MADIS Profiles at Airports Quality Control Checks
The level 1 validity checks restrict each observation to falling within a set of tolerance limits, which are based on those specified in the TSP, and on the limits used in the aircraft decoder. Observations not falling within the limits are flagged as failing the QC check. The following table lists the basic tolerance limits:
--------------------------------------------- Validity Checks --------------------------------------------- Track latitude -90 - 90 deg N Track longitude -180 - 180 deg E Air temperature -100 - 60 C Dewpoint temperature -100 - 60 C Altitude 100 - 1026 mb Wind Direction 0 - 360 deg Wind Speed 0 - 300 kts
The validity check on altitude is done by first converting the altitude to pressure using the U.S. standard atmosphere calculation. The gross limits shown in the table for air temperature, dewpoint temperature, and wind speed are only used if the altitude is missing or has failed the validity check. Otherwise, the limits used for these variables is a function of altitude:
------------------------------------------------------------------------------------- Validity Checks - Air and Dewpoint Temperatures Altitude(ft) Minimum(C) Maximum(C) ------------------------------------------------------------------------------------- < 18000 -60 60 - (80 * (altitude/35000)) 18000 - 35000 -60 - (40 * (altitude/18000)/17000) 60 - (80 * (altitude/35000)) > 35000 -100 -20
------------------------------------------------------------------------------------- Validity Checks - Wind Speed Altitude(ft) Minimum(kts) Maximum(kts) ------------------------------------------------------------------------------------- < 30000 0 70 + (230 * altitude)/30000 30000 - 40000 0 300 40000 - 45000 0 300 - (100 * (altitude - 40000)/5000) > 45000 0 200
The level 1 position consistency check compares location and time to previous reports to insure that an aircraft's position is consistent with the reported movement. An inconsistency is identified as an unrealistic speed. Speed is calculated from the distance traveled and the time elapsed since the previous report. Distance, in miles on a spherical Earth, is calculated from the reported latitude and longitude coordinates using the Haversine formula expressed in terms of a two-argument inverse tangent function (Sinnot, 1984).
The minimum allowable speed is 0 m/s because aircraft sometimes report from the ground, or while circling. The maximum allowable speed (600 m/s) is faster than actual speeds, but is used to take into account the 1 minute time resolution of the high-resolution reports taken during ascents and descents.
The position consistency check also expects an aircraft that's not sitting on the ground to move from one report to another. Therefore, if the altitude is > 2000 meters and the position hasn't changed, the check is failed.
The level 1 provider QC checks (TAMDAR only) flag the data that fail simple validity checks (no information is currently available on what thresholds are used), or data that are acquired while the de-icing heater is on.
The level 2 internal consistency check enforces reasonable, meteorological relationships among observations measured in a single report. For aircraft data, the dewpoint temperature observation must not exceed the corresponding temperature observation. If it does, both the dewpoint and temperature observation are flagged as failing their internal consistency check.
The level 2 temporal consistency check applied to altitude and temperature restricts the temporal rate of change of each observation to calculated tolerance limits that model the expected rate of change for either ascent/descent or flight level operations. To examine the rate of change for the current observation, comparisons are made with the preceeding and succeeding observation from the same aircraft.
The departure value (or observation difference) for the report at time j is calculated over the time interval i-k:
Then if | Odiff | > a threshold, the observation at point j fails the check.Odiff = Oj - ( Oi * DELTjk / DELTik ) + ( Ok * DELTij / DELTik )
where,
Odiff : departure value (deg C for temperature, m for altitude) j : index of observation being checked i : index of previous observation (in time) k : index of successive observation (in time) Oi : observation at point i (or j, k) DELTij : change in time between points ij (or ik, jk) (seconds)
The basic idea is to weight the previous and successive observations by how close they are in time to the observation being checked. If the three points are equidistant in time, and the observed value hasn't changed, the estimated value will equal the observed value, and the observation difference will be 0.
