The Bendix/King RDR-4A Weather Radar and Its RTA-4A Receiver/Transmitter

1. What the system is and what it was for

In briefThe RDR-4A is a weather radar that sits in an airliner's nose and shows the crew where rain and storms are ahead, how heavy they are and, within 40 NM, where the air in those storms is turbulent [3][2]. It was an airline product, not a military one. It came in with the first "digital" airliners, the Boeing 757 and 767 [4], and was the first Bendix/King airline radar with a solid-state, fully Doppler measurement.">coherent transmitter [2]. The RTA-4A is its receiver/transmitter box.

An airborne weather radar sends short bursts of microwave energy ahead of the aircraft and listens for the faint echoes that raindrops send back. The stronger the echo, the heavier the rain, and heavy rain usually means a strong storm. The RDR-4A painted those echoes on a cockpit display in colors, green for light rain through red for heavy rain, so pilots could pick a path around the worst weather [3]. It also measured how fast the raindrops were moving toward or away from the aircraft (the Doppler effect). When those speeds varied a lot over a small area, which is a sign of turbulence, it colored that area magenta [3][2].

In a 1990 paper, Bendix/King said it had been making airborne weather radars since 1954 and had delivered more than 35,000 radars for air transport aircraft. It listed three models then in airline service: the RDR-1E and RDR-1F, both with magnetron transmitters, and the RDR-4A, the "latest generation" with a "solid-state transmitter, fully coherent, Doppler turbulence detection capability" [2] Confirmed. Two years later, Allied-Signal reported more than 4,500 RDR-4A systems delivered [1].

1.1 Airborne, civil, and not military

Everything found places the RDR-4A in commercial air transport. The platforms named are all airliners: the 767, 757, A300, MD-80, DC-10 and L-1011 in Forecast International's 1990s report [5], and the 737, 747, 757, 767, A300, A310, A320, DC-10 and MD-80 in a current repair-shop catalog [9]. Continental Airlines flew RDR-4A hardware in an A300 during windshear testing [1]. British Airways tried an RDR-4A on a Concorde, G-BOAE, for about a year, but the radar "failed its British Airways trial and was removed" [7] Confirmed.

No evidence was found of the RDR-4A being used on U.S. military aircraft or in a ground-based role Not found. Ontic now supports the RDR-4A/4B line under license from Honeywell and describes it as fitted to airliner families such as the 737, 757, 777, A320, A330 and A340 [15]. Military derivatives of airliners could in principle carry the same equipment, but no source was found that says so, and this document does not assume it. The RDR-4A is an airline radar, and the documents found do not say where any individual technician worked on it.

→ See the training certificate (companion page with the scanned certificate).

↑ Back to top

2. History: Bendix, Bendix/King, Allied-Signal, AlliedSignal, Honeywell

In briefThe radar carried several company names during its life. Bendix built airborne weather radars from 1954 [2]. Allied Corporation bought Bendix in 1983 [25][24], and the avionics lines of Bendix and King Radio were joined under the Bendix/King name [26]. Allied merged with The Signal Companies in 1985 to form Allied-Signal, which became AlliedSignal in 1993 and took the Honeywell name after merging with Honeywell in 1999 [24]. The RDR-4A came out with the 757 and 767 in the early 1980s [4]. The RDR-4B, which added forward-looking windshear detection, followed in 1994 [5][6].

The RDR-4A was an important product for the company. The 1990 Bendix/King paper describes airborne weather radar as a core business, with the RDR-4A as the flagship [2], and by 2000 Honeywell's director of radar business development claimed a lead in air-transport weather radar "because of our extensive retrofit business" [6]. The windshear work of 1990–1994 turned the RDR-4A into the RDR-4B. The antenna needed no changes, and the receiver/transmitter, control panel and indicator each gained windshear functions [2][1]. Forecast International records that AlliedSignal's RDR-4B was certified in September 1994 and flight-tested on a Convair 580 and a Continental A300 [5].

↑ Back to top

3. The system units and how they connect

In briefAn RDR-4A installation has four kinds of unit. The RTA-4A receiver/transmitter sits in the electronics bay below the cockpit. The DAA-4A antenna drive and its flat-plate array sit behind the nose radome. The CON-4A control panel and one or two PPI indicators are on the flight deck [3][1]. The control panel sends commands to the R/T over an ARINC 429 data bus, and the R/T sends finished radar pictures to the displays over an ARINC 453 bus [1][2]. Microwave energy travels between the R/T and the antenna through a metal waveguide [4].

Data buses. In Allied-Signal's block diagram, the "Control Unit and Online Displays" send commands to the R/T's timing, control and monitoring section over an ARINC 429 control bus. The R/T's video memory sends the picture to the "Online Displays" over an ARINC 453 display data bus [1][2] Confirmed. The ASI test panel for the CON-4A decodes "ARINC 429 control words 1 and 2" from the control panel [13]. Honeywell notes that the ARINC 453 picture was sent in the same sweep order as the antenna, "rho-theta format" [4]. ARINC 708 defines the weather-radar display data word, with 512 range bins per radial and a 3-bit color code for each bin [23]. ASI describes the RTA-4A as an "ARINC 708 (RTA-4A) series" radar [13].

Antenna drive. According to ASI's description of the AST-4A antenna simulator, which stands in for the DAA-4A on the test bench, the scan and elevation drive signals originate in the RTA-4A, and the DAA-4A returns synchro azimuth and elevation feedback [13] Confirmed. That matches the "Antenna Stabilization Processor" and "Antenna Drive" blocks in Allied-Signal's diagram [1].

