: The Naval Research Laboratory (NRL) conducts comprehensive analyses of the Global Positioning System (GPS) atomic frequency standards under the sponsorship of the 2nd Space Operations Squadron (2SOPS) at the Master Control Station (MCS) in Colorado Springs, Colorado. The analysis is based on clock estimates that NRL computes from GPS monitor station carrier-derived pseudo-range measurements and National Geospatial-Intelligence Agency (NGA) computed precise post-fit orbit ephemerides. The purpose of the analyses is to determine the performance of the timing signals originating from the atomic frequency standards onboard the space vehicles. Metrics used in the analyses include frequency, drift, stability profiles, and stability histories based on the Allan and Hadamard variances. The relative performance of the space vehicle clocks is ranked and presented according to the clock types flown in GPS. An overview of the GPS constellation with respect to the lifetimes of space vehicles and space vehicle clocks, both active and deactivated, is also presented.

The Naval Research Laboratory (NRL) conducts comprehensive analyses of the Global Po- sitioning System (GPS) atomic frequency standards under the sponsorship of the 2nd Space Operations Squadron (2SOPS) at the Master Control Station (MCS) in Colorado Springs, Colo- rado. The analysis is based on clock estimates that NRL computes from GPS monitor station carrier-derived pseudo-range measurements and National Geospatial-Intelligence Agency (NGA) computed precise post-fit orbit ephemerides. The purpose of the analyses is to determine the performance of the timing signals originating from the atomic frequency standards onboard the space vehicles. Metrics used in the analyses include frequency, drift, stability profiles, and stability histories based on the Allan and Hadamard variances. The relative performance of the space vehicle clocks is ranked and presented according to the clock types flown in GPS. An overview of the GPS constellation with respect to the lifetimes of space vehicles and space ve- hicle clocks, both active and deactivated, is also presented.

The U.S. Naval Research Laboratory (NRL) has collected data and analyzed GPS space vehicle atomic clock performance since the beginning of the GPS Program. These analyses have largely been based on pseudorange observations and precise post-fit ephemerides provided by the National Geospatial-Intelligence Agency (NGA). Observational data and clock comparisons were limited to data collected within the GPS system due primarily to the lack of global tracking resources and effective means of gathering the data. With the successful conclusion of the International GPS Service (IGS) and Bureau International Poids and et Mesures (BIPM) Pilot Project, new capabilities to associate IGS geophysical data to universal coordinated time (UTC), and integration of timing centers to contribute to the determination of UTC are evolving. Data collection from participating timing centers and analysis to form the IGS timescales and Clock Products has established new capabilities for analysis of orbiting precision atomic clocks. NRL leads the IGS Clock Products Working Group and generates the IGS timescales. These efforts joined together with the GPS on-orbit analyses offer new possibilities for analysis and improved performance for both GPS and IGS. This paper discusses these new resources for analysis and possible improved analytical capabilities

: The Naval Research Laboratory (NRL) has, for many years, determined the GPS space vehicle clock offsets by differencing the pseudorange observations and the post-fit orbit provided by the National Geospatial-Intelligence Agency (NGA). On 14 March 2004, NGA put into production a new class of pseudorange observations called carrier-derived pseudorange observations (CDP). Because the measurement noise of the new CDP observations is much lower than that of the code-phase pseudorange observations, NRL has begun to use the newer CDP observations in its analysis of GPS clock performance. A number of differences between the two types of data are pointed out, and a comparative analysis of clock performance with respect to frequency stability is reviewed. This work was performed under the sponsorship of the GPS Joint Program Office.

The life test of two Perkin-Elmer rubidium atomic frequency standards (RAFS) at the US Naval Research Laboratory (NRL) in a simulated space environment and flight configuration began 31 March 1997. These clocks were production clocks from the Global Positioning System (GPS) Block IIR build and especially provided for this test by the GPS Joint Program Office and the Block IIR satellite contractor team led by Lockheed Martin. This continuously running test was designed to provide information on long term performance, operation, and potential unique characteristics of these clocks. This included potential failure mechanisms, and day-to-day characteristics of the clocks and their internal monitors. GPS Block IIR was the first on-orbit use of this design and there was initially significant concern over their lack of actual space experience. This paper summarizes the test results to date and compares those results with the observed on-orbit performance of the Perkin-Elmer rubidium clocks in the GPS constellation. The utility of life testing in supporting operation and new space clock introduction into the system is highlighted.

An overview of past accomplishments is presented that shows the development of satellite time transfer techniques and capabilities that are used today. The development is traced from the concepts and early demonstrations using a single satellite to the global coverage now pro- vided by a constellation of satellites. Predictions of future technology are compared with what has been accomplished over the last 25 years. Co-operative experiments performed jointly by colleagues of the PTTI Meetings were key to the development. These experiments demonstrated the potential of time transfer by satellites having global coverage. Some of the results are re- viewed and compared to current capabilities.

