Deriving Meteorological Data from free-to-air Mode-S broadcasts in an Australian Context.

Transcription

1 Deriving Meteorological Data from free-to-air Mode-S broadcasts in an Australian Context. Douglas Body Bureau of Meteorology, Melbourne, Australia ABSTRACT Using free to air Automatic Dependent Surveillance (ADS)-B/Mode-S(urveillance) messages it is possible to derive atmospheric temperature, wind speed and direction from messages that are transmitted by aircraft to air traffic control. This paper presents the results from a trial that compared 44 days of ADS-B/Mode-S measurements with sonde data. The results showed that more than 65% of the derived temperatures and 78% of the derived winds meet the WMO Goal criteria for aeronautical meteorology. 1 Introduction Under CASA mandate (Civil Aviation Safety Authority, n.d.), from 12 Dec 2013, all aircraft that fly at or above FL290 (~29000ft) in Australian airspace must be ADS-B equipped. Similarly, any new aircraft that is registered after 6 Feb 2014 and is operated under Instrument Flight Rules (IFR) must carry serviceable ADS-B transmitting equipment. While there are ADS-B/Mode-S data messages dedicated to meteorological information, they are not mandatory. Hence, experience in Australia and overseas (Stone & Pearce, 2016; de Haan, 2011) has shown that, they are rarely implemented. However, it is possible to use the information available in the non-meteorological messages to derive wind speed, wind direction and temperature. This has already been implemented by both KNMI and the UK Met Office (Stone & Pearce, 2016; de Haan, 2011). The data required comes from two different mechanisms: Mode-S messages: These are only broadcast by the equipped aircraft in response to an interrogation by suitable ATC radar. For the derivation of meteorological data, the DF20/21 Track Turn [BDS50] and DF20/21 Heading Speed [BDS60] reports are required. ADS-B messages: These are broadcast automatically by all equipped aircraft, multiple times a minute. While they are not directly used in the derivation of meteorological data, the DF17 Airborne Position reports are used to determine the aircraft position. For the remainder of this report, the shorthand Mode-S will be used to cover both ADS-B and Mode- S messages. The Australian Bureau of Meteorology receives data from 82 aircraft flying domestic and international routes as part of its AMDAR program (World Meteorological Organisation, 2016). These aircraft provide observations of temperature, wind speed and direction every 7 minutes during level flight and at least every 50hPa during take-off and landing. AMDAR requires dedicated software to be installed into the avionics which interrogates the aircraft systems and downlinks the observations using the airline s data communications system. Hence, it requires a minimum capability for the aircraft avionics, an arrangement between each airline and NMHS, and a data communications cost for each observation. In Australia, the result is that the

2 majority of AMDAR observations come from Qantas Boeing 737/747 aircraft, which limits the available data to routes flown by these large jet aircraft. In contrast, ADS-B/Mode-S data is available from smaller aircraft (flying at different altitudes to the larger jets), from a greater range of routes (giving a greater geographic coverage), for free, and from airlines (both domestic and international) for which the Bureau does have any contractual arrangements. 2 Derivation of Meteorological Observations 2.1 Wind Speed and Direction From the Mode-S BDS50 message (International Civil Aviation Organization, 2004), the following data is available (de Haan, et al., 2013; Royal Netherlands Meteorological Institute, 2013): True Airspeed Ground Speed True Track Angle From the BDS60 message, the following data is available: Magnetic Heading Once the Magnetic Heading has been corrected for the local difference between Magnetic North and True North (NOAA: National Centers for Environmental Information, 2016), the Wind Speed and Direction can be calculated according to Figure 1. True Airspeed Corrected Magnetic Heading Figure 1: Wind Speed and Direction can be calculated from the difference of the Ground Speed and True Airspeed vectors 2.2 Temperature The Mode-S BDS60 report also contains the Mach (number) M. Since MM = vv aaaaaaaaaaaaaaaa cc aaaaaa where v aircraft is the airspeed of the aircraft and c air is the speed of sound in air. Also (1) cc aaaaaa = TT where T is the air temperature in degrees Celsius. Substituting for c air and rearranging gives TT = vv aaaaaaaaaaaaaaaa 331.3MM 2 1 (3) 3 WMO Requirements The WMO maintains the Observing Systems Capability Analysis and Review (OSCAR) tool, which lists requirements for a range of meteorological variables. The requirements for Aeronautical Meteorology (WMO, 2016) are shown in Table 1. Threshold represents the minimum requirement to ensure the data is useful. Goal is an ideal requirement, above which further improvement is not (2)