Dual installations. Wide-body and some other aircraft carried two R/Ts sharing one antenna and one control panel, with switch selection between them; the CON-4B is the dual-system panel [3][4]. The Continental A300 test setup below shows exactly this arrangement, with an RTA-4A as R/T #1 and the prototype windshear R/T as #2 [1].

↑ Back to top

4. The RTA-4A receiver/transmitter in depth

In briefThe RTA-4A is the core of the system. It is a single box, about 24.5 lb by Allied-Signal's figure [1], that generates a very stable 9.345 GHz signal, amplifies it with transistors (not a magnetron) to about 125 W, sends it to the antenna, and then amplifies and analyzes the echoes [2][1]. Everything comes from one crystal reference, which makes it "coherent": the radar knows the exact phase of each pulse it sends, so it can measure Doppler shifts in the echoes. It also stabilizes and drives the antenna, checks itself continuously and stores fault codes [3][11]. The author remembers it as a modular unit with a power supply, a transmitter, a linear receiver and a logarithmic receiver, plus 13 plug-in circuit cards.

4.1 Characteristics and block diagram

Two Allied-Signal presentations at NASA/FAA windshear conferences, from 1990 and 1992, give the best public figures for the RTA-4A. Both are U.S. Government-hosted documents cleared for public use [2][1]. The 1992 slide titled "RTA-4A Characteristics" is shown below. It gives 125 W nominal peak power, 6 and 18 µs alternating pulse widths, a PRF of 380 Hz in weather mode and 1600 Hz in Doppler mode, 320 NM maximum range, pulsed coherent operation at 9.345 GHz, a 5 dB minimum noise figure, 50 dB minimum dynamic range, 60 dB minimum gain control, and a weight of 24.5 lb [1] Confirmed.

4.2 Frequency generation: why "fully coherent" matters

Bendix/King's 1990 "RDR-4A Frequency Generation" diagram shows a single crystal oscillator feeding two dividers and a multiplier. Their outputs are mixed and multiplied up to microwave frequency on two paths: one drives the power amplifier (the transmitter), and the other provides the receiver's local oscillator for the mixer that converts echoes down to the intermediate frequency (IF) [2] Confirmed. Because the transmit signal and the receiver's reference come from the same crystal, their phase relationship is fixed from pulse to pulse. That is what "fully coherent" means, and it is what lets the radar measure the small Doppler shift of moving raindrops. Kuntman's windshear patent explains the requirement: "Coherent means that the transmitter and receive frequencies are offset from each other by a fixed frequency," and the variation of that offset must be kept very small [20]. The older magnetron radars (RDR-1E/1F) could not meet that requirement for windshear detection [2].

4.3 The transmitter module

The RDR-4A transmitter is solid-state [2][1] Confirmed. Honeywell's RDR-4B manual adds that, because the system "employs a solid-state transmitter, no warm-up period is necessary" [3]. A current repair shop describes the RTA-4B as "solid-state, including transmitter output device" [8]. The nominal peak output is 125 W [1][2], far below the tens of kilowatts typical of magnetron weather radars. A PPRuNe post quoting an accident report says older parabolic-antenna radars had "up to 15 times the power of modern radars" [29] Unverified.

How does a 125 W transmitter see storms 320 NM away? Largely through long pulses. Radar detection depends on the energy in each pulse, which is peak power × pulse length. At 125 W, an 18 µs pulse carries about 2.25 mJ and a 6 µs pulse about 0.75 mJ Calculated. The RDR-4A alternated 6 and 18 µs pulses in weather and map modes [1][2], and used the shorter 6 µs pulse at 1600 Hz for turbulence detection. Coherent processing and a low-noise receiver (5 dB noise figure) help as well [1]. Allied-Signal's slides do not state the reasoning behind alternating the two pulse widths, so it is not asserted here Not found.

4.4 Receivers: linear and logarithmic

In Allied-Signal's block diagram, the echo passes from the RF front end through an "RF preamplifier and first mixer" to the "receiver," then to the "video and Doppler processor" [1]. The published drawings stop there: they do not show separate linear and logarithmic receivers. Those come from the firsthand account Firsthand. They fit the published data, though. The RTA-4A had to handle at least 50 dB of dynamic range (a power ratio of 100,000 to 1) with at least 60 dB of gain control [1], and it had to supply both rain intensity and Doppler information [2].

Why a radar would carry both. A logarithmic receiver compresses a huge range of echo strengths into a manageable output. Light drizzle and a severe thunderstorm core can both be measured without the strong echo overloading the circuit, and each step in output corresponds to a fixed number of decibels, which suits a display calibrated in rain levels. A linear receiver keeps the echo's amplitude and phase intact, which Doppler processing needs. A 2005 weather-radar study states the tradeoff directly: reflectivity products are "usually … based on the logarithmic receiver output, because of the large dynamic range provided by the logarithmic receiver," while "the linear receiver data also provides phase information which a logarithmic receiver does not" [27]. An earlier Bendix turbulence patent also discusses IF amplifiers that may be "linear or logarithmic" [21]. It is therefore reasonable to infer that the RTA-4A's log receiver fed the weather (reflectivity) display and its linear receiver fed the coherent I/Q detection used for Doppler turbulence detection Inferred. No Bendix document found confirms that split.