Analysis of the performance of all on-orbit Navstar space vehicle clocks and Global Po- sitioning System (GPS) monitor station reference clocks is pelfonned by the Naval Research Laboratory (NRL), in cooperation with the GPS Master Control Station, under the sponsorship of the GPS Joint Program Once. The measurements are collected by multi-channel GPS re- ceivers located at the Air Force and National Imagery and Mapping Agency (NIMA) monitor stations. The offset of each Navstar clock, computed every 15 minutes, is referenced to the De- partment of Defense Master Clock The resultant Navstar clock offsets are then used to com- pute frequency offset, drifr offset, frequency stability profiles, and frequency stability histories. The beginning-of-life, steady state,and end-of-life per$onnance of selected cesium and rubid- ium atomic clocks is presented_ Frequency stability results are presented using sample times &at vary from 15 minutes to several days. The stability for sample times of less than one day characterizes rhe measurement noise, while the stability for sample times in excess of one day characterizes both the periodic effects in the on-orbit data and the long-term performance of the Navstar atomic clocks.

The US. Naval Research Laboratory (NRW conducts comprehensive anulyses of the Global Positioning System (GPS) atomic frequency stand~rd~v under the sponsorslzip oj'the GPS Joint Program Ofice (JPO) and in cooperation with the 2nd Space Operations Squadron (2SOPS) ut the Muster C,'ontrol Station (MCS) in Colorado Springs, Colorado. Included in the analysis are the on-orbit Navslar space vehicle cloclcs and the ground reference clocks at each uf the jive Air Force and seven National 11nage~ and Mapping Agency (NIMA) GPS monitor stations. A presentation will be made of the performuncr of the Nuvstar clocks currentlv operating in the constellation, which are characterized through the use of phase, jkequency, drift and stahiliQ histories in addition to frequency stability profiles hased an (he Allun und Hudamard variances. Clock performance is analyzed using a multi-year databuse comprised of pseudorange measurements collected by each of the 12 GPS monitor stations. Resulis oj'these ana(vses are routine4~ used by the MCS in optimizing the q 's in the Kulman$lter. Continuous 15-minute measurements of the phase offset of each monitor station time reference from the DoD Master Clock are obtained @om Linked Common- View Time Transfir ,fmn? WoD Master Clock, which is the reference clock at the NIM Washington, D.C. monitor station. The method zs extended to obtain continuous 15-minute measurements ofthe phase offset of each act~ve Nuvstar space vehicle clockJrom the DUD Master Clock. Hence, the performance of all space and control segment cloclcs is referenced to the DoD Master Clock, Discontinuities in the phase andfiequenc); ofthe clocks are removed to yield the unperturbedperformunce of the clocks. The corrections, together with the probable cause of the discontinuiv, are summarized. Examples of the frequency histo~ and the exhaustive calculation, .for evely multiple of the sample period oj'I.5 minutes fiom 15 minutes to 18 days, oj-thejrequency stabilit?, profile jor swerul ~V~m;s/nr spuce vehicle clocks and,for the time reference at two of the GPS monitor stations will be presented, ilululysis of the performance ofthe"first on-orbit Block IIR operational rubidium clock will also be presented.

: The U.S. Naval Research Laboratory (NRL) conducts comprehensive analyses of the Global Positioning System (GPS) atomic frequency standards under the sponsorship of the GPS Joint Program Office (JPO) and in cooperation with the 2nd Space Operations Squadron (2SOPS) at the Master Control Station (MCS) in Colorado Springs, Colorado. Included in the analysis are the on-orbit Navstar space vehicle clocks and the ground reference clocks at each of the five Air Force and seven National Imagery and Mapping Agency (NIMA) GPS monitor stations. A presentation will be made o f the performance of the Navstar clocks currently operating in the constellation, which are characterized through the use of phase, frequency, drift and stability histories in addition to frequency stability profiles based an the Allan and Hadamard variances. Clock performance is analyzed using a multi-year database comprised of pseudorange measurements collected by each of the 12 GPS monitor stations. Results of these analyses are routinely used by the MCS in optimizing the q's in the Kalman filter. Continuous 15-minute measurements of the phase offset of each monitor station time reference from the DoD Master Clock are obtained from Linked Common-View Time Transfer from DoD Master Clock, which is the reference clock at the NIMA Washington, D.C. monitor station. The method is extended to obtain continuous 15-minute measurements of the phase offset of each active Navstar space vehicle clock from the DoD Master Clock. Hence, the performance of all space and control segment clocks is referenced to the DoD Master Clock, Discontinuities in the phase and frequency of the clocks are removed to yield the unperturbed performance of the clocks. The corrections, together with the probable cause of the discontinuity, are summarized.

