Date: June 30, 2009

Title: It’s All About the Reference Frames

Podcaster: Kenneth Johnston

Organization: United States Naval Observatory

Description: We’re all used to finding directions here on Earth. We orient ourselves based on our local experience of “up-down”, “left-right”, “front-back”. But how do you orient yourself in space? You need a reference frame, and the most precise reference frame we know is provided by the U.S. Naval Observatory.

Bio: Dr. Kenneth Johnston was born in New York City. He received a Bachelor’s degree in Electrical Engineering in 1964 from Manhattan College and a Ph.D. in Astronomy from Georgetown University in 1969.

While at Georgetown, he was a summer student at the Naval Research Laboratory (NRL), then a Postdoctoral Associate at NRL in the Radio Astronomy Branch of the Astronomy and Atmospheric Physics Division from 1969 through 1971. Dr. Johnston formally joined this branch in 1971 as a radio astronomer.

In 1980, Dr. Johnston became the Branch Head of the Radio and IR Astronomy Branch at NRL. He developed a program that applied interferometric techniques for high resolution imaging at optical and radio wavelengths.

In 1993, Dr. Johnston became the Scientific Director for the U.S. Naval Observatory. He is responsible for the scientific oversight of the precise time, time interval, and astrometry programs, developing the first imaging optical interferometer, the Navy Prototype Optical Interferometer (NPOI) located at Flagstaff, AZ.

He is at present developing the areas of radio and optical interferometry for astrometric and imaging applications with both ground and space instruments.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Professor Astronomy, a blog chronicling the day-to-day life and thoughts of a research astronomer, online at blog.professorastronomy.com. Professor Astronomy, wishing a very happy anniversary to Mrs. Astronomy.

Transcript:

Hello, I’m Dr. Ken Johnston, Scientific Director of the United States Naval Observatory in Washington, DC.

A character in a popular movie from the mid-1980’s once said “No matter where you go, there you are”. The ability to determine where “there” is describes a uniquely human trait that has allowed us to explore and inhabit not only nearly every niche of our home planet, but to also leave footprints on our nearest celestial neighbor and send robot emissaries to other worlds in our solar system.

Traveling almost anywhere raises a fundamental problem: in order to determine where you are going, you first have to know where you are, and in order to know where you are, you need to have a frame of reference. Since ancient times, navigators have used the stars to go places. We all know that the Sun rises in the East and sets in the West, which helps us when looking at a map and discerning the direction of North. Early navigators used the altitudes of stars to travel along parallels of latitude to reach islands over vast ocean regions such as the Pacific. Shortly after Galileo’s discovery of the moons of Jupiter, astronomers looked into determining longitude using the positions of Jupiter’s moons. With John Harrison’s development of the chronometer in the 18th century, the stars and a chronometer determined positions. Later, in the 20th century, low frequency radio wave broadcasts from multiple locations were used for coastal navigation. Today the Global Positioning System, or GPS, is used to pinpoint one’s location. GPS relies on precisely defined satellite orbits determined via a solution of signals received at many ground stations distributed worldwide. The time for these signals is generated by very accurate on-board atomic clocks, which are kept in synch with the U.S. Naval Observatory’s Master Clock to a precision of better than 10 billionths of a second. So it would seem that today there is no need for using stars to determine positions.

In looking at the furniture in your office and house, it appears that everything is at a fixed position relative to you. However, you are located on a small planet in the Solar System which is moving through space. While seemingly standing still on the Earth you’re actually on the surface of a rotating sphere. If you are on the Equator you are moving at a velocity of over 1,000 miles per hour due to this rotation. This sphere is revolving about the Sun at 19 miles per second. The Sun revolves about the center of the Milky Way galaxy at a velocity of 138 miles per second, and the galaxy itself is moving at 394 miles per second in the direction of the constellation of Hydra. And you don’t even feel dizzy.

On a more subtle scale, there is a twice-a-day variation of 20 inches in the height on the equator with respect to the center of the Earth due to the tidal pull of the Moon and Sun. There are many other motions, such as continental drift at 0.4 inches per year, the 26,000 year precession of the planet’s rotation axis, small semi-periodic “nutations” in the precession cycle, and small but significant changes in the positions of Earth’s rotational poles that also take place. So how do we determine where we are in time and space?

