25.09.15 - Draft AWR Requirements Document (1).docx

Requirements for an Airport Weather Radar (AWR) 18 August 2025

1 Introduction Low-level wind shear, especially a microburst, is very hazardous to aircraft departing or approaching an airport. The danger became especially clear in a series of fatal commercial airliner accidents in the 1970s and 1980s at U.S. airports. In response, the Federal Aviation Agency (FAA) developed and deployed three ground-based low altitude wind shear detection systems: Terminal Doppler Weather Radar (TDWR), Low Level Wind Shear Alert System (LLWAS), and Airport Surveillance Radar-9 (ASR-9) Weather Systems Processor (WSP). Since deployment, there have been no documented commercial airline accidents attributed to wind shear at any of the airports serviced by these sensors since they have been operational. This dramatic decrease in accidents caused by wind shear appears to confirm the safety benefits provided by these detection systems.

The WSP is currently used by Air Traffic Control (ATC) at 34 airports nationwide, most of them of medium traffic capacity. Less costly than the TDWR, the WSP was developed and enhanced by MIT Lincoln Laboratory as an add-on modification to the ASR-9. The technology was then transitioned to a commercial contractor for manufacturing. Commissioned from August 2002 to April 2005, the fielded WSP systems are each maintained by the FAA Weather Systems Processor Service (WSPS) and provides wind shear and microburst data to the Display Function (DF) subsystem located at an ATC Tower (ATCT) and Terminal Radar Approach Control (TRACON) facility.

As part of a larger FAA program of replacing all ASRs, the ASR-9 is planned to be replaced by a new system. This program, however, does not have weather observation (including airport wind-shear detection) as a requirement. Thus, in order to maintain wind-shear detection services at the 34 ASR-9 WSP airports, the FAA is considering acquiring a new weather radar for those sites. This new Airport Weather Radar (AWR) would provide base data output to the Integrated Terminal Weather System (ITWS), which would then generate the required wind-shear detection products. This document defines requirements for the AWR such that it would provide base data output to ITWS with sufficient data quality and resolution for terminal wind-shear product generation.

2 Mission Need The primary mission of the AWR is to enable ITWS to enhance the safety of air travel through the timely detection and reporting of hazardous wind shear in and near the terminal approach and departure zones at the 34 airports currently serviced by WSP. Specific sources of the hazardous wind shear that are to be detected are microbursts and gust fronts. Secondary missions to be met by the AWR are to enable ITWS to improve the management of air traffic in the terminal area through the forecast of gust-front-induced wind shifts at the airport, the detection and tracking of precipitation, and the censoring of false weather echoes caused by Anomalous Propagation (AP).

The end users of AWR/ITWS outputs are local air traffic controllers, their supervisors, and traffic management unit specialists. Air carrier operations personnel and airport operators would also benefit from the information provided by AWR/ITWS.

3 Requirements Statement Certain requirements listed in this document include more demanding objective requirements indicated as [OBJECTIVE]. The Government’s intention is to receive input from industry on the relative cost of achieving the objective requirements over the threshold requirements.

At this time, there is no particular operational band specified for the AWR. On the one hand, a number of issues favor S band (2700-2900 MHz): (1) The severe interference from wireless communication devices at C band (5600-5650 MHz) experienced by TDWR (and other weather radars worldwide) is not likely to decrease (and may even increase) in the future [1]. (2) X band attenuation due to propagation through precipitation and a wet radome can be quite significant, and may lead to loss of wind-shear signal. (3) Range-velocity ambiguity constraints, important for accurate velocity measurements and protection from out-of-trip range contamination, favor S band over higher frequency bands. On the other hand, there may be spectrum allocation issues to navigate with S band, with the incumbent ASRs (and likely their replacements) operating in this band, and the government’s interest in potentially auctioning off some or all of the 2700-2900 MHz band [2]. Respondents are encouraged to provide input on the choice and rationale of operational band for the AWR.

The AWR system must have the following capabilities and characteristics:

3.1 Operation

The AWR must be a stand-alone weather radar to be located within the airport property. The performance requirements in this section are largely derived from [3].

