MBT Electronics offers Repair and Overhaul Service for the RT-1115/APN-209 Radar Altimeter System.
- APN-209 Family History
- Reinstalling APN-209 Radar Altimeter After Maintenance
- AN/APN-194 Radar Altimeter System
- King KRA-10A and KRA-10 Systems
MBT Electronics specializes in servicing components of the APN-209 radar altimeter system. This equipment has been installed in Army helicopters since the late 1970s. It is a self contained, panel mounted instrument with the display and receiver-transmitter contained in one unit. There is an optional remote (co-pilots) display which repeats the output of the master unit. Several remote instruments can be installed in a system if necessary.
The APN-209 system is a pulse type radar altimeter utilizing two antennas for the transmit and receive functions. It transmits a short pulse and receives the reflected signal while the tracking system measures the time delay. The aircraft antennas point straight down and the signal bounces off the ground under the aircraft. The time delay is converted to a digital and analog readout in feet. Depending on the height above ground and the received signal level, the tracking circuitry controls the transmit power, pulse width, and receiver sensitivity. It will maintain the correct power and gain for reliable operation over all types of terrain. When the signal is reliable, the off flag pulls out of view and the digital display is enabled.
A problem with all radar altimeters is the tendency to lock-on to stray signals. If a signal is reflected from one antenna directly to the other without bouncing off the ground, the tracking system may be fooled. Usually this results in a constant reading of zero feet. The skids or other items under a helicopter can reflect the signal directly from one antenna to the other. The APN-209 has extensive circuitry to prevent this problem. This equipment was designed for helicopters. Internal adjustments can be customized to compensate for situations such as external (sling) loads or poor installation.
MBT Electronics Offers the Following Services
- Bench test and evaluate
- Written teardown report
- Test data sheet
- Overhaul and refinishing
- Upgrade red LED display to green LED display
- Build up good units from junk or de-militarized units
- Rental of working units
- Troubleshooting assistance
- Fabricate wiring harness
- Supply antennas and RF cables
- Buy and sell units
APN-209 Family History
The following information is not from official sources. It is observations and experience that I have collected while repairing APN-209 units. Many different part numbers have been assigned to these units. It is confusing. I have grouped them according to the RT number suffix which they resemble.
All the APN-209 units are interchangeable in the aircraft and will provide similar performance. The difference is the internal technology which has changed greatly over the 30 year existence of this equipment. The early units contain a vacuum tube and the latest units have micro-processor circuitry.
Detailed breakdown of part numbers:
Straight (No Suffix)
The “straight” model seems to be the earliest example of APN-209 and dates from about 1975. It can be identified by the gold coax connector on the back. One connector is gold and one is steel. This model incorporates a vacuum tube transmitter and requires a warm up period when first turned on. The LED display and lighting is red.
The A model has a solid state transmitter and turns on instantly. The receiver AGC, transmit power control, and pulse width control circuits have been improved. The LED display and lighting is red.
The B model is similar to the A model except it is night vision compatible. Most units have a green LED display and the High and Low warning lamps are blue / green. This makes the unit more compatible with NVIG (night vision) equipment. Strangely, some units also have red instrument lamps. NVIG lamps are available to replace these red bulbs if required.
The C model is similar to the B model in form and function. These units date from about 1982. The LED display and lighting is green. There are circuitry improvements including a change in the analog pointer servo system to give better resolution. The servo motor gives off an audible tone when the unit is turned on in a quiet environment.
Note: In the A, B, and C models, the power supply runs very hot and requires some airflow around the instrument case. In some failed units, the solder has melted on the power supply board. For best reliability, be sure that the aircraft installation provides sufficient airflow.
