jgray@zianet.com said: > What about this scenario - the GPSDO has a fixed position on initial > powerup, but then it is moved periodically or even continuously, without > updating the GPS coordinates. The distance from the original coordinates > could be anywhere from close by, to across the state. Periodically, a new > survey could be run to keep things from getting too far out in left field. Most commercial GPSDOs assume they are running at a fixed location and won't work very well unless they are near their surveyed location. Near is measured in feet, not miles. That allows them to work with fewer satellites which was important with the GPS receivers in old gear like the TBolt and Z3801A. But that's all software/firmware. If you build your own GPSDO you can use whatever GPS receiver you like as long as it puts out a PPS or something that you can lock to. I think Jackson Labs makes some non-survey GPSDOs. http://www.jackson-labs.com/index.php/products/lte_lite The FAQ says there is a non-survey mode. You might find something interesting in time-nuts archives from about 2 years ago. -- These are my opinions. I hate spam.

http://www.jackson-labs.com/index.php/products/lte_lite

Hal Murray wrote: I think Jackson Labs makes some non-survey GPSDOs. http://www.jackson-labs.com/index.php/products/lte_lite The FAQ says there is a non-survey mode. You might find something interesting in time-nuts archives from about 2 years ago. Here are some comments we put together regarding mobile GPSDO's: Using GPSDOs in mobile applications is a fascinating and very complex topic. There are three main issues affecting 3D mobile mode performance: 1) the oscillator sensitivity to acceleration, 2) the GNSS receiver sensitivities, and Kalman time-constants wrt motion and multipath, and its performance difference in 3D versus Position Hold mode, 3) the products’ sensitivity to possible airflow or temperature changes during motion (for example when operated in a UAV that has no airflow shielding). Jackson Labs Technologies, Inc. indeed makes a large number of mobile-capable GPSDO’s, some with vibration- and acceleration-hardened oscillators. This typically involves hardening of the crystal mounts and low-g sensitivity operation, and/or some sort of electronic g-compensation, and proper “steering” of the GPS receivers’ Kalman filter modes (time constants) and Doppler versus Carrier Phase tracking modes, but all of this depends on what “mobile” really means, e.g. what type of dynamics can be expected. There is a huge difference in expected operation between walking and a jet pulling 5g maneuvers. In our GPSDO’s and GNSS receivers starting with the low-cost $220 M12M Replacement Receiver the Kalman and tracking mode can be auto-set and auto-sensed via the GPS:DYMANIC command. The software will recognize acceleration, velocity, and change of altitude and set the GPS receiver to the proper mode that is optimal for a particular environment. There is a huge difference between operating while walking, or the aircraft is taxiing on the ground and getting all this multipath from the angled adjacent hangar roofs, versus what needs to happen once the aircraft is 200+ feet off the ground for example. In terms of sensitivity to acceleration, and GPS performance while in motion, we continuously do flight testing as well as drive testing and have found that the modern GPS receivers are not very sensitive at all to high-velocity airborne operation. Driving on freeways does present challenges due to overpass shading, as well as multipath caused by highway signs passing by. Usually this is not an issue because modern receivers are able to track such a large number of satellites. Also, it is impressive to see how quickly the modern GNSS receivers recover from their TCXO’s being accelerated in aerobatic flight for example. GPSDO Oscillator sensitivity to acceleration depends on the type of crystal used. TCXO’s typically have the highest g-sensitivity (2 to 3ppb or more per g per axis), while SC-cut DOCXOs we use have sensitivities of 1 to 1.5ppb per g typically, and low-g ordering options have less to much less than 0.3ppb per g per axis sensitivity. During our flight testing in jets and propeller aircraft for example we found that our “standard” DOCXOs as used on the Fury DOCXO and other products exhibit negligible effects on frequency stability and timing stability even when flying acrobatics (zero g, 2g steep turns, etc). These maneuvers are typically short (less than 30 seconds) and the phase/frequency deviation experienced does not cause a lot of drift, and once you pull out of the maneuver the crystal will go back to its nominal frequency. These events are also short enough typically that they fall well within the time constant of the loop filter, so the filter does not “chase” the frequency offset. Thermal and airflow sensitivity seems to be much more of an issue for TCXO’s and single-oven oscillators than acceleration sensitivity during driving/flying, while warmup and retrace errors seem to trump the g-sensitivity in DOCXO products in airborne applications. However if you are talking about dismounted applications (someone walking and wearing a unit in a back pack