On 8/21/12 9:53 AM, Sarah White wrote:
Wow. Okay. The user manual actual considers this cable delay to be worth
mention?
I can see why the trimble thunderbolt is a favorite among time nuts <3
I'm sold.
Cable time offset is in basically all GPSes. An awful lot of GPS
receivers (for timing) are installed where there's a long run of coax to
the antenna. 100 ft wouldn't be unusual, and that's 120-150 ns in
typical coax. Since even the lamest GPS receivers put out a 1 pps that's
good to tens of ns, you might as well offer the correction.
If all you're interested in is frequency, then you may not care about
the correction. If all you're interested is time to, say, microseconds,
likewise (until you get up into the 1000 ft of cable range).
But, since it's a basic parameter in the GPS engine's calculations, and
some applications need it, you might as well always have it. I always
thought it was weird to have that feature available in a handheld GPS
with a fixed antenna, but I was at a tradeshow asking a Trimble guy
about it some 20 years ago.. Here is a paraphrase of the reasons:
1) Our competitor has a removable antenna and they need the cable
correction, so we need the cable correction so that in a comparison
chart we both have a check mark in that "Cable Length Correction".
2) We use the same chip set and software in multiple receivers, and it's
easier to just keep it the same.
3) We have a reradiator thing that lets you use an external antenna a
cable, and a coupler to the handheld, and for that, the length
correction is useful.
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