AMDAR – the unknown giant
The airborne communication domain is currently subject to a significant re-design and very interesting applications are on the horizon (see article Future communication).
In all of this, one tends to forget that there have already been very innovative and quite sophisticated airborne data-sharing applications in the past. One of them is the Aircraft Meteorological Data Relay (AMDAR), which falls into the larger category of airborne weather measurements, called Aircraft-Based Observations (ABO) . Every day, several thousand aircraft are travelling around the planet and measuring air data for their own flight systems. Why not share this information with meteorologists on the ground? This is exactly what AMDAR does!
The idea of measuring meteorological data by airplanes can be traced back to around 1910 (!) when early military biplanes made recordings of the atmosphere using devices called “meteographs” . They were later replaced by balloons, a technique still in use today. It was only with the advent of “modern” navigation and communication systems in the 1970’s that widespread airborne measurements using airplanes became a valuable option again. An early prototype was the Aircraft-to-Satellite Data Relay (ASDAR), created by the World Meteorological Organization (WMO) and involved special equipment installed on transport category airplanes. The data was transmitted via the WMO’s own Global Telecommunication System (GTS) . ASDAR was operational on a large scale from around 1991 until 2007. The introduction of digital avionics on transport airplanes during the 1980’s paved the way for an even simpler observation method: As the typical avionics of a transport airplane now measured air-data in a digital format and the airlines had implemented the Aircraft Communication Addressing and Reporting System (ACARS), it was possible to forward the air-data to the ground without additional hardware on the airplanes. AMDAR was born.
Today’s AMDAR installations on airplanes are essentially a software package that collects the required data and forms a standardized AMDAR report, which is transmitted automatically at given intervals without flight crew interaction. These transmissions occur more frequently, when the aircraft is climbing or descending . The sub-networks involved are primarily ACARS, but also Mode S EHS combined with ADS-B and in remote areas ADS-C .
Figure 1: Simplified AMDAR reporting path 
WMO recently designated a global data center in order to improve the collection and distribution of AMDAR data and ensure quality control. This function is performed by the National Oceanic and Atmospheric Administration (NOAA) in the US .
What is being reported?
AMDAR data reports typically include :
- Aircraft identifier
- Pressure altitude
- Wind speed / direction
- Static air temperature
- Turbulence (if available, based on load factor n)
- Icing (if available, based on ice detector)
- Humidity (if available, special sensor)
AMDAR measurements “fill the gap” between satellite data and radiosonde observations and have become a crucial component for global weather modelling . As an example, it has been demonstrated that errors in wind/temperature forecasts have been reduced by 50% during the last three decades thanks to AMDAR data .
Figure 2 depicts the data points of a single day, showing that AMDAR data is generally available in Europe, the US and over the Atlantic but is sparsely reported elsewhere. It is one of the current undertakings of the WMO to improve that over the next years .
Figure 2: AMDAR reports of a single day (16th July 2019) 
Specially modified airliners
Some airlines have modified a small number of aircraft to further enhance the global understanding of weather. Below are some examples.
Humidity – a critical parameter
It turns out, that one of the most important parameters in meteorology, the water vapor content (humidity), is not routinely measured by airplanes. In the US and Europe, several operators provide this crucial value using special sensors. Figure 3 shows a Lufthansa A320 installation. The system incorporates a small data processor and interface to the aircraft avionics and works without flight crew interaction .
Figure 3: Humidity sensor installed on a Lufthansa A320 
Even more data: IAGOS and CARIBIC
Lufthansa operates some aircraft with considerable modifications under the In-service Aircraft for a Global Observing System (IAGOS) and the Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container (CARIBIC) programs . A detailed description of the more than 100 parameters that are recorded on these aircraft would be beyond the scope of this article, but these initiatives show that a commercial airline can make a significant contribution to climate research.
Figure 4: IAGOS and CARIBIC sensors (upper left/right pictures) on a Lufthansa A340. The measuring equipment is stored in a cargo container (lower picture) 
IAGOS and CARIBIC aircraft are scheduled to cover areas of meteorological interest and therefore often change the city-pairs they connect. Data analysis is performed by universities and special institutes across Europe. This kind of specially modified aircraft have been operated by Lufthansa since 2004 .
The number of aircraft fitted with such sensors is very small compared to the entire “AMDAR-fleet”, but they have significantly improved the understanding of meteorological processes over the last years .
Useful for air traffic management
The implementation of Continuous Climb Operation (CCO) and Continuous Descent Operation (CDO) relies on accurate wind data for trajectory planning. Where possible, AMDAR data is already used for such purposes. Additional efforts have been made to collect more reliable wind data in terminal airspace. Within the SESAR program in Europe, this manifested itself in the form of the European Meteorological Aircraft Derived Data Center (EMADDC), a project led by the UK and the Netherlands . The Maastricht Upper Area Control Center (MUAC) forwards Mode S EHS data to generate an accurate wind data set. Initial results are very promising, and it is foreseen to make EMADDC part of the EUMETNET ABO program by 2021 .
As in many other fields, the consequences of the pandemic are also very apparent here. The global number of daily ABO-reports decreased dramatically in spring 2020, as Figure 5 depicts:
Figure 5: Breakdown of aircraft-based observations in the course of the COVID-19 outbreak .
A further perspective on the impact of COVID-19 can be found when studying Figure 6 and 7. These two pictures compare the distribution and number of AMDAR reports of a 24-hour period before and during the pandemic.
Figure 6: AMDAR data before COVID-19 outbreak (January 2020) 
Figure 7: AMDAR data during COVID-19 outbreak (May 2020) 
So, when you encountered somewhat less accurate wind forecasts these days, it might me related to the decrease in data points being fed into global weather models.
The use of Mode S EHS data continues to grow in Europe and can significantly improve the number of measurements, especially with regard to smaller aircraft without ACARS. WMO and IATA are currently working together to establish an EFB-based AMDAR solution which could also contribute to a more accurate picture . This will further improve weather awareness of flight crews.
 WMO, “Guide to Aircraft-based Observations”, doc. 1200, 2017
 WMO, “WIGOS, WMO Integrated Global Observing System”, technical report 2018-01, 2018
 Hoff, “Humidity Measurements by Aircraft of the E-AMDAR Fleet”, DWD
 WMO, “AMDAR newsletter”, https://sites.google.com/a/wmo.int/amdar-news-and-events/newsletters/volume-19-may-2020, 28/09/2020