An unknown antenna model can lead to biases in your trajectory solution since the antenna phase center (APC) to antenna reference point (ARP) offsets cannot be taken into consideration. If you are using a GNSS base antenna you have not used before, you should check the Base Station Information section of the POSPac report (Figure 1) to make sure POSPac recognized your GNSS antenna and that it is not “unknown”. If these fields shown in figure 1 say Unknown, then a calibration file is not available and your trajectory solution could contain biases that range from minor to quite significant depending upon the specific antenna model.
If you are seeing UNKNOWN, but there is a model specified in your RINEX or T02 file, and it is a newer model, then you may need to update the antenna database on your machine. Update the antenna database using the Trimble Office Configuration File Update Utility.