GLOBAL ATOMIC CLOCK SYNC AND GPS CALIBRATION
First developments and implementations of global high-accuracy two-way satellite-based time-synchronization facilities known as Two-way Satellite Time Transfer which provides the most accurate synchronization between widely separated clocks as the primary means for the maintenance of Universal Coordinated Time (UTC) and GPS-time.
Two-way satellite time transfer, or TWSTT, is the most accurate and definitive global method of coordinating primary standards laboratories that maintain UTC. Howe pioneered TWSTT in the mid-1980’s, being responsible for managing, developing, procuring, and deploying the first global method that could compare primary Cs atomic standards. Howe’s most refined methods first appeared in a 1987 IEEE publication entitled “Progress toward One Nanosecond Two-Way Time Transfer Accuracy Using Ku-Band Geostationary Satellites”. To assure best timing through satellite transponders, he led development of an ultra-wideband (UWB) direct-sequence spread-spectrum low-noise modem for satellite timing. In connection with low-noise microwave equipment and Ku-band uplink dish antennas and using international geostationary communications satellites in a global network, the synchronization proved to be far superior to GPS time synchronization. Howe’s TWSTT had significantly reduced sensitivity to satellite motion, interference, radio propagation effects, and other transmission anomalies so that world-wide accuracy achieved an astounding one nanosecond level and precision of +0.1 nanoseconds (or +100 picoseconds). This was 10,000 times better than GPS at that time and so became a method of calibrating GPS time transfer. TWSTT still remains the state-of-the-art and 20-times better than GPS time transfer today. Noting that Howe greatly respects IEEE editorial rigor, the Director of the National Research Council Time Division of Canada was one anonymous reviewer of Howe’s groundbreaking work. He later acknowledged Howe’s papers as the best definitive description of the future of coordinated time synchronization among laboratories participating in the realization of UTC. Howe’s original technical scheme without changes was adopted by the International Telecommunication Union (ITU) Standards Working Group 7 in Geneva in 1994 as the primary time transfer method among the approximate two hundred National Measurement Institutes worldwide. For his pioneering, scientific work, Howe was given the highest award by the U. S. Department of Commerce, the Gold Medal.
Howe’s highly precise satellite timing technique incorporates UWB modulation to and from users through each satellite in two-way communications. Because of the transcontinental coverage using high radio frequency satellites (12 to 14 Gigahertz, or Ku-band), small earth transceivers called “satellite earth terminals” are used. To use any geostationary satellite, the original NIST setup had a large, elevated, high-gain 6.1-meter master dish antenna (shown) with a computer-controlled positioner, two small 1.8-meter remote earth terminals, and one mobile uplink station which was used for calibration of remote sites far from NIST.
Howe’s highly precise satellite timing technique incorporates UWB modulation to and from users through each satellite in two-way communications. To use any geostationary satellite, the original NIST setup had a large, elevated, high-gain 6.1-meter master dish antenna (shown) with a computer-controlled positioner.
For perspective, 0.1 nanosecond is the time it takes a radio signal to travel only one inch in free space. Thus, remarkably, intercontinental-separated Cs atomic standards with Howe’s TWSTT could detect inch-level length fluctuations tens of thousands of miles away from the NIST Boulder Labs for the first time.
PULSED RAMSEY ATOMIC VELOCITY SELECTION
Original research, development and implementation of an atomic velocity selector for an automated accuracy evaluation system of Cs atomic-beam standards.
The Cs beam atomic standard has been the workhorse of systems needing accurate time for over 50 years. This is primarily because (1) these standards are available commercially and (2) the SI second is defined by Cs RF (magnetic-hyperfine) quantum transition at 9.2 GHz. All beam standards use two-interaction Ramsey atom interferometry to narrow the atomic linewidth. The most notable accuracy errors in such a scheme are due to a combination of the second-order Doppler shift and cavity phase shift between the two interactions, usually difficult measurements. Calculations of the Doppler shift are made based on the velocity distribution profile of the atomic beam while the cavity phase shift is minimized by stringent dimensional accuracy of the microwave RF interrogation of the beam. Howe was the first to build an automated system for creating velocity-selected atomic beams. He was first to measure the entire beam velocity distribution using a technique in which the RF is pulsed and the interval between these pulses corresponds to completion of a Cs quantum transition only at a specific atomic velocity. See “VELOCITY DISTRIBUTION MEASUREMENTS OF CESIUM BEAM TUBES” and “PRELIMINARY RESEARCH AND DEVELOPMENT OF THE CESIUM TUBE ACCURACY EVALUATION SYSTEM”. Norman Ramsey was closely interested in Howe’s work which later opened up several developments related to quantum superposition states, more reductions in Ramsey interferometric inaccuracies, atom interferometry for acceleration sensing, and stability improvements in commercially available standards.
COMPACT HYDROGEN MASER
Development of a hydrogen maser physics package that is eight-times smaller and more rugged than conventional masers.
The purpose of this project was to a develop small, rugged, high-stability atomic clock for potential use in the next generation of clocks in outer-space probes, future military work related to the Milstar program, and civilian navigation needs, notably the Global Positioning System. Several hydrogen masers provide the best ”flywheel” oscillators in NIST’s realization of UTC (universal coordinated time) since NIST’s very accurate primary Cs frequency standards (F-1 and F-2) are not run continuously as a clock and are not as stable as the hydrogen masers. This fundamental methodology was ideal for the constellation of GPS satellites and the ground-control segment. Six compact masers were successfully built, tested, and delivered to the Naval Research Lab in Wash, D.C., and to NASA JPL Pasadena, CA. With Howe’s focused development of the so-called physics package, these compact masers performed as well as their full-size counterparts, a major achievement of the project. See “A Compact Hydrogen Maser with Exceptional Long-Term Stability”. The maser design was subsequently transferred to the private sector (Ball Aerospace) for custom applications to NASA and for integration into various other product lines.
