Due to the ubiquity of Orthogonal Frequency Division Multiplexing (OFDM) based communications sta... more Due to the ubiquity of Orthogonal Frequency Division Multiplexing (OFDM) based communications standards such as IEEE 802.11 a/g/n and 3GPP Long Term Evolution (LTE), a growing interest has developed in techniques for reliably detecting the presence of these signals in dynamic radio systems. A popular approach for detection is to exploit the cyclostationary nature of OFDM communications signals. In this paper, we focus on a frequency domain cyclostationary detection algorithm first introduced by Giannakis and Dandawate and study its performance in detecting IEEE 802.11a OFDM signals in the presence of practical radio impairments such as Carrier Frequency offset (CFO), Phase Noise, I/Q Imbalance, Multipath Fading and DC offset. We then present a hardware implementation of this algorithm developed using MathWorks HDL Coder and provide implementation results after targeting to a Xilinx 7 Series FPGA device.
One of the key challenges for state of the art radio systems is enabling efficient utilisation of... more One of the key challenges for state of the art radio systems is enabling efficient utilisation of the Radio Frequency (RF) spectrum. Licensed frequency bands are often under-utilised in both time and geographical location and thus the opportunity exists for secondary users to transmit in these bands, provided that they do not interfere significantly with the operation of the primary licensed user. A proposed method for exploiting this opportunity is Cognitive Radio (CR) wherein the secondary user is able to modify its transmissions based on observation of the operating RF environment. Orthogonal Frequency Division Multiplexing (OFDM) is the enabling technology for many modern communications standards such as IEEE 802.11a (WiFi) and 4G Long Term Evolution (LTE). Therefore, facilitating robust and cost effective detection of OFDM signals is a key problem for the design of secondary user CR systems. In this paper, we derive and assess the performance of a low complexity detection scheme that exploits the inherent cyclostationarity of OFDM signals. We then present details of its implementation on a Xilinx Artix 7 FPGA and compare the resource cost of the proposed detector with another low complexity detection algorithm found in the literature.
We use simulation to compare the confidence in estimating frequency drift in the presence of stoc... more We use simulation to compare the confidence in estimating frequency drift in the presence of stochastic noise between two different estimators: (1) a mean second difference using neighboring data, and (2) a single second difference over the entire data set. In each case we simulate 100 s data sampling and
IEEE Transactions on Instrumentation and Measurement, Jun 1, 1987
The National Bureau of Standards (NBS) Time and Frequency Division now performs precision time an... more The National Bureau of Standards (NBS) Time and Frequency Division now performs precision time and frequency transfer using common view measurements of Global Positioning System (GPS) satellites as a calibration service. Using this service, we have been able to transfer time with time stabilities of a few nanoseconds, time accuracies of the order of 10 ns, and frequency stabilities of one part in 10 1 4, or better, for measurement times of about four days and longer. The full accuracy of the NBS primary frequency standard is now available at a remote site. This paper describes the technique used for weighting and smoothing the data to produce these levels of stability and accuracy. All of the primary frequency standards used in the generation of International Atomic Time (TAl) now use this technique.
IEEE Transactions on Instrumentation and Measurement, Jun 1, 1985
Frequency differences between major national timing centers are being resolved with uncertainty o... more Frequency differences between major national timing centers are being resolved with uncertainty of less than 1 part in 1014',using satellites of the Global Positioning System (GPS) in common-view. Portable clock and GPS time differences are in excellent agreement. Around the world GPS measurement between three laboratories had a time residual of 5.1 ns.
The internafional clock ensemble, which contributes to the generation of International Atomic T h... more The internafional clock ensemble, which contributes to the generation of International Atomic T h e (TAI and UTC), has improved d r d c d y over the last few years. The main change has been the introduction o f a significant number of HP 5071A clocks. Of the 313 clocks conhr'buting to TAIIUTC during 1994, 94 of these were HP 5071A.v. The environmental insensitivity of the HP 5071A clocks is more than an order of magnitude better than that of previous contributing clocks. This environmental insensitivity translates to outstanding long-term stabilitywith a typicalpicker floor of a few xl0-'" In addition, there are now several hydrogen masers with cavity tuning contributing to TAIIUTC. These not only have outstanding short-term stabilily, but comparatively low frequency drifi and esceUent intermediate-term jrequency stability. By analyzing the data available from the international ensemble, we have obtained two important results. First, the frequency stabilify obtainable with an optimum algorithm is about 10-'"or both the intermediate and long-term regions. It could be as good in the short-term if time transfer measurement instabilities were reduced sulfieiently. Second, with cooperation, this performance can be made available on an internakiond basis in near real time. The recent enhancements in the contributing clocks are already providing a significant improvement in the accuracy with which UTC is made available to the world from several of the national timing centers, such as NIST and USNO. Recently, a cesium fountain primary frequency standard was reported to have an accuracy of : 3 x lo-'! The development of the new HP 5071A cesium-beam commercial frequency standard is, as will be shown, a major contributor after its introduction a few years ago.