The D model is completely redesigned internally. These date from about 1993. They can be identified by the position of the analog and digital “zero” trimpot openings on the sides of the unit. The D unit has one opening on the side and one on the top. The previous models have both openings on the side. Otherwise the form and function is the same as the earlier units. They have slightly better performance on the bench than the older models and I suspect that they work a little better in the aircraft. There are noticeable improvements in the transmitter power control and the receiver AGC. The heat problem of the earlier units has been solved and these units are very reliable. The display section has been redesigned for NVIG operations. The lighting is very carefully isolated with rubber boots and green filters. In my opinion this is the best model of the APN-209.
The J model is a completely new design internally. This is the current model, as far as I know. It has a bare metal case which makes it easy to recognize. The internal construction consists of multi-layer circuit boards with surface mount components. It incorporates micro-processor controllers with proprietary software installed. This model has a verbal warning system. When wired into the aircraft audio system, a voice will sound off when the “Low” or “High” lights come on. This unit runs cool and the power drain is lower than the previous units.
In my opinion, this unit seems to be less robust than the earlier units. The electronic components are very vulnerable to ESD damage. The circuit cards, with tiny gauge circuit runs, are easily damaged when any short circuit occurs, whether inside or outside of the instrument. This damage is difficult to repair when the burned run is deep inside a multilayer board. The micro-controllers are easily destroyed by stray voltages or accidental shorts which may exist in the aircraft installation. Extreme care should be exercised to check for shorts or stray voltages on the connectors in the aircraft before plugging the unit in.
Remotely Mounted RT Units
These units contain the receiver-transmitter section of the APN-209 without the instrument section. They are compatible with any of the ID-1917/APN-209 remote indicators. Any number of remote indicators can be connected in a daisy chain fashion. The case is designed to mount in a remote location in the aircraft and thereby simplify the installation. The unit is controlled by the ID-1917 instrument on/off switch and push-to-test switch. The antennas and electrical connections are identical to the APN-209 units.
The remote indicators are all compatible electrically and work with any model of the APN-209. The difference is the internal design and the NVIG lighting.
This is the earliest version of the remote indicator. Normally it would be installed in the co-pilots position. It repeats the information from the RT unit. In the aircraft, it looks the same as the RT unit and can control the on-off and test functions. It has its own “high” and “low” bugs and dimming system. This version has a red LED display and lighting.
Remote indicator similar to “straight” model but probably incorporates subtle improvements.
Remote indicator similar to the ID-1917 units. This model has green LED display and lighting and normally would be used with the B or C model RT units.
This remote indicator is similar to the JG1209AC01 and is used with the D or J models.
Antenna. This antenna is a flat (micro-strip) antenna and mounts flat on a surface. The shape is square.
Can other antenna types be used?
All radar altimeters operate at 4.3 Ghz and have similar requirements for antenna performance. The APN-209 radar altimeter will electrically match, and function with, any 4.3 Ghz radar altimeter antenna. Any good quality antenna should work with the APN-209 system but for optimum performance, other factors should be considered.
The beam shape and gain is different with different types of antennas and should be selected to match the type and mission of the aircraft. Some aircraft missions involve roll and pitch maneuvers and would work best with a wide beam antenna. Other missions will do better with a narrow beam antenna.
Another factor is the style of mounting the antenna. The pyramid shaped antennas require a large reinforced hole in the aircraft while the flat micro-strip style can be installed on the skin with a simple backing plate and minimal structural changes.
If a previous system was installed in an aircraft, and it worked well, that antenna installation should work with the APN-209 system. The safest procedure is to use the type of antenna and the mounting location which is successful in another of the same type of aircraft.
The APN-209 system has no installation manual available. Refer to the manuals for other high quality radar altimeters such as Collins and King for information on antenna installation. The suggested location and spacing of antennas will be applicable to the APN-209 system.
Reinstalling APN-209 Radar Altimeter After Maintenance
Two adjustment ports are available on the side of the APN-209 unit. These are small “doors” that slide open to allow access to screwdriver adjustments. These should be adjusted before putting the system back in service.
