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GPS, EGNOS and Vista HCx

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.

Andy
MM0FMF

In reality the I estimate the benefit of EGNOS very low: To receive the EGONS sats you need a very good view to the southern sky. But in this case the large number of GPS sats in view is sufficient for good accuracy. With only a few GPS sats in the forest when the fix gets bad do not dare to hope for an EGNOS signal.

In reply to MM0FMF:

strengh bars. Without EGNOS the accuracy of the Vista HCx is +/- 17m
horizontal at best. With EGNOS that is cut to +/- 3m.
Did you read the accuracy display of the Vista HCx or check with a point of known position?
17 m displayed accuracy is not really overwhelming for GPS. My Garmin Oregon 450t under free sky displays values below 10 m typically. I did a quick test today and had 3 to 6 m displayed without EGNOS and 3m with EGNOS (Ds on the sat bars).
Andy, I suspect your increase of displayed accuracy was solely due to the increasing number of GPS sats locked and not due to EGNOS.

73 de Michael, DB7MM - using GPS since 1995

In reply to DB7MM:

Ack.

My old Vista Cx shows a normal accuracy of ±8m. As you know, it uses a less sensitive RX.

The Legend HCx is down to ±5m, usually.

I never used EGNOS, I don’t see much gain in it.

73 Norby

In reply to LX1NO:
Well I have tried egnos today at my qth using the garmin 60csx. I’m at 57 degrees north and 150m asl, standing still facing the same direction I seen no difference with this mode switched on or off +- 4m in both cases.

Adrian
MM0TAI

In reply to DB7MM:

Andy, I suspect your increase of displayed accuracy was solely due to the
increasing number of GPS sats locked and not due to EGNOS.

You could well be right. I think I was more impressed to have been able to receive the EGNOS transmissions from an occluded southern aspect than for any accuracy benefits. Today I was on Creag nan Gabhar (Crag of the goats) GM/ES-022. This is an 834m summit surrounded by many much bigger mountains. My route up was via the North ridge. So for the entire walk I had higher ground to the South. The GPS did not have any EGNOS reception on the way up.

Andy
MM0FMF