Author Archives: WQ

About WQ

I received my PhD (2013) in Remote Sensing, Earth and Space Science at the Dept. of Aerospace Engineering Sciences, University of Colorado, Boulder, USA, under a Fulbright fellowship. Currently, I'm an Assistant Professor in the Dept. of Space Science at Institute of Space Technology (IST), Islamabad, Pakistan, where I have been a founding member of the Geospatial Research & Education Lab (GREL). My general expertise is in Remote Sensing where I have worked with various remote sensing datasets through my career, while for my PhD thesis I specifically worked on Remote Sensing using SAR (Synthetic Aperture Radar) and Oceanography, working extensively on development of techniques to measure ocean surface currents from space-borne SAR intensity images and interferometric data. My research interests are: Remote sensing, Synthetic Aperture Radar (SAR) imagery and interferometric data processing & analysis, Visible/Infrared/High-resolution satellite image processing & analysis, Oceanography, Earth system study and modelling, LIDAR data processing and analysis, Scientific programming. I am a reviewer for IEEE Transactions on Geoscience & Remote Sensing, Forest Ecosystems, GIScience & Remote Sensing, Journal of African Earth Sciences, and Italian Journal of Agronomy. I am an alumnus of Pakistan National Physics Talent Contest (NPTC), an alumnus of the Lindau Nobel Laureate Meetings, a Fulbright alumnus, and the Pakistan National Point of Contact for Space Generation Advisory Council (SGAC). I was an invited speaker at the TEDxIslamabad event held in Nov., 2014. I've served as mentor in the NASA International Space App Challenge Islamabad events in April 2015 and April 2016.

Difference in “date” function implementation in Mac OSX and Ubuntu bash

A few weeks ago, I had written some bash code in Max OSX terminal to identify the current date, and then defining the date a few days back in time. When I tried to run the same code in Ubuntu bash terminal, the code line for identifying the previous date fails. Some brief time spent on Google told me that there is some difference in how the “date” function is implemented in Mac OSX and Ubuntu bash. The correct usage is as follows:

curr_date=$(date +”%Y%m%d”)
echo ‘Current date: ‘$curr_date

# Find the date 3 days ago – For OSX bash:
prev_date_OSX=$(date -v-3d +”%Y%m%d”)
echo ‘Previous date: ‘$prev_date

# Find the date 3 days ago – For Linux Ubuntu bash:
prev_date=$(date -d “3 days ago” +”%Y%m%d”)
echo ‘Previous date: ‘$prev_date


Uninstalling User-Installed Python from macOS Sierra

macOS Sierra comes with a built-in default Python installation. On macOS Sierra 10.12.6, this default installation is on the /System/Library/Frameworks folder (which, by the way, is a critical system folder and should not be touched). macOS Sierra 10.12.6 comes with Python 2.7, which is getting outdated very fast, and also Apple itself recommends officially that to run a coding project, users should install their own updated version of Python with their own dependencies setup.

I had installed Python 2.7 myself last year when I was in a mood to start working on Python, and at that time I didn’t know that Python comes installed in macOS by default. Now I have to start working with Python in earnest for a project, and wanted to uninstall the custom Python 2.7 installation, so that I can start from scratch on a new installation of Python 3. This required some web-surfing, and some hours to figure out how to do it properly. After reading and deciphering some posts by others, I can now give an updated clean solution.

This assumes you have a good knowledge of shell / bash usage. This works for Python 2.7 installed by the user, but I am sure it works the same if you want to uninstall Python 3 from macOS Sierra too.

  • Step 1: Manually remove Python 2.7 folder from Applications (drag to Trash).
  • Step 2: Remove the Python 2.7 framework from /Library through the terminal:   sudo rm -rf /Library/Frameworks/Python.framework
  • Step 3: Clear python files from /usr/local/bin:      sudo rm -rf /usr/local/bin/python*
  • Step 4: Clear symbolic links to deleted Python files. If you have Homebrew installed already (highly recommended), then simply run brew doctor first, which will show you the broken symbolic links. Then just run brew prune to fix them (you can check it by running brew doctor again). If you don’t have Homebrew installed, then follow Step 3 here.

For more discussions, see this, this, and this.

Now we are ready for a fresh Python install from scratch!

NOTE: After uninstallation, I do need to fix the system to call the default python version installed in macOS Sierra. Probably need to revise some path specifications. But I am more concerned with the new Python3 installation at this point :). See here for more on this.

CAUTION: Under no circumstances should you try to delete or touch anything in the /System or /usr/bin/python folders. This can cause your macOS to malfunction, your Macbook could self-destruct, and there is a possibility of an alien invasion as well. If you don’t believe me, just do a web search on why not to touch anything in the macOS /System folder.

