Earth Observation Using Python. Rebekah B. Esmaili

Geostationary platforms orbit the Earth at 35,700 km above the Earth’s surface. GEO satellites are designed to continuously monitor the same region on Earth, and thus can provide many images over a short period of time to monitor change. National Oceanic and Atmospheric Administration (NOAA) operates the Geostationary Environmental Satellite System (GOES) satellites for monitoring North and South America. GOES‐16 and ‐17 have an advanced baseline instrument (ABI) onboard to create high‐resolution imagery in visible and infrared (IR) wavelengths. The GOES‐16 and ‐17 satellites are also equipped with the Geostationary Lightning Mapper (GLM) to detect lightning. Instruments designed for space weather include the Solar Ultraviolet Imager (SUVI) and X‐ray Irradiance Sensors (EXIS). The European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) operates and maintains the Meteosat series GEO satellites that monitor Europe and Africa. The Japan Aerospace Exploration Agency (JAXA) operates and maintains the Himawari satellite that monitors Asia and Oceania.

       Low‐Earth orbit (LEO) satellites. Polar orbiting satellites provide approximately twice daily global observations at the equator (with more observations per day at the poles). Figure 1.3 displays the equatorial crossing time for historic and existing LEO satellites, which refers to the local time at the equator when observations are made. Overpasses from some LEO satellites shift during a mission, while others are periodically adjusted back to maintain a consistent overpass time throughout the duration of a mission. Polar orbiting satellites are called low‐Earth orbit satellites because they are much closer to the Earth’s surface (at 400–900 km) than GEO satellites, which are approximately 40 times further away from the earth or at ~35,000 km. The lower altitude of LEO satellites facilitates their higher spatial resolution relative to GEO, although the temporal resolution tends to be lower for LEO satellites. The Suomi‐NPP and NOAA‐20 are two satellites that were developed and maintained by NASA and NOAA, respectively. They are each equipped with an imager, the Visible Infrared Imaging Radiometer Suite (VIIRS), and infrared and microwave sounders, the Cross‐track Infrared Sounder (CrIS) and an Advanced Technology Microwave Sounder (ATMS). The MetOp series of LEO satellites (named MetOp‐A, ‐B, and ‐C) were developed by the European Space Agency (ESA) and are operated by EUMETSAT.

Figure 1.3 Equatorial crossing times for various LEO satellites displayed using Python.

      1.2.2 Hydrology

      Because water is sensitive to microwave frequencies, microwave instruments and sounders are useful for detecting water vapor, precipitation, and ground moisture. The Global Precipitation Mission (GPM) uses the core GPM satellite along with a constellation of microwave imagers and sounders to estimate global precipitation. The SMAP satellite mission uses active and passive microwave sensors to observe surface soil moisture every two to three days. The GRACE‐FO satellite measures gravitational anomalies, that can be used to infer changes in global sea levels and soil moisture. All three hydrology missions were developed and operated by NASA.

      1.2.3 Oceanography and Biogeosciences

      Both GEO and LEO satellites can provide sea surface temperature (SST) observations. The GOES series of GEO satellites provides continuous sampling of SSTs over the Atlantic and Pacific Ocean basins. The MODIS instrument on the Aqua satellite has been providing daily, global SST observations continuously since the year 2000. Visible wavelengths are useful for detecting ocean color, particularly from LEO satellites, which are often observed at very high resolutions.

Additionally, LEO satellites can detect global sea‐surface anomaly parameters. Jason‐3 is a low‐Earth satellite developed as a partnership between EUMETSAT, NOAA, NASA, and CNES. The radar altimeter instrument on Jason‐3 is sensitive to height changes less than 4 cm and completes a full Earth scan every 10 days (Vaze et al., 2010).

      1.2.4 Cryosphere

      ICESat‐2 (Ice, Cloud, and land Elevation Satellite 2) is a LEO satellite mission designed to measure ice sheet elevation and sea ice thickness. The GRACE‐FO satellite mission can also monitor changes in glaciers and ice sheets.

      

1.3 The Flow of Data from Satellites to Computer

The missions mentioned in the previous section provide open and free data to all users. However, data delivery, the process of downloading sensor data from the satellite and converting it into a usable form, is not trivial. Raw sensor data are first acquired on the satellite, then the data must be relayed to the Earth’s ground system, often at speeds around 30 Mbits/second. For example, GOES satellite data are acquired by NASA’s Wallops Flight Facility in Virginia; data from the Suomi NPP satellite is downloaded to the ground receiving station in Svalbard, Norway (Figure 1.4). Once downloaded, the observations are calibrated and several corrections are applied, such as an atmospheric correction to reduce haze in the image or topographical corrections to adjust changes in pixel brightness on complex terrain. The corrected data are then incorporated into physical products using satellite retrieval algorithms. Altogether, the speed of data download and processing can impact the data latency, or the difference between the time the physical observation is made and the time it becomes available to the data user.

      Data can be accessed in several ways. The timeliest data can be downloaded using a direct broadcast (DB) antenna, which can immediately receive data when the satellite is in range. This equipment is expensive to purchase and maintain, so usually only weather and hazard forecasting offices install them. Most users will access data via the internet. FTP websites post data in near real time, providing the data within a few hours of the observation. Not all data must be timely – research‐grade data can take months to calibrate to ensure accuracy. In this case, ordering through an online data portal will grant users access to long records of data.

While data can be easily accessed online, they are rarely analysis ready. Software and web‐based tools allow for quick visualization, but to create custom analyses and visualizations, coding is necessary. To combine multiple datasets, each must be gridded to the same resolution for an apples‐to‐apples comparison. Further, data providers use quality flags to indicate the likelihood of a suitable retrieval. However, the meaning and appropriateness of these flags are not always well communicated to data users. Moreover, understanding how such datasets are organized can be cumbersome to new users. This text thus aims to identify specific Python routines that enable custom preparation, analysis, and visualization of satellite datasets.

Figure 1.4 NOAA‐20 satellite downlink.

1.4 Learning Using Real Data and Case Studies

      I have structured this book so that you can learn Python through a series of examples featuring real phenomena and public datasets. Some of the datasets and visualizations are useful for studying wildfires and smoke, dust plumes, and hurricanes. I will not cover all scenarios encountered in Earth science, but the skills you learn should be transferrable to your field. Some of these case studies include:

       California Camp Fire (2018). California Camp Fire was a forest fire that began on November 8, 2018, and burned for 17 days over a 621 km2 area. It was primarily caused by very low regional humidity due to strong gusting wind events and