In this work we present an implementation of a fully functional IEEE 802.11p transmitter in software-defined radio. We describe the rapid-prototyping methodology that was used to implement the frame-encoder within the open-source GNU Software Radio (GNURadio) platform [1]. The encoder generates OFDM frames in digital complex base-band representation and uses the USRP2 [2] as digital-to-analog front-end for up-conversion and final transmission. Since the actual encoding process involves a large number of complex steps we split the development approach into three sequential stages. First, a reference-encoder in a high-level language (MATLAB) is derived from the IEEE standard documents. Second, the individual blocks of the MATLAB encoding chain are progressively ported to GNURadio, cross-checking with the reference after each step. Finally, standard compliance is verified by conducting comparative over-the-air measurements with an early prototype of a commercial 11p transceiver. Initial measurement results indicate that the fidelity of the resulting GNURadio implementation is on par with non-software-defined radio industry solutions and capable of generating truly standard-compliant OFDM frames. The encoder presented here has been released under GPLv3 and is also capable of encoding frames according to the 11a and 11g amendments, thus making it a valuable building block for upcoming software-defined radio projects.