The temperature threshold (Tdiff) is calculated as:
Tdiff = ( XD * D ) + ( XA * 6.5 * | DELA | / 1000 )
where,
Tdiff : maximum allowable difference between observed and estimated temperature (deg C) D : distance over the three observations used in the QC (report being checked and preceeding & succeeding reports) (miles) DELA : altitude change over the three observations used in the QC -- the quantity 6.5 * |DELA|/1000 is the standard atmosphere lapse rate for the altitude range (m) XD : coefficient to weight the expected difference resulting from change in distance (expected to dominate the calculation at flight level) [ XD = 0.25 ] XA : coefficient to weight the expected difference resulting from change in altitude (expected to dominate the calculation for ascents/descents) [ XA = 1.97 ]
The altitude threshold (Adiff) is calculated as:
Adiff = C * DELT
where,
Adiff : maximum allowable difference between observed and estimated altitude (m) DELT : time change over the three observations used in the QC (seconds) C : 5.84 for ascents/descents, 2.80 for flight level
The determination of the value of the coefficient C is made based on the speed calculated at each of the three reports used in the QC. If the speed over all three reports is > 500 mph then the tighter flight level value is used, otherwise the ascent/descent value is used. (Speed is calculated as described above in the position consistency check discussion.)
*It should be noted that while the QC checks discussed here are generally applied to the form of the variable stored in the database, the QC results will also be applied to any forms of the variable that are requested by the user and are derived from the primary variable. For example, specific humidity will get the QC results from the checks applied to dewpoint temperature.
Subjective Intervention
Two text files, a "reject" and an "accept" list provide the capability to subjectively override the results of the automated QC checks. The reject list is a list of aircraft and associated input observations that will be labeled as bad, regardless of the outcome of the QC checks; the accept list is the corresponding list of stations that will be labeled as good, regardless of the outcome of the QC checks. In both cases, observations associated with the stations in the lists can be individually flagged. For example, wind observations for a particular aircraft may be added to the reject list, but not the temperature observations.
Here are the current subjective intervention lists in use:
QC Data Structures
The MADIS QC information available for each variable includes the following QC structures: a single-character "data descriptor", intended to define an overall opinion of the quality of each observation by combining the information from the various QC checks, and for users desiring detailed information, a "QC applied" bitmap indicating which QC checks were applied to each observation, and a "QC results" bitmap indicating the results of the various QC checks.
The following table provides a complete list of the data descriptors and the bits used in the bitmaps:
-----------------------------------------
MADIS QC Information - Aircraft-Based Aircraft
-----------------------------------------
QC Data Descriptor Values
-------------------------
No QC available:
Z - Preliminary, no QC
Automated QC checks:
C - Coarse pass, passed level 1
S - Screened, passed levels 1 and 2
V - Verified, passed levels 1, 2, and 3
X - Rejected/erroneous, failed level 1
Q - Questioned, passed level 1, failed 2 or 3
where level 1 = validity, position consistency, and provider QC checks
level 2 = internal and temporal consistency checks
level 3 = N/A
Subjective intervention:
G - Subjective good
B - Subjective bad
Interpolated/Corrected observations:
T - Virtual temperature could not be calculated, air temperature passing all QC
checks has been returned
Bitmask for QC Applied and QC Results
-------------------------------------
Bit QC Check Decimal Value
--- -------- -------------
1 Master Check 1
2 Validity Check 2
3 Position Consistency Check 4
4 Internal Consistency Check 8
5 Temporal Consistency Check 16
6 Reserved 32
7 Reserved 64
8 Reserved 128
9 Reserved 256
10 Reserved 512
11 Reserved 1024
12 Provider QC Check 2048
The QC bitmask is used in the QC applied and QC result "words" returned along with the QC data descriptor. By examining the individual bits, the user can determine which checks were actually applied, and the pass/fail status of each check that was applied.
In the QC applied word, a bit value of 1 means the corresponding check was applied, a bit value of 0 indicates the check wasn't applied.
In the QC results word, a bit value of 1 means the corresponding check was applied and failed, a bit value of 0 indicates the check passed (given that the check was applied).
The "Master Check" is used to summarize all of the checks in a single bit. If any check at all was applied, this bit will be set in the QC applied word. If the observation failed any QC check, it will be set in the QC results word.
When read as decimal numbers, the different bits that are set in the bitmask are summed together. For example, a QC applied value of 19 should be interpreted as 1 + 2 + 16, meaning the validity and temporal consistency checks were applied.
References
Sinnott, R.W., 1984: Virtues of the Haversine, Sky and Telescope, 68, 159-170.
Technique Specification Package 88-21-R2 For AWIPS-90 RFP Appendix G Requirements Numbers: Quality Control Incoming Data, 1994. AWIPS Document Number TSP-032-1992R2, NOAA, National Weather Service, Office of Systems Development.
Last updated 16 March 2017