4.5 The power supply

No public source describes the RTA-4A's power supply: its input power (for example 115 V AC 400 Hz, common in airline avionics), its output voltages or its current draw Not found. The ASI RCT-4A system test panel, which powers and exercises a complete RDR-4A on the bench, has a power-supply monitor section on its front panel [10], but its labels are not legible enough in the catalog scan to quote. The ASI COT-4A equivalent (ASI-252-2) that tests the CON-4A runs from 115 V AC 400 Hz [13], but that is the test panel, not the RTA-4A.

4.6 Processing, self-test and fault memory

The "video and Doppler processor" turns receiver output into rain levels and turbulence flags, and the "video memory" holds the picture that goes out on the ARINC 453 bus [1]. The "timing, control and monitoring" block takes commands from the 429 bus and supervises the unit [1]. For the RDR-4B, Honeywell says the R/T contains "the system integrity monitoring, self test and fault memory circuits" and the circuits that interface to the aircraft attitude reference for antenna stabilization [3]. ASI's RTA-4A test set confirms the RTA-4A had an equivalent fault memory. The ASI-169-2, a copy of the Bendix RST-4A (P/N 2041797-0401), plugs into the RTA-4A's ATE connectors and displays "all circuit module fault codes stored in the RTA-4A fault memory," starting with the most recent. It can erase them and can stop fault codes being sent to the PPI during system test [11] Confirmed. The phrase "circuit module fault codes" implies the BITE reported faults down to individual modules or cards Inferred.

4.7 The 13 circuit cards

What was searched. The search covered: the RTA-4A component maintenance manual (Bendix instruction books in the 1104 series, ATA 34-41); ASI and other test-equipment catalogs; repair-station capability lists (Ontic, PAG); parts brokers (ILSmart, SE Aerospace, ODS, Aeroval); NASA NTRS papers; patents; and FAA TSO and STC data. No public listing of the RTA-4A's circuit cards, their names, their part numbers or their slot order was found Not found. The RTA-4A CMM itself was not found online. The RTA-4B's CMM is ATA 34-41-36 (instruction book 1104A-3) [14], so the RTA-4A's is probably a neighboring 34-41 number in the same 1104 family, but that is unconfirmed Unverified.

What the sources do support is the list of functions the cards and modules together had to perform. These are the blocks in Allied-Signal's diagrams [1][2]: frequency sources (crystal reference, dividers, multipliers), RF preamplifier and first mixer, IF receiver, video and Doppler processing, video memory and ARINC 453 output, timing and control with ARINC 429 input, monitoring and fault memory [11], and antenna stabilization with scan/tilt drive outputs [13]. How those functions were divided among the cards is not public.

According to the author's firsthand recollection, the RTA-4A used 13 plug-in circuit cards alongside its power supply, transmitter, linear receiver and logarithmic receiver modules Firsthand. No public documentation of the individual cards (their names, part numbers, slot order or functions) was found Not found. Card-level detail may be added later.

4.8 Part numbers, variants and the RTA-4A / RDR-4A / RDR-4B relationship

RDR-4A is the system and RTA-4A is its receiver/transmitter. Honeywell describes the RDR-4B system as "the Receiver/Transmitter (RTA-4B), the Display Unit (PPI-4B), the Antenna Assembly (DAA-4A or -4B drive unit and REA-4A or -4B Array), and the control panel (CON-4A/B)" [3]. The RDR-4A used the same scheme with the RTA-4A [1][12] Confirmed. The RTA-4A is the R/T only; the antenna is the separate DAA-4A plus REA array [1].

RTA-4A part numbers. The RTA-4A is in the 2041217 series: "2041217 (SERIES) RTA-4A RADAR XCVR" [9]. Dash numbers seen include -0401 (listed for the A320) [9], -0411, -0413, -0414, -0415, -0416, -0417, -0418, -0419, -0421, -0424 and -0428 [18], and -0424 in the 1992 Continental test [1]. An ILSmart listing pairs 2041217-0416 with "Alternate Part Number RTA-4A" [19]. What distinguishes each dash number was not found Not found. The 066-50008 numbers belong to the later RTA-4B (066-50008-01xx through -04xx) [8][14][7], not the RTA-4A. A Concorde document lists an alternative transceiver as "TRA-4A 2041217-0422"; the 2041217 prefix identifies it as an RTA-4A, and "TRA-4A" is probably a typo [7] Inferred.

From RDR-4A to RDR-4B. Allied-Signal's 1990 and 1992 plans made the RDR-4B a modification of the RDR-4A. The R/T gained windshear detection hardware and software, windshear mode control and windshear data on its output buses. The indicator gained windshear display, and the control panel gained windshear mode selection (in some cases). The antenna, radome and aircraft structure needed no changes, although some wiring might [2][1]. The Concorde addendum puts the difference simply: the RDR-4B has more processing power and gain, plus predictive windshear [7].

↑ Back to top

5. The PPI-4A indicator

In briefThe PPI-4A was a Bendix radar indicator, the cockpit screen that showed the radar picture. Two current installation documents confirm the name: a Honeywell EGPWS installation guide lists "Bendix PPI-4A/4B" as a supported display type on the ARINC 453 bus [16], and a Collins STC data sheet for MD-80 aircraft "assumes Honeywell PPI-4A/4B Weather Radar equipped aircraft" [17]. Beyond the name, almost nothing about the PPI-4A was found online: no part number, no description of its screen and no photo. Its close relative, the PPI-4B color indicator, is well documented.

