Base Station Selection & Switching

There are a number factors that go into the network either selecting a station upon initial connection or also switching to a new / alternate station over the period of the session.

1. Data availability - the primary determining factor for what station a rover / client will using during a connection session is the delivery of raw data from the stations in the immediate area of the connection request.  If the closest station is delivering data, then it is the most likely candidate to be
   • The 'master' station in the cell being produced for the rover when using a network solution or
   • To be the 'nearest' station for the rover when using a single-baseline nearest solution.
2. Number of satellites - another consideration factoring into what station a rover / client will use during a connection session is
   • The number of satellites that are either fixed at the cluster level for a station when using a network solution or
   • The raw number of satellites that are available for a station when using a single-baseline nearest solution
   • For example if station XXXX is the closest station by distance, but station YYYY is only 5 km further way and is tracking / fixed on 2+ additional satellites, then the network will use station YYYY as the 'master' or 'nearest' station in its corresponding solution
3. Overall Station Health / Performance - the final set of factors into what station a rover / client will use during a connection session is
   • Fixing status of the stations in the immediate area of the connection request, i.e. if a station is not fixing on the cluster and a rover / client is requesting a network correction then this station will not be used in the provided solution
   • Data stream consistency, i.e. if there are large number of data breaks in the raw data from a station in the immediate area of a connection request the network will not typically deliver corrections from this station until this data stream is more predictably available
4. Distance moved by rover - To attempt to mitigate constant switching of stations mid-session, the network also evaluates the distance moved by a rover / client during the session.  And assuming the variables above have not material changed, then the network will maintain its site selection and will not re-evaluate its selection until the rover has move more than 1 km

All of the factors above are constantly evaluated by the algorithms in the system to attempt to provide the best balance of availability, distance to rover / client, etc. during a connection session for a rover / client.  Below are few example situations and are by no means a complete accounting of all the permutations that may happen in the field.

 

Example 1:

Conditions:

• A rover / client (roverABC) connects and is 23 km from station XXXX and 26 km from station YYYY
• Station YYYY is tracking 5 additional satellites compared to station XXXX
• Station YYYY has 3 additional satellites fixed at the cluster level compared to station XXX

Evaluation:

• The network evaluates the position of the rover / client in relation to the two stations
• The network evaluates the number of tracked satellites by each station
• The network evaluates the number of fixed satellites by each station

Delivery:

• Nearest Correction
  • Because station YYYY is within the 5 km buffer prescribed on the network and because it is tracking 5 additional satellites
    • The network will provide Station YYYY in the single-baseline nearest correction for the rover / client
• Network Correction
  • Because station YYYY is within the 5 km buffer prescribed on the network andbecause it is fixed on 3 additional satellites
    • The network will create a cell with Station YYYY being the 'master' and provide corrections for the rover / client for this cell in network correction operations

Example 2:

Conditions:

• The roverABC from previous example moves just over 1 km from its original location
• The roverABC is now 22 km from station XXXX and 26.6 km from station YYYY
• Station YYYY is tracking 4 additional satellite compared to station XXXX
• Station YYYY has 2 additional satellites fixed at the cluster level compared to station XXX

Evaluation:

• The network evaluated the distance travelled by the rover / client from the initial position
• The network evaluates the position of the rover / client in relation to the two stations
• The network evaluates the number of tracked satellites by each station
• The network evaluates the number of fixed satellites by each station

Delivery:

• Nearest Correction
  • Because nothing has materially changed the network will continue to provide Station YYYY in the single-baseline nearest correction for the rover / client
• Network Correction
  • Because nothing has materially changed the network will continue to provide a cell with Station YYYY being the 'master' and provide corrections for the rover / client for this cell in network correction operations

Example 3:

Conditions:

• The roverABC from previous example moves just over 1 km from its new location
• The roverABC is now 21 km from station XXXX and 26.1 km from station YYYY
• Station YYYY is tracking 5 additional satellite compared to station XXXX
• Station XXXX is no longer fixed at the cluster level compared

Evaluation:

• The network evaluated the distance travelled by the rover / client from the initial position
• The network evaluates the position of the rover / client in relation to the two stations
• The network evaluates the number of tracked satellites by each station
• The network evaluates the number of fixed satellites by each station

Delivery:

• Nearest Correction
  • Because Station YYYY is now > 5 km further from the rover the network will provide Station XXXX in the single-baseline nearest correction for the rover / client
• Network Correction
  • Because Station XXXX is not fixed at the cluster level the network will continue to provide a cell with Station YYYY being the 'master' and provide corrections for the rover / client for this cell in network correction operations even though the 5 km buffer distance is no longer valid
  • If Station XXXX does regain a fixed status at the cluster level the network will update the cell configuration and begin delivering corrections with Station XXXX being the 'master' because both the satellite count and buffer distance rules are no longer valid

The number of different outcomes on station selection and switching are very complex and can lead to sometimes unexpected outcomes from a rover / client perspective.  But the simplest way to view this is that the network is always trying to determine what the best overall correction based on all the available factors, with the key criteria always being delivering the highest quality and highest availability correction to allow the rover / client continue to operate without interruption.