EarthNow! Landsat Image Viewer
This page contains a list of the most frequently asked questions (FAQs) about the EarthNow! Landsat Image Viewer and the satellite data it displays. More help on using EarthNow! may be found on the Help page.
What is Landsat?
Why is Landsat imagery jagged along the edges?
What is the resolution of the imagery on screen?
What does "Server Unavailable" mean?
Why might a pass be late?
Why might a pass be missing?
Why might the applet fail to run (the screen is just gray)?
Why does the applet sometimes hang, go completely blank, or elements within the applet fail to function properly?
Q: What is Landsat?
A: Landsat is the name of a series of satellites that have been used by governments, scientists and educators to monitor changes of the Earth's land surface. Landsat satellites provide a means for measuring the change in the amount of healthy vegetation, extent of damage and rate of regeneration following a forest fire, and various other applications of interest to professionals in the defense and agriculture industries. Since the launch of Landsat 1 in 1972, these satellites have continuously monitored the Earth. The USGS currently operates Landsat 5 and Landsat 7. These satellites contribute over 400 images per day to the EROS data archive.
The EarthNow! Landsat Image Viewer displays the images in the 5, 4, 3 (Red, Green, Blue) band combination for both the TM and ETM+ sensors.
Q: Why is Landsat imagery jagged along the edges?
|A: The edges of Landsat imagery appear jagged because of the way the Landsat sensors collect image data. As the satellite travels above the Earth's surface, the sensor rapidly scans the viewing area from side to side. The imaging detectors are turned off momentarily at the edge of the image. They are turned on at a slightly different point in the return scan than where they were turned off, giving the imagery a jagged edge.|
Q: What is the resolution of the imagery on screen?
A: Each pixel of Landsat data in the Moving Window Display covers about 250 meters by 250 meters.
Q: What does "Server Unavailable" mean?
A: This message will be displayed if EarthNow! is unable to connect to the server at USGS EROS. This could be caused by a network error between your personal computer and the server at EROS or if too many users are connected to EarthNow! simultaneously.
Q: Why might a pass be late?
A: The time specified as the next pass time is when the ground station expects to begin receiving new data from the satellite. Technical difficulties may cause data to be delayed or missed entirely.
Q: Why might a pass be missing?
A: Image data passes through several pieces of hardware between the satellite antenna and EarthNow!. There is redundant backup for each stage of processing. In the event of a hardware failure, the ground station operators may have to switch to receiving from the backup data stream. When that happens, the data will not reach EarthNow! because it is only connected to the primary data stream. Also, scheduling conflicts and other types of hardware or software failure may prevent a pass from being received.
Q: Why might EarthNow! fail to run (the screen is just gray)?
A: EarthNow! requires Java version 1.4 or greater to run. If you do not have Java installed or have an older version, please upgrade to the latest version by going to http://www.java.com. If this fails to fix your problem, please contact us for further assistance.
Q: Why does EarthNow! sometimes hang, go completely blank, or elements within the applet fail to function properly?
A: We have found that older versions of Java have problems that cause various errors in the applet. Upgrading Java to the latest version should fix these issues. You may upgrade your version of Java by going to http://www.java.com. If this fails to fix your problem, please contact us for further assistance.