AbstractA test of a Hydrogen Maser designed for long term operation in space is in preparation for installation on the Russian space station, Mir, in late 1997. The U.S. Space Shuttle will deliver the payload to Mir which will then be transferred from the shuttle cargo bay to the exterior of Mir by U.S. astronauts. Pulsed laser time transfer with a resolution of 10 picoseconds from the primary laser site at the NASA Godddard Space Flight Center at Greenbelt MD will be used to measure the H-Maser's frequency stability. Daily time comparisons made with a precision of better than 86 picoseconds will allow an assessment of the long term stability of the space maser at a level of better than 1 part in 1015. The arrival time of laser pulses will be recorded with a resolution of 10 picoseconds by an event timer on the HMC package and transmitted to earth. The pulses will be reflected back to the earth station and timed with similar resolution to allow removal of the transit time. To provide data for relativistic and gravitational frequency corrections, tracking of Mir with a precision of 1 meter in altitude and 1 mm sec-1 will be done with an on board GPS receiver located in the HMC package. We expect other laser sites in addition to the one at GSFC that have access to high precision time to participate in using this opportunity to demonstrate high precision world wide time transfer.
(i) Comparison of the maser's frequency relative to international time scales, using laser pulse timing from a satellite laser ranging station equipped with clocks and timing equipment. If possible, high precision time transfer tests with other laser timing stations will be conducted.
(ii) Observation of the maser's operation by adjusting the maser's functions and monitoring its environment and internal operating parameters. Recovery of monitoring and timing data will be performed using Mir's telemetry to earth stations in Russia; command will be done by having a Cosmonaut instigate pre-programmed operations with an onboard computer.
(iii) Determination of the effects of ambient space conditions that could affect the maser's long-term frequency performance.
Expended hydrogen is absorbed by two sorption cartridges that capture only hydrogen. Two small ion pumps with self-contained high voltage supplies, remove other outgassing products.
The cylindrical vacuum tank, made of titanium alloy, contains a cylindrical TE011 mode microwave resonant cavity, within which is mounted a quartz hydrogen storage bulb. The cylinder and end plates of the resonator are made of internally silvered Cer-Vit, a mechanically stable glass-ceramic material having a very low thermal coefficient of expansion. A double Belleville spring clamps the cavity endplates to the cylinder with an axial force of approximately 480 lbs. It is adjusted so that the compressive force is nominally independent of the length of the holddown can and thus the cavity's resonance frequency is approximately independent of the thermal expansion of the hold-down can. The cavity mounting baseplate is attached in cantilever at its center to the base of the vacuum tank for isolation from dimensional changes in the outer vacuum envelope.
The maser signal is picked up by a coupling loop at a level of approximately -100
The source of H2 is about 50 grams of lithium aluminum hydride contained in a heated stainless steel container whose temperature is controlled to maintain a constant hydrogen pressure within the container. Hydrogen flow into the maser is controlled by permeation through a heated palladium silver diaphragm, sensing the pressure in the dissociator by means of a thermistor Pirani gauge, and regulating the diaphragm temperature.
Molecular hydrogen at a pressure of approximately 10-1 torr is led to a cylindrical glass bulb (6 cm long x 3 cm diameter) mounted within the vacuum chamber. A plasma discharge in the bulb is maintained by about 4 watts of 75 MHz RF power to dissociate molecular hydrogen. Atomic hydrogen is collimated into a beam of about 1014 H atoms per second into the state selection magnet that focuses atoms in the upper hyperfine quantum state into the storage bulb.
Frequency shifts from variation of the magnetic field within the maser storage bulb are controlled by 4 layers of passive shielding and by active field compensation. Leakage through the first shield is sensed by a flux-gate magnetometer and nulled by a compensating coil wound on the next innermost magnetic shield. This combination provides a shielding factor, S = dBext/dBint > 2 x 106, for external field variations of ± 0.5 Gauss. A two-layer flexible printed circuit solenoid closely fitted to the inside of the innermost shield produces a 0.5 milliGauss uniform axial magnetic field within the cavity. With the available shielding factor, we can limit the fractional frequency effects of external field variations to less than 1 part in 1015.
The temperature dependence of the resonance frequency of the cavity bulb combination is about -800
Laser pulses arriving at the spacecraft will be sensed and their arrival times will be recorded in terms of the time scale maintained by the space maser. Pulses will be reflected back to the ground station by the corner reflectors and their round-trip time interval will be recorded at the earth stations with a similar event timer. A complete account of the retroreflector detector event timer system is the topic of a separate paper.2
Each retroreflector array contains twenty solid fused-quartz cube corners mounted in an aluminum housing along with an array of optical fibers that receive laser light and transmit it, via a fiber bundle a few centimeters long, to the optical filter and photodetector and event timer circuit enclosed in the same housing. The photo detector's output pulse goes to a constant-fraction discriminator and then to the event timer and recorded with a resolution of 10 picoseconds. Because the expected pulse length is considerably longer than the desired measurement precision of 10 ps, and the pulse height can vary from pulse to pulse, the constant-fraction discriminator circuit is needed to produce a logic pulse at a time that is largely independent of the laser pulse's amplitude.
Each event timer operates from a 100 MHz maser signal. The event timer consists of standard digital gates and registers, and a hybrid analog time interpolation circuit. The interpolator charges a capacitor by a constant-current source triggered by the incoming pulse, and discharges it at a slower rate through a second constant-current circuit. The discharge time is measured in terms of 10 ns clock periods, with the ratio of discharge-to-charge currents providing a 1000-to-1 "expansion" of time, thus yielding the 10 ps measurement resolution. Timing data from the GPS will also be recorded. Timing data will be sent to the HMC dedicated experiment processor for formatting with other data transmitted to earth.
Standard digital circuitry provides the interface between the HMC and the spacecraft's telemetry and telecommand functions. The dedicated experiment computer buffers signals between the HMC and the spacecraft, receiving telecommands and sending data to the IBM 750C. Pre-recorded command sequences will be stored in the laptop for execution by keystroke entry by a Russian Cosmonaut. These commands include the maser power sequence, cavity resonator tuning, RF dissociator operating level, automated magnetic field measurement (Zeeman sweep) and functions for various diagnostic programs.
The entire experiment will consume on average approximately 156 to 188 watts of 28 VDC power, depending on thermal conditions owing to the orientation of Mir as it orbits the earth.
The HMC contract is supported by NASA's George C. Marshall Space Flight Center, Huntsville, Alabama.
"Precise Temperature Control for Precision Frequency Standards"
Hydrogen Maser Clock Project home page
