[time-nuts] Newbie questions

Bob Camp kb8tq at n1k.org
Wed Jan 27 18:31:39 EST 2016


Hi

(a few minor additions ..)

> On Jan 27, 2016, at 4:04 PM, Attila Kinali <attila at kinali.ch> wrote:
> 
> Moin,
> 
> As no-one seems to want to answer the GPS related questions....
> 
> 
> On Tue, 26 Jan 2016 19:04:10 -0500
> timenut at metachaos.net wrote:
> 
>> 
>> Paradoxically, I have no interest in time. As in time of day, day of week,
>> etc.. I have never had a job where I got to work on time. My philosophy has
>> always been "go to bed when sleepy, get up when not". I was notorious in high
>> school for only showing up on test day. But, I am interested in being able to
>> timestamp events accurately and in measuring time (and other things). I am
>> also interested in how a very accurate frequency source can be used in
>> other applications and test instruments. That brings me to my desire to build
>> a GPSDO and my questions.
> 
> I have wirtten a couple of mails with references to popular GPSDO's
> a couple of times in the past. You might want to have a look at them
> and the discussions they were in:
> 
> https://www.febo.com/pipermail/time-nuts/2012-March/064873.html
> https://www.febo.com/pipermail/time-nuts/2013-June/077368.html
> 
> 
>> I have also been researching GPS antennas. From what I can see there are two
>> basic types - the flat puck and the helical. 
> 
> There are actually many more, but the cross dipole, the spiral antenna
> and the two you mentioned are by far the most popular ones.
> 
> 
>> I have not seen anything to
>> distinguish the two types based on performance or usage or to indicate that
>> one or the other might be better for GPS timing.
> 
> What makes good antennas and what not is actually not that easy to define.
> There was a discussion on this last spring. look for
> "Important parameters for a GPS/GNSS antenna" in the archives.
> 
>> However, I have seen "GPS
>> Timing Reference Antennas" advertised. Most or all of those appear to be
>> helical. But, I have not seen anything that specifies the difference between 
>> an active GPS antenna and an active GPS Timing Reference Antenna.
> 
>>   1. What is the difference between a "normal" GPS antenna and a GPS Timing
>>      Reference antenna? What features are of interest?
> 
> The timing reference antennas are usually build by manufacturers with the
> intended use for high precision instruments. Either timing or geodetic survey.
> Their parameters are more strictly controlled, they have (usually) a more
> stable phase center and some of them are also dual band (L1/L2).
> 
> Unless you need sub-ns stability and accuracy or have a dual band receiver,
> go for a standard antenna.

One more parameter - they are designed to be outdoors pretty much forever and ever. 
The “rain sealing” details are much better addressed than a normal mag mount antenna. 

> 
> 
>>   2. Is there anything extra needed besides a GPS antenna to enable the use
>>      of WAAS or other services? Apparently the ubolt receivers can make use
>>      of some of that, but it is not clear what is needed to provide that
>>      information to them, or if they just pick it up automatically using a
>>      standard GPS antenna.
> 
> I am not sure what the LEA modules need to active WAAS/EGNOS/etc, it's
> probably listed under one of the commands in the protocol reference.
> 
> You don't need any special antenna for those, as these augmentation
> services use the same frequency as L1 GPS.
> 
>> Also, from what I have read, using carrier phase for timing is potentially
>> more accurate by a couple orders of magnitude. Are there any GPS timing
>> receivers available that use carrier phase? 
> 
> Yes, but they are darn expensive.
> 
>> Or use both L1 and L2 for increased accuracy? 
> 
> Yes, but again, expensive.


As in your “GPS module” goes from $40 to $4,000. Your antenna goes from 
$40 to $1.5K if bought new. 

> 
>> I see that the ubolt receivers can report some carrier
>> phase information, but that doesn't appear to translate to increased accuracy.
> 
> If anything, it would lead to higher precision ;-)
> 
> The reason why carrier phase tracking does not lead to large improvements
> is because most receivers are single frequency receivers which cannot
> directly calculate the ionospheric corrections. Thus they have to rely
> on predictions send out by the GPS satellites (and by WAAS/EGNOS).
> But even with those predictions, the ionospheric terms will dominate
> the error. Thus there is very little gain from using carrier phase
> tracking over code phase only tracking for an L1 only receiver.

It *can* lead to higher performance, but then you need to correlate against a local 
precision site. That turns this into a pretty exotic / single task sort of device. 

> 
>> And the LEA M8T use dual channels, but don't appear to mix GPS and GLASNOS to
>> improve accuracy. Do any receivers do that?
> 
> I did not have a look at the M8T yet, so i cannot answer this.
> I am pretty sure that there are dual system timing receivers out there.


Yes you can set up M8’s to track multiple time standards. There is not much to gain by 
it at the moment due to the low(er) precision of the Glonass time system. 

=====

The other thing to consider is the need for sawtooth correction. This is what takes you 
from the (sort of) 10 ns level to the (maybe) sub 1 ns level. 

Bob

> 
> 
>> I suspect that building a GPS
>> receiver is probably more complex than can be easily handled by an amateur so
>> I am most likely restricted by available receivers.
> 
> Not really. I know of three people on this mailinglist who are
> doing exactly this. It is just a lot of work and takes a lot of time.
> The basics of a GPS receiver can be taught in an afternoon and would
> enable you to build your own. But, doing it right and and time-nut
> grade, with support for more than just L1 C/A requires quite a bit
> more understanding and even more time.
> 
> 
>> I have also read, more than once, statements in this forum that something or
>> another could be had for some low, low price so why build it yourself? I think
>> that there are several reasons, including but not limited to the following.
> 
> We don't ask here "why". If it's fun for you to do, go ahead. We wont
> question your reasons :-)
> 
> 
> 				Attila Kinali
> 
> -- 
> Reading can seriously damage your ignorance.
> 		-- unknown
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