The combo of GPS+[secondary satellite system] seems to be the key. Once Galileo is completely fleshed out it should be the go-to for most consumer-grade navigation as its accuracy is far higher than GPS, Glonass, or Baidu at the non-military implementation level. Until then, combining two or more systems seems to work quite well.
Living in the District of Columbia, the GPS system is purposely more crippled than in other parts of the U.S. due to the density of high-importance governmental facilities. It takes a long time for the GPS-only Edge 500 to acquire a strong enough signal to start my rides (I live approximately equidistant from the White House and the Naval Observatory, where the VP lives). I'd often have to wait at least 4-6 minutes to get a signal lock.
The Edge 520, on the other hand, uses GPS+Glonass. It locks onto a strong signal within 40-50 seconds, and often less than that. Sure, the GPS chipset in the unit is more modern than in the 500, so it should be faster to acquire a signal. But testing the unit with only GPS in play shows that it still takes 3-4 minutes to get a strong enough signal to go - so there's obviously some obfuscation muddying the triangulation.