  Space Very-Long-Baseline-Interferometry (S-VLBI) observations at high
frequencies hold the prospect of achieving the highest angular resolutions and
astrometric accuracies, resulting from the long baselines between ground and
satellite telescopes. Nevertheless, space-specific issues, such as limited
accuracy in the satellite orbit reconstruction and constraints on the satellite
antenna pointing operations, limit the application of conventional phase
referencing. We investigate the feasibility of an alternative technique, source
frequency phase referencing (SFPR), to the S-VLBI domain. With these
investigations we aim to contribute to the design of the next-generation of
S-VLBI missions. We have used both analytical and simulation studies to
characterize the performance of SFPR in S-VLBI observations, applied to
astrometry and increased coherence time, and compared these to results obtained
using conventional phase referencing. The observing configurations use the
specifications of the ASTRO-G mission for their starting point. Our results
show that the SFPR technique enables astrometry at 43 GHz, using alternating
observations with 22 GHz, regardless of the orbit errors, for most weathers and
under a wide variety of conditions. The same applies to the increased coherence
time for the detection of weak sources. Our studies show that the capability to
carry out simultaneous dual frequency observations enables the application to
higher frequencies, and a general improvement of the performance in all cases,
hence we recommend its consideration for S-VLBI programs.