3.1.1 Reliability

The AWR system reliability program must be in accordance with MIL-STD-785. The AWR system hardware reliability requirements must be specified in terms of Full Mission Capability (FMC), Mean Time Between Critical Failure (MTBCF), and Mean Time Between Failure (MTBF).

3.1.1.1 Mean Time Between Critical Failure

The AWR system hardware must have a MTBCF of 1500 hours or greater.

3.1.1.2 Mean Time Between Failure

The AWR system hardware must have a MTBF of 550 hours or greater.

3.1.2 Maintainability

The AWR system maintainability program must be in accordance with MIL-STD-470. The AWR system maintainability program must ensure that the following Mean Time to Repair (MTTR) and Maximum Corrective Maintenance Time (MAXCT) requirements are achieved:

3.1.2.1 Mean Time to Repair

The AWR system must have a MTTR of 0.5 hour or less.

3.1.2.2 Maximum Corrective Maintenance Time

The AWR system must have a MAXCT of 2 hours.

3.1.2.3 Mean Bench Repair

The AWR line replaceable units (LRUs) will have a Maximum Bench Repair (MBR) design goal of 1 hour or less, with 95% of all bench repairs completed in less than 4 labor hours.

3.1.3 Availability

The AWR must have an inherent availability of 0.99967.

3.2 Radar Performance

In order for ITWS to meet its wind-shear detection performance requirements, the AWR must have certain performance characteristics. This section lays out these requirements, based on past experience and the WSP-like terminal weather surveillance performance requirements developed for the Spectrum Efficient National Surveillance Radar (SENSR) program [4] [5]. At this time, the required base data output fields for ITWS ingest are reflectivity, radial velocity, velocity spectrum width, and signal-to-noise ratio (SNR). However, in the future, polarimetric base data fields of differential reflectivity, correlation coefficient, and specific differential phase may also become requirements in order to accommodate enhancements to the ITWS product generation algorithms.

The AWR must provide output reflectivity, radial velocity, velocity spectrum width, and SNR data with the following characteristics:

3.2.1 Sensitivity

Minimum detectable reflectivity of no more than -6 dBZ for a single-pulse SNR of 0 dB, within 6 nmi range of the airport reference point (ARP). [OBJECTIVE: Minimum detectable reflectivity of no more than -17 dBZ for a single-pulse SNR of 0 dB, within 6 nmi range of the ARP.]

3.2.2 Data Output Range

The AWR must output reflectivity, radial velocity, velocity spectrum width, and SNR data meeting all requirements that follow in this section between the slant ranges of 0.25 nmi and 48 nmi from the radar antenna. In addition, the AWR must output surface-scan reflectivity and SNR data that are unambiguous in range between the slant ranges of 0.25 nmi and 250 nmi, once at the beginning of every volumetric scan; this long-range surveillance scan must meet all requirements that are applicable to reflectivity and SNR in Section 3.2, except for out-of-trip signals (Section 3.2.22).

3.2.3 Bias

(a) Reflectivity bias of less than 2 dBZ for SNR > 10 dB and true velocity spectrum width = 1 to 4 m/s.

(b) Radial velocity bias of less than 1 m/s for SNR > 10 dB and true velocity spectrum width = 1 to 4 m/s.

(c) Velocity spectrum width bias of less than 1 m/s for SNR > 10 dB and true velocity spectrum width = 1 to 4 m/s.

(d) SNR bias of less than 2 dB for SNR > 0 dB.

3.2.4 Standard Deviation

(a) Reflectivity standard deviation of no more than 2 dBZ for SNR > 10 dB and true velocity spectrum width = 1 to 4 m/s.

(b) Radial velocity standard deviation of no more than 1 m/s for SNR > 8 dB and true velocity spectrum width of 4 m/s.

(c) Velocity spectrum width standard deviation of no more than 1 m/s for SNR > 10 dB and true velocity spectrum width = 1 to 4 m/s.

(d) SNR standard deviation of no more than 2 dBZ for SNR > 10 dB and true velocity spectrum width = 1 to 4 m/s.

3.2.5 Quantization

(a) Reflectivity quantization of no more than 1 dBZ.

(b) Radial velocity quantization of no more than 0.1 m/s.

(c) Velocity spectrum width quantization of no more than 0.1 m/s.

