With all the interest in Garmin Vista HCx devices recently I’ve been Googling a bit for comments on the unit. One that I saw was speculation that EGNOS doesn’t work with it.
EGNOS is the European Geostationary Navigation Overlay Service. Essentially a number of base stations in known locations receive the normal commercial GPS signals and calculate the current error between their known and reported positions. This error correction signal is uplinked to the EGNOS satellites and is retransmitted for reception on any EGNOS capable GPS receiver. Unlike the GPS satellites which are in a MEO and hence moving across the sky, the EGNOS satellites are fixed. The higher your latitude, the closer to the Southern horizon the EGNOS satellite will be. This means you need a good view of the Southern horizon which can be hard to get in a built up area.
On the Vista, you need to enable WAAS/EGNOS in the setup menu. (WAAS is the US Wide Area Augmentation Service) When you receive an EGNOS signal, a small letter D appears at the bottom of the satellite signal strength bars. Without EGNOS the accuracy of the Vista HCx is +/- 17m horizontal at best. With EGNOS that is cut to +/- 3m.
At my latitude (56N) the satellite is only 34deg above the horizon. I live 170mASL on the North facing side of a hill, so anything below 10deg is obscured. Also there are 2 storey houses around me. However, stood in the front garden it took around 30secs for my GPS to gain lock (it hasn’t been used since Jan 1st) and 8 satellites were visible. The satellite display showed no satellites visible to the Southern aspect which was what I expected as my house was in that direction. However, after about 3 mins, a small D appeared at the bottom of each strength bar and the reported error changed to +/3m. I was impressed at this as the L band signal for the EGNOS satellite would have to travel through either mine or my neighbours’ houses. Or possibly I was receiving a reflection from the houses in front of me. Either way with some obscuring of the Southern aspect, EGNOS was receivable and did exactly what it says on the tin. I was suitably impressed.