[GPS_Standard] strange exponents, GPS tips

[Bert, VE2ZAZ]

Hello to All,

Regarding the very small value displayed in the accumulated average field, I
saw it too yesterday evening on my system. I will look at the Montrol code
and try to figure out what is happening. In my honest opinion, since it is a
rather simple math formula, it could be a glitch in the way that Delphi
deals with floating point calculation and divisions. Butr I'll try to find a
fix.

Best Regards,

Bert, VE2ZAZ

----- Original Message -----
From: "SP2IQW" <michal at e2000.gdynia.pl>
To: "Bert, VE2ZAZ" <ve2zaz at sympatico.ca>
Cc: <gps_standard at mailman.qth.net>
Sent: Wednesday, April 01, 2009 7:39 PM
Subject: strange exponents, GPS tips


Hi Bert and all,

Few times I have also noticed irrationally small numbers as 10-27. I'm
enclosing few printscreens but unfortunately log was not switched on
during any of these issues.

Soon I will assemble next version of your GPSDO now with Oscilloquartz
8663 DOCXO. I have added a clean PWM voltage reference with CMOS buffer
so it is no suffered from voltage fluctuation coming from rest of
digital ciruits. Other modifications added by Rafal SQ4AVS are 5/10MHz
doubler and different output buffering with transistor voltage
followers. Our schematic is enclosed too.
I would recommend Trimble Lassen iQ receiver which has really good
±50ppm PPS specification. This number is even better in static mode. Low
and stable current consumption (below 30mA at 3.3V) is a big benefit
especially in portable operation. Many GPS draws over 100-150mA during
acquisition. It concerns most of SiRF based as well latest uBlox Antaris
5 (Atmel chipset) modules.
I doubt if expensive timing GPS receivers will give any significant
improvement to VE2ZAZ design compared to iQ receiver. Counter is feeded
with 100ns (10MHz) period so PPS with 15-25ns insted of 50ns accuracy
should give every time the same count.

Here are few GPS configuration tips to achive best timing performance:

- use static navigation mode
- set elevation mask >5°
- set signal level mask C/No >30 dBHz
- use fix-based PPS (generated only on position fix)

Be sure that GPS antenna will see rather unobstructed sky. Window
operation with half of sky shadowed don't warranty that GPS will have
all the time enough good located sattelites to make a fix.

By the way, interesting information which is often missed.
Within NMEA frames only ZDA is strictly related with PPS pulse it
follows! The time information in RMC or GGA frames may be 1s late in
some cases.


73, Michal SP2IQW

PS - I'm not in any relation with Trimble, just using proffesionally
from years.



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