Teacher Notes - Geodesy

Information in Radio Waves

Teacher Notes for the “Geodesy” Presentation:

Possible discussion questions before presentation: - If you didn’t know the size and shape of the Earth, how would you go about figuring it out?

Geodesy - Focus of this presentation is on the basics of Geodesy and what it is, the history of Geodesy, and the modern techniques used in the field of Geodesy.

What is Geodesy? - This slide provides a general definition of Geodesy with an even simpler description of the definition. - Good opener for discussion: Where do you see elements of Geodesy? How do you think Geodetic measurements are made?

History - Meant to give students a foundation of what the study of Geodesy is. Geodesy evolved as people became more and more concerned with the overall world around them.

History continued - Introduces flat vs round Earth. Have students discuss how it was determined Earth was round. - Use this opportunity to ask students what they know about the shape and size of the Earth. Ask your students if they think the Earth is perfect sphere, shy or why not?

Dawn of the Sphere - Explanations used by first people to propose the idea of a spherical Earth.

As science often does... - Ask students what is meant by science starting with “pure conjecture.” ○ Make note to them that “pure conjecture” is in quotes because even though they are making a guess to a value, logic a reasoning are being used to come to that conclusion. ○ Good opportunity to relate back to the general scientific method

and 8: Increasing Accuracy and Eratosthenes cont. - First activity of this unit is to recreate the measurements done by Eratosthenes with slightly more modern tools. Hopefully the students are able to get an even more accurate measurement. Information in Radio Waves

and 10: Others of note - Meant to be addressed quickly, might be fruitful to discuss lightly the methods each used to get their measurements. Each can be found briefly summarized in their respective wikipedia pages. * Good Point to pause presentation and do the “Recreating Eratosthenes’ Experiment” Activity *

“Modern” Additions - The meridian arc method was to measure the land distance of a change in degree latitude between two points of equal longitude. ○ Cassini’s measurements were localized and therefore off. His arc predicted the Earth to be egg-shaped ○ Oblate spheroid was confirmed by the French Geodesic Mission in the 1730s. - This method showed conclusively that the length of degree as you go north becomes longer, meaning the Earth was not perfectly spherical but an oblate spheroid with a flattening of around 1: 210

Video - Good video done by NASA detailing the history of Geodesy

So, What next... - Recap where the picture of Earth is up to this point. - Quick discussion on how to get a more accurate picture.

Space! - Before you get into the nitty-gritty details of the space based observations of Earth, make note that from the times of the meridian arc measurements until the first satellites that were launched, the study of Geodesy didn’t pause. - Methods of triangulation still occurred. Cartography still continued. Better and better systems of latitudes and longitudes continued to be made.

Space Age - When covering this slide, make sure to highlight the importance of timing in satellite based geodesic studies. By knowing the orbit of the satellite and by measuring the range and doppler shift of the signal, very accurate measurements of the size and rotational movements of Earth could be calculated.

Modern Geodesy - Worth mentioning again that each of these tools / techniques in their own way rely on timing.

1.) GPS and GNSS - Maximum of 32 satellites in orbit, allowing for a minimum signal of 4 satellites at any point. - if animation is functional, should show how the number of satellite signals available changes for a single point Information in Radio Waves

GPS background - Each satellite relies on a highly accurate clock. These clocks are maintained and updated by the Navy. - The signal that each GPS satellite transmits contains both the timing information as well as the satellites position. Transmitted on at least 2 frequencies. Each satellite has its own characteristic signal as well.

GPS satellite signal structure - SV = space vehicle - The carrier signal that is sent out gets combined with the code modulation and the navigation data. The overall is sent out as the GPS signal. - The carrier wave is used to determine the range and speed of the satellite. - The code is used to identify the satellite and its range. - The navigation data transmits the orbit and health of the satellite as well as any error corrections

Range Measurements - Range is determined by the phase difference or time difference between the sent and received signals.

- Shows the method of triangulation done by at least 3 satellites in determining true position on Earth’s surface. Most often 4 satellites is used, the fourth being for correction in the clock bias.

What is GPS used to do? - TEC is a measure of the total electron content of the atmosphere. Shows the extent to which the atmosphere is ionized at the moment and therefore gives a picture of the overall effect the sun is having on the Earth. - See “space weather” unit and activity.

Ionospheric parameters - Shows the types of ionospheric information that can be gathered through GPS

2.) VLBI - Introduces the basics of VLBI. VLBi made it possible to greatly increase the size of the veiwing aperture to the distance between radio satellites. ○ In a way, makes the largest satellite possible the diameter of the Earth

and 26: VLBI basics - Each individual satellite will observe the same source. The signal from the source will be recorded and tied to a highly accurate clock (hydrogen masers are most often used as they are the most accurate). Information in Radio Waves

- The signals are then correlated with one another allowing for reduction in overall noise and greater resolution Slide 27: Geodetic VLBI - Timing data can also be used to measure very precisely the movements of the Earth system.

Picture of VLBI - Discuss with the students how when the Earth rotates the angle of the viewed source changes at the individual sites while the time delay also changes at each site. When this data is correlated together it reveals a very accurate picture of the exact positioning of each site along with a clear representation of how the Earth was moving.

VLBI contributions - Precession, nutation, and polar wandering: be sure to explain what each of these things are. - Every 100 yrs the Earth’s spin slows down by about 2 milliseconds. - Ask the students why : Due to the gravitational interactions with the moon.

and 31: VLBI contributions (2) - Biggest contribution is direct evidence of plate tectonics. ○ Change in baseline distance between telescopes revealed this.

Video - Demonstrates and explains the history and techniques behind VLBI. Made by NASA.

- Graph showing this direct evidence of plate movement.

3.) SLR - SLR is very similar to radar, but using light. Again, highlight to the students that this technique is all about timing. - The link is a document provided by NASA that would work well as a handout for students. There are 11 pages to choose from explaining all of the functions and uses of SLR.

Satellite Altimetry - Like SLR’s altimetry capabilities, satellite altimetry focuses on surface heights. Unlike SLR, satellite altimetry’s primary focus is the surface height of the oceans and seas. - Very useful in tracking global patterns like El Nino and La Nina. - Has revealed a hard to argue global trend in a rising sea level.

Is This Important? - Many students will have their own “opinions” on global warming, whether or not its happening and what’s causing it to happen. This graph at least makes it hard to argue that an overall trend of a rising sea level is occurring. Information in Radio Waves

Most Important Detail in Geodesy: - The purpose of this slide is to once again instill the notion that time is extremely important in Geodesic techniques. - Use this opportunity to review why timing is important in each of the techniques covered in this presentation.