ERC Decision of 10 March 1999 on the Adoption of Approval Regulations for Equipment to be Used for Sub-STM1 Digital Radio Relay Systems (DRRS) Operating in the 13 GHz, 15 GHz and 18 GHz Frequency Bands with About 28 MHz Co-Polar and 14 MHz Cross-Polar Channel Spacing, Based on the European Telecommunications Standard (ETS) 300 639