the bit patterns are allocated to different commands and or operating states the computing unit 22 of the mobile unit 16 generates different correlation bit patterns for the cross correlation during an operation in fig4 a possible operating sequence is shown after an initialization 54 the computing unit 22 begins a search 56 for an operating bit pattern by means of the correlation with an operating bit pattern in this context the computing unit 22 stops the mobile unit 16 via the drive unit 40 when the computing unit 22 has found 58 the operating bit pattern the mobile unit 16 begins to work 60 in this context the computing unit 22 continues to correlate the received signal with the operating bit pattern when the computing unit 22 no longer finds 62 the operating bit pattern in the received signal the computing unit 22 stops the mobile unit 16 the computing unit 22 begins the search 56 for the operating bit pattern again when the computing unit 22 does not find 62 the operating bit pattern the computing unit 22 checks whether a maximum search period provided has elapsed 64 when the maximum search period has not elapsed 66 the computing unit 22 continues to search 56 for the operating bit pattern in the boundary signal 14 when the maximum search period for the operating bit pattern in this case for example one second has elapsed 68 the computing unit 22 searches 70 by means of the correlation with a command bit pattern for a further command for example for a return command during the search of the command bit pattern the mobile unit 16 is stopped when the computing unit.