Deep Dive: Geodesy from Space
Here’s something different from my usual Rocket History. We were short on content for the rocket roundup on 9/29 so i wrote this deep dive.
Geodesy is the study of the Earth’s shape, orientation in space, gravitational field and how all of those change over time. Measurements are made with several terrestrial and space based methods to come up with a reference frame of the Earth’s surface, from which different points on the surface can be assigned coordinates.
Traditional methods, which have their roots based on techniques developed thousands of years ago, relied on using landmarks such as the peaks of mountains, big rocks, and artificial landmarks like cairns made out of stones and brass markers. Using fairly simple tools and techniques, it’s possible to calculate distances between landmarks. If you were to combine those methods with celestial navigation techniques, you could figure out where something is.
Around the world, landmarks have been replaced by brass survey markers of various types. If you keep your eyes open, you might be able to spot one when you’re out and about.
Modern techniques build on these traditional techniques and use more precise and accurate tools to very precisely measure the location of something in three dimensions. These methods include Satellite Laser Ranging, Very Long Baseline Interferometry, and the Global Navigation Satellite System.
Satellite Laser Ranging is pretty much what it sounds like: a laser on the ground is fired at a satellite. The laser then bounces off fancy mirrors called retroreflectors on the satellite and returns to the source (the thing that fired a beam of photons into space to begin with). The time from sending to receiving the laser pulse from the spacecraft is plugged into a complicated equation which determines where the satellite is in space. Laser ranging on a navigation satellite is especially important because that’s one of the independent ways to determine precisely what orbit it is in, which affects the accuracy of the navigation signal. Special geodetic satellites are also launched occasionally, these are basically disco balls; solid metal balls covered in retroreflectors, designed specifically for laser ranging.
Another technique, Very Long Baseline Interferometry, uses the time difference between observation of a radio signal across many radio telescopes on Earth from a Quay zaar quasar, an active black hole that shoots off jets bright in radio light. These antennas have a known position to a few millimeters, so it is very easy to mark the Earth’s precise orientation in space from observation of a few quasars by several radio telescopes. The International Celestial Reference Frame is generated from this orientation.
You are probably familiar with this next technique: Global Navigation Satellite System, shortened to GNSS. In the US, we typically just call it GPS, for Global Positioning System. GNSS uses the slight difference in timing between signals on two different frequencies sent by a navigation satellite to determine the position of the receiver. This method is less accurate, because the navigation signal itself relies on laser calibration of the satellites orbit.
An important application of these coordinates is, of course, navigation on or just above the Earth’s surface, but the geodetic coordinates are also used to monitor how the Earth itself is moving. For example, the fact that the Earth is composed of several plates which are constantly moving, plate tectonics, was a hotly debated topic in Earth sciences for most of the 20th century. Evidence for plate movements was provided by satellite geodetic measurements and other methods. Satellite geodesy is also used to measure the sea level, necessary for all sorts of oceanographic measurements like how melting ice is affecting the sea level globally and in certain regions and cycles like the El Niño.
Beyond Earth, laser ranging is also used to track the Moon’s orbit using retroreflectors placed on the surface by Apollo missions.