If these are not adjusted, the radar altimeter will indicate a height above ground when the aircraft is actually on the ground. This represents the electrical delay in the coax cables and the distance from the antennas to the ground. This can be as much as 50 feet in some aircraft.
The two adjustments are independent. One adjustment controls the analog pointer and the other controls the digital readout. Unfortunately the adjustment ports, on the sides of the unit, are hard to reach in a typical aircraft installation. Before installation, the unit can be plugged in to the aircraft connectors and held in the hand while adjusting the trimpots. This also makes it easier for the technician to see the dial while doing the adjustment.
The analog dial is adjusted to read zero. In most units, the dial will hit a mechanical stop just past zero, so be sure that it is not adjusted on the negative side of zero and against the stop.
The digital zero control will adjust the readout to zero and a negative sign will appear if it is adjusted too far. The numbers that would represent the negative direction will not appear. If you see a negative sign, you don’t know how far on the negative side the adjustment actually is. For this reason it is best to adjust it for 1 foot or ticking between 1 and zero. This will insure that it is not adjusted past zero.
If the reading is unstable, it might mean that the ground under the aircraft has poor RF reflective properties. You can wet the ground under the aircraft to get around this problem.
After adjustment, the digital and analog readouts should agree. To test the system, hold a sheet of metal under the antennas at an angle which causes the signal to reflect against a nearby car or other object. The altimeter will indicate a distance to this object.
MBT Electronics Offers Repair / Overhaul Service for the AN/APN-194 Radar Altimeter System.
The APN-194 radar altimeter system consists of a remotely mounted RT (receiver-transmitter), two antennas, and one or more indicators. This system has been installed in numerous aircraft, mostly large turbine types, since about 1974.
This is a pulse type radar altimeter which utilizes solid state electronics except for the transmitter which is a vacuum tube cavity oscillator. The output power is 200 watts peak. Due to the ample transmit power; the performance of the system is very good. The maximum range is 5000 feet and the system will read down to zero feet.
There are several compatible indicators available for this system which is detailed below. The difference between the indicators is mainly the scale factors and zero position. Some have zero at the top, and some have zero at the bottom. One indicator has a 1000 foot range with an expanded scale. The indicators which were used with the older AN/APN-141 radar altimeter system are also compatible.
The output of the RT unit is an analog DC voltage which the indicator converts into a pointer position. The analog voltage is electrically expanded below 400 feet by circuitry in the RT unit. The indicators have a scale which is also expanded below 400 feet. The pointer drive is a servo which is not affected by aircraft attitude or G force. The indicator servo receives a reference voltage, along with the analog range voltage, from the RT unit. This improves the accuracy of the system because the indicator does not contain a precision voltage reference which could go out of tolerance. The indicator performs a “ratio” measurement comparing the analog range voltage to the reference voltage. The indicator does not need to be calibrated with a specific RT unit. A low altitude warning lamp, controlled by a moveable pointer or “bug”, and a test function are also provided.
The aircraft 115 VAC / 400 cycle power is supplied to the indicator which has its own internal power supply. The on / off switch on the indicator also controls the power to the RT unit. With this design, the indicator has independent operating voltages and will not cause a load on the RT power supply. A failure in the indicator will not damage the RT unit.
An optional digital output module is used in the RT-1015 configuration of the APN-194. This outputs a digital form of the altitude data in addition to the analog outputs.
RT Unit Part Numbers
Receiver transmitter with digital module installed.
Receiver transmitter (most common configuration)
Remote Indicator Part Numbers
Indicator. Zero at bottom. 5000 foot scale.
Indicator. Zero at top. 5000 foot scale.
Indicator. Zero at bottom. 5000 foot scale.
Indicator. Zero at bottom. 5000 foot scale.
Indicator. Zero at bottom. 1000 foot scale.
Antenna Part Numbers
MBT Electronics offers the following services:
- Bench test and evaluate
- Written tear down report
- Test data sheet
- Overhaul and refinishing
- Build up good units from junk or de-militarized components
- Rental of working units
- Troubleshooting assistance
- Buy and sell parts and units
MBT Electronics offers repair and overhaul of the King KRA-10A and KRA-10 systems.