The international clock ensemble, which contributes to the generation of International Atomic Tim... more The international clock ensemble, which contributes to the generation of International Atomic Time (TAI and UTC), has improved dramatically over the last few years. The main change has been the introduction of a significant number of HP 5071A docks. Of the 313 clocks contributing to TAI/UTC during 1994, 94 of these were HP 5071As. The environmental insensitivity of the HP 5071A clocks is more than an order of magnitude better than that of previous contributing clocks. This environmental insensitivity translates to outstanding long-term stability-with a typical flicker floor of a Jew x 10-is. In addition, there are now several hydrogen masers with cavity tuning contributing to TAI/UTC. These not only have outstanding short-term stability, but comparatively low frequency drifts and excellent intermediate-term frequency stability. By analyzing the data available from the international ensemble, we have obtained two important results. First, the frequency stability obtainable with an optimum algorithm is about 10-is for both the intermediate and long-term regions. It could be as good in the short.term if time transfer measurement instabilities were reduced su#iciengy. Second, with cooperation, this performance can be made available on an international basis in near real time. The recent enhancements in the contributing clocks are already providing a significant improvement in the accuracy with which UTC is made available to the world from several of the national timing centers, such as NIST and USNO.
The reference time scale for all scientific and technologic applications on the Earth, the Univer... more The reference time scale for all scientific and technologic applications on the Earth, the Universal Coordinated Time UTC, must be as stable, reliable and accurate as possible. With this in view the BIPM and before it the BIH, have always calculated and then disseminated UTC with a delay of about 80 days. There are three fundamental reasons for doing this. i) It takes some weeks for data, gathered from some 200 clocks spread worldwide , to be collected and for errors to be eliminated. performances of contributing clocks, has improved the stability of UTC, namely Oy(,) _NG ,l_a,%E BLA]_LK NOT F'ILMEO 217 9.4(0 to the order of 8x10 "15 for averaging times of 40 days, and this could also help to decrease the delay of access to UTC. iii) The SA is now overcome both for GPS common-views and for real-time access to GPS time. In addition, other time dissemination methods, which are not intentionally degraded, are now very promising. iv) Modern computer capability and improvements in data communication capability allow quicker and more efficient automatic data transmission.
Most workers i n the f i e l d o f atomic clocks encounter frequency and time i n s t a b i l i t... more Most workers i n the f i e l d o f atomic clocks encounter frequency and time i n s t a b i l i t i zed (or model led) as random fluctuations t y p i c a l l y display a which varies as a power-law over
NASA is planning a , Space Shuttle experiment to demonstrate high-accuracy global time and freque... more NASA is planning a , Space Shuttle experiment to demonstrate high-accuracy global time and frequency transfer. A hydrogen maser clock on board the Space Shuttle will be compared with clocks on the ground using two-way microwave and short-pulse laser signals. The accuracy, goal for the experiment is 1 nsec or better for the time transfer and 10~1 4 for the frequency comparison. A direct frequency comparison of primary standards at the lO" 1^ accuracy level is a unique feature of the proposed system. Both time and frequency transfer will be accomplished by microwave transmission, while the laser signals provide calibration of the system as well as sub-nanosecond time transfer. Following the demonstration with the Space Shuttle, an operational system could be implemented in a free-flying satellite to provide permanent global time and frequency transfer.
IEEE Transactions on Instrumentation and Measurement, 1978
Snider, "Methods for production screening for anomalous responses in quartz crystals intended for... more Snider, "Methods for production screening for anomalous responses in quartz crystals intended for high reliability applications," in Proc. 31st AFCS, June 1977. [14] H. F. Tiersten, "Analysis of intermodulation in rotated Y-cut quartz thickness-shear resonators," in Proc. 28th AFCS, pp. 1-4, May 1974.
