Shuttle Radar Topography Mission (SRTM) 1 Arc-Second Global— [1] Story.

Halo!

I hope all of you are staying safe and healthy.

I have been looking for open-source data with an adequate resolution to exercise R Language for spatial analyst purposes. At first, I simply wanted to use Shuttle Radar Topography Mission (SRTM) 3 Ar-Second that I am familiar with since college. It has an approximate 90 meter/pixel spatial resolution. However, I changed my mind when I was exploring the data from earthexplorer.gov.us. The reason was it has greater resolution/pixel and I haven’t played with it until now.

So, I started to download the data for Sumatera and Malaya Region. It turned out that I needed to download over 100 datasets and took some time to store them on my laptop. Meanwhile, I read the product overview from the USGS Earth Resources Observation and Science (EROS) Center archive and They mentioned in the overview section that:

The Shuttle Radar Topography Mission (SRTM) was flown aboard the space shuttle Endeavour February 11–22, 2000. The National Aeronautics and Space Administration (NASA) and the National Geospatial-Intelligence Agency (NGA) participated in an international project to acquire radar data which were used to create the first near-global set of land elevations.

The radars used during the SRTM mission were actually developed and flown on two Endeavour missions in 1994. The C-band Spaceborne Imaging Radar and the X-Band Synthetic Aperture Radar (X-SAR) hardware were used on board the space shuttle in April and October 1994 to gather data about Earth’s environment.

It took my interest to get more stories about this paragraph and elaborate on this platform. In addition, I was surprised after reading the story because I had a wrong assumption about the SRTM background story until this day.

First thing first, I started typing several keywords on google like “SRTM, SRTM mission, SRTM 1 Arc-Second”. I found a lot of sources published by the Jet Propulsion Laboratory of The National Aeronautics and Space Administration (JPL NASA) of the U.S. federal government. Here is the list of links that I found and read:

1. USGS EROS Archive — Digital Elevation — Shuttle Radar Topography Mission (SRTM) 1 Arc-Second Global. July 30, 2018. https://www.usgs.gov/centers/eros/science/usgs-eros-archive-digital-elevation-shuttle-radar-topography-mission-srtm-1#overview
2. SRTM mission overview. Updated January 16, 2022. https://www2.jpl.nasa.gov/srtm/missionoverview.html
3. Shuttle Radar Topography Mission (SRTM). https://www.jpl.nasa.gov/missions/shuttle-radar-topography-mission-srtm
4. SRTM project status. Updated February 26, 2016. https://www2.jpl.nasa.gov/srtm/p_status.htm
5. U.S. Releases Enhance Shuttle Land Elevation Data. September 23, 2014. https://www.jpl.nasa.gov/news/us-releases-enhanced-shuttle-land-elevation-data
6. The Shuttle Radar Topography Mission. https://www2.jpl.nasa.gov/srtm/SRTM_paper.pdf

From the SRTM paper, I found they already compiled the information from the idea of the mission to the data utilization. I would summarize it with the same structure related to the context and in brief explanation yet informative.

INTRODUCTION — The Need for Global Topography

At the foundation of modern geosciences, quite literally, is knowledge of the shape of the Earth’s surface.

The idea of this mission was to capture the surface of the Earth, with the most complete and higher resolution (at that time) digital elevation models. They said it would be a benefit to have the elevation models of the Earth in the form of topographic maps which provide a base and context for airborne navigation systems and for a range of field activities in the civilian and military sectors.

They were highlighting several constraints using conventional topographic mapping technologies that will lead to initializing SRTM. The constraints that have been mentioned were as follows:

1. Conventional topographic mapping technologies have produced maps of uneven quality — some with astounding accuracy, some far less adequate.
2. Most industrial countries have created and maintained national cartographic databases and the map products derived from these databases have demonstrated the idiosyncrasies of topographic data: The maps are at a variety of scales and resolutions, often referenced to country-specific datums and thus inconsistent across national boundaries.
3. The global coverage has been uneven. In many parts of the world, particularly cloudy parts of South America and Africa, very little quality topographic data exists.
4. It has proven exceedingly difficult and expensive to produce a global map set or digital elevation model of consistent scale and resolution by conventional means.
5. The cost of deploying aircraft globally is prohibitive, where many areas are inaccessible politically.
6. Optical stereo mapping systems suffer from poor control and matching difficulties in areas of low contrast, and from persistent cloud cover in many important areas of the world.

Then they said, In the 1990s, the emergence of synthetic aperture radar (SAR) interferometry gave a possibility of efficient and affordable fashion to create a global digital elevation model within the grasp of spacefaring nations.

INTRODUCTION — Genesis of SRTM: The Shuttle Imaging Radar Program

Remote sensing missions from low earth orbit ensued after the space shuttle became operational. It was aboard a reusable spacecraft that had onboard accommodations, unlike any spacecraft that had flown at that moment. On its flight in 1981, the Shuttle carried the first science payload, OSTA-1 (Office of Space and Terrestrial Applications-1), including a synthetic aperture radar, designated Shuttle Imaging Radar-A (SIR-A).

