Non-line-of-sight (NLOS) reception is the major cause of large GNSS position errors in dense urban environments. Multipath interference is also a severe problem. With a multi-constellation GNSS receiver there is scope to select the best signals for positioning and reject the rest. However, the challenge is to identify which signals are contaminated by NLOS reception or severe multipath errors and which are not. Under this project, a new NLOS detection technique using a dual-polarisation antenna has been developed and demonstrated. A new consistency-checking technique based on subset comparison has been developed and shown to perform better than the conventional sequential elimination approach. A new multipath detection technique using three-frequency carrier-power-to-noise-density ratio measurements has also been developed. This activity is part of the Innovative Navigation using new GNSS Signals with Hybridised Technologies (INSIGHT) collaborative research program (www.insight-gnss.org). Project duration: 2010-2013; Sponsor: EPSRC; Researcher: Dr Ziyi Jiang.