The National Bureau of Standards (NBS), Boulder, Colorado, uses an algorithm which generates UTC(... more The National Bureau of Standards (NBS), Boulder, Colorado, uses an algorithm which generates UTC(NBS) from its ensemble of clocks and automatically, optimally, and dynamically weights each clock in the ensemble. The same algorithm was used at the Master Control Station (MCS) of the Global Positioning System (GPS) to generate a time scale from a small ensemble of cesium clocks. Time transfer employing the GPS common view technique between NBS (Boulder) and MCS (Colorado Springs) was used to evaluate the stability of the MCS ensemble relative to UTC(NBS). The results demonstrate the power of the NBS algorithm in prwiding a stable time scale from a small ensemble of clocks. The resulting scale is, in principle, more stable than the best clock and a poor clock need not degrade the ensemble.
Managers ar9 often required to make key program decisions based on the performance of some elemen... more Managers ar9 often required to make key program decisions based on the performance of some elements of a large system. This paper is intended to assist the manager in this important task in so far as it relates to the proper use of precise and accurate clocks. An intuitive approach will be used to show how a clock's stability is measured, why it is measured the way it is, and why it is described the way it is. 4n intuitive explanation of the meaning of time domain and frequency domain measures as well as why they are used will also be given.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, Apr 1, 2016
Over the past 50 years, variances have been developed for characterizing the instabilities of pre... more Over the past 50 years, variances have been developed for characterizing the instabilities of precision clocks and oscillators. These instabilities are often modeled as nonstationary processes, and the variances have been shown to be well-behaved and to be unbiased, efficient descriptors of these types of processes. This paper presents a historical overview of the development of these variances. The time-domain and frequency-domain formulations are presented and their development is described. The strengths and weaknesses of these characterization metrics are discussed. These variances are also shown to be useful in other applications, such as in telecommunication. Index Terms-Allan variances (AVARs), atomic clocks, nonstationary processes, precision analysis, time series analysis. I. INTRODUCTION N ATURE gives us many nonstationary and chaotic processes. If we can properly characterize these processes, then we can use optimal procedures for estimating, smoothing, and predicting them. During the 1960s through the 1980s, the Allan variance (AVAR), the modified Allan variance (MVAR), and the time variance (TVAR) were developed to this end for the timing and the telecommunication communities. Since that time, refinements of these techniques have been developed. The strengths and weaknesses of these variances will be discussed in the following text. The applicability of these variances has been recognized in other areas of metrology as well. Knowing the strengths and weaknesses is important so that they can be properly used. Prior to the 1960s, before atomic clocks were commercially available, quartz-crystal oscillators were used for timekeeping. The largest contribution to the long-term-frequency instability of these oscillators was frequency drift. It was generally recognized that the stochastic contribution to the long-term performance could be modeled by flicker-noise frequency modulation (FM), which is a nonstationary process, because it has a power-spectral-density proportional to 1/f , where f is the Fourier frequency. The integral of the power spectral density for this type of process diverges logarithmically, so that the data cannot be characterized by a well-defined classical variance. In real experiments, the upper and lower limits on the Manuscript
Recent improvements in the atomic time scales of the National Bureau of Standards
Coincident with the installation of a new measurement system, the national Bureau of Standards ha... more Coincident with the installation of a new measurement system, the national Bureau of Standards has also developed a new philosophy for the generation of both UTV(NBS) and atomic time, TA(NBS). Several benefits have resulted from this new direction, First, a more uniform UTC(NBS) scale was achieved in order to meet the increased requirements of our users. Second, improved synchronization of
1993 IEEE International Frequency Control Symposium
INMARSAT has designed a GPS (L1) transponder that will be included in their third generation sate... more INMARSAT has designed a GPS (L1) transponder that will be included in their third generation satellites. This transponder will broadcast a pseudo-GPS signal that can be used for navigation and also for disseminating integrity data and/or differential corrections for the GPS satellites. This INMARSAT Geostationary Overlay (IGO) service will be used to enhance the performance of the GPS navigation service for civil aviation and other users. The IGO service can also be used as a method of disseminating a precise time reference. Since the IGO signal is compatible with GPS, conventional GPS timing receivers can easily be modified to utilize the proposed service. Preliminary test results taken through the IGO satellite have demonstrated a timing accuracy of 10 nanoseconds.
Since 1967 the National Bureau of Standards has improved its traditional radio broadcast services... more Since 1967 the National Bureau of Standards has improved its traditional radio broadcast services from radio stations WWV, WWVH, and WWVB and introduced several new services to meet changing needs. The new services are described briefly, including the GOES satellite time code, the Frequency Measurement Service using Loran-C and WWVB, and the Global Time Service based on the CPS satellite common-view technique.