What is confirmed about the PPI-4A. It is a real Bendix/Honeywell unit name, grouped with the PPI-4B in two independent installation documents [16][17] Confirmed. The Honeywell guide treats the PPI-4A and PPI-4B as one "display configuration group" that receives terrain data on an ARINC 453 bus and sends range data (label 271) back on ARINC 429 [16]. That implies the PPI-4A used the same ARINC 453 display interface as the rest of the RDR-4 family Inferred. It also notes that Peaks terrain display requires a "Bendix PPI 4B with blue water capability," which suggests the PPI-4A lacked some later PPI-4B features Inferred.

What was not found. The PPI-4A's part-number series, screen type (color or monochrome CRT), size, weight, test-equipment references and CMM number were not found Not found. The PPI-4B uses the 2041222 series [8][9]. By analogy the PPI-4A might sit nearby, but no number was found, and none is suggested here.

The PPI-4B, for comparison. Honeywell's RDR-4B manual describes the PPI-4B as receiving and processing video data from the R/T and presenting "a continuous display of weather or terrain mapping" in up to four colors: green for the weakest returns, then yellow and red, with magenta for moderate-to-heavy turbulence within 40 NM when turbulence detection is selected [3]. In some installations it doubled as a multifunction display for checklists, ACARS, EGPWS and TCAS [3]. PAG lists PPI-4B variants: "PPI-4B Color Radar Indicator" (2041222-24xx), "with TCAS II" and "with Wind Shear" (2041222-34xx to -64xx) [8]. PPI-4B indicators were driven by an RTA-4A in the 1992 Continental A300 tests [1], so an RDR-4A installation could use a PPI-4B Confirmed. In glass-cockpit aircraft (EFIS), the radar picture could go to the electronic displays instead of, or as well as, a separate indicator [3].

↑ Back to top

6. The CON-4A control panel

In briefThe CON-4A is the cockpit control panel for a single radar system. The CON-4B is the version for dual systems [3]. It sends the pilot's settings (mode, gain, tilt, stabilization and, in some installations, range) to the RTA-4A as digital ARINC 429 words [13][3]. Its part numbers run from 2041223-0401 to 2041223-3136 [8], and it outlived the RDR-4A: the RDR-4B also uses it [3].

Identity and numbers. Honeywell: "The CON-4A/4B contains the controls for operating the radar system except for those located on the display unit. The CON-4A is used in a single system configuration while the CON-4B is the dual system control panel" [3] Confirmed. Its part-number range is 2041223-0401 through 2041223-3136 [8], and Ontic lists the "Controller CON-4A," 2041223-****, under CMM 34-41-12 (instruction book 1104D), with the COT-4A test set (2041593-0401) as major equipment [14].

What it sends. The ASI-252-2, a copy of the Bendix COT-4A radar control test panel (Bendix instruction book I.B. 1104ET / 34-41-59), "receives and decodes ARINC 429 control words 1 and 2 from the CON-4( ) and displays the decoded data" [13][10] Confirmed. In other words, the CON-4A has no analog wiring to the radar. It is a small digital encoder that turns knob and switch positions into two ARINC 429 words.

Controls. From the RDR-4B manual, a typical CON-4A/4B has: a mode selector (TEST, WX, WX/TURB or TURB, MAP; in 4B installations also windshear-related positions); GAIN (AUTO/CAL or manual); TILT (±15°); ANTENNA STAB ON/OFF (if provided); and, depending on the installation, range and system (L/R) select [3]. Exact panel layouts vary by dash number and aircraft [3]. An RDR-4A-specific panel description was not found Not found.

↑ Back to top

7. The DAA-4A antenna drive and REA arrays

In briefThe DAA-4A is the antenna drive unit, not a data adapter. It is the motorized mount behind the nose radome that sweeps the antenna 90° left and right and tilts it up and down, and it keeps the beam level as the aircraft pitches and rolls [3][1]. Bolted to it is a flat-plate antenna, the REA-4B (30 in) or REA-4A (24 in) [1]. The same DAA-4A was carried over unchanged into the RDR-4B [2][3].

Identity. Honeywell: "The component parts of the antenna assembly are the DAA-4A Antenna Drive Assembly and the REA-4B flat-plate array" [3]. PAG calls it the "DAA-4A Antenna Drive Unit," 2041444-0401 through -0424 [8]. The PAT catalog lists "2041444 (SERIES) DAA-4A RADAR ANTENNA" on the 737, 747, 757, 767, A300, A320, DC-10 and MD-80 [9] Confirmed. Ontic's list pairs the DAA-4A (2041444-04*) with REA planar array assemblies "4A" (2041445-04*), "4B" (2041446-04*) and "4D" (3614070-0401), all under CMM 34-41-11 (instruction book 1104C) [14]. The 1992 Continental test used a DAA-4A 2041444-0404 with an REA-4B 2041446-0401 [1]. The lighter DAA-4B (3614071-0401) came later, for smaller aircraft [8][3].

How it works. The antenna forms "a 3 degree conical shaped beam that sweeps 90 degrees to the left and 90 degrees to the right," with tilt coverage of ±15° [3]. It uses line-of-sight stabilization [1]: the R/T's antenna stabilization processor uses aircraft pitch and roll to keep the beam at the selected angle to the horizon [1][3]. The drive signals come from the RTA-4A, and the DAA-4A reports its position back through synchros [13]. The ASI antenna simulator exists so a technician can test an RTA-4A on the bench without a real DAA-4A, by simulating the motor load and the synchro feedback [13].

