ADS-B WEATHER
The Flight Information Service Broadcast (FIS-B), commonly referred to as ADS-B weather, is
a continuous ground-based weather and NOTAM broadcast. FIS-B also includes station
information: latitude, longitude, tower type, and tower status. ForeFlight can display ADS-B
weather, NOTAMs, and tower information on the map when connected to Sentry, Sentry Mini,
or Sentry Plus.
Reception of ADS-B weather requires direct (line-of-sight) communication with a minimum of
one ground-based transmitter. Aircraft on the ground or at low altitudes may not be able to
receive ADS-B weather due to an inability to achieve line-of-sight communication.
ForeFlight displays a label in the upper left corner of the map when a weather or NOTAM layer
is selected. The label indicates the source of data, the status of the source, and the last time
the data was updated. When connected to an ADS-B receiver, the label indicates how many
towers are currently being received, not how strong the signal is. ForeFlight will display one of
the following four statuses:
•
No Towers - Sentry is not receiving any tower data.
•
No Data - Sentry receiving tower data, but not weather
or NOTAM data.
•
Marginal - Sentry receiving FIS-B data from one tower.
•
Good - Sentry receiving FIS-B data from more than one
tower.
ADS-B Ground Based Towers
Hundreds of towers throughout the United States broadcast FIS-B data. There are four types
of FIS-B ground-based towers: surface, low altitude, medium altitude, and high altitude. Each
tower type provides a range of aeronautical information (see table in this chapter for additional
details). Approximately two-thirds of towers in the United States are low altitude tier towers.
Low altitude towers provide less information but can be received at lower altitudes.
The amount of information broadcast by a tower is determined by its look-ahead range. ADS-B
tower look-ahead range specifies how far and the type of data a tower transmits. For example,
a low altitude tower broadcasts METARs for airports within 250 nm, whereas a medium altitude
tower broadcasts METARs for airports within 375 nm.
Sentry is capable of receiving data from multiple towers simultaneously. What is shown in
ForeFlight is a combination of data from all the towers being received.
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