icd_gps_202c-1.pdf

ICD-GPS-202 Rev C 27 March 2003

NAVSTAR GPS CONTROL SEGMENT/

U.S. NAVAL OBSERVATORY

TIME TRANSFER INTERFACES

Revision C

DISTRIBUTION STATEMENT C. Distribution authorized to US Government agencies and their contractors for Administrative or Operational Use (Date of Determination – May 1, 1996). Other requests for this document shall be referred to Navstar GPS Joint Program Office (SMC Det 11/CZGI).

WARNING – This document contains technical data whose export is restricted by the Arms Export Control Act (Title 22, U.S.C. 2571 et seq.) or Executive Order 12470. Violation of these export-control laws subject to severe criminal penalties. Dissemination of this document is controlled under DoD Directive 5230.25 and AFI 61-204.

DESTRUCTION NOTICE: - For classified documents, follow the procedures in DoD 5220.22M or DoD 5200.1-R.

For unclassified, limited documents, destroy of any method that will prevent disclosure of contents or reconstruction of the document.

Office responsible for the basic preparation, distribution, and retention of this ICD:

SMC CZE

2435 Vela Way, Suite 1613

El Segundo, LA AFB, CA 90245-5500

PAGE REVISION RECORD

PAGES REVISION COMMENTS

All pages Revision A (960501) This revision incorporates numerous changes to ICD GPS

202. Of particular note is the change to the Time Transfer Error Budget (Table 1). Additionally, minor corrections were made to the text throughout the ICD. Lastly, corrections were made to the Communications Formats addressed in Appendix I.

All pages IRN-202A-001 (011031)

This Interface Revision Notice (IRN) updates ICD-GPS-202A to Revision B. The updates reflect interface changes due to the Architecture Evolution Plan (AEP) upgrades to the Operational Control System, the combining of this ICD with ICD-GPS-231 (Navstar GPS Control Segment/USNO Frequency Standard Interface), the Block IIR Space Vehicles, and the equipment upgrade supporting direct data communications between the MCS & USNO. These changes include adding USNO Alternate Master Clock Characteristics and the Monitor Station Frequency Standard Element Output Signals sections. The Earth Orientation Processing and Polar Motion Estimate sections were removed from the ICD. Earth Orientation data and Polar Motion Estimate data are provided to the GPS Control Segment by the National Imagery and Mapping Agency (NIMA), as documented in ICD-GPS-211B Navstar GPS Control Segment/NIMA Interfaces.

Timing information from Section 3 that went beyond the scope of requirements was moved to Section 6, Related Timing Information.

This IRN also includes GPS Block IIF requirements in support of the retirement of IRS-GPS-CS/OI-04 Navstar GPS Block IIF Control Segment to Other Internal and External Interfaces, Appendix B (GPS IIF CS to the USNO) and Appendix D (GPS IIF CS to USNO Multiple Frequency Standard System).

All pages IRN-202B-001 (5 December 2002)

This IRN updates ICD-GPS-202B to Revision C. It incorporates the changes in hardware, software, download format, and procedures for the time transfer interface between USNO and the GPS MCS. Numerous grammatical and format updates have also been included in this IRN.

