[time-nuts] Strange GPS behaviour

Bob Camp lists at rtty.us
Sun Dec 30 02:34:44 UTC 2012


Hi

The gotcha is that often the "navigation" and "timing" receivers are identical in terms of hardware. There is no upgraded hardware in the timing device.

When you put a receiver into position hold, you are telling it "I don't care about the location solution". It reduces the weight of that part of the filter. Yes, that's only one way to look at it and there are other ways to look at it.

Bob

On Dec 29, 2012, at 5:42 PM, bg at lysator.liu.se wrote:

> Hi Bob,
> 
> I am curious about the sacrifice time and get better position. I have not
> seen any discussions about that in manuals, books or papers. Do you have
> reference?
> 
> What would be the difference between timing and navigation versions of
> cheap commercial receivers?
> 
> 1) Timing receivers are often stationary, navigation receivers are
> typically not stationary for extended periods of time. So a timing
> receiver can benefit from a time only solution. Ie you have a known
> (x,y,z)/(lat,lon,h) position and solve only for time, reducing the unknown
> from (x,y,z,t) to only (t). The known position can be had from a) full 4D
> single fix solution or better from b) a site survey taking the mean
> position from many hours of measurements or also good from c) an
> independently surveyed position of the antenna position. This mode will of
> cause improve time when there are fewer than 4 satellites tracked.
> 
> 2) Timing versions might get an upgraded oscillator. Maybe a TCXO instead
> of a standard XO.
> 
> 3) Navigation receivers integrations will not require high accuracy PPS
> output from the receiver. If you can save some cents on PPS generation
> hardware by having 500ns resolution of the PPS instead of 1-10ns
> resolution - the navigation receiver will not even have hardware resources
> to generate a decent 1PPS.
> 
> To my knowledge, the accuracy of (x,y,z,t) is given by the "geometrics" of
> the satellites tracked. The receiver reports this in the DOP (dilution of
> precision). There is PDOP (3d position), HDOP (horizontal position), HDOP
> (vertical position) and TDOP (Time). There is not much the receiver can do
> in normal operation with four or more tracked satellites to select between
> position and time accuracy. The relationship is given by the geometry of
> the measurements!
> 
> For a navigation receiver, there must be a time solution that is (scaled
> by c) more or less at the same accuracy as the position. Having 3 meter
> accuracy means the receiver time is good to 10ns. However the navigation
> receiver does not need to communicate this timing accuracy to the outside.
> It does not have to calibrate filter/processing delays. Slowly varying
> delays in analog filters due to temperature changes can also be ignored.
> Delays due to LNA and cables can also be ignored if you have a spec at
> 1PPS of 1us.
> 
> --
> 
>    Björn
> 
> 
>> Hi
>> 
>> Indeed the solution is done once per second or so. In the solution they
>> weight the significance of position versus time. If you accept a larger
>> time error in the solution, you can come up with a smaller location error.
>> Is that a bit of mathematical sleight of hand? - of course it is. Can you
>> keep doing it forever? - no you can't. Eventually you need to get the time
>> back up to date.
>> 
>> Put another way - if all that was going on was the same solution process
>> in every receiver - there would be no differences in results. Software is
>> software. The hardware in a low cost timing receiver is the same as the
>> hardware in a low cost "location only" receiver. The difference is the
>> firmware. You can indeed shoot timing firmware into some location
>> receivers and turn them in to timing receivers.
>> 
>> Bob
>> 
>> On Dec 29, 2012, at 12:14 AM, Michael Perrett <mkperrett at gmail.com> wrote:
>> 
>>> Bob,
>>> That is simply not accurate - if the solution rate is 1/second, then all
>>> parameters are solved in that time frame. There are 4 indpendent
>>> variables
>>> and minimal processing power is required to solve all four equations.
>>> Although I am not very familiar with commercial receivers, that is what
>>> happens in the Rockwell, Trimble and IEC military units. If the output
>>> is
>>> more than once per second it is *usually* an output of the Kalman
>>> Filter,
>>> not a "true" measurement.
>>> 
>>> Michael / K7HIL
>>> 
>>> On Fri, Dec 28, 2012 at 6:20 PM, Bob Camp <lists at rtty.us> wrote:
>>> 
>>>> Hi
>>>> 
>>>> …. except… A navigation GPS doesn't care much about the time solution.
>>>> Updating the location is a much higher priority than updating the time.
>>>> The
>>>> typical "solution" is to let the time estimate coast for a while and
>>>> update
>>>> it much less often than the location.
>>>> 
>>>> Bob
>>>> 
>>>> On Dec 28, 2012, at 7:18 PM, Magnus Danielson
>>>> <magnus at rubidium.dyndns.org>
>>>> wrote:
>>>> 
>>>>> On 28/12/12 23:35, Bob Camp wrote:
>>>>>> Hi
>>>>>> 
>>>>>> The GPS does an estimate against the local crystal frequency. It
>>>> generates the PPS off of it's estimate. The less often it updates the
>>>> estimate the more odd things you see as the crystal drifts.
>>>>> 
>>>>> A typical GPS off the shelf solves the position solution every second,
>>>> having a 1 Hz report rate. This includes clock corrections. Some GPSes
>>>> is
>>>> capable of higher report-rates.
>>>>> 
>>>>>> Of course, the crystal can have trouble all it's own. If the crystal
>>>> has a rapid rate of frequency change over a narrow temperature range,
>>>> the
>>>> GPS simply can't keep up with the crystal.
>>>>> 
>>>>> Most GPS receivers only have TCXOs, and even if tossing in an OCXO,
>>>> excessive heat can throw the frequency and hence the GPS solution way
>>>> of
>>>> the mark. For many GPS reference stations, rubidiums is used to steer
>>>> the
>>>> internal clock, and the quality of that lock can affect how well it
>>>> tracks
>>>> it and have secondary frequency issues.
>>>>> 
>>>>> So, it comes as no surprise that the GPS module is temperature
>>>> sensitive. The metrology labs measure and compare the temperature
>>>> stability
>>>> of various GPS-receivers,
>>>>> 
>>>>> There are also filters that can provide temperature effects, but the
>>>> TCXO is where it usually hurts most.
>>>>> 
>>>>> Cheers,
>>>>> Magnus
>>>>> 
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> 
> 
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