I'm still utilizing the Datum 9390-6000 and it's still locking on to 6-8 SV's everyday! Good manufacturing. We recently bought the Fluke 910R to replace the Datum. Still waiting on the maintenance department to run the cable and replace the antenna with Fluke's. I think the 910R meets the specifications you require. It's a GPS that can also be used as a stand alone Rubidium standard that's pretty accurate itself.
I find it very interesting how the cesium atomic resonance occurs and how stable cesium 133 works with the atomic clock. I saw a picture of an old atomic clock in a huge laboratory. It was pretty massive. It's incredible that every GPS satellite in space presently contains four atomic clocks!!
I calibrate and repair a bunch of Spirent (GNSS) GPS Simulators here that peak at 1575.42MHz. They actually simulate satellite constellations. These simulators cost around $600,000! It's crazy to me to be working on a piece of equipment that is worth a massive house depending on where you live!