There's a variety of inexpensive little GNSS receiver chips. The UBLOX F9P is often used by DIY autosteer systems. It has a USB connection, 2x UART serial connections, and an I2C connection. You can buy a variety of little hobby type boards that have some basic circuitry to power and break out the UBLOX F9P to IO headers. I've got like 5 of them, base station, tractors, a portable/spare one.

There's a bunch of open source autosteer PCB's onto which you can plug in F9P's for autosteer.
This one is probably the most widely supported: https://www.ardusimple.com/product/simplertk2b/
This one is quickly catching up and I predict will soon exceed support. Same chip, smaller footprint: https://www.ardusimple.com/product/simplertk2b-micro/

UBLOX has a software called U-Center. Using U-Center you can configure which port does what at what speed, what messages are sent, etc. So you can configure one serial port for corrections and the other port for NMEA data.

Ardusimple also sells weatherproof patch antennas (ANN-MB) and little plastic boxes. The first F9P I linked uses an Arduino footprint, so you can use an Arduino box. Questionable weatherproof-ness on those boxes, you are probably better off making your own.

I have the second one I linked and a Pi mounted in a metal box on my chimney. The box has 6 screws and an oring seal, proper cable glands, and desiccant packs inside. It seems to be holding up well.



I don't know what you are planning to do, but if all you are trying to do is make some RTK corrected straight lines or something, then check out AgOpenGPS. Its a free windows autosteer program. You can just connect an F9P via USB cable and use the program as a lightbar. If you have NTRIP corrections then AgOpenGPS can sort that out and get the corrections into the F9P without messing around with a serial wires.