[GPS_Standard] GPS_Standard Digest, Vol 35, Issue 6

Thu Jun 17 03:36:03 EDT 2010

Michal, thank you very much for the interesting and detailed info.

Roberto EB4EQA

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Today's Topics:

   1. Re: preferred GPS - Trimble modules, GPS tips (SP2IQW)

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Message: 1
Date: Wed, 16 Jun 2010 00:14:14 +0200
From: SP2IQW <michal at e2000.gdynia.pl>
Subject: Re: [GPS_Standard] preferred GPS - Trimble modules, GPS tips
Cc: gps_standard at mailman.qth.net
Message-ID: <4C17FB36.60600 at e2000.gdynia.pl>
Content-Type: text/plain; charset=UTF-8; format=flowed

Hi Bert and Group,

I'm sorry for a bit long text but I would like to share my expierience
gained when I have worked with the GPS receivers.
Working with AVL systems, I have designed a few boards with the latest
Trimble GPS modules. A new types are very interesting as a low cost, low
power 1pps engines.

Timing GPS receivers are more expensive so in my opinion for typical HAM
radio applications up to milimeter waves most important is to get a good low
noise (D)OCXO.  Please remember that short time stability as well low phase
noise depends mainly on quality of OCXO. Added GPS support takes care about
long time stability only.

First module I have used for GPSDO application was the Trimble's Lassen iQ.
Sized 26x26x6mm, features timing accuracy of ?50 nanoseconds when valid
position fixes are being reported.  Very interesting is power consumption
100mW which makes it ideal type for portable application.

The next module from Trimble launched on the market is Copernicus II.
Thumbnail-sized 19x19x2.54 mm, less than 94mW power consumption with
increased sensitivity is specified as ?60 ns RMS (in static mode, please
note explicity stated this type of operation) and ?350 ns RMS (in indoor
aplication, static mode -145dBm signal). But don't use indoor mode to
discipline your standard !

The most promising is the latest Trimble's Condor receiver. It is available
in different shapes for easy use as an upgrade in an old design PCB's (as
for example replacing Lassen-iQ). Very promising is ?25 nanoseconds PPS
specification, close to dedicated timing models (although without T-RAIM
functionality).

All these types as well as mentioned by Stig OZ3XO Resolution-T are 3.3V
CMOS output devices. There are many ways to interface 3.3V CMOS logic to 5V
CMOS compatible input of a PIC microprocessor. In my implementation of the
iQ receiver I have used two inverters - the first, the 74HCT series and the
second, the 74HC series with 1k pullup resistor between them.
In another place I have used common base transistor (BC847B/2N2369A)
shifter, with base biased at 1V by 2.2k/560 voltage devider decoupled by
.1uF capacitor, emitter connected to output of the GPS module and colector
to 5V level logic input with 1.5k - 2.7k pullup resistor. It is rather
tricky so better is to use logic level shifters.
Limited rise time is not a critical case as eventual jitter is considerable
lower than 1pps granularity. Important is to get proper voltage swing.

One cent about PPS accuracy. Nicely looking numbers as for example ?25ns or
?60ns don`t mean that pulse by pulse it will be within this specification.
It describes statistic value which is just of our interest in GPSDO
application.
The granularity of PPS pulses depends on the internal GPS clock. For a 16MHz
clock the garnularity is about 67ns so it is the worst case pulse position
innacuracy or half of this number in some cases.

Regardless of which GPS type you choose there are several tips you have to
keep in mind to get most from the GPS receiver:

- provide good visibility of satellites, it is worth to place an active
antenna on the roof and go down with signal with low loss cable.

Low cost antennas are terminated with a RG174 cable which adds 1dB/m
attenuation. Shortening this cable to 20-50cm and expanding lengh with
10-20m of good quality eg. RG8-series cable will still provide a good signal
(use SMA or N connectors).

- don't think that a window position will give you reliable GPS operation!
GPS constelation provides at least 6 satellites in your area but don't
guarantee that 3 satellites will be "visible" from your window.

GPS receiver, configure as follow:

- use static navigation mode (Dynamics > stationary)

- limit use of low elevation sattelites by setting "elevation mask" at least
5-7deg

- use only sattelites with good signal by setting the Signal Level Mask
(C/N) over 30dB-Hz  (Trimble use different units AMU - set it for 2.2)

- configure PPS output as "Fixed-based" in other words, the PPS pulses will
be available on the output only when the GPS has a valid navigation
solution.
   Without synchronizing to satellites 1pps is taken from the standard grade
crystal RTC oscillator.

- as a default in most GPS receiver modules, the precision timing
information is carried on a rising edge of PPS pulse, so no matter how long
it is.

If you have doubts check and eventually change GPS configuration using
software provided by manufacturer.

- the performance of a GPS module depends on the availability and accuracy
of navigation data as sattelite ephemeris and almanac. To keep them valid
during a power failure as well to keep real time clock (RTC), a backup
battery should be used. Some data are transmitted every 12.5 minutes but
valid for a few hours or even a few days so the reacquisition of position
and later of precise timing is much more faster when battery backup is used.

If you really need better timings, it is worth to look for timing models as
for example an old Motorola Oncore Timing moduls.
Less known are Trimble timing receivers mentioned below but with the
exception of Thunderbolt model, don't expect you will get them as a second
hand.

Resolution-T,  ?15ns (1?)
Resolution-SMT (same size as Copernicus, probably reasonably priced), ?15ns
(1?) Mini-T GPS  - small child of Thunderbolt with 10MHz OCXO on board,
?15ns (1?)

I hope these GPS timing tips will be useful. They apply also to the timing
modules but their specific consideration (T-RAIM functionality) are not
covered here.

vy 73, Michal
sp2iqw

W dniu 2010-06-02 18:54, Bob Bownes pisze:
> So, do folks have a preferred GPS module to discipline clocks? Clearly 
> some are better than others. I'd like one that will output in NMEA so 
> I can use it to drive some other things as well, but other than that, 
> it has become clear that the Rockwell MicroTrack TU00 just isn't going 
> to cut it as it only locks to 4 satellites, has quite a bit of jitter, 
> doesn't hold a very good 3D lock (+/-100m just isn't good enough for
> me...)
>
> Thanks,
> Bob
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