[Thumbgps-devel] Article -- Macx-1: GPS receiver with standard USB connector and PPS support

[Eric S. Raymond]

Ron Frazier (NTP) <timekeepingntplist at techstarship.com>:
> * Hi sensitivity internal antenna / chipset that can be used indoors
> in a residential type structure.

The Macx-1 already has this.  It's sitting on my desk 5 feet from a window 
blocked by large trees and hasn't lost lock once since it got here. Gary
says it's working just as well in his basement.

Truly, this is the most indoor-friendly GPS I've ever seen.  Puts the
general run of SiRF mice to shame, and raises my opinion of uBlox a
*lot*.

> * External antenna port in case you're in a steel building or
> basement or something with no signal.  Use commonly available,
> generic, possibly powered, antennas.

Doesn't have it.  Doesn't seem to need it!

> * GPS status led.  (My BU-353 slowly flashes when it has a fix,
> shows steady with power but no fix.)  The led should be synchronized
> to the pps.

Has it.

> * Supercap preferably, or battery which is easily user replaceable
> and a common part number, such as CR2032 for example.  That battery
> may be too big, but you want something readily available.

Doesn't have it.  But the unit price is low enough that I don't care;
the widget is disposable.

> * If a battery is involved, a small flashing led to indicate a weak
> battery alert, kind of like what is on a smoke alarm.

Doesn't have it.

> * My BU-353 has a magnetic base.  Something like that might be
> useful for positioning the antenna.

Has it.

> * May or may not be relevant, but, would 5 - 10 Hz position sensing
> improve accuracy and stability for your purposes?

No, not for a timing GPS.  And modules with multiHz sampling are tres
expensive.

> * You may wish to expose the pps signal on a screw terminal or
> connector in case you want to attach other instrumentation to that
> signal.

You may wish it.  I don't.  I *like* the sealed case, the low parts 
count, and the rock-bottom cost.  For the kind of deployment we're
doing, this design is optimal.

> Here's a possible alternative to providing an external antenna port,
> and all the hassles of piping RF around.  With my BU-353, the "puck"
> device contains both the electronics and the antenna.  The only cord
> coming from it is the USB cable.  So, it might be preferable to have
> a sealed weather proof unit, and just use extension cords for USB
> rather than extending the RF section.  I know you can easily extend
> USB 10 feet or so.  I think you can go much further with powered
> hubs and such.  Just a thought.

Way ahead of you.  This is why I *didn't* spec an antenna.  One cable
is simpler (therefore better) than two.

> Here's a wild idea, maybe not practical, but, you could attach a
> solar cell to the external antenna and / or the thumb-gps case.
> This could feed power back to the GPS board and prolong the life of
> the battery or slow the drain of the supercap while powering the
> antenna if the antenna is in the sun.  You could even draw some
> energy from ambient indoor lighting.  I have a solar powered
> "atomic" watch from Casio that does this.  It never needs a battery,
> never needs charging, and never needs setting.  Very handy.  I also
> have two "atomic" wall clocks that do the same thing.  I think they
> use a rechargeable battery instead of a supercap.
>
> Here's another idea, which may or may not have already been
> discussed.  Do you want to have an on board RTC, which can keep
> outputting valid time for a while, if the satellites are not
> available due to weather, jamming, interference, etc.?

Custom hardware.  Expensive.  Not suitable for bufferbloat deployment.
-- 
		<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>