TABLE OF CONTENTS

1. SCOPE

1.1 ICD APPROVAL AND CHANGES

2. APPLICABLE DOCUMENTS

2.1 GOVERNMENT DOCUMENTS

2.1.1 Specifications:

2.1.2 Standards:

2.1.3 Drawings:

2.1.4 Other Publications:

2.2 NON-GOVERNMENT DOCUMENTS

2.2.1 Industrial Standards

3. REQUIREMENTS

3.1 INTERFACE DEFINITION

3.1.1 Master Control Station

3.1.2 U.S. Naval Observatory

3.1.3 Communications

3.1.4 Satellite Navigation Message

3.2 PERFORMANCE CHARACTERISTICS

3.2.1 MCS Characteristics

3.2.1.1 MCS Equipment

3.2.1.2 MCS Operations

3.2.1.2.1 MCS Time Difference Processing

3.2.1.2.2 Processing of USNO Measurements

3.2.1.2.3 Steering of GPS Time

3.2.2 USNO Characteristics

3.2.2.1 USNO Equipment

3.2.2.2 USNO Operations

3.2.2.2.1 USNO Time Difference Processing

3.2.3 Navigation Message Characteristics

3.2.4 USNO Error Budget

3.2.5 USNO Alternate Master Clock (AMC) Characteristics

3.2.5.1 USNO AMC Output

3.2.5.1.1 Frequency

3.2.5.1.2 Voltage and Impedance

3.2.5.1.3 Frequency Stability

3.2.5.1.4 Steering

3.2.5.1.5 Phase Noise

3.2.5.1.6 Harmonic Distortion

3.2.5.1.7 Non-Harmonic Distortion (spurious output)

3.2.5.1.8 Phase Signal-to-Noise Ratio

3.2.5.1.9 Accuracy

3.2.5.2 USNO AMC Output Clock Monitor Signals

3.2.5.2.1 USNO AMC Clock Monitor Signal Definition

3.2.5.2.2 USNO AMC Output TTL Signal Characteristics

3.2.6 Monitor Station Frequency Standard Element (MSFSE) Output Signals i

3.2.6.1 Frequency

3.2.6.2 Signal Level and Impedance

3.2.6.3 Frequency Stability

3.2.6.4 Phase Noise

3.2.6.5 Harmonic Distortion

3.2.6.6 Non-Harmonic Distortion (spurious output)

3.2.6.7 Phase Signal-to-Noise Ratio

3.2.6.8 Accuracy

3.2.6.9 Short Circuit Protection

3.2.7 USNO AMC I/O Panel

3.2.7.1 Monitor Signal Connector

3.2.7.2 RF Frequency Signal Connector

3.3 MESSAGE DEFINITIONS

3.3.1 Navigation Message Format

3.3.2 Communications Characteristics

3.3.2.1 Communications Equipment

3.3.2.2 Communications Procedures and Protocol

3.3.2.3 Communications Message Content

3.3.2.3.1 Time Difference Messages

3.3.2.3.1.1 Single Pass Data DFR24

3.3.2.3.1.2 Aggregate Data DFRMF

3.3.2.3.1.3 Aggregate Data DFRHV

3.3.2.3.2 Leap Second Advisory

3.3.2.3.3 Satellite Status Advisories

3.3.2.3.4 Abnormal Satellite Status Advisories

3.4 SECURITY CONSIDERATION

3.5 BACKUP PROCEDURES

3.6 ANOMALY RESOLUTION

4.0 ACRONYMS AND GLOSSARY

4.1 ACRONYMS AND ABBREVIATIONS

4.2 GLOSSARY

5.0 APPENDIX 1 – SAMPLE OUTPUTS

5.1 COMMUNICATIONS FORMATS

5.1.1 Time Difference Data

ii

TABLE OF TABLES

TABLE 1. USNO AMC FREQUENCY STABILITY CHARACTERISTICS

TABLE 2. MSFSE FREQUENCY STABILITY CHARACTERISTICS

TABLE 3. USNO AMC I/O PANEL MONITOR SIGNAL CONNECTOR (J1) (MS 3470L22-41SW).. 15

TABLE 4. SINGLE PASS DATA FILE DFR24

TABLE 5. AGGREGATE DATA FILE DFRMF

TABLE 6. AGGREGATE DATA FILE DFRHV

TABLE 7. DFR24 OUTPUT EXAMPLE

TABLE 8. DFRMF OUTPUT EXAMPLE

TABLE 9. DFRHV OUTPUT EXAMPLE

TABLE OF FIGURES

FIGURE 1. USNO TO MCS TO SPACE SEGMENT TO USNO TIMING DATA FLOW

FIGURE 2. PHASE NOISE CHARACTERISTIC OF THE USNO AMC

FIGURE 3. USNO AMC (USNO AMC I/O) AND COSPM (MSFSE) INTERFACE

FIGURE 4. MCS TO SIPRNET CONNECTION DIAGRAM

iii

THIS PAGE IS BLANK

iv

1. SCOPE

This Interface Control Document (ICD) defines the interface between the Navstar Global Positioning System (GPS) Control Segment and the U.S. Naval Observatory (USNO) regarding the dissemination of precise time. This interface includes operations at USNO in Washington, DC and the USNO Alternate Master Clock (AMC) at Schriever Air Force Base.

1.1 ICD Approval and Changes

Generation and modification of this ICD shall be in accordance with Navstar GPS Interface Control Working Group Charter (YEN 75-13B). Space and Missiles Center (SMC) /CZE is the designated Interface Control Agency (ICA)

The following signatories must approve this ICD to make it effective:

1. U.S. Naval Observatory (USNO)

2. GPS Joint Program Office (SMC/CZ)

3. Air Force Space Command, GPS Program Lead, (AFSPC/DOO)

4. GPS Control Segment Contractor (Boeing)

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 ICA is responsible for the preparation of the change pages, change coordination, and the change approval by all signatories in accordance with YEN 75-13B. 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 ICA for inclusion in the released version of the approved ICD.

2. APPLICABLE DOCUMENTS

2.1 Government Documents

The following documents of the current issue and all approved changes form a part of this ICD to the extent specified herein.

2.1.1 Specifications:

Federal None

Military None

Other-Government Activity

ICD-GPS-200

Current Issue

Navstar GPS Space Segment/Navigation User Interfaces

2.1.2 Standards:

Federal None

Military None

2.1.3 Drawings:

None

2.1.4 Other Publications:

YEN 75-13B with Change Notice Number 3 dated 21 October 1988

Navstar GPS Interface Control Working Group Charter

2.2 Non-Government Documents

2.2.1 Industrial Standards

Internet Protocols are defined by various documents called Request for Comments (RFCs) which are the working notes of the Internet research and development community. The following RFCs are listed for reference only, they are not compliance documents. Specific RFCs relevant to this interface are:

RFC 1600, March 1994 Internet Official Protocol Standards developed by the Internet Architecture Board

RFC 791, September 1981 DoD Standard Internet Protocol (IP) developed for the Defense Advanced Research Projects Agency (DARPA) Internet Program Protocol Specification

RFC 793 September 1981 DoD Standard Transmission Control Protocol (TCP) developed for the DARPA Internet Program Protocol Specification

3. REQUIREMENTS

3.1 Interface Definition

This ICD defines two types of interface between the GPS Control Segment and USNO. The first is the time transfer interface between the USNO in Washington, DC, and the GPS Master Control Station (MCS), and the second is the signal interface between the USNO Alternate Master Clock (AMC) and the Colorado Springs Monitor Station Frequency Standard Element (MSFSE). The physical interface between the MSFSE and the USNO AMC is at the USNO AMC input/output (I/O) panel.

3.1.1 Master Control Station

The primary GPS point of contact is the Master Control Station (MCS) of the Control Segment (CS). The MCS is the central control point for GPS and performs most of its data collection, data processing and data generation functions. It consists of data processing communications and control and display equipment with associated software and operations and support personnel. It has the prime responsibility for command and control of the GPS satellite constellation for the purpose of achieving the GPS navigation mission. This includes uploading GPS satellites with navigation messages, which report precision ephemeris and clock offset data. Among the clock offset data provided are parameters which relate GPS Time to Coordinated Universal Time (UTC) as maintained at the U.S. Naval Observatory (UTC(USNO)).

3.1.2 U.S. Naval Observatory

The mission of USNO is to determine the positions and motions of celestial bodies, the motion of the Earth, and to maintain the physical time reference for DoD activities. To ensure uniformity and interoperability between the services, USNO has been designated to act as the Precise Time and Time Interval (PTTI) manager for all DoD time keeping and time dissemination operations. In support of this, USNO serves as the UTC time reference for the GPS system and provides technical expertise to the GPS program in areas of time keeping and time dissemination.

As PTTI manager, USNO works closely with international standards organizations to insure interoperability between US and international timing systems. Coordinated Universal Time (UTC) is established by the Bureau International des Poids et Mesures (BIPM), located in Paris, France and UTC(USNO) is the real-time realization of UTC for DoD.

The USNO Time Service Department, located in Washington, DC, monitors GPS via monitor station GPS receivers. The USNO is responsible for computing from these measurements the GPS time offset and providing these corrections to the GPS MCS. A backup time monitor station capability exists at the USNO Alternate Master Clock (USNO/AMC) in Schriever AFB. Additionally, the USNO/AMC provides a frequency reference for the GPS Operational Control System (OCS) monitor station in Colorado Springs.