3 necessary. Breakthrough is an intermediate level which provides significant improvement in the target application. Variable Goal Breakthrough Threshold Temperature (K) Wind Horizontal (knots) Table 1: WMO Requirements for Accuracy No requirements for Wind Direction could be determined. 4 Direct Reception Broadmeadows The Mode-S data is broadcast free to air. The data is unencrypted and the format documentation is readily available (International Civil Aviation Organization, 2004). Hence, with the right antenna/receiver, it is possible for a NMHS to directly receive and decode the relevant information. A Radarcape kit (receiver, antenna, cabling) (Kollner, 2014) was purchased and installed on the roof of the Physics annexe at the Bureau Meteorological Training Centre, Broadmeadows, Victoria in May A coverage map, based on received data is shown in Figure 2. This shows that at least 100km range is available in all directions, and more than 300km is possible. After the data was received, it was decoded and stored, by python based software, on a dedicated linux (virtual) server. Figure 2: Range of ADS-B/Mode S data received at Broadmeadows. Range rings are 50km. Red 10000ft (altitude) range, Yellow 20000ft (altitude) range, Green ft (altitude) range Data Received The software was run during business hours on 14 separate days between 16 Jun and 28 Aug. The software was then run continuously from 1-17 Oct 2014 and 28 Jan 12 Feb Data storage issues limited the volume of data that could be collected. From this data, a total of 597,224 meteorological observations were derived. 1 OSCAR requirements are in SI units (in this case m/s). The requirements have been converted to knots to match the received variables.

4 Total Raw Messages Received 114,676,704 DF17 Airborne Position Messages 17,983,804 BDS50 Heading/Speed (Mode-S) Messages 7,839,341 BDS60 Track/Turn (Mode-S) Messages 7,707,032 Table 2: Available Message Statistics Sonde flights are made twice a day at Melbourne Airport, which is ~10km from the ADS-B/Mode-S antenna site. Sonde and Mode-S derived data were considered to be collocated (and hence, comparable) if the difference between sonde observations and derived Mode-S value was:- <0.1 degree latitude and longitude. The available sonde data did not include the sonde position, so the sonde was assumed to rise vertically above the launch site. <5000ft (1500m) altitude <600s observation time. From the available collocated data, the sonde observation closest to the aircraft was chosen. 5 Results The Mode-S data, on which this observation relies, is produced as a response to an Air Traffic Control (ATC) Radar request to the aircraft. To conserve bandwidth, the aircraft Mode-S response provides no indication as to the type of data in the message. Instead, the calculation process involves three stages:- Status Bits: A necessary condition for a Mode-S message to be of a particular format is for the designated status bits to be set. For example, in a BDS50 Track Turn Report, bits 1/12/24/35/46 must be set. Data Sanity: Next, the message is decoded and the individual values are examined for their sanity/reality aircraft position within a possible range of the receiver, aircraft performance variables (speeds, roll angles) within reasonable limits. Data Consistency: Finally, related parameters [true airspeed/indicated airspeed/ground speed & magnetic heading/true track angle] are checked to ensure they are within credible limits of each other. All data that passes these 3 checks is then used in the calculation of temperature, wind speed and wind direction. The results for comparison of Mode S to sonde for: temperature; wind speed; wind direction; and wind direction (wind speed > 10knots) are shown in Table 3. The scatter charts and histograms of the differences are shown in Figure 3-Figure 5, in Appendix Section 9. The biases, standard deviations (SD) and the percentage of points that met the requirements for Aeronautical Meteorology, see Table 1, are shown in Table 3. For comparison, the performance data of the AMDAR fleet compared to Sondes is also included. Initial analysis also showed that the wind direction is less accurate when the wind speed < 10knots. Table 3 includes performance data for all wind direct values, and also for only those wind direction values for which the wind speed was 10 knots.

