Björn

In a message dated 12/11/2007 22:00:37 Pacific Standard Time, bg@lysator.liu.se writes: >I am surprised surveying mode is needed anymore. In a SA environment it >makes sense, but for me its hard to understand with the current accuracy >given by the GPS system. -- > Björn Hi Bjoern, even with a very good M12M or M12+ receiver we will typically see some variation of the averaged position over it's ~3 hour (default) auto-survey interval. I've seen altitude change by a meter or two for example. Those couple of nanoseconds may be insignificant when compared to older GPS receiver performance (VP Oncore etc), but if one has the time, why not get the best position average possible. I did also note that it does make a difference if the Auto Survey is done during the day, at dawn, or during the night etc. Might be caused by multipath etc. A 12 hour or 24 hour Auto Survey may reduce the Diurnal effects. We recently added a command to our Fury firmware that allows the user to set any time from 1s to 10,000s for the Auto Survey process to finish. This was done because the differences are so small between any given single-second position, and an auto-surveyed position (less than 5 meters or so). The error due to this is much less than 100ns which is more than adequate for a lot of applications. We have some customers that want the unit to do only 3 to 5 minutes of Auto Surveying for a particular mobile military application, and the resulting <<100ns UTC accuracy is way better than their requirement. In that case the Auto Survey time is insignificant when compared to the OCXO warmup time. It all depends on your particular accuracy requirement. bye, Said **************************************See AOL's top rated recipes (http://food.aol.com/top-rated-recipes?NCID=aoltop00030000000004)

Hi Said and All I have been following this thread with a lot of interest because my application is also in portable Microwave operations with narrowband modes such as WSJT. The main issue here is not so much long term frequency stability as one only has to find a signal in an SSB passband, but the short-term stability should preferably be better than a few Hz over a 46 second transmission. A second issue as others have mentioned is that one needs to have a stable signal at a new location within a relatively short period for many rover operations (say 15 mins). (Note: I earlier asked a question about short term stability with my Trimble Min-T and this has been solved by increasing the Time constant from the default value of 10 seconds to 200 seconds. The short-term stability is now better than one part in 10^10 which is the limit to which I can measure) Now to my Question: I note the importance of the self survey for absolute time but I wonder if a self survey is important for accurate frequency where the absolute time is not an issue? There are number of things that it seems one could do to achieve frequency stability as fast as possible at a new location. 1. Run the Oven off batteries full time - this can be typically only 3 to 5 watts - for the Mini-T or the Fury. 2. Record the EFC voltage at the home location and insert this in software as starting point at the portable location. 3. Insert the position information in software of the portable location as a staring point. 4. As Said suggests run a short self survey of only a few minutes. Now I am not sure which of any of the above points are necessary or important but I would be grateful for the thoughts of others. Regards Rex VK7MO We have some customers that want the unit to do only 3 to 5 minutes of Auto Surveying for a particular mobile military application, and the resulting <<100ns UTC accuracy is way better than their requirement. In that case the Auto Survey time is insignificant when compared to the OCXO warmup time. It all depends on your particular accuracy requirement. bye, Said **************************************See AOL's top rated recipes (http://food.aol.com/top-rated-recipes?NCID=aoltop00030000000004) _______________________________________________ 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.

Rex Moncur wrote: > Hi Said and All > > I have been following this thread with a lot of interest because my > application is also in portable Microwave operations with narrowband modes > such as WSJT. The main issue here is not so much long term frequency > stability as one only has to find a signal in an SSB passband, but the > short-term stability should preferably be better than a few Hz over a 46 > second transmission. A second issue as others have mentioned is that one > needs to have a stable signal at a new location within a relatively short > period for many rover operations (say 15 mins). > > (Note: I earlier asked a question about short term stability with my Trimble > Min-T and this has been solved by increasing the Time constant from the > default value of 10 seconds to 200 seconds. The short-term stability is now > better than one part in 10^10 which is the limit to which I can measure) > > Now to my Question: I note the importance of the self survey for absolute > time but I wonder if a self survey is important for accurate frequency where > the absolute time is not an issue? > > There are number of things that it seems one could do to achieve frequency > stability as fast as possible at a new location. > > 1. Run the Oven off batteries full time - this can be typically only 3 to 5 > watts - for the Mini-T or the Fury. > 2. Record the EFC voltage at the home location and insert this in software > as starting point at the portable location. > 3. Insert the position information in software of the portable location as a > staring point. > 4. As Said suggests run a short self survey of only a few minutes. > > Now I am not sure which of any of the above points are necessary or > important but I would be grateful for the thoughts of others. > > Regards Rex VK7MO > Rex If you make provision for multiple power sources with preassigned priority its not necessary to run of a battery all the time, however the battery needs to be present as a backup when the other power supplies are disconnected. Achieving this without disturbing the power supply voltages when switching between various power sources requires careful design, a simple diode OR circuit is in general inadequate particularly if more than 2 sources (mains, external dc, internal battery) are available, however the required circuity need not be overly complex. Bruce Bruce