The SAR Journal Webpage and Community

I just discovered this amazing Synthetic Aperture Radar (SAR) website and magazine site, so aptly named as The website and content in it is quite amazing, and being a SAR aficionado, I have immediately signed up for their newsletter. I wish someone sends me an invite to the “Community” also, it seems to be only by invitation 🙂

In their own words, the website managers “represent the worldwide airborne and spaceborne SAR community worldwide. We are operated, moderated and maintained by members of the SAR community.”

So take a look at the SAR Journal website and sign up for the newsletter:


GLaSS and EOMORES Inland Water Remote Sensing Projects

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Phander Lake in District Ghizer, Gilgit-Baltistan, Pakistan. Photo credits: Auhor

The EU collaborative project GlaSS (Global Lakes Sentinel Services) developed tools, algorithms and applications for the monitoring of global lakes and reservoirs using the Copernicus Sentinel-2 (S2) optical and Sentinel-3 (S3) satellite data, and also USGS Landsat 8 data. The great thing about this project is that the results and developed data processing methodology have been made available online as training material in a very detailed and systematic manner. I have gone through them briefly, and they are readily usable in undergraduate or graduate level courses in remote sensing, especially water & hydrology remote sensing focussed courses. There are 10 lessons in total. Take a look at the GlaSS training material here:

The GlaSS project has lead to various news reports and scientific publications. The project was finished few months ago, and in fact seems to have transitioned into the EU H2020 EOMORES (Earth Observation-Based Services For Monitoring And Reporting Of Ecological Status) project, which claims to be a project “aiming to develop commercial services for monitoring the quality of inland and coastal water bodies, using data from Earth Observation satellites and in situ sensors to measure, model and forecast water quality parameters.” The EOMORES project has just started few months ago, and we look forward to seeing what results it brings us in the future.


Imagery Downlinked from Satellite to Ground in 6 Minutes

Some months ago, I had written about the EDRS SpaceDataHighway and real-time provision of Sentinel-1 satellite imagery through laser link and satellite relay. Now, ESA has done an experiment with Sentinel-2B to deliver imagery to the ground moments after it was capture by the satellite. The captured image strip was downlinked in just 6 minutes. This experiment brings us closer to the amazing future when we would be able to access satellite imagery as non-defence / non-strategic users in near-real-time.

See more details here:

Aperture Synthesis and Azimuth Resolution in Synthetic Aperture Radar – Lecture Notes

Teaching the fundamentals of Synthetic Aperture Radar (SAR) system design and imaging mechanism to remote sensing students / professionals is always a difficult task. Remote sensing students / professionals generally do not have an in-depth background of signal processing and radar system design, and as an instructor, I always have to think over how much I need to tell them about SAR system design, without diving into the detailed mathematics of signal processing and imaging mechanism. Normally, I go in-depth towards the imaging geometry and an understanding of the Doppler history curve, and briefly go over the signal-processing heavy concepts like pulse compression and matched filtering. A good fundamental understanding of the SAR system design, imaging geometry, and image formation is essential for remote sensing students / professionals to have a background context knowledge when they select SAR data and process / analyze it for different remote sensing applications.

For the past few years, I have been teaching a graduate course in Radar Remote Sensing and also run an annual Summer School on Earth Remote Sensing with SAR at our research group GREL. One of the core issues in understanding the aperture synthesis process is the requirement for enhancement of the azimuth / along-track resolution. It is always interesting to discuss in class how in normal imaging radar the azimuth resolution depends inversely on the antenna along-track length, while in fully-focussed SAR the azimuth resolution becomes half of the antenna along-track length. This is a significant reversal: In normal imaging radar, we need a bigger antenna in along-track dimension to get better azimuth resolution, while in SAR, the smaller the antenna in the along-track dimension, the better the azimuth resolution.


To explain how aperture synthesis changes the azimuth resolution to half of the along-track antenna length, I have made some detailed notes for my ongoing graduate class on Radar Remote Sensing. These notes require just basic knowledge of geometry, algebra, and sum series in mathematics. I would like to share them with the wider scientific audience, please access the PDF notes here: Aperture Synthesis and Azimuth Resolution.


The synthetic aperture length is defined in the figure above. The azimuth resolution in fully-focussed SAR becomes half of the antenna along-track dimension.

I have taken the help of two excellent resources on SAR remote sensing in developing these notes:

For more in-depth understanding and analysis of how SAR is used for remote sensing, you can consider attending the next Summer School on Earth Remote Sensing with SAR, which I will be offering this summer. The summer school is coming up in July, 2018, and it will be open for international participants; formal dates will be announced soon. Keep watching the GREL website for updates.