The GPS/NAVSTAR Navigation Satellite System will be
the mainstay of the NSS and must provide a global full
coverage system through the late 90’s and beyond. The
heart of the GPS Satellite System is an atomic clock
which maintains synchronous system time to generate
range and range-rate data for global users. Current GPS
satellite Cesium frequency standards on which GPS timing
depends, were designed in the early to mid 1980s. While
these clocks have performed admirably on orbit, a new
design for a Cesium frequency standard utilizing modem
digital and RF technology offers significant advantages
with respect to stability and environmental insensitivity,
which taken together eliminate any perceived need for a
clock temperature controller in the satellite. In this paper
we describe a Cesium design that takes advantage of the
fact that the Cesium standard is the only primary standard.
The approach builds on the Cesium beam tube design and
flight worthyness of our previous successful GPS Cesium
standards married to Hewlett Packard’s world recognized
5071A Cesium standard electronic and software design to
achieve an optimized clock for GPS space application.
The design provides advancements in the areas of
performance optimization, stability, accuracy, extended
useful life, reduction in environmental sensitivities, state
of health diagnostics and clock system simplicity. In
particular, the new design provides: immunity to
environmental and aging effects, e.g., temperature, loop
gain variations, magnetic and RF stability, and radiation
response. Thus, the requirement for auxiliary
environmental temperature control on the GPS satellite can
be eliminated. Telemetry monitors that can indicate ciock
health and longevity, supplemented with additional data
available to telemetry on command will allow engineering
specialists to interpret trends and predict future
performance and could even permit on orbit optimization
of all performance determining operating parameters.
The theory of rf SQUID operation, including the effects of fluctuations, is discussed. The theory is in agreement with the large signal and small signal behavior of real devices under optimum conditions. The noise levels of many existing rf SQUID systems are substantially above those predicted on the basis of amplifier noise temperatures. All rf SQUIDs seem to exhibit excess noise of unknown origin at low input frequencies. The limitations of devices and future directions of development are considered.
The design of a simple self-balancing AC resistance bridge with digital readout is discussed. The instrument operates with an extremely small power dissipation in the unknown, and is particularly suitable for use with carbon resistance thermometers at low temperatures. An accuracy exceeding 0.5% is obtained over the range 20 Omega to 2M Omega .
Abstract : The 40.5 GHz hyperfine resonance of 199 Hg ions stored in an rf quadrupole trap has shown to have a very small fractional linewidth, suggesting its use as a high-precision frequency standard. The most significant offset in such a standard would be the second-order Doppler shift resulting from the motion of the stored ions. We have recently analyzed the situation in which the secular motion is cooled to a temperature of about 300 K by the presence of a light background gas at low pressure, resulting in an ion cloud whose density is almost completely determined by the balance of pseudopotential and space-charge forces. Under these circumstances we have found that the second-order Doppler shift can be calculated accurately from the trapping parameters, the temperature, and the total number and mass of the stored ions.
The spectral density of the low-frequency open-circuit intrinsic input noise of the hysteretic rf-biased SQUID is calculated. The noise level, which affects the optimized power sensitivity of the SQUID, is found to be strongly model dependent and thus also constitutes an interesting probe of device physics. The results of the calculation are significantly different from previous predictions. A direct measurement of the total input noise of a typical rf-biased SQUID system is reported. The results, which are consistent our calculation, correspond to a system noise temperature of 3.1±0.4 mK at a frequency of 1 kHz. This value is consistent with the Manley-Rowe equations and with previous experimentally determined upper limits.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)