I think the root cause is likely your use of the phrase "urban environment". I suspect you're experiencing multipath or EM interference. Not much you can do about those when surrounded by concrete taller than you. Field practices are key, as in trying to have sky obstructions to your north as the GPS constellation tends to have a hole to the north anyways. Your mileage may vary.
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Eric Collins, P.L.(Eng.), P.Tech.(Eng.)
Operations Technology, Chief Operations Engineering
Calgary, AB, Canada
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