[FOSS-GPS] Recommended receivers for precise fixed relative measurements

[Stewart Nelson]

I'd like to measure several points on a small (~ 70 m X 140 m) property, 
with 5 cm or better accuracy, relative to a reference receiver in the 
center of the field.

Can you please recommend suitable USB-powered receivers, compatible with 
RTKLIB or other free or inexpensive software?  They must be readily 
available (in stock in US) and if possible under $200 for the pair, 
including external antennas if required.

There is good Wi-Fi Internet coverage over the property.  The laptops 
that will power the receivers can boot to Windows 10 or Ubuntu 14.04.

If needed, the receiver pair can be left several minutes at each 
location to be measured.  Though absolute positions are not required, 
there is a recent survey marker whose coordinates could be entered.

I see some really inexpensive units that get good reviews (though not 
for precision use), e.g. GlobalSat BU-353-S4 at ~$30 each delivered.  
However, it appears that RTKLIB can't read the SiRF binary format, and 
its NEMA output doesn't include carrier phase info.  What accuracy could 
be expected from hardware like this?

Thanks,

Stewart