Due to the ubiquity of Orthogonal Frequency Division Multiplexing (OFDM) based communications sta... more Due to the ubiquity of Orthogonal Frequency Division Multiplexing (OFDM) based communications standards such as IEEE 802.11 a/g/n and 3GPP Long Term Evolution (LTE), a growing interest has developed in techniques for reliably detecting the presence of these signals in dynamic radio systems. A popular approach for detection is to exploit the cyclostationary nature of OFDM communications signals. In this paper, we focus on a frequency domain cyclostationary detection algorithm first introduced by Giannakis and Dandawate and study its performance in detecting IEEE 802.11a OFDM signals in the presence of practical radio impairments such as Carrier Frequency offset (CFO), Phase Noise, I/Q Imbalance, Multipath Fading and DC offset. We then present a hardware implementation of this algorithm developed using MathWorks HDL Coder and provide implementation results after targeting to a Xilinx 7 Series FPGA device.
One of the key challenges for state of the art radio systems is enabling efficient utilisation of... more One of the key challenges for state of the art radio systems is enabling efficient utilisation of the Radio Frequency (RF) spectrum. Licensed frequency bands are often under-utilised in both time and geographical location and thus the opportunity exists for secondary users to transmit in these bands, provided that they do not interfere significantly with the operation of the primary licensed user. A proposed method for exploiting this opportunity is Cognitive Radio (CR) wherein the secondary user is able to modify its transmissions based on observation of the operating RF environment. Orthogonal Frequency Division Multiplexing (OFDM) is the enabling technology for many modern communications standards such as IEEE 802.11a (WiFi) and 4G Long Term Evolution (LTE). Therefore, facilitating robust and cost effective detection of OFDM signals is a key problem for the design of secondary user CR systems. In this paper, we derive and assess the performance of a low complexity detection scheme that exploits the inherent cyclostationarity of OFDM signals. We then present details of its implementation on a Xilinx Artix 7 FPGA and compare the resource cost of the proposed detector with another low complexity detection algorithm found in the literature.
We use simulation to compare the confidence in estimating frequency drift in the presence of stoc... more We use simulation to compare the confidence in estimating frequency drift in the presence of stochastic noise between two different estimators: (1) a mean second difference using neighboring data, and (2) a single second difference over the entire data set. In each case we simulate 100 s data sampling and
IEEE Transactions on Instrumentation and Measurement, Jun 1, 1987
The National Bureau of Standards (NBS) Time and Frequency Division now performs precision time an... more The National Bureau of Standards (NBS) Time and Frequency Division now performs precision time and frequency transfer using common view measurements of Global Positioning System (GPS) satellites as a calibration service. Using this service, we have been able to transfer time with time stabilities of a few nanoseconds, time accuracies of the order of 10 ns, and frequency stabilities of one part in 10 1 4, or better, for measurement times of about four days and longer. The full accuracy of the NBS primary frequency standard is now available at a remote site. This paper describes the technique used for weighting and smoothing the data to produce these levels of stability and accuracy. All of the primary frequency standards used in the generation of International Atomic Time (TAl) now use this technique.
IEEE Transactions on Instrumentation and Measurement, Jun 1, 1985
Frequency differences between major national timing centers are being resolved with uncertainty o... more Frequency differences between major national timing centers are being resolved with uncertainty of less than 1 part in 1014',using satellites of the Global Positioning System (GPS) in common-view. Portable clock and GPS time differences are in excellent agreement. Around the world GPS measurement between three laboratories had a time residual of 5.1 ns.
The internafional clock ensemble, which contributes to the generation of International Atomic T h... more The internafional clock ensemble, which contributes to the generation of International Atomic T h e (TAI and UTC), has improved d r d c d y over the last few years. The main change has been the introduction o f a significant number of HP 5071A clocks. Of the 313 clocks conhr'buting to TAIIUTC during 1994, 94 of these were HP 5071A.v. The environmental insensitivity of the HP 5071A clocks is more than an order of magnitude better than that of previous contributing clocks. This environmental insensitivity translates to outstanding long-term stabilitywith a typicalpicker floor of a few xl0-'" In addition, there are now several hydrogen masers with cavity tuning contributing to TAIIUTC. These not only have outstanding short-term stabilily, but comparatively low frequency drifi and esceUent intermediate-term jrequency stability. By analyzing the data available from the international ensemble, we have obtained two important results. First, the frequency stabilify obtainable with an optimum algorithm is about 10-'"or both the intermediate and long-term regions. It could be as good in the short-term if time transfer measurement instabilities were reduced sulfieiently. Second, with cooperation, this performance can be made available on an internakiond basis in near real time. The recent enhancements in the contributing clocks are already providing a significant improvement in the accuracy with which UTC is made available to the world from several of the national timing centers, such as NIST and USNO. Recently, a cesium fountain primary frequency standard was reported to have an accuracy of : 3 x lo-'! The development of the new HP 5071A cesium-beam commercial frequency standard is, as will be shown, a major contributor after its introduction a few years ago.
