction on a given message using a seed of 0. The sequence of 24 bits
(p~1~,p~2~,\...,p~24~) is generated from the sequence of information
bits (m~1~,m~2~,\...,m~276~) in a given message. This is done by means
of a code that is generated by the polynomial

> $$g(X) = \sum_{i = 0}^{\text{24}}{g_{i}X^{i}}$$

where

> $$\begin{array}{r}
> g_{i} = 1\ \text{for}\ i = 0,\ 1,\ 3,\ 4\ ,\ 5,\ 6,\ 7,\ \text{10},\ \text{11},\ \text{14},\ \text{17},\ \text{18},\ \text{23},\ \text{24} \\
> 0\ \text{otherwise}
> \end{array}$$

This code is called CRC-24Q (Q for Qualcomm Corporation). The generator
polynomial of this code is in the following form (using binary
polynomial algebra):

> $$g(X) = (1 + X)p(X)$$

where p(X) is the primitive and irreducible polynomial

> $\begin{array}{r}
> p(X) = X^{\text{23}} + X^{\text{17}} + X^{\text{13}} + X^{\text{12}} \\
>  + X^{\text{11}} + X^{9} + X^{8} + X^{7} + X^{5} + X^{3} + 1\ 
> \end{array}$

When, by the application of binary polynomial algebra, the above g(X) is
divided into m(x)x^24^, where the information sequence m(x) is expressed
as

> $$m(X) = m_{k} + m_{k - 1}X + m_{k - 2}X^{2} + \cdot + m_{1}X^{k - 1}$$

The result is a quotient and a remainder R(x) of degree \< 24. The bit
sequence formed by this remainder represents the parity check sequence.
Parity bit p~i~, for any *i* from 1 to 24, is the coefficient of x^24-i^
in R(x).

This code has the following characteristics:

> 1\) It detects all single bit errors per code word.
>
> 2\) It detects all double bit error combinations in a codeword because
> the generator polynomial g(X) has a factor of at least three terms.
>
> 3\) It detects any odd number of errors because g(X) contains a factor
> 1+X.
>
> 4\) It detects any burst error for which the length of the burst is ≤
> 24 bits.
>
> 5\) It detects most large error bursts with length greater than the
> parity length r = 24 bits. The fraction of error bursts of length b \>
> 24 that are undetected is:
>
> a\) 2^-24^ = 5.96 × 10^-8^, if b \> 25 bits.
>
> b\) 2^-23^ = 1.19 × 10^-7^, if b = 25 bits.