The answer still lies in the sky. Stars in the Milky Way show motions due to their movement through the galaxy as well as the annual apparent reflex motion known as parallax, caused by the Earth revolving around the Sun. The simple fact is that the stars move. The familiar constellations of tonight’s sky will be completely distorted about 100 thousand years from now by these stellar motions. Basing a reference frame on moving targets means that every 50 years or so the reference stars and all the objects that relate to them must have their positions re-computed, an enormous task given the number of catalogued objects in the Universe.

The solution to this dilemma is to find a reference frame that does not move. Fortunately, Nature has given us the perfect objects to meet this need: quasars. Quasars were once very abundant when the Universe was very young. They are believed to have been enormous “black holes” in the centers of the first galaxies to form after the Big Bang. They are so far away that their high-energy X-ray and Gamma-Ray emissions have been shifted into the low-energy microwave portion of the electromagnetic spectrum, and their distances are thus measured in tens of billions of light years!

By arraying together individual, sensitive radio telescopes, typically separated by thousands of miles, and simultaneously observing individual quasars, astronomers can create a “grid” of these distant objects that can then serve as the fundamental reference frame of the Universe. The technique of combining remote radio telescopes is called “Very Long Baseline Interferometry”, or VLBI. The combination of data from a VLBI network is accomplished at the U.S. Naval Observatory’s Mark-V Correlator Facility in Washington, DC. The reduced data not only determine the precise place of a quasar source, they also tell us the precise orientation of Earth’s rotational pole and the instantaneous speed of its spin at the time of the observation. Using the precise positions of some 212 quasars, the resulting reference grid is known as the International Celestial Reference Frame, or ICRF, maintained and continuously updated by astronomers at the Naval Observatory.

It is this reference frame which allows us to determine precise positions on the Earth and in space. For the proper calibration of the Global Positioning System we need to know exactly where the Earth is in inertial space. In Einstein’s theory of special relativity, there is no preferred inertial frame of reference, as motion must always be specified with respect to another object. Observations of quasars made on the Earth’s surface by radio telescopes determine this by referencing their positions and the Earth’s pole and meridian. Astronomers can use this reference frame to measure the positions and distances of nearby stars, more distant nebulae, and radio emitting sources across the galaxy. They can also use it to fly space probes to precise destinations around distant planets.

How does this help spacecraft to get to Mars? Interplanetary probes are in constant touch with the Earth through the NASA Deep-Space Network, a trio of tracking facilities each spaced roughly one-third of the way around the world. By observing a spacecraft’s radio beacon with two stations simultaneously, NASA trackers can pinpoint the probe’s position relative to the ICRF using the tiny Doppler shifts in the beacon’s carrier frequency, which can also be measured to determine the spacecraft’s speed relative to the Earth.

However, even tiny variations in the velocity of Earth’s rotation and the orientation of its rotational pole can introduce a significant uncertainty into the exact positions of the tracking antennas relative to the space probe. This uncertainty must be factored out in order to yield the most accurate positions and velocities.

According to the NASA Mars Exploration Rover website, in order to hit the precise point in Mars’ atmosphere to ensure a successful landing, the exact location of each of the Deep-Space Network’s tracking antennas needed to be known to a precision of better than 2 inches on the surface of the Earth relative to each lander. Any uncertainty in position greater than this could build over the distance between Earth and Mars, leading to a quarter-mile location error at the red planet. Hitting a precise landing site target that is scientifically interesting on Mars is thus nearly impossible unless the Earth’s current rotation rate is known to a timing precision of better than two ten-thousandths of a second! In October 2008, using the same technique, mission navigators successfully steered the $3 billion Cassini orbiter to a 16-mile close encounter with Enceladus, one of the small but fascinating ice moons of Saturn.

So, next time you make a trip across town in your car using GPS or navigate your way through the jumble of rings and moons around a distant planet like Saturn, thank the stars. They’re still the best guides to steer by.

End of podcast:

365 Days of Astronomy
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