The King KRA-10A radar altimeter system is designed for small general aviation aircraft. It radiates a microwave signal downward and measures the time required for the signal to reflect from the terrain and return to the aircraft. This time interval is converted to feet and displayed on an analog dial with a maximum range of 2500 feet. This continuous measurement of altitude above ground level is not dependent on pressure altitude settings or GPS database. A bug on the instrument can be set to alert the pilot when the aircraft descends below the set level. This is useful during approach to landing in IFR conditions. The DH warning illuminates a lamp on the instrument bezel and produces a tone in the aircraft audio system.
The radar altimeter system consists of three assemblies: RT (receiver-transmitter), antenna, and pilot’s indicator instrument. The RT and antenna are usually installed in the tail or belly of the aircraft. Except for the DH bug setting, the system requires no tuning or adjustment by the pilot. When the system does not receive a valid signal, because of poor signal reflection from the terrain, the system hides the indicator pointer behind a mask at the top end of the scale. This “stow” position is also used to indicate a height above terrain greater than 2500 feet. If the system loses power, the pointer mechanically springs back to the zero feet position on the indicator which is labeled “off”.
This type of radar altimeter cannot read below about 50 feet. When the aircraft is on the ground, the indicator readout is disabled by a signal from the landing gear switch. This prevents the confusing readouts, during taxi, when the system signal bounces off of nearby objects and intermittently indicates a distance. The earlier KRA-10 units do not have this feature.
Many low cost radar altimeters have been designed for general aviation and most have failed to live up to expectations. Compromises in design cause these low cost units to lack adequate performance in the aircraft. Intermittent operation, with the pointer flipping in and out of the stow position, annoys the pilot and causes loss of confidence in the equipment. Since there is nothing blocking the signal between the aircraft and the ground, the pilot blames the equipment and questions whether to trust it during actual IFR conditions. For this reason, many poor performing designs have become extinct in the active fleet.
The King KRA-10A system has performed well enough to remain in production for many years. Although it is a low cost unit, which means compromises in design, it has adequate performance to satisfy most aircraft operators. It works when installed and functioning properly but any loss of performance will put it below the usefulness threshold. The correctness of the installation and the quality of maintenance make the difference.
To keep the cost low, the King KRA-10A outputs only about 15 mW. This is the power that is directed toward to the ground and reflected back to the receiver. When the aircraft is above terrain with poor reflective properties, and the reflected signal is below the minimum necessary to activate the receiver, the indicator needle goes to the stow position. All radar altimeters suffer from this situation sometimes, but more power results in better reliability. As an example, a professional corporate level radar altimeter, the Collins ALT-50, transmits 200 mW of power; more than ten times greater than the King KRA-10A. Naturally, the professional unit can be expected to operate more reliably over adverse terrain. This is only one parameter of the system but it illustrates the situation where low cost dictates minimal margins of performance.
With minimal power available, the antenna and coax cable become important issues. The effective radiated power and sensitivity of the system (loop gain) is strongly influenced by the antenna system. If there is a loss of efficiency in the coax or antenna, it detracts from both the transmitted and received signals.
The simple RF section in the King KRA-10A uses only one antenna and depends on correct antenna matching for proper operation. In this system, the transmitter and receiver operate simultaneously using a single input /output port. The transmit signal normally flows from the RT unit to the antenna without interacting with the received signal flowing back to the RT unit. This depends on the antenna and coax being correctly matched to the transmitter. According to transmission line theory, with correct matching, all the transmitter energy is radiated from the antenna and no transmitter energy returns to the receiver except the received signal reflected from the terrain. This is what we want. A poor antenna or coax causes strong transmit power to reflect back to the receiver. The unwanted reflected transmitter power overloads and de-sensitizes the receiver and distorts the receiver response. The distorted output from the receiver presents a broken-up signal to the processor system resulting in inaccurate and unsteady indicator readout.
