![]() |
![]() |
![]() |
![]() |
![]() |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
![]() |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
The GPS Reception Conditions Page gives you dynamic displays of the GPS Reception conditions. By default the last 48 hours are displayed. Different time intervals can be selected by changing the form on top of the display and clicking on Draw . The start time is specified in at(1) time format which can take dates and times in intuitive fashion, such as “1.4.2001 08:15”. For details see the rrdtool manual. The page shows the current UTC time and the UTC time at which the last value was recorded. If the results appear to be strange or if there are no data at all, it is a good idea to check these times. Maybe the data recording process is not running. Each display consists of a number of different graphs. At the end of this page you can find a description of the antenna site used to collect the example graphs The first graph presents the bottom line of GPS reception quality, the Clock Bias Uncertainty. This value is the GPS receiver’s estimation of how far its timing pulses (PPS) deviate from UTC time. The y-axis is in nanoseconds (10-9 s). The horizontal red line at 1000ns represents the threshold value at which the clock pulses are switched off and measurements cease. As long as the bias uncertainty remains below this threshold, GPS reception is good enough for measurements. With the ACUTIME 2000 receiver the typical value obtained with GPS selective availability switched off is 110.2ns as shown on the good but somewhat boring graph below:
The next graph is somewhat less boring. It shows the number of satellites in three different categories of usefulness to the receiver. The dark green area represents the number of usable satellites. These satellites can be used to calculate the time or position solution. The light green area represents the number of satellites that are received but not usable to calculate the solution. A particular satellite can be unusable for a number of reasons, such as too weak a signal or too low an elevation. The light red area represents the number of satellites being searched. During normal operation a satellite is being searched when it should be visible at the position of the receiver according to the GPS almanac, the orbital data broadcast by all satellites. The red line shows the minimum number of usable satellites for each time period. The reception conditions clearly have 24h period. This is caused by the orbital characteristics of the GPS satellites: the period of their orbits is approximately 12 hours. This means every 12 hours the satellite constellation in space is roughly the same. Since the earth rotates “underneath” the satellites once every 24 hours, any point on earth will observe a similar satellite constellation every 24 hours.
Site Suitability ConsiderationsOnce the receiver is in overdetermined timing (ODT) mode one usable satellite is sufficient to keep accurate time; more usable satellites provide more accurate time measurements. However before the receiver can go into ODT mode it needs to determine its position. In order to do this it needs at least four usable satellites for approximately 33 minutes. A dark green horizontal line represents this threshold value. The antenna site form which the example graph was taken is sufficient for timing because the minimum number of satellites never falls below 1. However it is not an ideal site because for most of the day there are not sufficient usable satellites available to initialise the receiver in case it has been reset or powered off. One can see from the graph that it may take up to 11h for the initialisation to complete.
The following graph shows the internal status of the receiver over time. Normally this will be status 13 which means that the receiver is in overdetermined timing mode.
![]() The Satellite Availability Overview graph shows which particular satellite has been usable, received or searched over time. There are up to 32 active satellites in the GPS constellation. Each one is represented by a horizontal line. The color coding is the same as in the Reception Conditions graph.
This graph shows the 12h period of the satellite orbits. Some satellites are also seen “over the pole”. A good example in the example above is satellite 1.
The Cumulative S/N Ratios graph is intended for Antenna fine tuning. It shows the “total amount of signal received”. The colored bands represent the channels of the receiver. In general it is safe to say that the antenna position gets better if the total value increases as long as the number of visible bands remains constant. A band appearing or disappearing represents a channel picking up or loosing a distinct satellite. This may be caused by antenna position changes but it may also be caused by the satellite’s change in position relative to the antenna. Use this graph as an indication when trying different antenna locations and orientations but do not try to optimise for too small variations here.
The S/N Ratios per Channel display provides the same information as the previous graph but nicely separated by channel. This cold be useful to spot receiver problems. Otherwise it looks nice, doesn’t it?
The Directional Reception Summary tries to give an insight into the amount of sky that is visible from the antenna location. The Y-axis represents the bearing from the antenna position towards the satellite being received. The color codes are the familiar ones from the reception conditions graphs. It would probably be better to represent this in an azimuth/elevation plot but I do not have the time to hand-code that now. Note that there is no elevation information information whatsoever in this graph.
As one can very clearly see this antenna location is facing roughly east and is very well covered towards the west. In fact this data was taken from an antenna located inside an East facing window on an Amsterdam canal one floor up from street level. The canal roughly runs North-South. The building above and behind the antenna fully shields the it from signals coming from the 160o-040o arc. Signals received from these directions are reflections off other buildings and correctly classified by the receiver as unusable. For more detailed data about satellite trajectories relative to the antenna position see the satellite tracking graphs. |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
![]() |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
![]() |