Before GPS, there was LORAN (http://en.wikipedia.org/wiki/LORAN-C), which operates on similar principles but uses radio towers instead of satellites. Before LORAN there was DECCA (http://en.wikipedia.org/wiki/Decca_Navigator_System), which works pretty much the same as LORAN but operates on slightly different radio wave properties. It was used during WWII.
For a self-contained device that can tell your your current location, there are various inertial navigation systems with varying levels of accuracy. The MK 39 MOD 3A Ring Laser Gyro (http://www.sperrymarine.northropgrumman.com/products/Inertial_Navigation/mk39) is an example of such a device. It's about the size of a small oven, though, not exactly pocket-portable. The more accurate models are closer to refrigerator-size. If I wanted a GPS-like device on my time-travelling vessel, though, something like a ring laser gyro inertial nav system is what I'd want. As long as your time machine transports you only in time and not spatially (relative to the earth) then it should be pretty accurate.
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timeasmymeasure
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Date: 2009-06-18 04:00 pm (UTC)For a self-contained device that can tell your your current location, there are various inertial navigation systems with varying levels of accuracy. The MK 39 MOD 3A Ring Laser Gyro (http://www.sperrymarine.northropgrumman.com/products/Inertial_Navigation/mk39) is an example of such a device. It's about the size of a small oven, though, not exactly pocket-portable. The more accurate models are closer to refrigerator-size. If I wanted a GPS-like device on my time-travelling vessel, though, something like a ring laser gyro inertial nav system is what I'd want. As long as your time machine transports you only in time and not spatially (relative to the earth) then it should be pretty accurate.