Context : There is growing evidence of relativistic jets in radio-loud narrow-line Seyfert 1 ( RL-NLS1 ) galaxies .

Aims : We constrain the observational properties of the radio emission in the first RL-NLS1 galaxy ever detected in gamma-rays , PMN J0948+0022 , i.e. , its flux density and structure in total intensity and in polarization , its compactness , and variability .

Methods : We performed three real-time e-VLBI observations of PMN J0948+0022 at 22 GHz , using a global array including telescopes in Europe , East Asia , and Australia .
These are the first e-VLBI science observations ever carried out with a global array , reaching a maximum baseline length of 12 458 km .
The observations were part of a large multiwavelength campaign in 2009 .

Results : The source is detected at all three epochs .
The structure is dominated by a bright component , more compact than 55 \mu as , with a fainter component at a position angle \theta \sim 35 ^ { \circ } .
Relativistic beaming is required by the observed brightness temperature of 3.4 \times 10 ^ { 11 } K. Polarization is detected at a level of about 1 % .

Conclusions : The parameters derived by the VLBI observations , in addition to the broad-band properties , confirm that PMN J0948+0022 is similar to flat spectrum radio quasars .
Global e-VLBI is a reliable and promising technique for future studies .