(d) SNR quantization of no more than 1 dB.

3.2.6 Range Resolution

(a) Reflectivity range resolution of no more than 150 m.

(b) Radial velocity range resolution of no more than 150 m.

(c) Velocity spectrum width range resolution of no more than 150 m.

(d) SNR range resolution of no more than 150 m.

3.2.7 Range Accuracy

(a) Reflectivity range accuracy of no more than 80 m.

(b) Radial velocity range accuracy of no more than 80 m.

(c) Velocity spectrum width range accuracy of no more than 80 m.

(d) SNR range accuracy of no more than 80 m.

3.2.8 Range Data Output Interval

(a) Reflectivity range data output interval of 150 m.

(b) Radial velocity range data output interval of 150 m.

(c) Velocity spectrum width range data output interval of 150 m.

(d) SNR range data output interval of 150 m.

3.2.9 Effective Azimuthal Resolution

(a) Reflectivity effective azimuthal resolution of no more than 2°, inclusive of beam scan smearing and data windowing effects. [OBJECTIVE: Reflectivity effective azimuthal resolution of no more than 1°, inclusive of beam scan smearing and data windowing effects.]

(b) Radial velocity effective azimuthal resolution of no more than 2°, inclusive of beam scan smearing and data windowing effects. [OBJECTIVE: Radial velocity effective azimuthal resolution of no more than 1°, inclusive of beam scan smearing and data windowing effects.]

(c) Velocity spectrum width effective azimuthal resolution of no more than 2°, inclusive of beam scan smearing and data windowing effects. [OBJECTIVE: Velocity spectrum width effective azimuthal resolution of no more than 1°, inclusive of beam scan smearing and data windowing effects.]

(d) SNR effective azimuthal resolution of no more than 2°, inclusive of beam scan smearing and data windowing effects. [OBJECTIVE: SNR effective azimuthal resolution of no more than 1°, inclusive of beam scan smearing and data windowing effects.]

3.2.10 Azimuthal Accuracy

(a) Reflectivity azimuthal accuracy of no more than 0.25°.

(b) Radial velocity azimuthal accuracy of no more than 0.25°.

(c) Velocity spectrum width azimuthal accuracy of no more than 0.25°.

(d) SNR azimuthal accuracy of no more than 0.25°.

3.2.11 Azimuthal Data Output Interval

(a) Reflectivity azimuthal data output interval of 1°.

(b) Radial velocity azimuthal data output interval of 1°.

(c) Velocity spectrum width azimuthal data output interval of 1°.

(d) SNR azimuthal data output interval of 1°.

3.2.12 Elevation Angle Resolution

(a) Reflectivity elevation angle resolution of no more than 2°. [OBJECTIVE: Reflectivity elevation angle resolution of no more than 1°.]

(b) Radial velocity elevation angle resolution of no more than 2°. [OBJECTIVE: Radial velocity elevation angle resolution of no more than 1°.]

(c) Velocity spectrum width elevation angle resolution of no more than 2°. [OBJECTIVE: Velocity spectrum width elevation angle resolution of no more than 1°.]

(d) SNR elevation angle resolution of no more than 2°. [OBJECTIVE: SNR elevation angle resolution of no more than 1°.]

3.2.13 Elevation Angle Accuracy

(a) Reflectivity elevation angle accuracy of no more than 0.25°.

(b) Radial velocity elevation angle accuracy of no more than 0.25°.

(c) Velocity spectrum width elevation angle accuracy of no more than 0.25°.

(d) SNR elevation angle accuracy of no more than 0.25°.

3.2.14 Data Update Period

(a) Reflectivity update period for the surface scan of no more than 60 s.

(b) Radial velocity update period for the surface scan of no more than 60 s.

(c) Velocity spectrum width update period for the surface scan of no more than 60 s.

(d) SNR update period for the surface scan of no more than 60 s.

(e) Reflectivity update period for the volumetric scan of no more than 150 s.

(f) Radial velocity update period for the volumetric scan of no more than 150 s.

(g) Velocity spectrum width update period for the volumetric scan of no more than 150 s.

(h) SNR update period for the volumetric scan of no more than 150 s.

3.2.15 Data Latency

(a) Reflectivity latency of no more than 5 s.