3.1.3 Communications

Communications between the MCS GFE communications equipment and the USNO utilizes the DoD-managed Secure Internet Protocol Router Network (SIPRNet), Figure 1 is a simplified block diagram showing this interface. Back-up communications are accomplished using DSN and commercial dial-up capabilities. The data file containing time difference data is supplied via the communications equipment described in section 3.3.2.1. Frequency data is transferred between the USNO AMC and the MSFSE using a direct cable connection. This cable provides a 5 MHz sinusoidal signal. The data download method between the USNO and the MCS GFE communications equipment uses TCP/IP protocol to support this data transfer interface and is accomplished via the SIPRNet.

GA

SV

L Band

S Band

GPS

WAN

USNO

Firewall

MCS

Firewall

SIPRNet

FIGURE 1. USNO TIMING DATA FLOW

3.1.4 Satellite Navigation Message

USNO monitors the same GPS broadcast data available to users for time and navigation. Specifically, USNO’s GPS timing receivers are used to make pseudorange measurements and read navigation message parameters to obtain GPS Time and GPS’s delivered prediction of UTC(USNO). Operational GPS space vehicles (SVs) transmit the message, at L1 and L2 frequencies, superimposed on a satellite-dependent code sequence. The UTC-related portion of the message is identical for all SVs, except for temporary differences resulting from staggered uploading of new data to the various SVs.

3.2 Performance Characteristics

3.2.1 MCS Characteristics

The applicable characteristics of the MCS are described in the following sections.

3.2.1.1 MCS Equipment

The MCS contains general-purpose high-speed processors, archival data storage units, direct access storage devices, display equipment, and controller terminals. Secure communication equipment links the MCS to the Control Segment Ground Antennas (which provide the command and control interface to the GPS SVs) and links the MCS to the Monitor Stations (MSs) (which measure and time-tag the satellite transmissions and collect the navigation messages). The interface to USNO is described in section 3.3.2.1.

3.2.1.2 MCS Operations

The MCS processors perform on-line, real-time functions to support the GPS mission. The processors collect Monitor Station measurements, estimate and predict satellite position, velocity, and clock offsets, and generate navigation messages for broadcast from each SV. They also maintain GPS Time. GPS Time – UTC(USNO) information is entered via a display terminal and is used to produce the predictions contained in the navigation message. Inputs at the display terminal also permit GPS Time, corrected for leap seconds, to be steered toward UTC(USNO).

3.2.1.2.1 MCS Time Difference Processing

The USNO provides the MCS with an estimated difference for a specified epoch between GPS Time and UTC(USNO).

This estimate is calculated at USNO by monitoring the difference between GPS Time as derived from each satellite monitored and UTC(USNO). The estimated difference is represented in the Navigation Message as:

1. ( )( )seconds, 604800AA t =t 10LSUTC totE WNWNtt −+−++∆∆ where ∆tLS, A0, A1, tot, WN, and WNt are the parameters provided in the Navigation Message computed by the MCS and as defined in ICD-GPS-200. The WNt value consists of the eight least significant bits (LSB) of the full week number and the truncated nature of this parameter must be accounted for in its use. tE is GPS Time as estimated by the user which is described below.

The correction ∆tLS is the number of leap seconds which have accumulated since the current GPS epoch of 0 hours, 6 January 1980 (UTC(USNO)). If LS represents the current number of leap seconds then

2. seconds. 19- LS=t LS∆

Should the GPS epoch be changed in the future, the expression for ∆tLS given above would have to be modified.

The derivation of GPS’s delivered prediction of UTC(USNO) is described in ICD-GPS-200 (in these mathematical equations, tUTC represents GPS’s delivered prediction of UTC(USNO)). It is given by

3. , ( )[ ]seconds 86400 Modulot - t t UTCEUTC ∆= where

4. ( ) ( ){ }IONOTROPO

ocSVf2ocSVf1f0SVE ttta t- ta a - t t εε −−+−++= , tSV is the effective SV PRN code phase at message transmission time, af0, af1, af2, toc, are clock correction parameters in the navigation message, ε is the correction accounting for relativity and selective availability (dither), tTROPO is the correction for tropospheric delay, and εIONO is the correction accounting for ionospheric delay and the effect of SV group delay differential between L1 and L2.

ICD-GPS-200 also provides information on the past and future relationships between GPS Time and UTC(USNO) when a leap second update occurs.

3.2.1.2.2 Processing of USNO Measurements

MCS will update and maintain estimates of the time differences represented by the parameters A

, A l and ∆tLS using the measurements supplied by USNO. It is assumed that these measurements represent for a specified epoch, t

GPS

, an estimate of the difference between GPS Time and UTC(USNO), as derived from the entire constellation. These measurements along with the estimated measurement errors are obtained from USNO at least daily and supplied to the MCS mission computer by a controller through a display terminal. MCS utilizes the new measurements to update estimates of the A and A

1 factors.

The new estimates of A and A

(and the associated WNt, tot, ∆tLS) are incorporated in subsequent uploads to the SVs.

3.2.1.2.3 Steering of GPS Time

The GPS Composite Clock, an ensemble of MS and SV frequency standards define GPS Time in the OCS. GPS Time can be “steered” into alignment with UTC(USNO). In particular, GPS Time will be steered, normally by introducing small changes in drift rate to maintain

5. | UTC(USNO) - t

GPS

| [Modulo one second] <= 1 microsecond.

That is, GPS Time normally will be steered in order to stay within one microsecond of UTC(USNO) after accounting for leap seconds.

Lara Schmidt Fixed italics.

3.2.2 USNO Characteristics

The characteristics of USNO, which are relevant to the GPS interfaces, are discussed below.

3.2.2.1 USNO Equipment

The primary data processing capability used for time keeping processing at USNO consists of a network of data acquisition and analysis computers. They are interfaced with nominally 50 cesium beam and 9 hydrogen maser frequency standards in ten clock vaults. One of four steered maser reference systems is designated as the USNO Master Clock. A redundant system of dedicated computers collect data from a redundant system of absolutely calibrated dual-frequency multi-channel Precise Positioning Service (PPS) GPS timing receivers. The antennas for these receivers are installed at the facility in an accurately surveyed location, and the signal transfer time from antenna to receiver has been calibrated.

3.2.2.2 USNO Operations

USNO evaluates and disseminates estimates of time differences between GPS and UTC(USNO). The U.S. Naval Observatory continues to improve the total data collection effort and the accuracy of its predictions for UTC, since the accuracy and promptness of these predictions remains critical for the highest precision users of the GPS. USNO also disseminates international decisions regarding when to incorporate a leap second.

