This is what @texaspyro meant when he said crank up the elevation and SNR mask for the most precise timing, it also works for positioning. But you need a good antenna, a timing antenna usually works really well.. if you use a patch antenna use a larger ground plane than people often use, because that will translate to a stronger signal, use the
elevation mask to eliminate
multipath (positional error from reflections)
Note the way the mask is set. This is in RTKlib, so not for timing, but you can set elevation and SNR mask similarly on any GPS, you just need a strong unobstructed signal.
http://www.navsparkforum.com.tw/viewtopic.php?f=26&t=214
A good antenna is very important for good RTK performance.
The reason for the ground plane around patch antennas is to give them a more stable phase center and to combat multipath reflections. To get a RTK solution the rover compares the phase differences between the base station and local receiver and there is only one point in space where the phases from all satellites happen to match up. Bad antennas can change there phase centers depending on what angle the signal comes from, this causes a bit of offset to be added to the phase of each satellite. Due to these phase measurement errors the RTK solution is no longer a single point in space, but it smeared out into a cloud where the phases are close to matching up but never match exactly. With the solution not being so firmly defined the algorithm can have problems finding where this cloud is located within the noise.
Low elevation satellites pose a similar problem because there signal goes trough a lot more of the atmosphere on the way to you. As you know speed of radio propagation trough air can vary ever so slightly and this again can introduce a phase offset large enough to confuse the algorithm.
These problems should be alleviated soon now that Ublox has made a RTK GPS receiver that also makes use of the L2 band. Its not actually decoding L2 but just using the phase information of the L2 carrier to better predict phase errors and fix them.
Multi band GNSS's are not ublox's invention, they are one of many companies making them. But hopefully ublox with their high volume will mean the price comes down. They have been available for the high end market for a while. They will need a broadband antenna with an accurate phase center as well. That may mean some kinds of 'frequency independent' gnss antennas that I have been interested in for awhile may become available at affordable prices. I hope so.
Yes the price is the kicker. This is the first affordable L2 capable GPS module and someone as big as them will certainly turn the market around.
There is nothing inherently expensive about being L2 capable, its just that everyone kept selling them for a few grand. Its a similar thing as is happening with thermal cameras. Yes the sensors in these cameras have been very difficult to manufacture, but after so many years of development they improved the manufacturing process a lot, allowing them to get good yields and produce bigger sensors. Yet few manufacturers make these sensors and they all keep selling them for half a grand and up (For anything with nice resolution).