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Hello,
I have an AF-5600 with a connected Navworx ADS600-B for weather and traffic. The displayed traffic shows a + or - number indicating relative altitude. My question is what is being compared to derive this number. Is it MSL or GPS altitude?
Thanks,
Dick
The ground stations report the relative altitude to the EFIS. ADS-B altitude is reported to the ground station based GPS altitude.
From the faa.gov adsb faq:Quote:
Originally Posted by Shawn McGinnis
The altimetry source used by the transponder must also be the source of altimetry information transmitted by ADS-B. If two different sources are used, a pilot may be flying at one altitude while the aircraft is transmitting a different altitude to the controller.
In fact, ADS-B reports two kinds of altitudes: barometric and geometric. Barometric or pressure altitude is the one pilots know best – this is the altitude that is displayed on the altimeter in the aircraft. Geometric altitude is calculated by GPS (Global Positioning Satellites) as the height of the aircraft above the earth ellipsoid. These two altitudes are not the same, but having both allows for applications that require one or the other as an altitude source and provides a means of verifying correct pressure altitude reporting from aircraft.
Hi Shawn,Quote:
Originally Posted by Shawn McGinnis
I'm surprised to hear that the ground stations are involved with calculating the relative altitude. What happens when the adsb data is received via a direct transmission from another aircraft transmitting adsb?