(b) Radial velocity latency of no more than 5 s.

(c) Velocity spectrum width latency of no more than 5 s.

(d) SNR latency of no more than 5 s.

3.2.16 Radial Velocity Dealiasing

The required radial velocity output range is ±40 m/s. If dealiasing is needed to cover this span, the dealiasing success rate must be at least 97%. [OBJECTIVE: The required radial velocity output range is ±50 m/s. If dealiasing is needed to cover this span, the dealiasing success rate must be at least 99.5%.]

Instances of dealiasing failure must be indicated via the corresponding base data quality flag.

3.2.17 Stationary Clutter

(a) The AWR must suppress at least 55 dB stationary clutter. [OBJECTIVE: The AWR must suppress at least 60 dB stationary clutter.]

(b) The AWR must detect stationary clutter, where clutter-to-noise ratio (CNR) > 0 dB and clutter-to-signal ratio (CSR) > -10 dB. The AWR must filter detected stationary clutter to reduce clutter power to at least noise level or 10 dB below weather signal level, whichever is greater, as allowed by the specified maximum suppression capability.

(c) When stationary clutter filtering is applied, the reflectivity and SNR biases introduced by the filtering process must be ≤ 2 dB.

(d) When stationary clutter filtering is applied, the radial velocity and velocity spectrum width biases introduced by the filtering process must be ≤ 1 m/s.

(e) When stationary clutter filtering is applied, the reflectivity and SNR standard deviations introduced by the filtering process must be ≤ 2 dB.

(f) When stationary clutter filtering is applied, the radial velocity and velocity spectrum width standard deviations introduced by the filtering process must be ≤ 1 m/s.

3.2.18 Receiver Dynamic Range

The baseband linear instantaneous dynamic range of the AWR’s receiver must be at least 95 dB. [OBJECTIVE: The baseband linear instantaneous dynamic range of the AWR’s receiver must be at least 100 dB.]

3.2.19 Polarization Mode

The radar component must operate in dual linear (horizontal and vertical) or linear horizontal polarization mode. If it is a phased array radar in linear polarization mode, the reflectivity must be corrected to within 1 dB of the reflectivity corresponding to true linear polarization at the scatterer (relative to Earth’s gravity) for all antenna beam pointing angles and absolute differential reflectivity ≤ 8 dB.

3.2.20 Antenna Sidelobes

(a) The two-dimensional (azimuth-elevation) integrated sidelobe level (two-way, for all pointing angles, radome effects included) must be no more than -26 dB. [OBJECTIVE: The two-dimensional (azimuth-elevation) integrated sidelobe level (two-way, for all pointing angles, radome effects included) must be no more than -32 dB.]

(b) The peak sidelobe level (two-way, for all pointing angles, radome effects included) must be no more than -48 dB. [OBJECTIVE: The peak sidelobe level (two-way, for all pointing angles, radome effects included) must be no more than -54 dB.]

3.2.21 Range Sidelobes

Range sidelobe requirements only apply to solution implementations that utilize radar pulse compression.

(a) The integrated sidelobe level must be no more than -46 dB for Doppler velocities from -80 m/s to 80 m/s, including sensitivity time control (STC) effects over the uncompressed pulse length. [OBJECTIVE: The integrated sidelobe level must be no more than -50 dB for Doppler velocities from -80 m/s to 80 m/s, including STC effects over the uncompressed pulse length.]

(b) The peak sidelobe level must be no more than -25 dB beyond 150 m of the peak response; no more than -35 dB beyond 175 m of the peak response; no more than -54 dB peak sidelobe, for Doppler velocities from -80 m/s to 80 m/s, including STC effects over the uncompressed pulse length. The null-to-null main-lobe width must not exceed 390 m. [OBJECTIVE: The peak sidelobe level must be no more than -25 dB beyond 150 m of the peak response; no more than -35 dB beyond 175 m of the peak response; no more than -58 dB peak sidelobe, for Doppler velocities from -80 m/s to 80 m/s, including STC effects over the uncompressed pulse length. The null-to-null main-lobe width must not exceed 390 m.]