The international clock ensemble, which contributes to the generation of International Atomic Tim... more The international clock ensemble, which contributes to the generation of International Atomic Time (TAI and UTC), has improved dramatically over the last few years. The main change has been the introduction of a significant number of HP 5071A docks. Of the 313 clocks contributing to TAI/UTC during 1994, 94 of these were HP 5071As. The environmental insensitivity of the HP 5071A clocks is more than an order of magnitude better than that of previous contributing clocks. This environmental insensitivity translates to outstanding long-term stability-with a typical flicker floor of a Jew x 10-is. In addition, there are now several hydrogen masers with cavity tuning contributing to TAI/UTC. These not only have outstanding short-term stability, but comparatively low frequency drifts and excellent intermediate-term frequency stability. By analyzing the data available from the international ensemble, we have obtained two important results. First, the frequency stability obtainable with an optimum algorithm is about 10-is for both the intermediate and long-term regions. It could be as good in the short.term if time transfer measurement instabilities were reduced su#iciengy. Second, with cooperation, this performance can be made available on an international basis in near real time. The recent enhancements in the contributing clocks are already providing a significant improvement in the accuracy with which UTC is made available to the world from several of the national timing centers, such as NIST and USNO.
The reference time scale for all scientific and technologic applications on the Earth, the Univer... more The reference time scale for all scientific and technologic applications on the Earth, the Universal Coordinated Time UTC, must be as stable, reliable and accurate as possible. With this in view the BIPM and before it the BIH, have always calculated and then disseminated UTC with a delay of about 80 days. There are three fundamental reasons for doing this. i) It takes some weeks for data, gathered from some 200 clocks spread worldwide , to be collected and for errors to be eliminated. performances of contributing clocks, has improved the stability of UTC, namely Oy(,) _NG ,l_a,%E BLA]_LK NOT F'ILMEO 217 9.4(0 to the order of 8x10 "15 for averaging times of 40 days, and this could also help to decrease the delay of access to UTC. iii) The SA is now overcome both for GPS common-views and for real-time access to GPS time. In addition, other time dissemination methods, which are not intentionally degraded, are now very promising. iv) Modern computer capability and improvements in data communication capability allow quicker and more efficient automatic data transmission.
Most workers i n the f i e l d o f atomic clocks encounter frequency and time i n s t a b i l i t... more Most workers i n the f i e l d o f atomic clocks encounter frequency and time i n s t a b i l i t i zed (or model led) as random fluctuations t y p i c a l l y display a which varies as a power-law over
NASA is planning a , Space Shuttle experiment to demonstrate high-accuracy global time and freque... more NASA is planning a , Space Shuttle experiment to demonstrate high-accuracy global time and frequency transfer. A hydrogen maser clock on board the Space Shuttle will be compared with clocks on the ground using two-way microwave and short-pulse laser signals. The accuracy, goal for the experiment is 1 nsec or better for the time transfer and 10~1 4 for the frequency comparison. A direct frequency comparison of primary standards at the lO" 1^ accuracy level is a unique feature of the proposed system. Both time and frequency transfer will be accomplished by microwave transmission, while the laser signals provide calibration of the system as well as sub-nanosecond time transfer. Following the demonstration with the Space Shuttle, an operational system could be implemented in a free-flying satellite to provide permanent global time and frequency transfer.
IEEE Transactions on Instrumentation and Measurement, 1978
Snider, "Methods for production screening for anomalous responses in quartz crystals intended for... more Snider, "Methods for production screening for anomalous responses in quartz crystals intended for high reliability applications," in Proc. 31st AFCS, June 1977. [14] H. F. Tiersten, "Analysis of intermodulation in rotated Y-cut quartz thickness-shear resonators," in Proc. 28th AFCS, pp. 1-4, May 1974.
