Why are some GPSs so much better than others?

From: Peter (peter_at_no-spam-please-6644.co.uk)
Date: 08/04/04 

Date: Wed, 04 Aug 2004 10:54:15 +0100

I don't have many data points but this is what I see:

I have a Honeywell KLN94B panel-mounted GPS which I have been flying
with for 350 hours and it has been 100% perfect, with even the
altitude being within 20ft of true airfield elevation most of the
time.

I have a Socket bluetooth GPS from socketcom.com (SIRS chipset??)
which returns the altitude about 300ft too high (always) but more
importantly it loses signal very easily. Even under a composite
aircraft roof it doesn't receive anything. This unit is frankly
rubbish; nothing short of a very good sky view works, and even
locating it outdoors but within a few feet of a house is likely to
make reception poor. (I am using it with Oziexplorer which has a
satellite display feature)

A friend has a cheap Garmin $100 camping-shop 12-channel GPS which he
can chuck in his bag and chuck that in the back of an all-metal
aircraft, with almost no view of the sky, and it works perfectly,
judging from the track it has recorded.

I also have a Skyforce (now Honeywell) Skymap 2 8-channel handheld
with an external (suction cup) antenna, which over about 100hrs has
been perfect.

Being an electronics hardware/software engineer I can understand a lot
of the issues with reception, but why such a huge variation in
sensitivity?

It appears the Garmins are very sensitive indeed, and perhaps as a
result they work "a bit too reliably" in that there is a persistent
group of aviation (light aircraft) users who have occassional failures
and tell everybody that GPS cannot be relied on. Most of the units in
aviation use are Garmins and nearly all of them have integral
antennas. The typical usage is in an all-metal aircraft with side
windows; hardly ideal.

I also wonder what is the point of a 12-channel GPS when most of the
time there are 8 or fewer satellites in the sky.

Peter. 

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