T&T: GPS Update Rates

[capteric36]

Hi All,

 Dave wrote:
<snip>
 I remember
when the first GPS's came out. They worked fine 
for taking "sights". Then
the next generation could be left on and would 
update every few minutes.
Then we got "continuous update" models. However, 
if you moved too fast the
GPS position was always well behind you. Aircraft 
units were always way more
expensive to allow faster processors to be used to 
keep the location in the
plane ;-
<snip>

 Well I was working for Trimble Navigation in 1987 
before the first GPS units came out. WE sold the 
first commercially available GPS receivers (The 
10X). They were available with both LORAN and GPS 
boards, for the simple reason that there were only 
3 "experimental" satellites in orbit at that time 
and one could only get a GPS "fix" about 1 or 2 
hours a day.
When the 10X was receiving data from 3 satellites 
you would get a 2 dimensional position fix once 
every second.
 There was jitter (there still IS jitter) but you 
had the ability to set the averaging period 
yourself to allow for smoothing of data, like SOG 
and COG (Speed Over Ground and Course Over 
Ground).
They were tested in cars at up to the speed limit 
(65 MPH then) and the position was between 60 and 
88 feet "behind" the car. That is because the 
process took almost the entire 1 second to compute 
the position. These tests were performed with the 
averaging set to zero.
Trimble Navigation developed the first 
GPS-on-a-chip. The deal with Siemens was that they 
could not steal our design for 10 years. I think 
they violated that deal by about 5 years, but 
who's counting.

 Modern GPS receivers have the luxury of 'seeing' 
satellites at all times, they have much faster 
CPUs, and more well developed software.
They could probably produce a position fix every 
100 milliseconds or faster. There's just no point 
in doing that though.. The 'averaging' is now 
built-in, heck with some GPS units you can't even 
change the DATUM to match your chart's DATUM I 
have heard. I think most of these units SUCK when 
compared to the Trimble GPS receivers I have on my 
boat today.

 But that's just me.

 My $0.02.


Eric Thompson
S/V Procrastinator
South San Francisco
capteric36 at sbcglobal.net