3.2.22 Out-of-Trip Signals

Out-of-trip signals must be filtered out while keeping the added errors due to this operation within the following limits. (P1 is the first-trip signal power, PO is the out-of-trip signal power, W1 is the true first-trip velocity spectrum width, and WO is the true out-of-trip velocity spectrum width.)

(a) Reflectivity and SNR biases < 1 dB for PO/P1 ≤ 50 dB, W1 ≤ 4 m/s, and WO ≤ 4 m/s.

(b) Radial velocity bias < 0.5 m/s for PO/P1 ≤ 40 dB, W1 ≤ 4 m/s, and WO ≤ 4 m/s.

(c) Velocity spectrum width bias < 0.5 m/s for PO/P1 ≤ 50 dB, W1 ≤ 4 m/s, and WO ≤ 4 m/s.

(d) Reflectivity and SNR standard deviations < 2 dB for PO/P1 ≤ 50 dB, W1 ≤ 4 m/s, and WO ≤ 4 m/s.

(e) Radial velocity standard deviation < 1 m/s for PO/P1 ≤ 40 dB, W1 ≤ 4 m/s, and WO ≤ 4 m/s.

(f) Velocity spectrum standard deviation < 1 m/s for PO/P1 ≤ 50 dB, W1 ≤ 4 m/s, and WO ≤ 4 m/s.

3.2.23 Fixed-Location Clutter with Non-Zero Radial Velocity

The AWR must detect and censor fixed-location clutter with non-zero radial velocity (e.g., roads, wind turbines), on a range-azimuth cell basis if CNR > 0 dB and CSR > -10 dB.

3.2.24 Point Targets

The AWR must detect and filter out point target returns, including multi-trip echoes, < 300 m in range extent, for point target signal power > 10 dB above the weather signal or noise power, whichever is greater.

3.2.25 Radio Frequency Interference (RFI)

For RFI sources with duty cycle < 20% and interference-to-noise or interference-to-weather-signal ratio (whichever is less) ≥ 10 dB, RFI must be detected and suppressed with detection probability ≥ 0.95 and false alarm probability ≤ 10-4.

3.2.26 Propagation Attenuation

(a) The two-way RF propagation attenuation due to air and rain within 6 nmi of the ARP must be less than 2.5 dB for rain rates not exceeded 99.9% of the time at the radar site.

(b) The two-way RF propagation attenuation due to air and rain within 55 nmi of the ARP must be less than 4 dB for rain rates not exceeded 99.9% of the time at the radar site.

3.2.27 Data Quality Flags

The AWR must output the following data quality flags on a range-azimuth cell basis. Flag meanings are explained in [6].

(a) Valid data flag.

(b) Valid velocity flag.

(c) Velocity dealias fail flag.

(d) Point target flag.

3.3 Monitoring Function (MF)

The AWR MF must monitor critical system performance data by providing the functions needed to perform monitoring, fault isolation, variable parameter adjustment, and control of the AWR system. Automatic fault detection and localization must be to the LRU level with the following possible exceptions: keyboards, point and click devices, and computer monitors. The AWR MF must allow equivalent control, monitoring, and troubleshooting of the AWR from a Maintenance Display Terminal (MDT). The AWR MF must employ a modern graphical user interface (GUI) to facilitate its use by NAS equipment specialists.

3.3.1 MF Performance and Functional Requirements

The AWR MF must perform the following control, monitoring, and troubleshooting functions:

(a) Control of AWR system operational functions and adjustable parameters.

(b) Monitor, collect, and process performance and status data to determine if the AWR system is operating within the performance limits defined in this requirements document.

(c) Provide alarms, alerts, and status of system elements needed for operation.

(d) Perform fault isolation and diagnostic tests on the AWR system to isolate failures.

Failure of the MF must not interfere with the operation of any other AWR subsystem, nor must any MF function interfere with the operation of the AWR.

3.3.1.2 Performance Monitoring

At periodic intervals, the MF must monitor, execute measurements, and collect hardware and software critical performance data of each AWR subsystem to determine whether each subsystem is operating within specified limits. Self-diagnostic capabilities included with COTS subsystems within the AWR must be used, processed, and reported by the MF. A candidate set of subsystem measurements and performance data must be developed that support the requirements of this section. The candidate set must include the capability to monitor and detect failure of all active external interfaces to the AWR. The candidate set will be subject to Government approval, following which it must be implemented.

