In this paper, a novel multi-channel waveform agile radar testbed - ASTAP (Advanced Space-Time Adaptive Processing) with eight transmit channels and a single receive channel to form a co-located Multiple-Input Single-Output (MISO) radar is presented. Based on current configuration, the ASTAP radar system possesses agility for synthesizing and transmitting independent waveforms via each transmit channel simultaneously via multi-channel Arbitrary Waveform Generator (AWG). Hardware imperfections influence on digitally created waveforms including limited-bits DAC (Digital-to-Analog Converter) quantization, inter-channel time- and phase-skew (and/or jitter), non-linearities introduced by high-frequency devices (RF amplifiers and mixers etc) and antenna dispersion effects has been studied experimentally.An end-to-end system-level digital calibration using Over-the-Air (OTA) channel measurements to minimize system hardware imperfections prior to transmission is proposed. Calibration accuracy for waveform transmit ambiguity functions, orthogonality in MISO transmissions, received signals separation and beamforming process for the synthesis of target azimuth distributions is examined and quantified. Front-end performance with different waveforms types for colored transmission and simultaneous MIMO is demonstrated and analysed. Extraction of the signals corresponding to each transmit channel from the composite received signal in a single receive channel exploiting waveforms orthogonality is studied experimentally.