US–Indian space mission maps extreme subsidence in Mexico City (opens in new tab)

Like the article hints at, some of the particular strengths of this new measurement:

- frequent revisit, so can track even sub-monthly changes

- the L-band radar is at a wavelength (24cm) that penetrates vegetation canopy, removing a confounder from the measurement

- excellent spatial resolution that is relevant to urban scenes

The data volume is exceptionally high and required a lot of engineering effort. All radars are demanding, but this one was a new high-water mark.

(https://www.earthdata.nasa.gov/news/now-that-nisar-launched-...)

I wouldn't trust that graphic at the top of the article to be very accurate. It has an obvious acquisition footprint that was not resolved in processing. Those WNW-ESE stripes should've been resolved before publishing by ground-truthing the stripes using benchmarks established inside the mapped area so that the end result wouldn't suggest higher/lower subsidence along tracks than seen on parallel offset from tracks. That's just sloppy.

The striping can have multiple sources so they need to study why there is an obvious footprint and then make the appropriate corrections.

"Objective:

NISAR is the first of its kind mission, jointly developed by ISRO and NASA. It is an L and S-band, global, microwave imaging mission, with capability to acquire fully polarimetric and interferometric data.

The unique dual-band Synthetic Aperture Radar of NISAR employs advanced, novel SweepSAR technique, which provides high resolution and large swath imagery. NISAR will image the global land and ice-covered surfaces, including islands, sea-ice and selected oceans every 12 days.

NISAR mission’s primary objectives are to study land & ice deformation, land ecosystems, and oceanic regions in areas of common interest to the US and Indian science communities.

NISAR mission will help to measure the woody biomass and its changes track changes in the extent of active crops understand the changes in wetlands’ extent map Greenland’s & Antarctica’s ice sheets, dynamics of sea ice and mountain glaciers characterize land surface deformation related to seismicity, volcanism, landslides, and subsidence & uplift associated with changes in subsurface aquifers, hydrocarbon reservoirs, etc.

Spacecraft Configuration

The Spacecraft is built around ISRO’s I-3K Structure. It carries two major Payloads viz., L & S- Band Synthetic Aperture Radar (SAR).

The S-band Radar system, data handling & high- speed downlink system, the spacecraft and the launch system are developed by ISRO. The L-band Radar system, high speed downlink system, the Solid-State Recorder, GPS receiver, the 9m Boom hoisting the 12m reflector are delivered by NASA.

Further, ISRO takes care of the satellite commanding and operations, NASA will provide the orbit maneuver plan and RADAR operations plan.

NISAR mission will be aided with ground station support of both ISRO and NASA for downloading of the acquired images, which after the necessary processing will be disseminated to the user community

The data acquired through S-band and L-band SAR from a single platform will help the scientists to understand the changes happening to Planet Earth."

https://www.isro.gov.in/Mission_GSLVF16_NISAR_Home.html

So, perhaps a dumb question, but the article mentions that 14 steps have been added to the base of the Angel of Independence monument, and the Wikipedia article mentions the same things:

> Originally, nine steps led to the base, but due to the sinking of the ground, an ongoing problem in Mexico City, fourteen more steps have been added.

So why didn't the monument itself also sink? Does it have piles going down to bedrock or something?

The amount of subsidence is quite dramatic, up to 25 cm per year!

What are the practical consequences of this today, and what is being done to remedy this?

Cloudflare: verification rejected. Accessing from Japan.

Thank you very much, Cloudlare.

The way that this article is written reads like American propaganda. This is already being done, and has been done for a long time, including at the same or better temporal and spatial resolution. NISAR is genuinely cool, do I don't know why they felt the need to write this way. The new capabilities are mainly being able to do this in highly vegetated areas. In urban areas, like mexico city, this is literally 'intro to SAR' stuff.

For the uninitiated, ISRO -> Indian Space Research Organization

For those unfamiliar, why is this an issue?

A lot of regions in the city rely on gravity to move water out of them. Floods have both increased in quantity and magnitude in recent years. If the city keeps sinking, more water will move in instead of out. Some areas of the city might become perma-flooded in a decade or two.

This is a very serious situation with no obvious solution to it.

Excessive groundwater extraction and urban development could be reasons for this. But these are common practice in almost all of the modern world, so why is this only showing up in Mexico City.

There must be other contributing factors too.

I really can't believe that an issue discovered in 1925 still isn't solved. A kind of issue which wont take a Nobel prize to be solved. This is sad.

I get that the article is primarily about the satellite capabilities, but it's rather annoying it doesn't mention what the future impact of the subsidence might be.

Uh, you know, from the original source - Nasa (2 points, 2 days ago) https://news.ycombinator.com/item?id=47970672

Real shame this re-report made the SCP

In parts of Central Valley CA, there's been over 30 ft / 9m of subsidence from ground water extraction over several decades. (30 cm/y) Lone pipes and drains that previously sat at ground level tower over the land.