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June 02, 2009

Chip Scale Atomic Clocks, GPS Towers



There is some concern that if there are delays in launching replacement GPS satellites that there could a degradation in the availability and accuracy of GPS (global positioning).

There are currently 31 operational GPS satellites.

According to the GAO report, if the Air Force keeps to its current launch schedule, the chance that we'll still have 24 working GPS satellites dips below 95 percent next year, and falls to about a 80 percent in the 2011-2012 timeframe. Those might sound like good odds, but consider this: The GAO warns that if the Air Force falls behind on its planned deployment of next-generation GPS III satellites (the first is slated to go up in 2014) by just two years (and there's a "considerable risk" that the Air Force will fall behind, according to the GAO's report), the delay could leave us with a mere 10 percent chance of 24 working GPS satellites by 2017.


If needed European or Russian launchers could be commissioned if there is a problem with launchers for the US Air Force and the satellites do start failing. There is ZERO chance that we will fall below 24 working satellites for any more than one year. The reason being is if the satellites start failing fast enough then the US military will throw money and the problem and get the launchers and some more version II satellites up to maintain performance for the US military.

For full accuracy with fewer satellites or enhanced accuracy, we will have chip scale atomic clocks (or larger but still affordable atomic clocks). Military receivers and civilians that have atomic clock enabled receivers could have 24 satellite accuracy even with 21 or possibly fewer satellites. Also, regional GPS could be broadcast from towers.

TV Towers for GPS alternative




Chip scale atomic clocks are being developed by DARPA


The goal of the Chip-Scale Atomic Clock program is to create ultra-miniaturized, low-power, atomic time and frequency reference units that will achieve, relative to present approaches:

* >200X reduction in size (from 230 cm3 to <1 cm3),
* >300X reduction in power consumption (from 10 W to <30 mW), and
* Matching performance (1 X 10^-11 accuracy ⇒ 1 µ/day). Example of future payoff is wristwatch size high-security UHF communicator / jam-resistant GPS receiver.








Benefits of Atomic Clock Augmentation for GPS

Background
Most GPS receivers use inexpensive quartz oscillators as a time reference. The receiver has a clock bias from GPS time but this bias is removed by treating it as an unknown when solving for position. In effect, the receiver clock is continually calibrated to GPS time. If a highly stable reference were used, however, the receiver time could be based on this clock without solving for a bias. "Clock coasting" (with no clock model), as it is referred to, requires an atomic clock with superior long term stability. The analysis in this chapter shows the
potential for clock coasting to improve vertical positioning accuracy.

4.2 Atomic Clock Benefits
Currently, GPS receivers need four measurements to solve for three-dimensional position and time. The receiver clocks are not synchronized with GPS time which necessitates the fourth measurement. Using an atomic clock synchronized to GPS time in the receiver would eliminate the need for one of the measurements. Clock coasting has been shown to provide a navigation solution during periods which otherwise might be declared GPS outages. Misra has proposed a clock model in order to make the receiver clock available as a measurement continuously. Although five or more satellites are visible at any time with the current 24-satellite constellation, availability can be compromised by satellite failures. Therefore, the negative impact of satellite failures could be reduced by atomic clock augmentation. Redundant oscillators could be used in the receiver to lower the probability of a receiver clock failure. Thus, availability of GPS positioning would be increased. van Graas has noted that adding an atomic clock improves availability more than adding three GPS satellites or a geostationary satellite. Also, a perfect clock helps more than including the altimeter as a measurement.


FURTHER READING
Strontium clock accurate to 1 second over 200 million years

Micromagic Clock: Microwave Clock Based on Atoms in an Engineered Optical Lattice

A new class of atomic microwave clocks is proposed based on the hyperfine transitions in the ground state of aluminum or gallium atoms trapped in optical lattices. For such elements magic wavelengths exist at which both levels of the hyperfine doublet are shifted at the same rate by the lattice laser field, canceling its effect on the clock transition. A similar mechanism for the magic wavelengths may work in microwave hyperfine transitions in other atoms which have the fine-structure multiplets in the ground state.


Andrei Derevianko and Kyle Beloy of the University of Nevada in Reno and colleagues have come up with the idea of trapping the atoms in place using lasers. This means their energy states could be monitored in an area only a few micrometres across, potentially leading to more accurate measurements. This is difficult to get right, though, because the lasers distort an atom's energy levels in a complex way, making it impossible to define a jump that equates to a second.

Derevianko's team overcome this problem by finding a laser frequency that alters both hyperfine states by exactly the same amount - a trick that works in aluminium and gallium but not as well in caesium (www.arxiv.org/abs/0808.2821). "Then, the energy difference between the levels is the same as if the atoms are in vacuum," says Derevianko.

Using this method, the team has calculated the second to be 1506 million cycles of microwaves for aluminium-27 and 2678 million cycles for gallium-69.

Although the atoms can be trapped in an area only a few micrometres across, the lasers, and cooling and computing equipment will add to the bulk. Nevertheless, the team say the clocks may be portable and could be used in space-based experiments that require extremely accurate timekeeping, such as those for detecting gravitational waves or for testing Einstein's theories.

Tom Heavner, who works on fountain clocks at NIST, describes the proposal as forward-thinking and original. "It is a really clever way to meld together the old-style clocks with new laser technology," he says.


A Manufacturable Chip-Scale Atomic Clock

Several factors are converging to enable atomic clocks to be manufactured with very small dimensions and run at low operating power. MOEMS technology, high-speed vcsels, microelectronics, wafer-scale packaging, and the all-optical CPT method of exciting atomic transitions are key ingredients in the quest to make precision time-keeping devices with chip-scale dimensions. In this paper we report on the design and process that enable an atomic clock to be made with a total volume of 1.7 cm3, a total power budget of 57 mWatts, and an Allan Deviation at 1 hour of 5E-12.


Current methods for achieving GPS Level navigation and location accuracy are with things like using robotic vision algorithms and coupled MEMS-based dead-reckoning modules.

The vast majority of these efforts have focused on the use of inexpensive MEMS-based dead reckoning units to provide position, navigation, and timing (PNT) information. Unfortunately, these approaches suffer from errors in estimating angular rotation and accrue PNT errors in a linear fashion. Small, low-cost implementations are limited to providing short duration benefit before the error accumulations render them useless. Magnetically hostile environments such as light industrial buildings further degrade PNT effectiveness. Attempts to provide useful MEMS-based error correction sources in an acceptable size, weight, and power consumption tend to be limited to techniques that are equally susceptible to distortion. Stride-based correction mechanisms require precise calibration to the individual and are immature when dealing with combat/first-responder body poses.

Rockwell Collins has addressed these issues by challenging the fundamental assumption that MEMS aiding will provide an acceptable solution. Instead, robotic vision algorithm-based navigation is aided by the MEMS. Current robotic vision algorithms such as Simultaneous Location and Mapping (SLAM) are through put intensive and are not presently practical for the dismounted user. The use of the MEMS provides excellent instantaneous sensor pointing information that reduces the SLAM processing requirements significantly. The use of Feature Constellation Tracking (FCT) algorithm improvements further reduce the processing requirements by allowing intelligent thinning of the features tracked. This approach has allowed the Ghostwalker IR&D project to demonstrate accurate GPS-denied indoor and outdoor navigation without prior knowledge of features or telltale emissions.


DARPA is also funding inertial navigation system development

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