3.2.2.2.1 USNO Time Difference Processing

USNO evaluates time differences by observing all satellites in view. USNO operates several multi-channel PPS GPS receivers, one of which is designated as primary. The other receivers function as hot spares. The primary timing receiver obtains the navigation message from all healthy GPS SVs in view, estimates the apparent range to those SVs (signal travel time), and applies the data in the navigation message to derive estimates of GPS Time and UTC from those SVs. These measurements are compared to UTC(USNO), and the differences are corrected for all known or estimable delays, offsets, and leap seconds. The results, referenced to UTC(USNO), are stored in the USNO computer and made available to the MCS via the communications equipment described in section 3.3.2.1.

The USNO timing receiver tracks each operational GPS satellite in view. The USNO computer collects time transfer data from the timing receiver at 6-second intervals. A linear fit is performed over each 15-minute satellite pass to provide the difference between GPS Time and UTC(USNO). The linear fit is determined using a 3-sigma filter for outlier removal.

The resulting output, provided for each SV, is its estimated value at the midpoint of the pass of the differences: GPS Time (SVi)-UTC(USNO). The output is described in detail in section 3.3.2.3.1.1.

These single pass data are used in a higher level of aggregation, applying the same filtered linear fit technique to all passes of known acceptable satellites, to obtain an overall value for the GPS constellation’s estimated difference between GPS Time and UTC(USNO). Satellites with known anomalies are not considered acceptable for the calculation. The results are referenced to a fixed epoch, currently the Modified Julian Date (MJD) and its corresponding Day of Year (DOY). The MJD is defined in terms of Julian Date (JD), such that MJD = JD –2400000.5; and the Julian Date (JD) is the continuous count of the days and fractions of a day that have elapsed since an epoch in the remote past: 1 January 4713 BCE (Before the Common Era). Thus JD is always a large positive number, currently 2,450,000+, and MJD exceeds 50,000. JD and MJD are different from DOY, the day of year, which cannot ever exceed 366.

3.2.3 Navigation Message Characteristics

The characteristics of the L1/L2 signals generated by the satellites, including detailed definition of the navigation message, are specified in ICD-GPS-200. The message is downlinked at a 50 bit-per-second rate, with a basic repeat cycle of 30 seconds. This cycle consists of five 300 bit subframes, of which the first three contain primarily the parameters necessary to describe the predicted orbit of the SV broadcasting them and the predicted deviations from GPS Time of the time source on that SV. (The SV time source is adjusted as necessary to keep it within one millisecond of GPS Time before applying the broadcast corrections.) The last two subframes of the cycle contain sub-multiplexed data: each of these subframes is time-shared by 25 pages which appear cyclically, each page being broadcast every 12.5 minutes.

Of particular relevance to this ICD are the UTC(USNO) parameters included in page 18 of subframe 4. These parameters include a current count of the accumulated leap seconds of difference between GPS Time and UTC. The effective date of a planned change in the leap second count (and its new value), and the parameters of a linear expression for the predicted difference (after leap second compensation) between GPS Time and UTC(USNO).

The data content of the various parts of the message is updated to maintain the required accuracy with different nominal update rates for different groups of fields. The MCS uploads several sets of data to the SV at once with instructions as to when each should be used.

Since the downlinked signal contains a precise indication of when it left the SV (in terms of the SV time source), GPS users can obtain a precise estimate of signal travel times and can use the navigation message to determine accurate SV position and GPS Time at the SV. Typical users observe four SVs simultaneously and use the data to determine their own position and current GPS Time; users at accurately surveyed locations can use a single SV to obtain GPS Time. In both cases, a user can apply the broadcast corrections to convert from GPS Time to GPS’s delivered prediction of UTC(USNO).

3.2.4 USNO Error Budget

The USNO time monitoring error budget is 3.0 ns (1 sigma). This error budget describes the extent to which the calibration level of the operational USNO time monitor GPS receiver can be duplicated by redundant hardware. This value is a very conservative estimate based upon both the live performance of daily values over the span of several years, and the results of temperature stability studies.

3.2.5 USNO Alternate Master Clock (AMC) Characteristics

The USNO AMC supplies a frequency signal and a clock monitor signal to the Monitor Station Frequency Standard Element (MSFSE). The USNO AMC makes these signals available at the I/O panel. The I/O panel is located no further than 200 feet from the MSFSE to limit cable losses. The characteristics of the USNO AMC, which are relevant to the GPS interface, are described in the following sections.

3.2.5.1 USNO AMC Output

The USNO AMC supplies the MSFSE with a sinusoidal signal having the following characteristics.

3.2.5.1.1 Frequency

The supplied signal has a frequency of 5-megahertz (MHz).

3.2.5.1.2 Voltage and Impedance

The supplied signal is 1 volt root mean square (VRMS) ±10 percent when measured in a 50-ohm circuit, and interfaces with a 50-ohm impedance at the Primary Distribution Unit (PDU) input port of the MSFSE without degradation.

3.2.5.1.3 Frequency Stability

The supplied signal has short and long-term frequency stabilities as summarized in Table 1.

Table 1. USNO AMC Frequency Stability Characteristics

Tau (seconds) Sigma y (Tau)

Short Term

10000

Long Term

1x105

1x106

1x107

2x10-12

2x10-13

2x10-14

1x10-14

3.2.5.1.4 Steering

Steering is defined as the adjustment of the normal output frequency of the USNO AMC to approximate UTC(USNO). The USNO AMC output frequency signal is steered such that the steering shall be below the noise level with respect to the stability specifications listed in section 3.2.6.3.

3.2.5.1.5 Phase Noise

The supplied signal satisfies the phase noise bound shown in Figure 2. Data are specifically collected at 1, 10, 100, 1000, and 5000 hertz (Hz) offset from the carrier; additional points may also be collected to approximate closely a continuous curve.

-150

-140

-130

-120

-110

-100

0.1 1 10 100 1000 10000

OFFSET FROM CARRIER (Hz)

S φ ( f

) d B

Figure 2. Phase Noise Characteristic of the USNO AMC

3.2.5.1.6 Harmonic Distortion

Harmonics of the supplied signal output frequency are 40 decibels (dB) or more below rated output.

3.2.5.1.7 Non-Harmonic Distortion (spurious output)

Spurious signals in the output that are not harmonics of the output frequency are below the rated output by at least 80 dB for offsets from the output frequency of 10 Hz or greater.