3.3.1.3 Status Data

The MF must monitor and collect operating status and built-in-test results, including system configuration, mode of operation, security and safety alarm system, run-time monitoring, and threshold parameter changes. A candidate set of status data must be developed that supports the requirements of this section. The candidate set will be subject to Government approval, and following approval, must be implemented.

3.3.1.4 MF Data Acquisition

(a) The collection of performance and status data must be performed automatically and periodically without need for an external command or interference with AWR operational requirements.

(b) Collected status data must be used by the MF to determine the equipment status for each element of the AWR system.

(c) Performance status data collected must be used by the MF to determine AWR system operability.

3.3.1.5 MF Alarms and Alerts

The MF must determine and report alarm and alert status for each AWR component and for parameters that are vital to AWR operation. An alert condition is any non-critical out-of-tolerance performance parameter that may cause the system to become non-operational. An alarm condition is any critical out-of-tolerance condition that requires immediate attention. The alarm and alert thresholds for these parameters, with the exception of those that are on/off only, must be programmable so non-system-critical parameters can be changed by the MF on command from an MDT. The MF must report alarms and alerts based on the set thresholds for conditions that affect critical system performance or data integrity.

3.3.1.5.1 Alarm and Alert Processing

The MF must perform a discriminating function to minimize the declaration of alarms and alerts caused by transient conditions. The MF must check every performance parameter value for an alarm or alert condition at least once during each general status cycle, and automatically report any alarm or alerts discovered. The MF must monitor and check every performance parameter value, detect any significant changes in value and report the changes. Out-of-tolerance and status conditions identified as alarms and alerts must be reported immediately, time tagged, and stored in order of occurrence.

3.3.1.5.2 Return to Normal Processing

The MF must automatically generate a System Notification message with a return to normal indication for each parameter previously reported as an alarm or alert when it is no longer in either the alarm or alert state.

3.3.1.6 Fault Isolation

The MF must initiate fault isolation and diagnostic routines from an MDT. The MF must initiate the corresponding diagnostic routine upon receiving a diagnostic command, execute the command by directing the diagnostic test, collect and analyze the data, and format and send a response to an MDT. The MF must permit the termination of fault isolation and maintenance tests at any time by operator action at an MDT.

3.4 Data Output Content and Formatting

The AWR must output data to ITWS conforming to the interface control specifications defined in [6].

3.5 Data Archiving

The AWR must provide the following automatic data recording and archiving capabilities:

3.5.1 Time Series Recording

While in operation, the AWR must continuously record in-phase and quadrature (I&Q) digital time series data output from the radar receiver, with at least the last 3 days of data available for downloading at any given time.

3.5.2 Base Data Archiving

While in operation, the AWR must continuously record base data output, with at least the last 15 calendar days of data available for downloading at any given time.

4 Acronyms and Abbreviations

5 Definitions Availability Availability is the probability that the AWR will be at Full Mission Capability (FMC) during any and all required operating times.

Base Data Weather radar base data refers to digital output fields of reflectivity, mean radial velocity, spectrum width, and, for dual polarization radars, differential reflectivity, differential phase, and correlation coefficient. For the purposes of the AWR, the minimum required base data fields are reflectivity, mean radial velocity, and spectrum width, along with SNR (which is usually not defined as part of weather radar base data).

Beam Filling Target A weather target that extends across the main beam to at least the half power points.

Corrective Maintenance Corrective maintenance consists of those actions directly related to correcting a failure. Corrective maintenance does not include administrative or travel activities.

Critical Failure A critical failure is any failure that degrades the AWR’s FMC.

Data Latency Data latency as defined for this document is the time elapsed between the end of a 360° azimuthal scan at a given elevation angle and the output of the corresponding base data from that scan by the AWR.

dBZ Radar reflectivity factor of weather scatterers expressed in decibels.

Effective Azimuthal Resolution Effective azimuthal resolution is the azimuthal resolution of a combination of the one-way antenna pattern, the azimuthal extent of the processed receiver samples, and any applied data windowing. The effective azimuthal resolution is computed by the convolution of the one-way antenna pattern with the square of the data window as defined over the azimuthal extent of the sample [7].

