Satellites Looking at Each Other: The Separation Measured by GRACE-FO's Laser

A satellite investigating groundwater need not point its main measurement system toward the ground. For GRACE-FO's two spacecraft, a crucial signal is a tiny change in the distance between them. Earth's distribution of mass affects their orbital motion, and that influence appears in their changing separation. The observation is not a photograph of underground water. It is a record of relative movement that can be connected with the changing gravitational influence of mass below.

GRACE Follow-On launched on May 22, 2018 to continue the approach established by GRACE. Alongside its microwave measurement system, the mission carried a laser ranging interferometer as a technology demonstration. The following account uses the 2018 design and first-light reports to explain the measurement and its early verification. The information cutoff is October 13, 2026. Expectations expressed before launch are kept separate from results subsequently demonstrated in orbit. [1][2]

The pair becomes one measurement arrangement

The original GRACE approach followed two spacecraft traveling one behind the other around Earth. As their positions changed relative to concentrations of mass, gravitational influences could change their motions differently. Their separation consequently provided information about the mass distribution. A mountain range and a body of groundwater contribute to that distribution in different ways, even though the measurement itself is made far above them. [1]

One separation curve from one pass is not a complete inventory of the water underneath. The mission descriptions explain that observations across orbits and over time support calculations of month-to-month mass changes. A persistent mountain and a changing reservoir of water do not contribute the same temporal behavior. Detecting a small orbital signal therefore begins an interpretation problem rather than eliminating the need for one.

The approximately 220-kilometer distance between the spacecraft is also distinct from the tiny fluctuations of interest. It is their separation, not their altitude above Earth's surface or the size of a feature in a map. Measuring minute changes across that long baseline is the technical challenge. A small fluctuation can carry useful information even when it is an extremely small fraction of the overall distance. [1][2]

Testing a laser without abandoning the microwave baseline

GRACE used microwave signals exchanged between the satellites. GRACE-FO retained this method and tested the laser system alongside it. Both arrangements observed the changing relative motion, providing an opportunity to compare a new measurement channel with the established approach. The mission did not suddenly produce its entire historical record from lasers or make the microwave instrument unnecessary at launch. [1][2]

Shorter optical wavelengths offer the potential to resolve smaller changes in separation. The prelaunch account described a goal of improving range-fluctuation measurement precision by at least a factor of ten for future missions using the technique. That factor should not be translated directly into a water map becoming ten times sharper. Instrument sensitivity to distance and the spatial resolution of a processed geophysical product are different performance measures.

Other observations and calculations stand between the two. Improving one measurement can be valuable without improving every downstream result by the same multiplier. Preserving this distinction is important because an impressive engineering specification can otherwise be mistaken for a demonstrated ability to identify an individual small reservoir or water body. [1]

First, the beams had to find their targets

Precise spacecraft assembly did not guarantee that a transmitted beam would immediately reach the receiving aperture on the other vehicle. Across hundreds of kilometers, a small pointing difference matters. Acquiring the optical link was therefore a separate task from producing laser light. The instrument needed a way to discover the settings under which the two ends could see one another. [1][2]

On June 13, 2018, the spacecraft began scanning their lasers in spiral patterns. Data returned the following day showed flashes received from the opposite spacecraft. Those encounters supplied information needed to establish a continuous link. The appropriate settings were calculated and uploaded, after which the first intersatellite range data were obtained. [2]

This sequence is more informative than saying that two lasers simply found each other without assistance. Scanning, examining downlinked observations and uploading settings all contributed to the initial acquisition. The prelaunch design envisaged continuous autonomous operation once the link had been established. That operational goal should not be confused with a claim that ground involvement was absent from the setup.

Autonomy in this case describes how a particular continuing activity was designed to proceed, not the removal of people from every phase of the mission. Likewise, the spacecraft's high orbital speed is not a measure of how fast the two vehicles were approaching one another. Their shared orbital travel and their relative range fluctuations are separate motions. [1][2]

A precise range still needs a physical explanation

The technical press kit describes processing the interferometer signal's phase as a measure of separation fluctuations. Motion has causes other than gravity, however. Accelerometers help identify nongravitational influences such as atmospheric drag and thruster firings, while GPS supplies orbital-position information. [3]

This division of work explains why a successful laser instrument is not, by itself, a complete gravity map. Even if separation is measured very precisely, assigning its changes to the wrong physical causes would undermine the interpretation. A finer measurement channel does not remove the need to characterize the spacecraft and its environment.

It also clarifies the purpose of the optical link. The central question here is not simply how quickly a file can be transmitted. The light is used to investigate how the relative range of two orbiters changes. Other missions can use optical links to carry data, but that shared technology does not make their scientific measurement targets identical. For GRACE-FO, the relevant product begins with the movement of the pair. [1][3]

What the first comparison established

The July 2, 2018 first-light report stated that the initial laser measurements agreed with the microwave observations as expected. This was early evidence that the acquired optical link could produce a meaningful range record. The same account explained that instrument operation and understanding of its data would continue to be refined in the following weeks and months. Establishing first light and completing every element of a measurement's error assessment are different milestones. [2]

The engineering achievement was to operate an approach developed through years of laboratory work between two separate spacecraft in orbit. Its broader scientific value depends on maintaining the chain from relative separation, through an account of other forces, to changes in Earth's mass distribution. Each link contributes something the others cannot simply replace.

GRACE-FO's laser demonstration is consequently best understood as a more precise comparison between two moving reference points, not a new camera revealing groundwater directly. The pair looks at itself to learn about the planet. That unusual geometry is the source of the method's usefulness, and the reason a distance measurement in space can become evidence about changing mass on Earth. [1][2]

Sources

[1] NASA Science (May 8, 2018). Lasers in space: Earth mission tests new technology. https://science.nasa.gov/technology/science-instruments/lasers-in-space-earth-mission-tests-new-technology/

[2] NASA Sea Level Change (July 2, 2018). First laser light for GRACE Follow-On. https://sealevel.nasa.gov/news/132/first-laser-light-for-grace-follow-on

[3] NASA/JPL (2018). GRACE-FO Launch Press Kit: Spacecraft and Instruments. https://www.jpl.nasa.gov/news/press_kits/grace-fo/mission/spacecraft/

NASA mission publications are sources for this original article; NASA did not author or approve this account.