↑ Back to top

8. How a weather radar sees storms (plain English)

In briefA weather radar works like shouting across a canyon and timing the echo, except that it uses microwaves and listens for echoes from raindrops. The time delay gives the distance, the antenna's direction gives the bearing, and the echo's strength gives the rain's intensity. A coherent radar such as the RDR-4A can also hear the tiny pitch change (Doppler shift) of moving raindrops, and wildly mixed speeds within a storm mean turbulence.

1. Timing the echo. Radio waves travel at about 300,000 km per second, so a radar pulse takes 12.36 microseconds to reach a target one nautical mile away and return. An echo that arrives 123.6 µs after the pulse therefore comes from rain 10 NM ahead [3]. The RDR-4A repeated this hundreds of times per second (380 pulses per second in weather mode) as the antenna swept back and forth [1].

2. Short waves, narrow beam. At 9.345 GHz the wavelength is only about 3.2 cm, which lets an antenna small enough for an airliner's nose form a narrow, searchlight-like beam, about 3° wide for the RDR-4 family [3][1]. Where the beam points tells the radar the storm's direction.

3. Echo strength means rain rate. Bigger and more numerous raindrops reflect more energy. Honeywell's color scale is: green for light returns (0.7–4 mm/hr), yellow for medium (4–12 mm/hr), red for strong (more than 12 mm/hr) and magenta for turbulence [3]. The radar also compensates for rain between it and a storm, which would otherwise make the far storm look weaker ("penetration compensation"), and normalizes for range [3].

4. Doppler reveals turbulence. Because the RDR-4A is coherent, it can measure how fast raindrops move toward or away from the aircraft. Bendix/King's windshear patent explains that turbulence-detecting radars "compare spectral bandwidth of the return signals against a threshold": a wide spread of speeds in one spot means turbulent air [20]. The RDR-4A did this in its turbulence mode, using 6 µs pulses at 1600 Hz out to 40 NM [2][1]. It cannot see clear-air turbulence, because without precipitation there is nothing to reflect the signal [3].

5. Tilt is everything. The beam is narrow, so where it points up or down determines what the pilot sees. Honeywell repeatedly calls "effective tilt management … the single, most important key to more informative weather radar displays" [3]. Point it too low and the ground fills the screen, which is what MAP mode is for. Point it too high and it overshoots the wet part of the storm.

↑ Back to top

9. Operation and maintenance

In briefIn the cockpit, the crew chose a mode (TEST, weather, weather plus turbulence, or ground map), a range, a tilt angle and a gain setting. The radar needed no warm-up because the transmitter was solid-state [3]. On the ground, built-in test equipment (BITE) watched the system continuously and stored fault codes for each circuit module, which a technician could read from the front of the RTA-4A with a small Bendix test set [11]. In the shop, a family of Bendix test panels could run a complete RDR-4A on the bench, each unit with its own instruction book [10][12].

9.1 Modes and test

The RDR-4A's weather/map and turbulence modes are documented by Allied-Signal [2][1]. The fullest description of the operating modes is Honeywell's RDR-4B manual, which describes the same family [3]:

• TEST: displays a test pattern and starts automatic test and fault isolation. If a unit has failed, the pattern is replaced by the name of that unit.
• WX: 180° of weather in green, yellow and red; gain control optional.
• WX/TURB or TURB: weather plus turbulence (magenta) out to 40 NM, weather only beyond. Gain affects weather but not turbulence.
• MAP: ground mapping with the same colors; gain control operative.
• GAIN AUTO/CAL: presets calibrated receiver sensitivity. A blue CAL advisory warns when gain is not calibrated.
• STAB ON/OFF: antenna stabilization.

The windshear functions described in that manual (automatic operation below 2,300 ft, alerts below 1,500 ft) belong to the RDR-4B, not the RDR-4A [3][5]. An RDR-4A pilot's guide exists: an "AlliedSignal Bendix King RDR-4A Weather Radar Pilot's Guide" has been offered for sale (see §13). It was not available to consult for this document.

9.2 Fault annunciations

Honeywell divides faults into soft failures, where the picture is kept but degraded ("typically a result of reduced transmitter power, a cooling problem, a stabilization fault"), and hard failures, where a major function is lost ("total loss of transmitter power, receiver gain or no antenna scan") and the display is replaced by the name of the failed unit [3]. The annunciations are CAL (R/T out of calibration), STAB (stabilization input lost), COOL (R/T cooling air), R/T FAULT, ANT FAULT, CON FAULT (control fault or waveguide), ATT FAULT (attitude) and IND FAULT [3]. These are from the RDR-4B manual; it is likely, but unconfirmed, that the RDR-4A used the same set Inferred.

9.3 Shop test equipment and manuals

Avionics Specialist, Inc. (ASI) makes form-fit-function replacements for the original Bendix test panels. Its catalog therefore preserves the Bendix model numbers, part numbers and instruction-book (I.B.) references [10]. Bendix instruction books for the RDR-4 family fall in the 1104 series under ATA 34-41. Ontic lists the CMMs as 34-41-11 (DAA-4A and REA arrays, I.B. 1104C), 34-41-12 (CON-4A/4B, I.B. 1104D) and 34-41-36 (RTA-4B, I.B. 1104A-3) [14].

Certification. Airborne weather radars are approved to FAA Technical Standard Order TSO-C63 [31][22]. Its current revision references RTCA DO-173 (weather and ground-mapping radar) and DO-220 (Doppler/windshear), and earlier revisions remain valid for equipment already approved [22]. The exact TSO-C63 revision the RTA-4A was approved to was not confirmed from an FAA source Not found.