3.2.5.1.8 Phase Signal-to-Noise Ratio

The supplied signal has a phase signal-to-noise ratio greater than 87 dB measured in a 30-kilohertz (kHz) bandwidth.

3.2.5.1.9 Accuracy

The supplied signal has an accuracy of ±3x10-12 with respect to the frequency reference, for which 9,192,631,770 cycles of the unperturbed frequency of the cesium atom define the time interval adopted as the second.

3.2.5.2 USNO AMC Output Clock Monitor Signals

The AMC supplies to the MSFSE a Transistor Transistor Logic (TTL) level signal that indicates the status of operation of the AMC.

3.2.5.2.1 USNO AMC Clock Monitor Signal Definition

The definition of the USNO AMC Monitor signal indication is as follows:

“Go” indicates that the 5 MHz output signal is present and its performance characteristics are as described in section 3.2.5.2.2.

“No Go” indicates that the signal does not meet the described performance characteristics or is not provided at the interface.

3.2.5.2.2 USNO AMC Output TTL Signal Characteristics

The TTL signal characteristics are as follows for the USNO AMC output monitor signal (V represents Volts).

0.0V ≤ "0" ≤ 1.0V = No Go.

2.5V ≤ "1" ≤ 5.5V = Go.

A voltage other than those defined by the logic state “0” (NoGo) and “1” (Go) is an undefined logic state and is an indication of a hardware failure in the driving circuit, connecting cable, or in the receiving circuit. The signal interfaces to a circuit in the MSFSE whose input impedance shall be no less than 44 kilohms (kOhms).

3.2.6 Monitor Station Frequency Standard Element (MSFSE) Output Signals The MSFSE shall provide frequency signals from MSFSE cesium beam frequency standards number 1 and number 2.

These signals shall interface with the USNO AMC at the USNO AMC I/O panel. The MSFSE shall provide two sinusoidal signals having the following characteristics.

3.2.6.1 Frequency

Supplied signals shall be at a frequency of 5 MHz.

3.2.6.2 Signal Level and Impedance

The supplied signals shall be 0.1 VRMS ± 10 % when measured in a 50-ohm circuit. Both signals shall interface with a 50-ohm impedance at the USNO AMC input port without degradation.

3.2.6.3 Frequency Stability

The supplied signals shall have frequency stabilities as summarized in Table 2.

Table 2. MSFSE Frequency Stability Characteristics

Tau (seconds) Sigma y (Tau) 1 5.0x10-12 10 2.7x10-12 100 8.5x10-13 1000 2.7x10-13 10000 8.5x10-14

3.2.6.4 Phase Noise

The supplied signals shall meet the phase noise bound shown in Figure 2. Data is specifically collected at 1, 10, 100, 1000, and 5000 Hz offset from the carrier; additional points may also be collected to approximate closely a continuous curve.

3.2.6.5 Harmonic Distortion

Harmonics of the supplied signals output frequency are 40 dB or more below rated output.

3.2.6.6 Non-Harmonic Distortion (spurious output)

Spurious signals in the output that are not harmonics of the output frequency shall be below the rated output by at least 80 dB for offsets from the output frequency of 10 Hz or greater.

3.2.6.7 Phase Signal-to-Noise Ratio

Supplied signals shall have a phase signal-to-noise ratio greater than 87 dB measured in a 30-KHz bandwidth.

3.2.6.8 Accuracy

The supplied signals shall have an accuracy of ± 1x10-11 with respect to the frequency reference, for which 9,192,631,770 cycles of the unperturbed frequency of the cesium atom define the time interval adopted as the second.

3.2.6.9 Short Circuit Protection

The MSFSE shall be able to withstand a short circuit of the output frequency ports without incurring damage or subsequent degraded performance.

3.2.7 USNO AMC I/O Panel

All electrical connections from the USNO AMC to the MSFSE are physically provided via the USNO AMC I/O panel for connection to a corresponding panel at the MSFSE (See Figure 3). J1 through J4 at the AMC constitutes the point of demarcation.

3.2.7.1 Monitor Signal Connector

The monitor signal connector shall be a type MS3470L22-41SW, which is designated "J8" on the USNO AMC I/O panel.

Connections shall be as indicated in Table 3.

Table 3. USNO AMC I/O Panel Monitor Signal Connector (J1) (MS 3470L22-41SW)

Signal Name Pin Number Normal Value

USNO AMC Status Go/NoGo* Return

A B

"1" (Go)

3.2.7.2 RF Frequency Signal Connector

Supplied frequency signal connectors on the USNO AMC I/O panel shall be type BNC female. The connector providing the USNO AMC 5-MHz output to the MSFSE is designated "J5" (See Figure 3). The connector providing the 5-MHz signal from the MSFSE cesium standard #1 to the USNO AMC is designated "J6". The connector providing the 5-MHz signal from the MSFSE cesium standard #2 to the USNO AMC is designated "J7". The connector designated “J1” gets the 5-MHz signal from the Primary Distribution Unit.

J1 –USNO AMC Clock Monitor Signal (GO/No GO)

J2 – 5 MHz Output from USNO AMC

J3 – 5 MHz Input from COSPM Cesium #1

J4 – 5 MHz Input from COSPM Cesium #2

USNO ALTERNATE MASTER CLOCK

USNO AMC I/O Panel

J1 –USNO AMC Clock Monitor Signal (GO/No GO)

J2 – 5 MHz Output from USNO AMC

J3 – 5 MHz Input from COSPM Cesium #1

J4 – 5 MHz Input from COSPM Cesium #2

USNO ALTERNATE MASTER CLOCK

USNO AMC I/O Panel

FIGURE 3. USNO AMC (USNO AMC I/O) and COSPM (MSFSE) Interface

3.3 Message Definitions

3.3.1 Navigation Message Format

The detailed format of the navigation message is specified in ICD-GPS-200. Each leap second count is represented as an integer number of seconds, in an 8-bit field in twos complement notation. The leap second effectivity date is represented as an integer week number (modulo 256) since the GPS epoch and an integer day of week. (The leap second takes effect at the end of the day so identified.) The reference time for the linear approximation to the UTC time difference is represented as a similar integer week number (modulo 256) and a time of week represented as an 8-bit integer with the least significant bit (LSB) equal to 4096 seconds. The bias term in the approximation (A

) is a 32-bit twos complement integer with the LSB equal to 2 -30 seconds, and the drift term (A l ) is a 24-bit twos complement integer with the LSB equal to 2

-50 seconds/second.