There was a sequent of development before SRTM flew into space. SIR-A was the first design of the instrument, then evolved to SIR-B, SIR-C, SRL(SIR-C/X-SAR), and ScanSAR. Then, ScanSAR interferometric operations became the basis of the SRTM topographic measurement scheme (Fig. 1). I will write the details instrument and mission that the paper has mentioned out below:

1. The SIR-A instrument was a singly-polarized (HH: horizontal send and receive) L-band (23.5 cm wavelength) SAR with a fixed look angle of 45° off-nadir.
2. SIR-B, which flew on Challenger mission 41-G (October 5–13, 1984), was the next step in the evolution of Shuttle-borne radars. System upgrades included a foldable antenna with the addition of a mechanical pointing system that allowed the beam to be steered over a look angle range of 15° to 60°. Like its predecessor, SIR-B operated at L-Band and was HH polarized.
3. SIR-C was proposed as a development tool to address the technical challenges posed by a multifrequency, multi-polarization SAR with wide swath capability. After considerable study and development through much of the 1980s the SIR-C instrument evolved into SIR-C/X-SAR, an L-band and C-band (5.6 cm) fully polarimetric radar with electronic scanning capability, coupled with an Xband (3.1 cm) single polarization (VV: vertical send and receive) mechanically steered radar supplied by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt, DLR) and the Italian Space Agency (Agenzia Spaziale Italiana, ASI).
4. SIR-C/X-SAR flew as the Space Radar Laboratory (SRL) in April and October 1994. SRL-1 and 2 gathered images over predesignated target sites and exercised several experimental SAR techniques. Among the SIR-C/X-SAR experiments were successful demonstrations of repeat-pass interferometry, where images of a target were obtained on repeat orbits (the difference in positions on each pass forming the interferometric baseline)
5. ScanSAR, where radar beams were electronically steered in elevation to increase the swath width.

Fig. 1. The Shuttle Imaging Radar Program Milestone.

INTRODUCTION — SRTM Mission Overview

The Shuttle Radar Topography Mission, flown on Space Shuttle Endeavour in February 2000 [STS-99](Fig. 2), was a joint project of the National Aeronautics and Space Administration, the National Geospatial- Intelligence Agency (NGA) (formerly National Imagery and Mapping Agency, NIMA) of the U.S. Department of Defense (DoD), and DLR. DLR worked in partnership with ASI. Endeavour was launched with a six-person crew from the Kennedy Space Center (KSC) on February 11, 2000, 12:44 p.m. EST. The nominal altitude was chosen to be 233 km, the orbital inclination 57°. With this geometry, the Shuttle would begin repeating in 159 orbits, in about 10 days. Since individual orbits were separated by 218 km at the equator and since, fortuitously, the width of the ScanSAR imaging swath was 225 km, Endeavour could map the target area−the strip between 60° north latitude (the southern tip of Greenland) and 56° south (Tierra del Fuego)−in a single cycle of 159 orbits.\

Fig. 2. The assembly of SRTM on the shuttle spacecraft (Image source: JPL NASA)

Following the launch, the first 12 hours of flight were taken up by On Orbit Checkout (OOCO), during which the payload bay doors were opened, the SRTM system activated and checked, and the orbiter maneuvered to the mapping attitude. With the successful acquisition and verification of test data, SRTM radar mapping began. Mapping continued for 149 orbits (222.4 hours). Data were acquired at the rate of 180 Mbps (C-RADAR) and at 90 Mbps (X-RADAR). Both rates were higher than the Shuttle’s downlink capacity (45 Mbps). This required the use of high-rate data tape recorders. Selected snapshots of data necessary for near-real-time performance assessment were downlinked via the Shuttle Ku-band and NASA’s Tracking and Data Relay System (TDRS) link to JPL in Pasadena and to the Payload Operations Control Center (POCC) in Houston. The total SRTM raw data volume amounted to 12.3 terabytes. About 99.96% of the targeted area was mapped by the C-RADAR at least once (Fig. 3a). Due to the loss of 10 orbits, a few patches of land in North America were missed. The X-RADAR data cover about 40% of the target area (Fig. 3b). Data gathering was concluded on flight day 10. Endeavour landed at KSC on February 22, 2000, 6:22 p.m. EST.

Fig. 3. a) The targeted area mapped by the C-RADAR. b) The targeted area mapped by the X-RADAR. (Image source: JPL NASA)

PERSPECTIVE — Get the Picture

From the above passages, I might get a new picture of the SRTM project. Frankly, I wasn’t seeing the forest for the trees. I only use the data without awareness of its story. This documentation would be an eye-opening knowledge to me. Below, I gather the things that break my assumption when I was in college about SRTM:

1. It was continuously updated data. The fact: SRTM is a project that was only held in February 2000. The data would only be coherent with the time-bound. However, we can use it as a baseline topographic data to assess the field that is linear to the main idea of SRTM that has been mentioned: the elevation models of the Earth in the form of topographic maps which provide a base and context for airborne navigation systems and for a range of field activities in the civilian and military sectors. Later, we need to verify and validate the data in our specific region of interest.
2. It assembled in a satellite. The fact: It was assembled to space shuttle called endeavour (STS-99). I wasn’t aware of the shuttle terms and led to misinterpretation that the project would be similar to Sentinel.
3. The project was originally developed and deployed around 1981 with an instrument called SIR-A. Nineteenth years later, SRTM launched and took 10 days of data acquisition.
4. Endeavour could map about 80 percent of Earth’s land surface−the strip between 60° north latitude (the southern tip of Greenland) and 56° south (Tierra del Fuego)−in a single cycle of 159 orbits.
5. The data became the first near-global set of land elevations.

That's all for this section, I hope you enjoy reading it and got an insight from SRTM project story. Furthermore, I am planning to explain the technology and its resolution for the next article.

So, Stay tuned!

Contributor: Rindang Muharza Viawan /// krnggo

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