T-45C TS, ADV & IUT ENG-24
GINA Operating Characteristics
5.
Differential-GPS signals are used by receiving
aircraft to “correct” standard GPS satellite signals to
the required degree of precision
INSTRUCTOR NOTES
A differential-GPS type of process is used by surveyors and
others, who require extremely accurate data. That process
employs a locally known benchmark coordinate to correlate and
correct the received GPS signals. Some systems accomplish
the correlation in “the field,” whereas others correlate the data
later in an office (post-processing). The in-the-field type GPS
signal correlation is similar to aviation GPS systems being
developed for use by aircraft. However, regional differential-
GPS transmission sites will be necessary, because the error
factor will vary geographically. For instance, a differential
correction signal valid for St. Louis will not be valid in Memphis.
Design-wise, most aviation systems being tested at this time
employ a GPS correction signal that is transmitted to receiving
aircraft, which then applies that “correction” to the GPS data
being received from satellites.
PROGRESS CHECK
Question 4 -- 1.7.7.6.4
What is the certified accuracy of the T-45C GPS system?
ANSWER: Since the T-45C is not a "keyed" GPS system, it is
certified to 100 meters. A new government policy affecting the
allowed accuracy of SPS (Standard Positioning Service) GPS
systems will likely enable the T-45C GPS to become much more
accurate, even without crypto-keying. And, a form of differential-
GPS will probably be approved by the FAA in the near future.
Then, GPS precision approaches will be possible for properly
equipped aircraft.
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