1984 TO 1994
In 1984, a recently installed chief of the Time and Frequency Division gave high priority to new, advanced systems of communicating the Division’s extremely high clock accuracy to industrial, commercial, and military users. He assigned me to lead a program to improve, update and modernize NIST time dissemination services.
Advanced Time Dissemination Research using Geostationary Communications Satellites
Existing short wave radio stations (WWV in Fort Collins, CO, and WWVH in Kauai, HI), long wave station WWVB, and GOES satellite services were very popular but lacking in precision and accuracy. Because of Howe’s intimate knowledge of the physics of timekeeping and metrology he was asked to start and lead two major project efforts: (1) build a time dissemination research test program to demonstrate and develop the best available precision and accuracy communications systems, and (2) develop a time broadcast service which equaled the performance of today’s atomic clocks and would continue into the next century.
His initialization of the R&D program represented the largest single effort by the NIST Division in terms of dollars, with total expenditures of over one million dollars in capital equipment alone. The new dissemination technique, called TWSTT (for two-way satellite time transfer), uses international and domestic stationary communications satellites in a global network. Superior to GPS in several aspects, the actual transmitted time information automatically compensates for all errors due to satellite motion, radio propagation effects, and other transmission delays. This novel, advanced system incorporates spread-spectrum modulation to and from users through each satellite in a two-way mode. Small earth transceivers called “satellite earth terminals” can be used because of the high radio frequency of transmission (12 to 14 Gigahertz, or Ku-band). The NIST facility which he constructed has a 6.1 meter satellite dish antenna with a computer-controlled positioner, two 1.8 meter earth terminals, and one mobile uplink system which is used for calibration of remote sites.
He has reported results of time transfer accuracy to the one nanosecond level and precision to 0.1 nanoseconds (or 100 picoseconds), 10,000 times better than the prevailing time transfer accuracy for that time. One nanosecond is the time it takes light to travel only one foot, so the system can easily detect the smallest fluctuations thousands of miles away from the NIST Boulder Labs. His 1987 IEEE publication entitled “Progress toward One Nanosecond Two-Way Time Transfer Accuracy Using Ku-Band Geostationary Satellites” was reviewed prior to publication by the Director of the National Research Council Time Division of Canada. He acknowledged this paper with a glowing recommendation as the best definitive description of the future of time synchronization among laboratories participating in the generation of UTC. The paper still stands as among the best outlines of ultimate limits on precision and accuracy of TWSTT to industry and future network communications in terms of fundamental calibration limits.
- High-Accuracy Time Transfer via Geostationary Satellites: Preliminary Results
- Progress Toward One-Ns Two-Way Time Transfer Accuracy Using Ku-Band Geostationary Satellites
- Spectrum Equipment Used for Two-Way Time Transfer
- Satellite Two-way Time Transfer: Fundamentals and Recent Progress
- Time Tracking Error in Direct-Sequence Spread-Spectrum Networks Due to Coherence Among Signals
Global Network Synchronization
Howe was assigned to develop a global time network in support of high-speed data communications to connect laboratories engaged in scientific and military experiments. This had the inherent requirement for the highest available timing accuracy. The experiments include (1) navigation of deep-space probes for Jet Propulsion Laboratories and NASA, (2) calibration and verification of the specifications of the military’s Global Positioning System, (3) coordination of primary frequency standards in Japan, England, Austria, Germany, France, and the U.S. (these are the locations of the principal standards of the free world), and (4) high-precision tracking of position and velocity of orbiting communications satellites. His program in TWSTT has created a large number of future R&D opportunities in industry and government. Several programs are being proposed which rely on recent TWSTT research. He received the Commerce Department’s highest achievement award in 1990, the Gold Medal, in a ceremony in Washington D.C. for his work. Other spin-off benefits are resulting now which include predicting fundamental synchronization limits in communications networks using signal standards such as the Sonet and wireless CDMA protocols. In addition, TWSTT is now used to certify timing specifications among master ground stations for the military’s GPS navigation system of satellites.
Spread Spectrum Modem Design and Development
To meet the specialized needs of a global satellite time synchronization project, Howe and another co-worker specified, designed, and developed a new spread spectrum modem from 1989 to 1991. Particular features of the modem were (1) that a variety of code types and chip rates could be programmed, (2) that the signal structure allowed for the exchange through the satellite of its own recovered time-residual data, (3) that the signal would have sufficiently low power density to exist alongside other types of signals which shared the same satellite transponder, (4) that precision would be 200 ps (picoseconds) or better, and (5) that the cost would be significantly below other commercially available modems. The design was successfully demonstrated in 1992 and led to contract development and procurement of other modems for laboratory and military use.