Precision time measurement

Precision Time Measurement

Why & How?

Early Precision Time Measurement

• Late 1630s – Galileo Galileiobserved that a pendulum took the same time to swing through a wide arc as a narrow arc.

• Mid 1720s – John Harrison improved accuracy of pendulum clock to 1/5 second per day.

• Pendulum clocks must be stationary, thus no precision timing at sea.

Navigation at SeaWhere Am I?

• Latitude – how far north or south of the equator? (easy)

• Longitude – how far east or west of the prime meridian? (hard)

• 1707 – Isles of Scilly –British fleet lost (1400+)

Methods of Determining Position

• Triangulation – Latitude (north/south)

– Sextant used to measure vertical angle between horizon and sun at highest point (or Polaris (a.k.a. the “north star”) at night)

Methods of Determining Position

• The “Longitude Problem”

– East/West location determination much harder than latitude. Longitude determination required knowing precise time at Greenwich (Prime Meridian = zero longitude).

– Voyages could last months.

– John Harrison worked from 1735 to 1772 (37 years) building 5 “sea clocks” (chronometers) for precision timing at sea. { 1/3 second per day }

Enter: Quartz

• 1880 - Piezoelectricity first demonstrated by brothers Pierre & Jacques Curie.

• When piezoelectric material is subjected to mechanical stress, (e.g. bending) it creates electrical charge.

• Inverse also true (apply charge to bend).

• “Feedback” employed to sustain resonance at very stable frequency & low power.

A quartz crystal “tuning fork” resonator in a wristwatch resonates with very stable

frequency (32,768 cycles per second. A counter is used to divide down to one pulse per

second.

Typical drift of quartz watch: ~ 1 second per day.

The “Gold Standard” of Precision Time Measurement: Atomic Clock

• The “cosmic oscillator” improved upon the quartz oscillator.– changes in the energy levels of atoms release

electromagnetic radiation of very specific frequencies.

– atoms don’t wear out, change their properties over time, or have small differences due to manufacturing imprecision.

• 1948 - First “atomic clock” built by Harold Lyons at National Bureau of Standards (NBS).

Atomic Clock Precision Proves Theories of Relativity

• Albert Einstein predicted Special Relativity (kinematic time dilation) in 1905, and General Relativity (gravitational time dilation) in 1915.– Kinematic: Time passes relatively

slower at high speed.

– Gravitational: Time passes slower in stronger gravitational fields.

• 1971 - Hafele-Keating Experiment– Used cesium clocks to prove both

theories

Precise Time Yields Position (again)

• Trilateration – determining locations of points by measurement of distances, using the geometry of circles, spheres or triangles.

Cell phone trilateration –• Measure transit time for radio signal between tower and phone. (Time yields distance).• Known distance to tower provides circle of possible locations.• Determine distance to more towers for more circles & find intersections of circles.

GPS – Precision Timing for All

• Cesium 133 atomic clockson-board each GPS satellite for extremely precise time synchronization.

• Constellation of satellites in circular orbits 20,000 km above earth surface.

• Radio signals broadcast from satellites include the exact time at which the signal was transmitted, as well as the ephemeris data for the satellite.

GPS Precision Navigation & Timing

• Exact location of satellite is calculated from ephemeris data included in the signal.

• Just like cellular towers, except in 3-D –replace circles of possible positions with spheres.

• 4th satellite provides receiver clock calibration.

• More GPS receivers in the world used for time synchronization than for determining position.