↑ Back to top

10. Specifications with sources

In briefAll the numbers in one place, with the source for each and a note where sources disagree. Where a value was searched for and not found, the table says so rather than guessing.

↑ Back to top

11. Firsthand account cross-check

In briefEach statement in the author's firsthand account, next to what the documents say.

↑ Back to top

12. Gaps in the public record

In briefWhat could not be found in public sources. The author's dates on the system (1989–1996) and his formal training (a sixty-hour Bendix/King RTA-4A course in June 1990) are documented [32].

Not found in public sources

• The RTA-4A component maintenance manual, and any list of its circuit cards (names, part numbers, slot order) Not found
• PPI-4A part number, screen type, size and photos Not found
• RTA-4A input power, dimensions, IF frequency, transmitter device type and stage count Not found
• Which features distinguish the RTA-4A dash numbers (-0401 through -0428) Not found
• The TSO-C63 revision the RTA-4A was approved under Not found
• Any military or ground-based use of the RDR-4A Not found
• Public-domain or Commons photographs of any RDR-4A unit Not found

↑ Back to top

13. Photos found elsewhere (not embedded)

In briefPhotos and drawings of these units exist online, but they belong to commercial sellers or to Honeywell, so they are linked here instead of copied into this page.

ILSmart listings for the RTA-4A (2041217-0416) and DAA-4A (2041444-0402) show only a generic stock image. SE Aerospace's RTA-4A, DAA-4A, PPI-4B and CON-4B pages show "no photo available." No current eBay listings for these units turned up in searches; eBay would need a manual check.

↑ Back to top

Glossary

In briefPlain-language definitions of the terms and abbreviations used in this document.

ARINC 429

The standard one-way digital data bus used across airliner avionics. The CON-4A sends its settings to the RTA-4A as ARINC 429 "control words." [1][13]

ARINC 453

A high-speed display data bus used to send the radar picture from the R/T to the indicators, one radial at a time in antenna-sweep order. [1][4]

ARINC 708

The airline standard for airborne weather radar and its display data word (512 range bins per radial, 3-bit color per bin). [23]

ATE connector

Automatic test equipment connector on the front of the RTA-4A, used by the RST-4A test set to read fault memory. [11]

BITE

Built-in test equipment: circuits that check the system continuously and store fault codes. The RTA-4A kept module fault codes in a fault memory. [3][11]

CFDS / MCDU

Central fault display system and multipurpose control display unit: aircraft maintenance computers that could show radar fault data on later airframes. [12]

CMM

Component maintenance manual: the shop manual for one unit, numbered by ATA chapter (34-41-xx for these radars); Bendix called them instruction books (I.B.). [14]

Coherent

A radar whose transmit signal and receiver reference come from one stable source, so the phase of every pulse is known. Required for Doppler measurement. [2][20]

DAA-4A

The RDR-4A antenna drive unit: the motorized, stabilized mount behind the radome that scans and tilts the flat-plate array. [3][8]

Doppler shift

The small frequency change of an echo from a moving target. It lets the radar measure how fast raindrops move toward or away from it. [20]

Dynamic range

The ratio between the strongest and weakest signals a receiver can handle. At least 50 dB (100,000 : 1) for the RTA-4A. [1]

EE bay

Electrical and electronics equipment bay: the avionics compartment below the cockpit where the RTA-4A was mounted. [4]

Flat-plate array

A flat antenna made of many radiating slots fed in phase. Compared with a dish it gives a narrow beam with low sidelobes. [1]

I/Q

In-phase and quadrature signals: two outputs of coherent detection that together carry an echo's amplitude and phase. Used for Doppler processing. [20]

IF

Intermediate frequency: the lower frequency the echo is converted to by the first mixer so it can be amplified and processed more easily. [2]

Linear receiver

A receiver whose output is proportional to the input and keeps phase information. Suited to Doppler processing. [27]

Logarithmic receiver

A receiver whose output rises with the logarithm of the input, compressing a very wide range of echo strengths. Suited to measuring rain intensity. [27]

LRU

Line-replaceable unit: a box that is swapped on the aircraft, such as the RTA-4A, DAA-4A, CON-4A or PPI. [3]

Magnetron

A vacuum-tube microwave oscillator used in older weather radars (e.g. Bendix RDR-1E/1F). Powerful, but its phase varies from pulse to pulse, so it is not coherent. [2]

MCU

Modular Concept Unit: the ARINC 600 box-width unit for airline avionics. The RTA-4A generation of R/Ts used an 8 MCU case. [4]

Noise figure

How much noise a receiver adds, in dB. Lower is more sensitive. 5 dB for the RDR-4A. [1][2]

PPI

Plan position indicator: a map-like radar display with the aircraft at the bottom center and the scanned sector fanning out ahead. [3]

PRF

Pulse repetition frequency: how many pulses are sent per second (380 Hz weather, 1600 Hz turbulence in the RDR-4A). [1]

Pulse width

How long each transmitted burst lasts (6 or 18 µs here). Longer pulses carry more energy; shorter pulses resolve finer detail. [1]

R/T

Receiver/transmitter: the RTA-4A. [3]

Radome

The nose cone that covers the antenna and lets radar energy through. [3]

REA-4A / REA-4B

The flat-plate (planar) antenna arrays used with the DAA-4A: 24 in (REA-4A) and 30 in (REA-4B). [1][14]

Stabilization

Using aircraft pitch and roll to keep the antenna beam at a fixed angle to the horizon while the aircraft maneuvers. [3][1]

STC

Supplemental Type Certificate: FAA approval for a modification to an aircraft type. [17]