3.3.2 Communications Characteristics

Data communications between USNO and the MCS utilize a computer-to-computer file transfer.

3.3.2.1 Communications Equipment

The MCS GFE SIPRNet communication equipment, consists of a Sidewinder Firewall hosted on a Pentium personal computer (PC), a Network Interface Card (NIC), an Ethernet-to-Token Ring conversion card hosted on a Sun Workstation, a SIPRNet switch, and assorted cabling and connectors. The demarcation point for where AFSPC begins maintenance and support is the point where the MCS’s firewall equipment plugs into the SIPRNet cloud. Figure 4 shows a more detailed view of the SideWinder Firewall’s CAT5 cable plug that connects the MCS communications equipment to the SIPRNet.

There is also a printer attached to the Sun Workstation to print out the time transfer information. This configuration is used for transmission of precise timing information as described in Appendix 1 from USNO to the MCS. The SIPRNet connection to the MCS communications equipment used to transmit USNO data is completely independent from (or isolated from) the OCS operational mission software in the MCS. In other words, there is no direct import of USNO data via the SIPRNet connection into the OCS mission software data system. The MCS operations includes procedures to manually input the USNO values into the MCS mission software data system. The configuration-controlled SIPRNet setup has been documented in the proper Air Force Technical Order (TO) manuals and other operational documents. The MCS communications equipment is maintained and supported by Air Force Space Command. The USNO equipment which supports this interface consists of a workstation with an IRIG-B timing interface to UTC(USNO), a router, a multiplexer, a KIV-7HS, and assorted cabling and connectors.

GFE

Workstation

FIGURE 4. MCS to SIPRNet Connection Diagram

3.3.2.2 Communications Procedures and Protocol

This data transfer utilizes the DoD-operated SIPRNet. This is a classified SECRET network used throughout the Department of Defense for a variety of mission applications. Data are transmitted on SIPRNet using TCP/IP protocol as defined in section 3.1.3. The USNO (and AMC as required and approved by procedure) places the timing data (defined in Appendix I) via the SIPRNet to the MCS Sidewinder Firewall. The intermediate MCS-operated computer automatically obtained the USNO data from the MCS Sidewinder Firewall. The data are retrieved from the intermediate MCS-operated computer and processed by MCS personnel, placed in the appropriate file prior to a navigation upload. USNO (AMC) makes available data files of the format specified in Appendix I, derived from the USNO operational GPS PPS receivers.

The exact details for the procedures and protocol used by MCS personnel are documented in this document, TOs, and other operational documents.

3.3.2.3 Communications Message Content

The minimum content of the messages required for this interface is identified below. Sample outputs of the messages are presented in Appendix 1.

3.3.2.3.1 Time Difference Messages

Single pass and aggregate time difference data shall be updated at least once per day during agreed-upon time window(s).

These data are provided in a single file containing USNO Time Monitor Estimates (UTiME), designated “UTiMEYYYYDOY.XX” where YYYY is the four digit year and DOY is the three digit day of year, where XX ranges from 01 to 99 and denotes a sequential file number for a given day. This file contains three separate formats identified as “DFR24”, “DFRMF”, and “DFRHV”. At least one week of such files will reside on the USNO computer in the event that the MCS requires the download of historical data. If files older than one week are required, MCS personnel will request that USNO make these files available when the need arises.

3.3.2.3.1.1 Single Pass Data DFR24

Single pass time difference data contain the parameters listed in Table 4 and each record represents a linear fit for the 15-minute period of the tracked PRN. All dates and times are referenced to GPS Time. The contents of DFR24 represent the most recent interval of single pass data. When no data are available for a reporting period, repeating nines (9’s) are reported in each column for each unavailable variable. See Table 4 for data file definitions and Table 7 for a sample output.

Table 4. Single Pass Data File DFR24

(See Sample Output in Table 7, Section 5.1.1) Parameter Column Resolution Range/Value Units PRN number (1) 1-2 1 1-32 N/A Space 3 N/A ‘space’ N/A

MJD (2) 4-14 .00001 52275.00000-

99999.99999 Days

Space 15 N/A ‘space’ N/A Year, DOY and Time

(YYYYDOY.DDDDD) (3)

16-28 .00001 2002001.00000- 9999366.99999

YearDay

Space 29 N/A ‘space’ N/A HHMMSS of beginning of pass (4)

30-35 1 000000-235959 N/A

Space 36 N/A ‘space’ N/A HHMMSS of midpoint of pass (5) 37-42 1 000000-235959 N/A Space 43 N/A ‘space’ N/A HHMMSS of end of pass (6) 44-49 1 000000-235959 N/A Space 50 N/A ‘space’ N/A

GPS-UTC(USNO)

15-minute fit (7)

51-58 .01 +/- 9999.99 Nanosecond s (ns)

Space 59 N/A ‘space’ N/A GPS- UTC(USNO) SLOPE (8) 60-66 .01 +/- 999.99 Picoseconds per second (ps/s)

Space 67 N/A ‘space’ N/A GPS- UTC(USNO) RMS (9) 68-72 .01 0.00-99.99 Nanosecond s (ns) Space 73 N/A ‘space’ N/A GPS-UTC(USNO) Sample Size (10)

74-77 1 10-9999 N/A

Space 78 N/A ‘space’ N/A Elevation 15-minute fit (11) 79-83 .01 0.00-90.00 Degrees Space 84 N/A ‘space’ N/A Azimuth 15-minute fit (12) 85-90 .01 0.00-359.99 Degrees Space 91 N/A ‘space’ N/A Measured Ionospheric Delay 15-minute fit (13)

92-98 .01 +/- 999.99 Nanosecond s (ns)

Space 99 N/A ‘space’ N/A Satellite Clock- UTC(USNO) 15-minute fit (14)

100-109 .01 +/- 999999.99 Nanosecond s (ns)

Space 110 N/A ‘space’ N/A Satellite Clock –UTC(USNO)

SLOPE (15)

111-117 .01 +/- 999.99 Picoseconds per second (ps/s)

Space 118 N/A ‘space’ N/A Modeled Troposphere Delay 15-minute fit (16)

119-124 .01 0.00-999.99 Nanosecond s (ns)

Space 125 N/A ‘space’ N/A Modeled Ionsopheric Delay 15-minute fit (17)

126-131 .01 0.00-999.99 Nanosecond s (ns)