Analysis of the on-orbit Navstar clocks and the Global Positioning System (GPS) monitor station reference clocks is performed by the Naval Research Laboratory using both broadcast and postprocessed precise ephemerides. The precise ephemerides are produced by the Defense Mapping Agency (DMA) for each of the GPS space vehicles from pseudo-range measurements collected at five GPS and at five DMA monitor stations spaced around the world. Recently, DMA established an additional site co-located with the US Naval Observatory precise time site. The time reference for the new DMA site is the DoD Master Clock. Now, for the first time, it is possible to transfer time every 15 minutes via common view from the DoD Master Clock to the 11 GPS and DMA monitor stations. The estimated precision of a single common-view time transfer measurement taken over a 15-minute interval was between 1.4 and 2.7 nanoseconds. Using the measurements from all Navstar space vehicles in common view during the 15-minute interval, typically 3-7 space vehicles, improved the estimate of the precision to between 0.65 and 1.13 nanoseconds. The mean phase error obtained from closure of the time transfer around the world using the 11 monitor stations and the 25 space vehicle clocks over a period of 4 months had a magnitude of 31 picoseconds. Analysis of the low noise time transfer from the DoD Master Clock to each of the monitor stations yields not only the bias in the time of the reference clock, but also focuses attention on structure in the behaviour of the reference clock not previously seen. Furthermore, the time transfer provides a a uniformly sampled database of 15-minute measurements that make possible, for the first time, the direct and exhaustive computation of the frequency stability of the monitor station reference clocks. To lend perspective to the analysis, a summary is given of the discontinuities in phase and frequency that occurred in the reference clock at the Master Control Station during the period covered by the analysis.

: With upcoming GPS Block IIR hunches scheduled, rubidium clock estimation will require more attention than ever before during the next decade of GPS operations GPS Master Control Station (MCS) estimation architecture relies on a three-state polynomial clock model, which does not include a time-variant decay parameter for frequency drift. Since current GPS rubidium frequency standard exhibit signifiant time-dependent frequency drift changes, the MCS is compelled to make precise utilization of the random run process noise parameter, known as q sub 3. The work of various scientists over the past three decades has shown the Hadamard variance to converge for random run FM. At PTTI '95, the 2d Space Operations Squadron (2 SOPS) introduced an algorithm that presented a simple, convergent polynomial relationship between the Hadamard variance and the MCS's Kalman filter process noise parameters. Until recently, however, neither the Hadamard variance nor the Hadamard-Q equation had actually been put to use in GPS. The Naval Research Laboratory (NRL) has now created analysis software designed to employ the Hadamard variance in their GPS clock analyses, to supplement their already existing software, which makes use of the Allan variance. This paper presents results of the NRL analysis using both the Allan and Hadamard variances for several operational GPS rubidium frequency standard, as well as results from the recent operational use of the Hadamard-Q equation, by 2 SOPS personnel, based on the NRL analysis data.

The Naval Research Laboratory (NRL) in the development of timing systems for remote locations, had a technical requirement for a Y code (SA/AS) Global Positioning System (GPS) precise time transfer receiver (TTR) which could be used both in a stationary mode or mobile mode. A contract was awarded to the Stanford Telecommunication Corporation (STEL) to build such a device. The Eastern Range (ER) als had a requirement for such a receiver and entered into the contract with NRL for the procurement of additional receivers. The Moving Vehicle Experiment (MVE) described in this paper is the first in situ test of the STEL Model 5401C Time Transfer System in both stationary and mobile operations. The primary objective of the MVE was to test the timing accuracy of the newly developed GPS TTR aboard a moving vessel. To accomplish this objective, a joint experiment was performed with personnel from NRL and the er at the Atlantic Undersea Test and Evaluation Center (AUTEC) test range at Andros Island. Results and discussion of the test are presented in this paper.

Abstract : Analysis of the performance of all on-orbit Navstar space vehicle clocks and Global Positioning System (GPS) monitor station reference clocks is performed by the Naval Research Laboratory (NRL), in cooperation with the GPS Master Control Station, under the sponsorship of the GPS Joint Program Office (JPO). The measurements used in the analysis are collected by multi-channel GPS receivers located at the Air Force and National Imagery and Mapping Agency (NIMA) monitor stations. The offset of each Navstar clock is computed with respect to the Department of Defense Master Clock. The resultant Navstar clock offsets are then used to compute frequency offset, drift offset, frequency stability profiles, and frequency stability histories. The performance of selected cesium and rubidium atomic clocks will be presented. Frequency stability results are calculated using sample times that vary from 15 minutes to several days. The operational Navstar timing signals will be ranked (according to frequency stability) for sample times of 6 hours and 1 day. In addition, the Block IIR clocks will be compared to results from an ongoing NRL laboratory life-test on two flight-qualified Block IIR rubidium atomic frequency standards. Observed clock behavior and anomalies will be discussed.

Frequency stability analysis of on-orbit Navstar clocks is performed by the Naval Research Laboratory using both the broadcast and the precise post-processed ephemerides. The phase offset between the Navstar clock and the reference clock is computed from pseudorange measurements obtained by dual-frequency GPS receivers at the five GPS monitor sites and at the U.S. Naval Observatory precise-time site. The broadcast ephemerides are generated at the GPS master control station by a Kalman filter using data collected from the five GPS monitor stations. The precise post-processed ephemerides are generated by the Defense Mapping Agency (DMA) using data collected from the GPS monitor sites and from five additional DMA monitor sites. In this paper the frequency stability is estimated for two Navstar caesium clocks-a Block I caesium clock (Navstar 9) and a Block II caesium clock (Navstar 23)-using both the broadcast and the precise ephemerides. A significant improvement in the estimate of the frequency stability of the Block II clocks has been achieved using the precise ephemeris.