OK so I've decided to sell my Magellan Meridian Platinum because it's complicated, big and uber clever. I prefer a map and compass and rarely use the gps unless I'm unsure of position and short on time to sort it out the 'real' way. So I went over to the local shop and bought a Garmin Foretrex which seemed to be everything I needed...It's small, it gives me a position if I need it. Or does it...?

I decided that I would start using the gps a bit more as a monitor of exactly how far I'm travelling, where I'm losing time etc etc. I suppose a lot of people do that...upload their route after a day out/weekend, and have a closer look at their movements and times. Of course you can make an effort to do all that stuff 'manually' but it's nice if its all done for you isnt it?

Anyway, I took the foretrex out for a couple of short local tests, in what must have been the best weather we've had for months. I didnt hide underground or push it down rabbit holes or anything else unfair, but it simply cant keep lock on the signals. OK fair enough, its midget, and cute, and clearly doesnt have a great big twitching antenna, but when it gets 'weak signal' it invariably sticks a track point literally kilometres away from where you are. If you download the track to a pc you can obviously remove the rogue track point. But say you actually wanted to use the backtrack function...You'd be ****ed. Garmin tech support were unable to provide a solution to this.

So I stretched, warmed up carefully and hefted the monster magellan onto my shoulder and set out on the same route. The magellan is not neccessarily 'more accurate' because at their best, the two units produce very similar positional inaccuracies, ie the usual few metres here or there. What does change is that firstly it's a lot keener to keep talking to the satellites, and secondly when you deliberately mess it about in whatever way, it doesnt produce any rogue track points.

So presumably this is a software issue no? The magellan must have a bit of code that tells it to wait until it has decent information before logging any more track points. Is this then a problem for ALL Garmin units. Maybe someone with one of the all singing all dancing ones could reassure me...They are smaller than the magellan and I might go for another, but not until I feel a bit more confident...

I rather feel like at the moment I really need the things, they wont actually work properly. A big part of me thinks 'bin the things. They're rubbish and you can do perfectly well without' and another part says 'buy the expensive gadget, you know you need it and want to play with it'. I think its only honest to admit that the voice of the evil gadget demon will shout louder.

Any theories?

J

My foretrex doesn't do this - holds lock well, and when it loses it there's no daft waypoints. Do you wait till it gets 6-8 satellites before starting your trip? This improves lock for me, even when running in Central London. can you swap it for a replacement unit?

Well I could, but when I raised the issue with Garmin support they seemed to think this was not a fault, but rather a fact of life.

I wonder.

J

I suspect that the manufacturers of smaller GPS units have to make huge compromises when it comes to the antenna.

The GPS receiver aerial has to be omni directional, ie it has to receive signals from horizon to zenith. Signals from the horizon will be attenuated compared to signals from the zenith. It is virtually impossible to build an aerial that will perform this function and respond equally to signals arriving from any angle, so any aerial design has to be a compromise that will allow a sufficiently usable signal to be presented to the receiver electronics. (“Sufficiently usable” is the operative phrase here).

GPS is broadcast in the L-band at 1500 MHz. This gives a wavelength of 19 centimetres. A quarter wave aerial works out at 4.75 cm, about a third of the long axis of my Magellan Sportrack! Magellan gets around this by using a "quadrifilar" aerial, basically a spiral, but this is also a compromise.

The manufacturers data do not allow a choice to be made, so the only way to determine which GPS is best for your needs is to read as many comparison reviews as you can. This led me to choose the Sportrak but I’m sure others will disagree!