Original research, development and implementation of digital modulation in commercial broadcast networks resulting in hi-accuracy <10ns time distribution and first expansion to digital communications.
The four-decade transition from analog commercial communications and broadcast standards to high-capacity, high-definition digital transmissions began in NIST’s Time Dissemination Research Section. Developers Dick Davis and David Howe designed, built, and tested the first-ever digital modulation sequences (ones and zeroes in BCD) into the unused time-interval between frames of moving images. Called a broadcast time code, synchronous code demodulation was possible to an unrivaled precision of 10 ns in 0.3 seconds, coast-to-coast, by using ABC, NBC, or CBS network feeds that were rebroadcast to user’s television receivers through local affiliates. He focused his research on improving and expanding the use of wideband broadcast resources as a means of inexpensive timing to transcontinental U.S. laboratories while Davis pursued expansion of closed captioning. His first results were published in Proc IEEE, May 1972, “Results of Active Line-1 TV Timing”. A full paper was published later in IEEE Trans IM in August, “Nationwide Precise Time and Frequency Distribution Utilizing an Active Code Within Network Television Broadcasts”.
AUTOMATIC VEHICLE LOCATION
Original research, development and implementation of Cs atomic clocks in commercial TV and FM broadcast stations that enabled sub-3m accuracy in car location.
Howe was the first to use atomic clocks to time high-power commercial “signals of opportunity” for unprecedented precision in radio location-based services in signal coverage, also known as PNT (position, navigation, timing). His methods and results using TV and FM stations were firsts, published in Navigation: Journal of the Institute of Navigation, 1974, “The Feasibility of Applying the Active TvTime System to Automatic Vehicle Location” and IEEE Trans on Broadcasting, March, 1974, “Precise Frequency Dissemination Using the 19-kHz Pilot Tone on Stereo FM Radio Stations”. Digital modulation did not interfere with the analog modulation. This property led him toward many subsequent publications to demonstrate that precise digital timing in wideband RF and optical links can greatly expand PNT and channel capacity using multiple orthogonal codes and modes. GPS satellites carry atomic clocks and operate as Howe’s original plan. His valuable, ground-breaking early results created new, game-changing roadmaps to replace older satellite-based navigation methods. He now has been involved in directing the research on hi-accuracy alternate-GPS methods, timing and radio location based services for 4G and 5G LTE cell phone challenges, competitions, and federal standards in NIST’s Public Safety Communications Research (PSCR) in the Advanced Communications Technology Laboratory in Boulder, CO.
Development of new classes of time-domain frequency-stability variances with substantially shorter testing time of atomic clocks and as much as a six-times confidence improvement compared to the Allan variance.
The most significant impairment to assuring UTC(NIST) is the accuracy of or confidence in each clock in the Atomic Time scale. Howe developed two new classes of variances called Total variance and later Theo variance which specify clock instability with unprecedented confidence, particularly for long averaging times. The variances are also tolerant to data gaps compared to any statistic including the sample Allan variance (Avar, and its square-root, Allan deviation, or Adev) which has expectedly poor long-term confidence and which requires there be no gaps in the data in order to compute without significant bias distortions. Howe’s advancement for national standards institutes, system developers, manufacturers and users gave widespread acceptance of the new variances virtually immediately.
The advantage of Howe’s new Theo variance is in estimating frequency stability with unprecedented confidence at long term τ that extends to 50 % longer than the definition of Avar. Rather than standardize a different, better variance, Howe came up with a clever scheme for an Allan-compatible, bias-removed version called TheoBR. The significance of it is a novel method of determining bias that does not require any a priori assumption of long-term type of random noise, normally determined by integer power-law functions of Avar(τ). TheoBR works with any noise type, i.e., any stochastic noise, even those with mixed, non-integer power-law dependence.
TheoH deviation (‘H’ to indicate a hybrid combination of TheoBR and Allan deviations) is the Allan deviation in short term and reverts to the TheoBR deviation in long term. Even in the presence of difficult to interpret, non-integer-power-law and mixed noise types, Howe’s comprehensive statistical approach is as effective and less cumbersome than past, multi-analysis approaches. TheoH requires no outside knowledge or human judgment of the data being analyzed. The substantially gained properties of TheoH are:
- The benefit of one statistic that yields frequency stability at large τ, 50 % beyond the longest possible τ using the Allan-deviation definition,
- Direct usability for comparisons with older Adev plots, since Allan-bias is automatically removed,
- Significantly narrower confidence intervals,
- Long-term frequency stability obtainable in one-third less time,
- ITU standard.
Item (4) is the principle benefit to industry and measurement institutes. TheoH substantially lowers clock-testing time and, hence, cost because, for example, a two-month clock stability can be obtained with three months of data, rather than the four months of data needed using Adev.
Howe’s statistical tools have extensive use in commercial software, user handbooks, and standardized clock statistics. They have been recently endorsed by IEEE STD 1139 committees that recommend standard methods for quantifying random instabilities for calibration purposes as the best analytics for high-confidence, dynamic characterizations of state-of-the-art atomic clocks and oscillators. US and Italian proposals are in place to formally add these statistics by the 2016 International Telecommunication Union (ITU) Standards Working Group 7 in Geneva.
D. A. Howe, “TheoH: a hybrid, high-confidence statistic that improves on the Allan deviation,” Metrologia, 43, pp 322-331, 2006.