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NASA Information Contact:
Michael Goodman, Global Hydrology and Climate Center

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The Coincidence Search Engine

The Coincidence Search Engine may be used to search for times when one to four satellites were within the same geographic location simultaneously. Searches may be constrained by time, geographic area, and/or distance between the sub-satellite points. Please note that this program does not take into account instrument angle of view.

The program uses an archived database of element set data to calculate the orbits, using a model supplied by the Colorado Center for Astrodynamic Research (CCAR).

Due to national security restrictions, the Coincidence Search Engine may no longer be used to predict satellite coincidences. It may, however, still be used to compute past coincidences.


Satellite Selection

The satellite selection area lists which satellites may be selected with a checkbox to the left of each satellite's name. To select a satellite for a search, click the mouse on the checkbox. Up to four satellites can be selected simultaneously. At least one satellite must be selected before pressing the "Execute" pushbutton.

A "hit" or coincidence will be recorded when all of the satellites selected satisfy the time and geographic search criteria. Each hit is listed as the date and time, satellite name, and longitude and latitude of the satellite.

Time Interval Selection

The time interval selection block is used to bound the time period of the search. It contains boxes for the start date, start time, end date, and end time, as well as a time interval.

Enter the start date for the search in the start date box. The default value is the current date. Use yyyy-mm-dd or yyyy/jjj format.

Enter the start time for the search in the start time box. The default value is 00:00:00.

Enter the end date for the search in the end date box. The default value is the current date. Use yyyy-mm-dd or yyyy/jjj format.

Enter the end time for the search in the end time box. The default value is 23:59:59.

Enter the time increment in the increment box. The default value is 00:00:10 (10 seconds). If the proximity criterion in the area of interest block is used ("Within (km)"), the increment value should be no greater than 0.075 times the proximity value to ensure that an overpassage is not missed.

Area of Interest Selection

The top and bottom boxes define the north and south boundaries of the area, respectively. Enter a latitude value in degrees, from -90.0 to 90.0. The northern boundary must be north of the southern boundary and the boundaries must not be equal.

The left and right boxes define the west and east boundaries of the area, respectively. Enter a longitude value in degrees, from -180.0 to 180.0. Negative longitude values are west, positive values are east. The area of interest extends from the west longitude eastward to the east longitude. So, for example, if west is -179 and east is 179, the area encompasses all of the world except a two-degree interval along the date line. However, if west is 179 and east is -179, the area only encompasses the two-degree interval along the date line. The west and east values may not be the same, nor may west be 180 while east is -180.

You may constrain the search to times when the great-circle distance between the subsatellite points does not exceed a given value. Enter a positive number of kilometers in the "Within (km)" box. No entry or a value of 0.0 disables the proximity criterion. If the proximity criterion is used, the increment value in the time interval box should be no greater than 0.075 times the proximity value.

Execute Pushbutton

When all of the search criteria are set up, press the "Execute" pushbutton to start the search. The program first checks the search criteria to make sure they are valid.

The elements databases for the selected satellites are opened and advanced to the most recent element set entry that does not exceed the selected starting date. If the database does not contain orbital elements prior to this date, an error message is displayed. If the end date extends past the life span of the satellite, a warning message is displayed, but the search continues. Extrapolations of more than a few weeks become increasingly inaccurate for most satellites.

The search is performed as follows:

  • The positions of the satellites are computed for the "current" time (initially the start time).
  • If any of the satellites are not within the area of interest, the search is not satisfied. Note that the satellites are always within the area of interest if it specifies the entire world.
  • If a proximity value was specified and all of the satellites are within the specified number of kilometers of each other (great-circle distance of their subsatellite points), then the search is satisfied.
  • If a proximity value was not specified, then the search is satisfied.
  • The "current" time is advanced by the increment and the process is repeated until the "current" time exceeds the end time.

Whenever the search is satisfied, an entry is made in the results area for each of the satellites that satisfied the search. This entry has the format

    yyyy-mm-dd hh:mm:ss yyy.yyy zzzzz.zz satellitename

"yyyy-mm-dd hh:mm:ss" is the UT, and "yyy.yyy" and "" specify the satellite's subsatellite latitude and longitude, respectively, and "zzzzz.zz" specifies its approximate altitude above sea level in kilometers.

You may cancel a long search by pressing your browser's "Stop" button.