Satellites of the GPS and other GNSS systems are usually not stationary, but circle around the globe on well-known tracks, so each individual satellite may be above or below the horizon at a given location and time. Satellites that are below the horizon can't be tracked anyway, so the receiver uses its last known position and almanac data from the satellites to determine which satellites are currently expected to be above the horizon at its geographic position, and can potentially be tracked. All these satellites are called to be in view.
However, even some the satellites that are in view may be shielded by buildings, mountains, etc., so the receiver may be unable to track these satellites. Also, individual satellites may be temporarily in maintenance mode, so they must not be used even if they can be tracked. Only satellites that can be tracked and are not in maintenance mode are considered good and used to determine the current position and time.
So the number of good satellites can never exceed the number of satellites in view, but it can be significantly less if the antenna has been installed in a location with limited view to the sky. In worst case this can lead to limited accuracy, or only temporary synchronization.
A big difference between Meinberg's own GPS technology and other GPS receivers / antennas is that the Meinberg GPS system has been designed from the ground up for time and frequency synchronization purposes.
We have made every effort to ensure that the time you get out of your Meinberg device is as close as possible to “real time”.
That means specifically, we prefer to rather not indicate a good reception status and use the time coming from our GPS receiver when we cannot be sure that the receiver is providing the correct time.
A lot of GPS receivers from our competition (and on smart phones and other non-timing related devices) are indicating a good reception status when they in fact picked up a reflected signal or has to average the position/time to a point where you have several microseconds or even milliseconds of time error (and the device is not telling you).
Another important point is that, unlike a pure navigational receiver, a timing receiver relies on the availability of UTC correction parameters that are periodically broadcast by the GPS satellites.
If those parameters are not yet available, the receiver first has to collect the associated information from the satellites.
See Warm Boot And Cold Boot Mode.
If a receiver has satellite almanac data and a last known position saved in its on-board memory then after power-up it computes which satellites are expected to be in view, assumes that all those satellites can be received, and tries to track all those satellites in Normal Operation mode. Satellites which can't be tracked are removed from list of good satellites, so the number of usable satellites can decrease quickly after power-up if the expected satellites can't be tracked.
If for some reason no satellites can't be tracked at all, i.e. the number of satellites in view decreases to 0, then the receiver switches to Warm Boot mode (immediately or after some delay) where it tries to detect one of the possible 32 satellites after each other. As soon as the receiver can track at least 4 satellites it can compute its own current geographic position, and switches back to Normal Operation. If none or less than 4 satellites can be tracked, the receiver stays in Warm Boot mode.
Cold Boot mode is basically the same as Warm Boot except that all the satellite data saved on-board in non-volatile memory have been cleared. As soon as at least one satellite can be received the navigational data from the satellites is collected again, which takes at least 12 minutes since the satellite data are broadcast repeatedly every 12 minutes. If the satellite data is complete the receiver switches to Warm Boot mode and continues as described above.
GPS satellite reception and tracking do not depend on any driver software. The satellite signals are exclusively decoded by the microcontroller installed on the GPS receiver. Driver software packages are only required to read the date and time from the receiver, monitor its status, and do some configuration, e.g. of the output signals provided by the device.
If a Meinberg GPS receiver is unable to track even a single satellite then there are several possible reasons:
The standard Meinberg antenna also includes a converter unit which converts the original 1.57 GHz L1 frequency down do some tens of MHz, which allows to use a very long antenna cable. 3rd party patch antennae don't provide such converter, and 3rd party antenna diplexers are not prepared to work with the converted frequency, so they can't be used directly with Meinberg GPS receivers.
The DC voltage for the antenna/converter unit can be measured at the receiver's antenna connector. Normally there should be about 15V DC between the inner and outer contact of the antenna BNC connector. If the DC voltage is 0V then the DC/DC converter component of the receiver is faulty and needs to be replaced. This happens only very rarely.
Please note the receivers are measuring the DC current drawn via the antenna connector. If no current is drawn then the card reports Antenna Failure. This is also the case if the receiver's DC output is faulty.
If this Antenna Failure message disappears when the antenna cable is connected then there must be something which draws the DC current, but eventually the RF signal doesn't get through, or there is a short-circuit in the antenna cable or one of the connectors. Also, as mentioned above, this can be a slave channel of an antenna diplexer where no other device is connected to the master channel, or the device at the master channel is powered off.
— Martin Burnicki martin.burnicki@meinberg.de, last updated 2020-04-21