King warns against altering or lengthening the special coax cable supplied with the antenna. The antenna and coax must be tested and adjusted for minimum reflected power as a unit. As explained above, excessive reflected power will reduce system performance. This test and adjustment is impossible to do in the aircraft. The reflected energy from the ground, under the aircraft, is indistinguishable from the possible reflected energy due to a bad antenna or coax. A special test set-up is necessary. Many aircraft operators live with less than optimum performance because the antenna system has deteriorated and no longer presents the correct match to the RT unit. A bench test of the RT unit alone will not reveal this situation.
A related issue is the gain and beam shape of the antenna. If there is corrosion or other damage to the antenna, the gain or beam shape may be compromised. The coax may also be bad due to age, loose connectors, or water intrusion. These conditions will reduce performance of the system due to attenuation and loss of gain. With any of these problems, the performance of the system will be poor.
The test procedure included in the King manual is written for facilities with minimal microwave test equipment. Although the antenna and coax are critical, there is no provision for a system test including the antenna. The microwave circuitry in the RT is treated as a replaceable component and not tested in detail. Using only the bench test procedure, it is easy to be fooled into replacing expensive components while following a process of elimination.
MBT Electronics has developed the capability to test and adjust the King KRA-10A system in detail including the antenna, coax, RT, and indicator. The components can be tested and operated together as a system on a special test setup. By testing and adjusting the system as a whole, the efficiency of the system can be assured and any abnormal conditions fixed. When a system with a poor antenna is operated on the test stand, abnormal test point readings will be seen within the RT. This is a sensitive test of antenna quality. The antenna system can be repaired or adjusted until the test point readings are normal.
The general procedure is to test, repair, and adjust the RT unit and indicator for correct operation. Then the components are installed on a test stand to verify the antenna and coax quality and impedance match. When the antenna is pointed skyward and no reflection from terrain is possible, any reflected energy reaching the receiver indicates a mismatched antenna. When the system antenna is pointed at the calibrated pickup antenna, the radiated power and antenna beam characteristics can be checked.
MBT Electronics recommends removing the antenna from the aircraft anytime poor operation of the KRA-10A is suspected. Removing the antenna is the only way to reveal possible poor bonding to the aircraft skin. This can be a source of intermittent operation. Like all bottom mounted aircraft antennas, the KRA-10A antenna is subject to water that sometimes finds its way into the belly or tail. The rubber and steel-mesh RF gasket gets wet and corrodes against the dissimilar aluminum skin. Sometimes the antenna is sealed around the outside perimeter by the installer. This is guaranteed to trap water between the gasket and skin which never dries out. Water can enter the antenna through cracked and warped plastic radome.
The earlier model King KRA-10
The KRA-10 is very similar to the KRA-10A. The only operational difference is the provision, in the KRA-10A, for a disabling signal to suppress operation when on the ground. This usually comes from a switch on the landing gear. Internally, one circuit card is different and completely redesigned in the KRA-10A. Presumably the designers at King corrected some defects in the original version by replacing the circuit card. MBT Electronics believes that with careful repair and adjustment, the KRA-10 units can be completely satisfactory. The same test setup is used and the issues with the antenna are the same.
The King KI-250 Instrument
The KI-250 contains the DH bug circuitry, the tone generator, and the meter drive circuitry. The meter drive circuit is a voltage to current converter. The analog altitude voltage from the RT unit is converted to current and applied to the meter drive motor. This motor torque counteracts a spring and results in a pointer position proportional to input voltage. When power is removed, the pointer springs back to the zero / off position. The indicators are compatible between the KRA-10 and KRA-10A units. The later serial number KI-250 indicators have modified and improved circuitry although the part number is the same. Many subtle faults can exist in the KI-250. It should be tested and adjusted with the system to insure best possible accuracy.