5 Comparison to Sonde AMDAR Mode-S Number of Comparison Points Bias SD Temperature ( C) Goal 93.7% 65.0% Breakthrough 97.4% 81.8% Threshold 99.1% 93.8% Bias SD Wind Speed (knots) Goal 63.6% 78.5% Breakthrough 80.3% 90.7% Threshold 94.0% 96.3% Wind Direction ( ) Bias SD Wind Direction ( ) Bias (Wind Speed > 10knots) SD Table 3: Summary of comparisons to Sonde data 6 Discussion Table 3 shows that 65% of temperature data meet the WMO s Goal range (where sonde temperature is the reference). Examination of Figure 3 shows the distribution has a very broad tails 6.2% data does not make the WMO threshold criteria. The largest differences occur at the higher temperatures when the aircraft and the comparison sonde are closest to the ground. In comparison to the AMDAR data, the Mode-S derived observations are significantly worse. Wind speed shows better overall performance than temperature 78.5% of observations meeting the WMO s Goal Range. The distribution in Figure 4 again shows a long tail. In contrast to the Temperature Data, the wind speed observations derived from Mode-S data are much closer to the comparison sondes. The Wind direction observations from Mode-S show a similar performance to AMDAR data. Initial examination of the data indicates that many of the observations in the tails of the temperature and wind distributions come from a small number of aircraft. Further work will treat these outlier in two ways. Initially, criteria for blacklisting aircraft will be developed. In the longer term, methods for correction of observations based on past performance will be investigated. 7 Conclusions Data collected over 44 days from free-to-air Mode-S messages in Melbourne shows that more than 65% of the derived temperatures and 78% of the derived winds meet the WMO Goal criteria for

6 aeronautical meteorology when compared to collocated sondes. In general, the Mode-S derived meteorological data shows similar accuracy to AMDAR data when compared to Sondes. Further work on individual aircraft corrections and/or removing poorly performing aircraft offers the ability to further improve the accuracy of the derived observations. 8 References Airservices Australia, ADS-B coverage Airservices. [Online] Available at: [Accessed 11 August 2016]. Civil Aviation Safety Authority, C., n.d. Civil Aviation Order Aircraft equipment - Basic operational requirements (02/12/2004). [Online] Available at: [Accessed August 2016]. de Haan, S., High-resolution wind and temperature observations from aircraft tracked by Mode-S air control radar. J. Geophys. Res, 116(D10), pp de Haan, S., de Haij, M. & Sondij, J., The use of a commercial ADS-B receiver to derive upper air wind and temperature observations from Mode-S EHS information in The Netherlands, De Bilt: Royal Netherlands Meteorological Institute. International Civil Aviation Organization, Manual on Mode S Specific Services, s.l.: s.n. Kollner, G., MODE-S BEAST - Your Airspace Observer. [Online] Available at: [Accessed 11 August 2016]. NOAA: National Centers for Environmental Information, NCEI Geomagnetic Calculators. [Online] Available at: [Accessed 17 August 2016]. Royal Netherlands Meteorological Institute, Aircraft as a meteorlogical sensor. De Bilt: s.n. Stone, E. K. & Pearce, G., A Network of Mode-S Receivers for Routine Acquisition of Aircraft- Derived Meteorological Data. J. Atmos. Oceanic Technology, Volume 33, pp WMO, OSCAR: Observing Systems Capability Analysis and Review tool. [Online] Available at: [Accessed August 2016]. World Meteorological Organisation, AMDAR Home Page. [Online] Available at: [Accessed 17 August 2016].

7 9 Appendix Figure 4: Comparison of Sonde and Mode-S derived Temperature: Dark Green [Goal], Light Green [Breakthrough], Orange [Threshold] Figure 3: Comparison of Sonde and Mode-S derived Wind Speed Dark Green [Goal], Light Green [Breakthrough], Orange [Threshold]

8 a) b) Figure 5: Comparison of Sonde and Mode-S derived Wind Direction a) Wind Direction, b) Wind Dirction (wind speed > 10 knots