Space 132 N/A ‘space’ N/A

Relativistic Delay 15-minute fit (18)

133-139 .01 +/- 999.99 Nanosecond s (ns)

Space 140 N/A ‘space’ N/A GPS’s delivered prediction of

UTC(USNO)-UTC(USNO) 15-

minute fit (19)

141-148 .01 +/- 9999.99 Nanosecond s (ns)

Space 149 N/A ‘space’ N/A

SPARE (20) 150-157 .01 +/- 9999.99 N/A

Space 158 N/A ‘space’ N/A

SPARE (21) 159-166 .01 +/- 9999.99 N/A

Space 167 N/A ‘space’ N/A

SPARE (22) 168-175 .01 +/- 9999.99 N/A

Space 176 N/A ‘space’ N/A

SPARE (23) 177-184 .01 +/- 9999.99 N/A

Space 185 N/A ‘space’ N/A SPARE Flag (24) 186-187 1 00-99 N/A Notes:

1. Satellite tracked.

2. MJD of midpoint of pass including fraction of the day.

3. Year and Day of Year at midpoint of pass where YYYY is the four-digit year, DOY is the Day of Year;

DDDDD is the decimal fraction of the DOY.

4. Time at start of pass where HH is hour, MM is minute, and SS is second.

5. Time at midpoint of pass where HH is hour, MM is minute, and SS is second.

6. Time at end of pass where HH is hour, MM is minute, and SS is second.

7. GPS Time (SVi) – UTC(USNO) 15-minute linear fit value represents the difference via the PRN between GPS

Time and UTC(USNO) at the midpoint of the pass.

8. Corresponding Slope of GPS Time (SVi) – UTC(USNO) for the pass.

9. Corresponding RMS deviation of GPS Time (SVi) – UTC(USNO) for the pass.

10. Sample size where N is the number of points used for the linear fit.

11. Elevation of the satellite at the midpoint of the pass.

12. Azimuth of the satellite at the midpoint of the pass.

13. Measured dual-frequency estimate of ionospheric delay, referenced to the midpoint of the pass.

14. Satellite Clock – UTC(USNO) 15-minute linear fit value represents the difference of the Satellite Clock

(SVCLK) and UTC(USNO) at the midpoint of the pass.

15. Slope of SVCLK – UTC(USNO) for the pass.

16. Modeled estimate of the tropospheric delay, referenced to the midpoint of the pass.

17. Modeled estimate of the ionospheric delay, referenced to the midpoint of the pass.

18. Relativistic delay estimate, referenced to the midpoint of the pass.

19. GPS’s delivered prediction of UTC(USNO) – UTC(USNO) represents the difference between the prediction of UTC(USNO) as delivered by the specified PRN and UTC(USNO) at the midpoint of the pass.

20. Spare reserved for future use. When no information is intended to be delivered, the column contains 9999.99.

21. Spare for future use. When no information is intended to be delivered, the column contains 9999.99.

22. Spare for future use. When no information is intended to be delivered, the column contains 9999.99.

23. Spare for future use. When no information is intended to be delivered, the column contains 9999.99.

24. Spare for future use. When no information is intended to be delivered, the column contains 99.

25. Columns 20-24 may be used on a temporary basis to convey timely information from the USNO to the MCS, or may be used to add additional variables to the standard download in the future without having to change data formats.

3.3.2.3.1.2 Aggregate Data DFRMF

An estimated offset between GPS Time and UTC(USNO) shall be provided to the MCS to predict GPS’s delivered prediction of UTC(USNO). The aggregate data contained in file DFRMF represent the last 16 days of the time difference between GPS Time and UTC(USNO), based on a linear solution of the entire constellation with corresponding slope, RMS about the fit, and number of passes used for the fit, and comments. All dates are referenced to midnight (GPS Time). When no data are available for a reporting period, repeating nines (9’s) are reported in each column for each unavailable variable.

See Table 5 for format and Table 8 for a sample output.

Table 5. Aggregate Data File DFRMF (See Sample Output in Table 8, Section 5.1.1)

Parameter Colum n Resolution Range/Value Units

Modified Julian Date (MJD) (1) 1-7 .1 52275.0 – 99999.0 Days Space 8-9 N/A ‘space’ N/A Year and DOY (YYYYDOY) (2) 10-18 .1 -2002001.0 – 9999366.0 YearDay Space 19 N/A ‘space’ N/A GPS Time – UTC(USNO) 1-day linear fit (3)

20-27 .01 +/-9999.99 ns

Space 28 N/A ‘space’ N/A GPS Time – UTC(USNO) Slope (4) 29-36 .01 +/-9999.99 ns/day Space 37 N/A ‘space’ N/A GPS Time – UTC(USNO) RMS About Fit (5)

38-45 .01 0.00 – 99999.99 ns

Space 46 N/A ‘space’ N/A GPS Time – UTC(USNO) Sample Size

(N) (6)

47-51 1 10 – 99999 N/A

Space 52 N/A ‘space’ N/A Comments 54-80 N/A text N/A

Notes:

1. MJD is the five-digit Modified Julian Date.

2. YYYY is the four-digit year and DOY is the Day of Year.

3. GPS Time – UTC(USNO) is the result of a linear fit of the single pass data for 24 hours (1200 GPS time of day 1 to 1200 GPS time of day 2) solving for 0000 GPS time of day 2.

4. Slope corresponding to the GPS Time – UTC(USNO) linear fit.

5. RMS about the GPS Time – UTC(USNO) linear fit, i.e. ∑ = n i ii YY n 1

2)ˆ(

1 where Yi represents the measurements of GPS Time – UTC(USNO), Y represents the linear fit associated with measurement Yî i, and n is the sample size contributing to the fit given in (6).

6. Sample size of the GPS Time – UTC(USNO) linear fit, where N is the number of single pass data used.

7. Optional comments describing observed anomalies, or warnings of observed data out of range.

3.3.2.3.1.3 Aggregate Data DFRHV

The mean difference between GPS’s delivered prediction of UTC(USNO) and UTC(USNO), utilizing a 24-hour average of tracks from the entire constellation shall be provided to the MCS to monitor the predicted UTC via GPS. The values of the difference are associated with the MJD and YYYYDOY. The aggregate data contained in file DFRHV represent the last 30 days of the 24-hour averages. When no data are available for a reporting period, repeating nines (9’s) are reported in each column for each unavailable variable. See Table 6 for definitions and Table 9 for an example.