Failure A failure is any event that causes the AWR system not to meet any requirement of this specification.

Full Mission Capability The level of performance which allows the AWR to perform its mission within the requirements of this specification.

Gust Fronts A moving wind shift zone separating ambient air from a thunderstorm outflow. Associated with gust fronts are strong updrafts ahead of the front, wind shear in the frontal (wind shift) zones, and strong turbulence in the frontal zone and above the outflow directly behind the frontal zone. The wind shift zone associated with a gust front is typically 0.5 nmi in width.

Hazardous Wind Shear A hazardous wind shear is a strong variation of the longitudinal wind speed along the flight path of an aircraft, sufficient to cause a loss of aircraft performance and sustain a significant loss of altitude.

Inherent Availability Inherent availability is a measure of the availability which excludes the logistics delay time and administrative delay time.

Line Replaceable Unit An LRU consists of one or more electronic/mechanical subassemblies and assemblies, as defined in MIL-STD-280 and applicable parts of MIL-STD-1388-2, and excludes items falling under the definition for a part as given in MIL-STD-280.

Maximum Corrective Maintenance Time MAXCT is the maximum time to isolate, fix, and retest any failed unit. This time is applicable to 95% of all possible failures.

Mean Bench Repair MBR is the total LRU bench repair time, to include set up and fault isolation time, repair time, and verification time divided by the total number or LRUs repaired.

Mean Time Between Critical Failures MTBCF is the total FMC time divided by the total number of critical failures.

Mean Time Between Failures MTBF is the total FMC time divided by the total number of failures that require corrective maintenance.

Mean Time to Repair MTTR is the total time to perform all on-site corrective maintenance divided by the total number of corrective maintenance activities required. MTTR includes faulty LRU identification, LRU removal, LRU replacement, system test, and system return to FMC.

Out-of-Trip Signals Signals resulting from targets beyond the unambiguous range of the radar (such as distant storms) that obscure first-trip weather returns due to range aliasing.

Preventive Maintenance Preventive maintenance activities are those maintenance activities performed to ensure that the AWR maintains its FMC and to prevent future in-service functional failure of equipment.

Radial Velocity The measured radial component of the velocity vector. Mean radial velocities are averaged over time and space (a range-azimuth bin). Outbound radial velocities are defined as positive velocities, and inbound radial velocities are defined as negative velocities.

Site Level Maintenance Maintenance is performed at this level on systems, system segments, and support equipment in direct support of the AWR System. It includes system maintenance monitoring, system fault isolation, and correction of system failures through the removal and replacement of LRUs and preventive maintenance, but does not include repair, service, calibration, and verification of the removed LRUs.

Surface Scan The surface scan is the 360° azimuthal scan at an elevation angle where the antenna main lobe is observing the lowest layer of the atmosphere and still capable of meeting all the data quality requirements in Section 3.2.

True Radial Velocity The true radial velocity is defined to be the actual radial velocity of the radar scatterer, as opposed to the measured value.

True Velocity Spectrum Width The true velocity spectrum width is defined to be the actual width (i.e., square root of the variance) of the radar scatterer velocity power distribution. This true width is distinguished from the measured width, which is just one realization of the true random process being observed.

Velocity Spectrum Width The velocity spectrum width is the square root of the second spectral moment about the mean radial velocity.

Volumetric Scan The volumetric scan is the set of multiple 360° azimuthal scans at various elevation angles that samples the entire column of precipitation present.

Wind Shear Any change in the wind speed and/or direction between two points in the atmosphere.

Wind Shift A wind shift is a boundary, straight or curved, that represents a transition zone of a sustained change in the horizontal wind speed and/or direction, but primarily the wind direction. The wind shift zone usually moves by or through a given location. When the wind shift zone moves in this fashion, often a change in active runways is required. The boundary usually separates air masses of different temperatures. Wind shifts are associated with thunderstorm gust fronts, cold fronts, warm fronts, and sea breeze fronts. The transition zone widths vary from 0.5 nmi (gust fronts and some cold fronts) to tens of miles (warm fronts and some cold fronts). All wind shifts are characterized by wind shear.

References