Synchro

An electromechanical position sensor that reports shaft angle over AC wiring. The DAA-4A reported azimuth and elevation this way. [13]

Tilt

The up/down angle of the antenna beam, ±15° on the RDR-4A. Correct tilt is critical to reading weather. [3]

TSO

Technical Standard Order: an FAA minimum-performance standard. Weather radars are approved under TSO-C63. [31][22]

Turbulence detection

Doppler processing that flags areas where raindrop speeds vary widely (a wide spectral width), shown in magenta within 40 NM. It cannot detect clear-air turbulence. [3][20]

Waveguide

A hollow rectangular metal pipe that carries microwave energy between the R/T and the antenna. [4]

Windshear

A sudden change in wind speed or direction, most dangerous near the ground in microbursts. Detected by the RDR-4B, not the RDR-4A. [5][20]

X-band

The 8–12 GHz radar band. The RDR-4A operates at 9.345 GHz. [1]

↑ Back to top

14. Image credits

In briefEvery image on this page is either public-domain or openly licensed material with a named source, or an original diagram drawn for this document. No commercial listing photos are embedded. The author's training certificate is shown on a separate companion page.

↑ Back to top

15. References

In briefAll sources cited in the text, numbered as they are cited. URLs were checked in September 2026.

1. [1]S. S. Grasley (Allied-Signal Aerospace, Bendix/King Air Transport Avionics Division), "RDR-4B Doppler Weather Radar With Forward Looking Wind Shear Detection Capability," viewgraphs in Airborne Wind Shear Detection and Warning Systems: Fourth Combined Manufacturers' and Technologists' Conference, Part 1, NASA Langley Research Center, Sept. 1992, pp. 317–338. NTRS 19930010412 (U.S. Government work, public use permitted). Includes the "RTA-4A Characteristics," "RDR-4 Antenna Characteristics," "RDR-4A Functional Block Diagram" and "Continental A300 Test Configuration" slides. https://ntrs.nasa.gov/citations/19930010412
2. [2]D. Kuntman (Bendix/King Air Transport Avionics Division, Allied-Signal Aerospace), "Doppler Weather Radar With Predictive Windshear Detection Capability," Oct. 18, 1990, in Airborne Wind Shear Detection and Warning Systems: Third Combined Manufacturers' and Technologists' Conference, Part 2, NASA Langley, 1991, pp. 755–766. NTRS 19910014837 (public use permitted). https://ntrs.nasa.gov/citations/19910014837
3. [3]Honeywell International, RDR-4B Forward Looking Windshear Detection/Weather Radar System User's Manual with Radar Operating Guidelines, ACS-5082, 006-18167-0000, Rev. 6, Feb. 2004 (106 pp.). http://hibdz.skydiving.co.uk/757/757_tech/flight_management_navigation/Honeywell_Radar_Manual.pdf
4. [4]S. Hammack, "8 Things You Might Not Understand About Airborne Weather Radar – Part 3," Honeywell Aerospace Radar Corner, Mar. 22, 2021. https://www.honeywellaerospace.com/us/en/insights/articles/radar-corner-8-things-airborne-weather-radar-part-3
5. [5]Forecast International, Airborne Windshear Detection Systems, Electronic Systems Forecast, archived report (archived Aug. 1999). https://www.forecastinternational.com/archive/disp_old_pdf.cfm?ARC_ID=529
6. [6]J. W. Ramsey, "Broadening Weather Radar's Scope," Avionics Magazine / Aviation Today, Aug. 1, 2000. https://www.aviationtoday.com/2000/08/01/broadening-weather-radars-scope/
7. [7]Heritage Concorde, "CAA: G-BOAA trial installation, radar" (British Airways modification 34D491 and addendum; RDR-4A/RDR-4B part numbers). https://www.heritageconcorde.com/caa-g-boaa-trial-installation-radar
8. [8]Precision Aviation Group, "Honeywell Weather Radar with Predictive Windshear, Model RTA-4B" repair-capability sheet (lists CON-4A, CON-4B, DAA-4A, DAA-4B and PPI-4B part-number ranges), 2024. https://www.precisionaviationgroup.com/wp-content/uploads/2024/07/PAG-RTA-4B-Weather-Radar-Airline-Support.pdf
9. [9]Precision Aero Technology (Precision Aviation Group), Commercial Capabilities catalog, Mar. 2024 (part number, model, ATA chapter and aircraft tables; product photos on p. 11). https://www.precisionaviationgroup.com/wp-content/uploads/2024/03/PAT-Commercial-Catalog.pdf
10. [10]Avionics Specialist, Inc. (ASI), Bendix test-equipment supplement catalog (OEM-equivalent model numbers, part numbers and Bendix instruction-book numbers for RST-4A, RCT-4A, COT-4A, PPT-4A, AST-4A, ATT-4A, RMT-4A). https://asitest.com/ASI_TE_SEARCH/BendixSupp.pdf
11. [11]Avionics Specialist, Inc., ASI-169-2 Portable Radar Test Set (Bendix RST-4A, P/N 2041797-0401) product page. https://www.asitest.com/cgi-bin/Results.pl?CatalogIndex=615
12. [12]Avionics Specialist, Inc., ASI-742 Radar System Test Panel (Bendix RCT-4A, P/N 2041589-0401) product page, with related ASI-799 fault data test unit and ASI-722-2 RTA-4A cooling mount. https://www.asitest.com/cgi-bin/Results.pl?CatalogIndex=2832