etc) it is a totally different ballgame because this backpack could be set onto the ground and turn over, the person could be walking or lying down, etc etc. The change of direction for dismounted use is much higher than for vehicle or aircraft use (if a vehicle turns upside down you are in big trouble anyways, and aircraft usually fly coordinated meaning the highest acceleration is always down the z-axis. This allows users to mount the GPSDO in the aircraft with the lowest crystal g-sensitivity pointing toward the z-axis to minimize errors). Thus for accurate operation in these types of applications usually low-g-sensitivity oscillators are used (less than 0.3ppb per g). Vibration due to turbulence etc is nominally sinusoidal and thus will only cause phase noise and short-term perturbations that average out over time. Rubidium and Cesium oscillators have the best g-sensitivity specs, in fact the Microsemi Rb’s and Cs’ we use typically have sensitivities below our measurable noise floor when doing a 2-g holdover test. There will be a quick frequency offset due to the TCXO g-sensitivity, then the atomic loop quickly pulls in the frequency error to near zero again. In terms of time-nuts applications, most of our units can operate in mobile mode without any large reduction in stability or accuracy. The only product that is not recommended for mobile operation is the Motorola M12M and M12+ receiver as used in the Fury for example. While there is a command to put the M12M GPS receiver into mobile mode, the performance when doing so is not optimal. The solution to that is to retrofit the unit with the M12M Replacement GNSS receiver which uses a u-blox M8T GNSS receiver that has absolutely charming timing performance in 3D mobile mode. That said, there is a real issue if you operate in challenged environments such as under foliage or indoors. In those environments the Position Hold mode operation with Auto Survey can increase stability by 10x or more. We had posted some example plots here some years ago showing mobile mode operation versus Position Hold mode operation when the GPS antenna location is extremely challenged and is only seeing about 10% of the sky from a window flanked by walls. The stability differences between 3D mobile and Position Hold mode were eye-opening. For those type of challenged applications using stationary Position Hold mode makes all the difference. So in summary: most of the higher end (SC cut) type oscillators will work very well in 3D mobile mode assuming that they are shielded from airflow (not necessary for the DOCXO oscillators). For the most stringent environments one should choose a ruggedized, low-g type oscillator. All our GPSDO’s have either mobile mode operation by default, or can be set to mobile versus Position Hold mode. Lower-end TCXOs can have significant g-sensitivities which can really cause issues if used in dismounted applications where the unit can be easily turned over, but in aircraft and vehicle usage these accelerations are typically temporary and can be easily compensated for by the loop or entirely fall inside of the loop time constant. Multipath presents significant challenges when driving on freeways or taxiing on an airfield. Modern GNSS receivers perform very well in mobile applications if the user sets the Kalman filter parameters and Doppler versus Carrier Phase tracking modes properly according to environmental dynamics. On most JLT products this setting can be chosen to be fully automated via the GPS:DYNAMIC SCPI commands. If the Kalman filter is set incorrectly (setting pedestrian mode in a vehicle for example) then there will be a very significant delay in the position versus actual position due to the low-pass effect of the Kalman filter. Some older GPS receivers do not perform well at all in mobile mode, one example is the M12+ and M12M receiver. Keith On Sun, Oct 16, 2016 at 11:56 PM, Hal Murray <hmurray@megapathdsl.net> wrote: > > jgray@zianet.com said: > > What about this scenario - the GPSDO has a fixed position on initial > > powerup, but then it is moved periodically or even continuously, without > > updating the GPS coordinates. The distance from the original coordinates > > could be anywhere from close by, to across the state. Periodically, a new > > survey could be run to keep things from getting too far out in left > field. > > Most commercial GPSDOs assume they are running at a fixed location and > won't > work very well unless they are near their surveyed location. Near is > measured in feet, not miles. That allows them to work with fewer > satellites > which was important with the GPS receivers in old gear like the TBolt and > Z3801A. > > But that's all software/firmware. If you build your own GPSDO you can use > whatever GPS receiver you like as long as it puts out a PPS or something > that > you can lock to. > > I think Jackson Labs makes some non-survey GPSDOs. > http://www.jackson-labs.com/index.php/products/lte_lite > The FAQ says there is a non-survey mode. > > You might find something interesting in time-nuts archives from about 2 > years > ago. > > > -- > These are my opinions. I hate spam. > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. >