The Hydrogen Maser Clock Project

The atomic hydrogen maser oscillator (H-maser) provides fractional frequency stability of approximately 1 part in 1016 for time intervals of a few hours to a day. The maser's high frequency stability makes it ideal for a variety of space applications including very long baseline interferometry (VLBI) from space, precision measurements of relativistic and gravitational effects and worldwide time and frequency transfer. Further, being a high stability active oscillator, the H-maser can serve as the required local, or "flywheel," oscillator signal for cooled cesium atom "fountain" frequency discriminators operating in the near zero "g" of a space platform. Frequency accuracy of 1 part in 1016 and a comparable level of frequency stability for one day intervals would be available from such a maser-frequency discriminator clock. By use of pulsed laser time transfer and high stability Doppler-canceled frequency transfer techniques, and making the appropriate corrections for effects of relativistic gravitation, such a space-borne clock system would provide the world's metrology laboratories the most precise representation of the Systeme International second of time. Global time synchronization at the picosecond level could also be achieved.

The Hydrogen Maser Clock (HMC) program was a NASA/SAO project to design, build and operate in space, a high-stability hydrogen maser atomic clock. This clock had its origin in the successful Gravity Probe A (GP-A) experiment flown in 1976, whose goal was to test Einstein's general theory of relativity. For GP-A, SAO developed a space-qualified hydrogen maser that was carried to an altitude of 10,000 km by a Scout D rocket in a two-hour sub-orbital flight. The experiment verified Einstein's predicted Gravitational Red Shift principle with a precision of 70 parts per million. In the years following GP-A, with funding from the U. S. Naval Research Laboratory and NASA, the SAO H-Maser Laboratory designed and tested subsystems for an improved hydrogen maser for long-term operation in space. These subsystems were the building blocks for the HMC maser.

The aim of the HMC experiment was to compare time kept by the clock in space with earth-based time scales by means of high precision pulsed laser-ranging techniques. A conceptual view of the program's components is shown below.

The original HMC contract, which began in 1992, called for the maser system to be flown in low earth orbit on the second flight of the European Space Agency's EURECA (EUropean Reusable CArrier) spacecraft. When ESA decided not to re-fly EURECA after its successful first mission, NASA transferred HMC to fly on the Russian space station Mir and be installed in late 1997. The mounting structure, power and communications systems were redesigned for Mir. All HMC hardware was designed and built to satisfy Space Shuttle safety requirements and space flight reliability criteria.

HMC involved new concepts of maser oscillator design and of control electronics. High precision time transfer was to be done by an event timer -- a sophisticated electronic stop-watch -- capable of timing the arrival of laser pulses with the unprecedented precision of 10 ps. This event timer was developed and built for HMC by the Los Alamos National Laboratory. SAO developed high precision temperature control systems to maintain the maser's internal temperature constant to within 100 micro-Kelvins. Testing of the flight maser began in March of 1996.

Budgetary problems and scheduling conflicts with other NASA experiments requiring Mir's facilities caused NASA to terminate HMC in February of 1997. All the HMC flight hardware and programmatic assets were transferred to SAO which has continued operation and testing of the maser. Continuous operation the HMC maser oscillator in SAO's vacuum test tank began in August 1996. The maser ran continously until it was shut down on 20 January 1999 to be refitted and tested for a new project with support from NASA.

In 1998 a project to develop a Primary Atomic Reference Clock in Space (PARCS) was begun with NASA support at the National Institute of Standards and Technology (NIST, in Boulder, Colorado). This device is an extremely accurate frequency discriminator that will control the output signal from the hydrogen maser oscillator. This frequency discriminator uses slowly moving groups of cesium atoms floating in the weightlessness of space that can be interrogated for very long time to obtain a very accurate measurement of the hyperfine separation by which the SI second is defined. For this process to work over the long times involved, the interrogating signal must have extremely high frequency stability as the line width of the 9,192,631,770 Hz transition frequency is in the order of tenths of a Hz. The hydrogen maser is capable of providing a signal of the necessary stability over these intervals. The PARCS frequency discriminator will provide corrections to the maser signal so that over one day both its frequency stability and frequency accuracy will be at the 1 part in 1016 level. This will be the most accurate manifestation of the SI second of time.

Work is now in progress to improve the short term (10 to 10,000 sec) stability of the space maser by equipping it with a preamplifier with a lower noise figure and better matched to the output signal from the maser resonator. Stability tests will be done using a pair of reference masers to obtain a evaluation of the performance specifically of the space maser rather than the relative stability of the space maser and its reference.

The HMC maser is now a thoroughly tested system that can be adapted to a variety of spacecraft. Use of the HMC maser in a new mission would require only designing and building a suitable mounting structure and re-configuring (without changing the circuit design) some control electronics in a format suitable for the new mission.

Some applications for a space-qualified hydrogen maser clock are:

Spacecraft tracking


Time transfer

Space-based VLBI (Very Long Baseline Interferometry)

Gravitation and relativity experiments

Some papers that describe the SAO/NASA 1976 Gravity Probe-A experiment are:

"A test of the equivalence principle using a space-borne clock"
R.F.C. Vessot and M.W. Levine, General Relativity and Gravitation, Vol. 10, No. 3, pp. 181­204.

"Tests of relativistic gravitation with a space­borne hydrogen maser"
R.F.C. Vessot, M.W. Levine, E.M. Mattison, E.L. Blomberg, T.E. Hoffman, G.U. Nystrom, B.F. Farrell, R. Decher P.B. Eby, C.R. Baugher, J.W. Watts, D.L. Teuber and F.D. Wills, Physical Review Letters, Vol. 45, Dec. 1980, pp. 2081­2084.

"Tests of gravitation and relativity" invited paper
R.F.C. Vessot, Contemporary Physics, Vol. 25, pp. 355-380.

Some papers that describe the HMC experiment are:
"High Precision Time Transfer to Test an H-Maser on Mir" invited paper
R. F. C. Vessot, E.M. Mattison, G. U. Nystrom, L. M. Coyle, David Boyd, and T. E. Hoffman,
in Proceedings of the 5th Symposium on Frequency Standards and Metrology (Woods Hole, MA, 16-20 Oct 1995), pp 39-45
or download: 212K Postscript file / 44K PDF file
"Precise Temperature Control for Precision Frequency Standards"
E.M. Mattison, D. Boyd, L. M. Coyle and R. F. C. Vessot,
in Proceedings of the 5th Symposium on Frequency Standards and Metrology (Woods Hole, MA, 16-20 Oct 1995), pp. 389-391
or download: 144K Postscript file / 28K PDF file
"A 10 ps Event Timer for Precise Time Transfer in Space"
E.M. Mattison, D. Boyd, L.M. Coyle, R.C. Smith and R. F. C. Vessot,
in Proceedings of the 5th Symposium on Frequency Standards and Metrology (Woods Hole, MA, 16-20 Oct 1995), pp. 475-477
or download: 88K Postscript file / 36K PDF file

Find out about the HMC Project astrophysicists

For more information on HMC and the Hydrogen Maser Laboratory activities, you can contact Dr. Robert F.C. Vessot at or Dr. Edward M. Mattison at


The HMC Project

The Harvard-Smithsonian Center for Astrophysics

60 Garden Street

Cambridge, Massachusetts 02138

telephone (617) 495-7276


Links to related institutional sites:

The Walsworth Group
The Harvard-Smithsonian Center for Astrophysics
The Radio and Geoastronomy Division
The Smithsonian Astrophysical Observatory
The Harvard College Observatory
The Smithsonian Institution

last update 9.23.99 by Kristi Armstrong