The National Bureau of Standards (NBS), Boulder, Colorado, uses an algorithm which generates UTC(... more The National Bureau of Standards (NBS), Boulder, Colorado, uses an algorithm which generates UTC(NBS) from its ensemble of clocks and automatically, optimally, and dynamically weights each clock in the ensemble. The same algorithm was used at the Master Control Station (MCS) of the Global Positioning System (GPS) to generate a time scale from a small ensemble of cesium clocks. Time transfer employing the GPS common view technique between NBS (Boulder) and MCS (Colorado Springs) was used to evaluate the stability of the MCS ensemble relative to UTC(NBS). The results demonstrate the power of the NBS algorithm in prwiding a stable time scale from a small ensemble of clocks. The resulting scale is, in principle, more stable than the best clock and a poor clock need not degrade the ensemble.
Managers ar9 often required to make key program decisions based on the performance of some elemen... more Managers ar9 often required to make key program decisions based on the performance of some elements of a large system. This paper is intended to assist the manager in this important task in so far as it relates to the proper use of precise and accurate clocks. An intuitive approach will be used to show how a clock's stability is measured, why it is measured the way it is, and why it is described the way it is. 4n intuitive explanation of the meaning of time domain and frequency domain measures as well as why they are used will also be given.
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, Apr 1, 2016
Over the past 50 years, variances have been developed for characterizing the instabilities of pre... more Over the past 50 years, variances have been developed for characterizing the instabilities of precision clocks and oscillators. These instabilities are often modeled as nonstationary processes, and the variances have been shown to be well-behaved and to be unbiased, efficient descriptors of these types of processes. This paper presents a historical overview of the development of these variances. The time-domain and frequency-domain formulations are presented and their development is described. The strengths and weaknesses of these characterization metrics are discussed. These variances are also shown to be useful in other applications, such as in telecommunication. Index Terms-Allan variances (AVARs), atomic clocks, nonstationary processes, precision analysis, time series analysis. I. INTRODUCTION N ATURE gives us many nonstationary and chaotic processes. If we can properly characterize these processes, then we can use optimal procedures for estimating, smoothing, and predicting them. During the 1960s through the 1980s, the Allan variance (AVAR), the modified Allan variance (MVAR), and the time variance (TVAR) were developed to this end for the timing and the telecommunication communities. Since that time, refinements of these techniques have been developed. The strengths and weaknesses of these variances will be discussed in the following text. The applicability of these variances has been recognized in other areas of metrology as well. Knowing the strengths and weaknesses is important so that they can be properly used. Prior to the 1960s, before atomic clocks were commercially available, quartz-crystal oscillators were used for timekeeping. The largest contribution to the long-term-frequency instability of these oscillators was frequency drift. It was generally recognized that the stochastic contribution to the long-term performance could be modeled by flicker-noise frequency modulation (FM), which is a nonstationary process, because it has a power-spectral-density proportional to 1/f , where f is the Fourier frequency. The integral of the power spectral density for this type of process diverges logarithmically, so that the data cannot be characterized by a well-defined classical variance. In real experiments, the upper and lower limits on the Manuscript
Recent improvements in the atomic time scales of the National Bureau of Standards
Coincident with the installation of a new measurement system, the national Bureau of Standards ha... more Coincident with the installation of a new measurement system, the national Bureau of Standards has also developed a new philosophy for the generation of both UTV(NBS) and atomic time, TA(NBS). Several benefits have resulted from this new direction, First, a more uniform UTC(NBS) scale was achieved in order to meet the increased requirements of our users. Second, improved synchronization of
1993 IEEE International Frequency Control Symposium
INMARSAT has designed a GPS (L1) transponder that will be included in their third generation sate... more INMARSAT has designed a GPS (L1) transponder that will be included in their third generation satellites. This transponder will broadcast a pseudo-GPS signal that can be used for navigation and also for disseminating integrity data and/or differential corrections for the GPS satellites. This INMARSAT Geostationary Overlay (IGO) service will be used to enhance the performance of the GPS navigation service for civil aviation and other users. The IGO service can also be used as a method of disseminating a precise time reference. Since the IGO signal is compatible with GPS, conventional GPS timing receivers can easily be modified to utilize the proposed service. Preliminary test results taken through the IGO satellite have demonstrated a timing accuracy of 10 nanoseconds.
Since 1967 the National Bureau of Standards has improved its traditional radio broadcast services... more Since 1967 the National Bureau of Standards has improved its traditional radio broadcast services from radio stations WWV, WWVH, and WWVB and introduced several new services to meet changing needs. The new services are described briefly, including the GOES satellite time code, the Frequency Measurement Service using Loran-C and WWVB, and the Global Time Service based on the CPS satellite common-view technique.
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Papers by David W. Allan