## When I create a scenario how can I limit the GDOP value?

 Answer There is no simple way to control GDOP (or any type of DOP such as HDOP, VDOP, PDOP or TDOP; see SOL10131 for a definition of "DOP"). DOP is a function of SV geometry as seen from your antenna, so to control the DOP you really must control (edit) the complete GNSS constellation (see Footnote 1). An understanding of the DOP type you are attempting to control, coupled with a trial and error approach, is suggested here. Let us assume we want to maintain a GDOP value >1.7 for 3 hours. First create and run a scenario to get an understanding of the GDOP and how it changes during the scenario run, whilst using a 'normal' constellation configuration. We might find that the GDOP >1.7 condition is met for all but 20 minutes of the 3 hour duration, somewhere in the middle of the run. Knowing that a good GDOP (a low value) is obtained when we have at least one SV directly overhead as well as at least 3 SV's at lower elevations (separated in azimuth), we could choose to identify the SV that is directly overhead during this period of good GDOP. We must manually manipulate the orbit of this one SV so that it no longer passes directly overhead at that time into run (you could of course move your antenna but this would result in a change in geometry for entire constellation, which would likely take us back to square one). In this case PosApp (SimGEN) can be used to make changes to the SV orbits via the scenario tree (for GPS, for example, go to [GPS constellation-->GPS signal sources file-->Motion-->Orbits]). Any parameter here can be changed however Mo,Wo or Reference time will prove most productive (see the screenshot below which shows the GPS control). (Also see Footnote 2).   FOOTNOTES Since DOP is a parameter that the receiver calculates based on the geometry of the received signals, obscuration and multipath effects could also be used to manipulate the final DOP calculation. Equally reducing the transmit power of an SV so that it is below the tracking sensitivity of the receiver would be an acceptable method of DOP manipulation here (and the simplest) if your test criteria permits. PosApp (SimGEN) also allows you to schedule satellite "Off times" (for GPS, for example, go to the scenaro tree [GPS constellation-->GPS signal sources file-->Signal Control-->Off times]). Rather than manipulating an SV orbit you might prefer to carefully schedule an SV 'off time' so that it temporarily 'disappears' from the constellation at the appropriate time into run.

Product : Positioning,Position Apps,SimGEN,PosApp,GLONASS,Galileo,GPS,GNSS