Table 6. Aggregate Data File DFRHV (See Sample Output in Table 9, Section 5.1.1)

Parameter Column Resolution Range/Value Units

Modified Julian Date (MJD) (1) 1-5 1 52275 - 99999 Days Space 6-7 N/A ‘space’ N/A Year and DOY (YYYYDOY) (2) 8-14 1 2002001 - 9999366 YearDay Space 15 N/A ‘space’ N/A GPS’s delivered prediction of UTC(USNO) – UTC(USNO) Daily Average (3)

16-23 .01 +/- 9999.99 ns

Space 24 N/A ‘space’ N/A GPS’s delivered prediction of UTC(USNO) – UTC(USNO) Sigma (4)

25-32 .01 0.00 – 99999.99 ns

Space 33 N/A ‘space’ N/A GPS’s delivered prediction of UTC(USNO) – UTC(USNO) RMS About Zero (5)

34-41 .01 0.00– 99999.99 ns

Space 42 N/A ‘space’ N/A GPS’s delivered prediction of UTC(USNO) – UTC(USNO) All-In-View RMS About Zero (6)

43-51 .01 0.00– 999999.99 ns

Space 52 N/A ‘space’ N/A GPS’s delivered prediction of UTC(USNO) – UTC(USNO) Sample Size (N) (7)

53-58 1 10 – 999999 N/A

Space 59 N/A ‘space’ N/A Comments 60-80 N/A text N/A

Notes:

1. MJD is the five-digit Modified Julian Date.

2. YYYY is the four-digit year and DOY is the Day of Year.

3. The difference between GPS’s delivered prediction of UTC(USNO) and UTC(USNO) is the result of an average of the single pass data over 24 hours (1200 GPS Time of day 1 to 1200 GPS Time of day 2) solving for 0000 GPS Time of day 2.

4. Corresponding Sigma (sample standard deviation) of the GPS’s delivered prediction of UTC(USNO) –

UTC(USNO) 24-hour average. That is, ∑ = n i i YY n 1

2)(

1 where Yi represents the measurements of GPS’s delivered prediction of UTC(USNO) – UTC(USNO), Y is the average given in (3), and n is sample size given in (7).

5. Corresponding RMS about zero for the GPS’s delivered prediction of UTC(USNO) – UTC(USNO) 24-hour average. That is, ∑ = n i i YYY n 1

22)(

1 where Y is the quantity given in (3) and ∑ = n i i YY n 1

2)(

1 is the square of the quantity given in (4).

6. Corresponding RMS about zero for the GPS’s delivered prediction of UTC(USNO) – UTC(USNO) 24-hour average as seen by a multi-channel user tracking up to 12 satellites concurrently in All-In-View mode. This quantity is calculated as in (5) by replacing Yi with Zi where Zi is the timestamp-average of all satellites tracked concurrently, that is, ∑ = k j ji Y k 1

1Z where k represents the number of satellites tracked concurrently.

7. Sample size of the GPS’s delivered prediction of UTC(USNO) – UTC(USNO) 24-hour average, where N is the number of passes used.

8. Optional comments describing observed anomalies, or warnings of observed data out of range.

3.3.2.3.2 Leap Second Advisory

Leap second advisory data are provided to MCS personnel at least sixty days prior to a leap second. If a leap second is currently scheduled (future or recent past) the data provided should include the date of the transition and the value of the leap second count after the change. Absence of this data from USNO constitutes notice that there will be no leap second for at least sixty days. Time Series Announcement Series 14 supplies any anticipated leap second changes.

3.3.2.3.3 Satellite Status Advisories

Satellite status advisories shall be provided by MCS personnel as required, and shall include:

a. Satellite number (PRN code),

b. Nominal effectivity date and time,

c. Estimated date and time of the end of the condition being reported, if available,

d. Satellite condition,

e. Expected impact, if known.

3.3.2.3.4 Abnormal Satellite Status Advisories

Information regarding abnormal situations with any satellite being monitored by USNO will be communicated to MCS personnel. This will be done either through personal contact between USNO personnel and MCS personnel or by telephone contact between USNO personnel and MCS personnel. Details of data provided by GPS to USNO will be resolved between MCS and USNO personnel either through personal contact between USNO personnel and MCS personnel or by telephone contact between USNO personnel and MCS personnel.

3.4 Security Consideration

USNO will be a GPS Authorized User consistent with the importance of maintaining time coordination during various applied levels of Selective Availability and Anti-Spoofing, and will provide secure facilities and implement procedures appropriate for use of the timing receivers in this mode. Communication of time transfer observations between the USNO and the MCS currently utilize the SIPRNet. Communication of leap second advisories is unclassified.

3.5 Backup Procedures

Ordinarily the communications interface described above is between the USNO computer at Washington, D.C., and a terminal at the MCS. However, in the event of equipment malfunction at either the USNO or MCS, USNO personnel may relay the timing corrections by voice instruction over phone lines. In the event of extended equipment malfunction at the USNO located in Washington DC, the USNO AMC located at Schriever AFB, CO, will become the primary time monitor site. The transfer of operations from USNO Washington to USNO AMC will be transparent to the MCS, but USNO personnel will notify MCS personnel of the change. If electronic communication to either site is not feasible, the data may be transferred in hard copy form by mail. While its use will then be less timely, the resultant system degradation will be small. In all cases, voice communication may be utilized as needed to coordinate or supplement the digital data interface.

Lara Schmidt Persue this with 2SOPS coders.

3.6 Anomaly Resolution

The USNO time transfer data collection and processing are automated and are unattended outside regular business hours. If missing or anomalous data are detected, they will be flagged in the Comments columns of data files provided to the MCS.

USNO contact names and phone numbers are provided to the MCS on a routine basis. When operationally possible, the MCS shall initiate contact between 1500 UTC and 1600 UTC to be assured that USNO personnel are available for anomaly resolution. On call USNO personnel will respond to these requests when contacted between 1500 UTC and 1600 UTC.

Outside of this period of time window, USNO personnel will respond when operationally feasible.

4.0 ACRONYMS AND GLOSSARY

4.1 Acronyms and Abbreviations

2 SOPS 2nd Space Operations Squadron

AMC USNO Alternate Master Clock ASCII American National Standard Code for Information Interchange AS Anti-spoofing

BCE Before Common Era BIPM Bureau International des Poids et Mesures

COSPM Colorado Springs Monitor Station CPCI Computer Program Configuration Item CS…