GPS Hints and Tips.
These Notes, Hints and Tips are provided as a forum for user's observations.
Submissions for consideration by the editor should be e-mailed to the we-master; see top row navigation tab.
| No. | Subject | Content | Date |
| 3 | . | The latest entry is at the top. | . |
| 2 | Cold starting | The Furuno receiver will output its 1PPS in Estimated Position Mode (normal mode) when it has a position fix with PDOP (Percentage Dilution Of Precision - based on satellite geometric positions) of less than 6.0 and a loaded almanac. This almanac is a database giving the positions of all the satellites, atmospheric details and other information like the offset between GPS time and UTC. From a cold start the almanac can take 12.5 minutes to download and longer if satellite coverage isn't good. The downlink data channel carries data at 50 Baud and the almanac is quite big. It is likely that not all the almanac is required and observed variable times to 'fix' of less than 12.5 minutes may have been obtained when switch-on occured at an optimum time in the cycle of downloading; this may depend on which satellite is used to download from first.It is possible to ignore the almanac issue and force a 1PPS output immediately the receiver is switched on; the 1PPS will start at a random phase relative to a receiver with 'fix'. Because of the nature of the need for phase coherent 1PPS in the application equipment this 'force' option is not used. Knowledge of the almanac will be lost when power is removed from the GPS Rx. | June 07 |
| 1 | GPS Ae Polar Diagram | If you are using ABDA and/or Satsync Frequency Generators with a GPS Aerial (Model 40642) supplied by Dalman then you may be interested in a means of checking the polar diagram of your installation. Loosing GPS 'fix' is not a crisis but it may be an advantage for you to know the efficiency of your installation. A tool has been developed - consisting of some freeware and a hardware interface - which enables you to gather data from an installation while the system remains operational and display these data in the form of a polar diagram on your ET(PC) - the term used in the trade is 'Elevation Mask Angle'. The freeware is from ApolloCom and you need VisualGPS.exe; the hardware circuit is available here (the 1PPS output is not necessary), Dalman can supply the hardware as product/kit No: 40644. Using this tool to record 24 hours of data on the GPS Ae installation at the factory brought results which we found surprising. Our GPS Ae faces approximately SSE (pictures in 'report' below) and is in that position because the installation was easy; we loose GPS 'fix' for a few minutes 2 or 3 times a day, not a surprise with its location - the 35dB S/N ratio polar diagram (see 'report') confirmed our expectation. However when you look at the 'trails' (see 'report') of all available satellites you will see that there is a large 'hole' in the northern sky that has never been visited by a satellite. See GPS Rx/Ae Installation Report 240K pdf. The obvious conclusion is that if our building had been the other way round our installation would have rarely provided a 'fix'. The moral is - put your GPS Ae on the south* side of the pole/mast/wall. *I assume there is also a 'hole' over the South Pole, so if you are south of the equator have your GPS Ae on the north side of the fixture. Acknowledgements to Dave Wilcox (NTL - Surrey Police) for directions to the freeware. |
21.02.02 |