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 gridcal -- compute the great-circle distance between two grid locators
 ======================================================================
 
-`gridcal` computed the short-path great-circle distance between two grid
+`gridcal` computes the short-path great-circle distance between two grid
 locators. The Grid Locator System divides Earth into a series of
 rectangles identified by a combination of letters and numbers, so that
 the approximate position of a point can be expressed with a short
 identifier like IO91vl.
 
+Usage: gridcal <loc1> [<loc2> [<pwr>]] 
+
+If `gridcal` is given only one locator, it prints on output the latitude
+and longitude of the centre of the corresponding grid square: 
+
+    gridcal IO91vl
+    IO91vl 51.4792 -0.208333 0.898481 -0.0036361  
+
+With two locators as input, `gridcal` prints on output the two locators
+and their great-circle distance, both in km and miles:
+
+    gridcal IO91vl JN65od
+    IO91vl JN65od 1213.71 754.164
+
+If `gridcal` is given as third numerical argument, that number is
+considered a "power" value in Watts, and `gridcal` will print on output
+a longer line containing the two locators, the distance in km and miles,
+the power value, and the corresponding km/W and miles/W:
+
+    gridcal IO91vl JN65od 1.2
+    IO91vl JN65od 1213.71 754.164 1.2 1011.42 628.47
+
+The latter can be useful to check if a QSO with another ham radio
+station qualifies for one of the thousand miles per Watt awards.
+
+
+Dependencies
+============
+
+`gridcal` is written in ANSI C has no dependencies.
+ 
+