13. [13]Avionics Specialist, Inc., product pages for ASI-741 Doppler signal generator (RMT-4A, 8055325-0401), ASI-770 radar antenna simulator (AST-4A, 2041886-0401; replaces the DAA-4A on the bench) and ASI-252-2 radar control test panel (COT-4A, 2041593-0401). https://www.asitest.com/cgi-bin/Results.pl?CatalogIndex=2930
14. [14]Ontic Engineering & Manufacturing, CAAC Capability List, QC-130A Rev. F, Dec. 18, 2023 (RTA-4B, CON-4A, CON-4B, DAA-4A and REA array entries with CMM/ATA numbers). https://ontic-legacy.files.svdcdn.com/production/img/CAAC-Certificate-Capability-List_2025-11-21-113756_rdvi.pdf?dm=1763725077
15. [15]Ontic, "Ontic signs license agreement with Honeywell for the RDR4A/B weather radar systems," July 28, 2022; and "Ontic licenses Honeywell RDR4A/B weather radar systems product line," Oct. 19, 2022. https://www.ontic.com/news-events/news/ontic-licenses-honeywell-rdr4a-b-weather-radar-systems-product-line
16. [16]Honeywell, MK VI / MK VIII EGPWS Installation Design Guide, 060-4314-150, Table 5.3.6.1-14 "Honeywell/Bendix PPI-4A/4B without Auto Range," sheet 213 (as reproduced by ManualsDir). https://www.manualsdir.com/manuals/95396/honeywell-mk-vi-mk-viii.html?page=213
17. [17]Collins Aerospace, STC Information Sheet, FAA STC ST02928AT (MD-80/MD-90 EGPWS), rev. Nov. 8, 2023: "This configuration assumes Honeywell PPI-4A/4B Weather Radar equipped aircraft." https://prd-sc102-cdn.rtx.com/collinsaerospace/-/media/ca/product-assets/marketing/s/stcs/be-stcs/stc-st02928at-md-80-90.pdf
18. [18]SE Aerospace, "Honeywell RTA-4A" parts listing (dash numbers 2041217-0411 through -0428). https://www.seaerospace.com/sales/product/Honeywell/RTA-4A
19. [19]ILS Aviation Auctions listing, "2041217-0416, Alternate Part Number RTA-4A, Radar Transceiver." https://auction.ilsmart.com/Listing/Details/1698030/20412170416-RADAR-TRANSCEIVER
20. [20]D. Kuntman, U.S. Patent 5,077,558, "Airborne wind shear detection weather radar," filed Dec. 14, 1990, issued Dec. 31, 1991 (assignee AlliedSignal Inc.). https://patents.google.com/patent/US5077558A/en
21. [21]D. V. Payne, U.S. Patent 4,223,309, "Turbulence detector for non-coherent pulse radar," filed Sept. 25, 1978, issued Sept. 16, 1980 (assignee The Bendix Corporation). https://patents.google.com/patent/US4223309A/en
22. [22]Federal Aviation Administration, TSO-C63d, Airborne Weather Radar Equipment (references RTCA DO-173 and DO-220; earlier revisions no longer effective for new applications). http://www.atr-usa.com/documents/TSO-C63d.pdf
23. [23]Wikipedia, "ARINC 708." https://en.wikipedia.org/wiki/ARINC_708
24. [24]Wikipedia, "AlliedSignal." https://en.wikipedia.org/wiki/AlliedSignal
25. [25]Wikipedia, "Bendix Corporation." https://en.wikipedia.org/wiki/Bendix_Corporation
26. [26]Wikipedia, "King Radio (company)." https://en.wikipedia.org/wiki/King_Radio_(company)
27. [27]R. Tracksdorf, M. Ghorbani, M. Chandra, M. Hagen and D. Bebbington, "Comparison of linear and logarithmic receiver signals from polarimetric weather radar echoes and their temporal decorrelation properties," Advances in Radio Science 3, 401, 2005. https://ars.copernicus.org/articles/3/401/2005/ars-3-401-2005.html
28. [28]Wikimedia Commons, "Airborne weather radar NASA.jpg" (NASA, 1992; from Airborne Trailblazer, ch. 5; public domain). https://commons.wikimedia.org/wiki/File:Airborne_weather_radar_NASA.jpg
29. [29]PPRuNe Tech Log forum thread, "Newer Wx radar vs. older units" (user post stating the RDR-4A entered service in 1982 on the 767; forum source, unverified). https://www.pprune.org/tech-log/401358-newer-wx-radar-vs-older-units.html
30. [30]Wikimedia Commons, "Being 737MAX weather radar (54844309191).jpg," photo by Ian Abbott, June 22, 2023, CC BY 4.0. https://commons.wikimedia.org/wiki/File:Being_737MAX_weather_radar_(54844309191).jpg
31. [31]Federal Aviation Administration, "Technical Standard Orders (TSO)" overview page. https://www.faa.gov/aircraft/air_cert/design_approvals/tso
32. [32]Certificate of Achievement, The Bendix/King Air Transport Avionics Division, Allied-Signal Aerospace Company, for successful completion of a sixty-hour training class covering the RTA-4A Weather Radar Transceiver, June 4–15, 1990; signed by the President, the Director of Product Support and the Instructor. Issued to the author; scan from the author's personal collection, supplied September 2026. Primary source; not published online.

Written by Rick Childers, September 2026, for GeeksForge Research Projects. Firsthand material is the author's own. Specifications are quoted from the sources shown; where sources disagree, both values are given.