The searching tool is called gsc. It is a c-program designed to operate with the compressed version of the GSC. The structure of gsc is the following.
First, commands and parameters on the command line are parsed and controlled for validity. Then the list of all the regions intercepted by the searching cone is found. In this phase gsc makes use of one of the auxiliary tables provided with the original catalogue (the list of regions), previously indexed on declination as part of the loading process.
The next step is to decode the selected regions. To speed-up the process only ra and dec are decoded at this stage. Only the records with coordinates within the searching cone are extracted and fully decoded. Additional manipulation of the extracted records is performed according to the options given on the command line.
The response time (elapsed time) for a search in a field of 10 arcmin in radius is usually of the order of 0.1 to 0.3 seconds, depending on the number of regions involved and the number of objects retrieved.
In order to allow an access to this compressed version of the GSC from any computer without logging in at cdsarc, a client/server mechanism was installed: the server search program is located on the cdsarc machine, and only the query (field center and radius, plus additional options) and the results (found stars) are trasmitted over the network. The client can be anywhere on Internet. The typical answer time for a query as described above from the US west coast is around 5s. The client program allowing the query from abroad computers is available for distribution at CDS (contact person F.O.).
The syntax of the gsc command is the following:
gsc -options parameters
or
gsc ra dec radius[1, 2]
(in this case: ra in decimal hours, dec in decimal degrees)
Options and parameters available for the query are described in the following table.
| options | parameters | format/units |
| c | ra +/- dec | free format |
| r | radius[1, radius2] | arcmin (default radius =10') |
| f | file-name | file with coordinates |
| (available only for local users) | ||
| p | 0 | fields as in the GSC, separated |
| by spaces (default); | ||
| 1 | as above, with ra dec in string format; | |
| 2 | summary of GSC record | |
| (ID, RA, DEC, mag); | ||
| 3 | as 2, with ra dec in string format. | |
| h | - | prints header for output columns |
| s | [field no.] | sorts output according to specified |
| field. Field numbers are from 1 to 12. | ||
| Default field is 11 (= sort by distance) | ||
| If field>0 sorting order is ascending, | ||
| if field<0 order is descending | ||
| n | no.of lines out | Number of sorted lines in output. |
| Max lines set by option l. | ||
| Option n forces option s. | ||
| l | max no.of lines | size of buffer for sorted output. |
| Default = 1000 lines. | ||
| m | mag1 mag2 | limits in magnitude (mag1<mag2). |
If coordinates (or file with coordinates) are not specified, ra and dec are read from standard input. Distance from centre (d in arcmin) and position angle (pa in degrees East of North) are always provided at the end of the printed record.