MLS Technical Information
The Microwave Landing System (MLS) originated in the early 1970's. The MLS is a precision approach and landing guidance system which provides two or three dimensional position information and various ground to air data. International standardization for the MLS Time Reference Scanning Beam (TRSB) concept was reached by the International Civil Aviation Organization (ICAO) in 1978.
The ICAO - Annex 10 (1985) Instrument Landing System (ILS) to MLS transition plan is slated to begin in 1998. Commercial air carrier representatives and other navigation system developers at the fourteenth meeting of the ICAO All-Weather Operations Panel (AWOP) have urged the review of other navigation systems as alternative position information sources for approach, landing, and departure operations. The global navigation satellite system (GNSS) is under close review for the next working group meeting as a possible alternative. Review of the new technology systems may result in an amendment to the 1985 ILS to MLS transition plan. Full replacement of existing ILS systems may not be implemented at all airports as predicted in the original transition plan. MLS is the scheduled standard for all runways supporting international operations beginning in 1998.
Replacement of the Instrument Landing System (ILS) is straightforward with the installation of an MLS. The MLS duplicates and augments the capabilities of the ILS, which provides a ± 0.7º proportional guidance region around the glide slope angle and a region of approximately ± 3.0° azimuth about the centerline approach of the instrumented runway.
The MLS is capable of providing a maximum ± 62.0° azimuth (AZ) coverage region with a typical installation using only ± 40.0° azimuth coverage region. The MLS elevation (EL) signal can provide coverage up to + 30.0° above ground level. A fully MLS equipped runway has four signal transmitters (Figure 1). Two of the transmitters provide MLS azimuth functions; they are located at each end of the runway, and are positioned facing the runway. With both runway directions equipped, the azimuth antenna facing the approaching aircraft is configured as the approach azimuth transmitter and the opposite antenna becomes the back azimuth transmitter. The approach azimuth transmitter is used to guide the aircraft during an instrument (non-visual) approach to the active runway. It may also be used in precision area navigation (RNAV) when coupled with the elevation and distance measuring equipment that are the remaining transmitted signals.
The AZ and BAZ signal beam shapes are both fan-shaped in a vertical plane formed along any of the antenna's radials. As viewed by the pilot of an aircraft on final approach to the runway, the azimuth beam is swept from the right-most coverage angle to the left-most in the "TO" scan and is then returned from the left to right coverage angle in the "FRO" scan after a specified delay at the left-most limit. The time difference between reception of the two scans is determined. Information derived from the data words transmitted to the aircraft by the MLS gives the aircraft MLS receiver specific information with regard to site geometry. This information is used along with beam timing to determine accurately the azimuth angle of the aircraft. Aircraft preparing for departure will be positioned on the active runway in front of the BAZ transmitter. The pilot would view the "TO" scan of the BAZ beam moving from the left to right. The "FRO" scan is from the pilot's right to left.
BAZ guidance is transmitted from either an azimuth antenna that has been configured to perform the BAZ function or from a special antenna that only performs the BAZ transmissions. The reconfiguration is accomplished by changing the data preamble that precedes each signal transmission. BAZ signals are used to guide the aircraft during departures from the active runway; they are also used to guide the aircraft through the departure airspace when a missed-approach has been declared.
A third signal transmitted is the elevation (EL) signal. This fan-shaped beam is transmitted from the EL antenna located abeam the MLS datum point. The beam originates at an angle near horizontal (minimum elevation angle), scans to the upper elevation angle limit in an upward direction (the "TO" scan), and then returns (the "FRO" scan). The time interval between the "TO" and "FRO" scans is measured in the receiver and based on the data transmitted from the ground (concerning site geometry and configuration) the elevation angle of the aircraft is determined.
The remaining signal is produced by the distance measuring equipment (DME). The DME is the MLS ranging element, and is responsible for providing the aircraft's slant range (line-of-sight distance) to a specified ground position. Precision distance measuring equipment (DME/P) provides for two modes of operation for approaching aircraft. The initial approach (IA) mode is active in the region from 7 nautical miles (NM) to 22 NM from the DME/P transponder. The final approach (FA) region is from the DME/P transponder to a range of 7 NM. The region between 7 NM and 8 NM is defined as the transition region. All DME/P equipment transitions from IA to FA mode within this region. The FA mode has enhanced precision and tolerances when compared to the IA mode. The Narrow-spectrum distance measuring equipment (DME/N) is compatible with the IA mode. DME transponders operate on assigned frequencies that are paired with specific frequencies of the MLS angle transmitters. Installation of a DME/P is preferred when possible.
MLS operating frequencies (5.03 to 5.09 GHz) help to reduce sensitivity to interfering signals and site topography. The equipment size is reduced because of the use of microwave frequencies. This reduction in size introduces installation procedures that are less labor intensive than ILS installations. MLS systems exist that are transportable by aircraft and are easily installed by a team of four people in a matter of hours. The relative ease of installation and transportation of this precision landing system is highly desirable in remote areas that may not need to be instrumented throughout the year. The permanent installations benefit from the reduced size as well.
The greatest benefit of upgrading to MLS-equipped airports is the use of RNAV. The MLS is capable of providing coverage to aircraft in azimuth angles to ± 62º about the runway centerline extended, elevation angles to + 30º, and ranges to 22.5 NM from runway threshold. Installing an additional computer on-board the aircraft in order to use this extra coverage allows the MLS to determine a three-dimensional position-in-space. Courses in congested airways and around obstacles can now be designed and flown with increased accuracy and airspace capacity. Figure 2 is an example of approach RNAV use. RNAV operation in MLS coverage regions allow more than one approach pattern at any given time. This benefits all users by allowing smaller aircraft to join final approach at distances closer to the runway, thus not requiring larger commercial air traffic to reduce airspeed to accommodate the smaller aircraft.
Ohio University's Avionics Engineering Center has since 1963 used general aviation (GA) aircraft and custom data collection packages to evaluate ILS and MLS installations at various sites throughout the United States. This work describes the latest data collection package, the airborne data collection system (ADCS). The ADCS incorporates basic MLS evaluation functions, with the additional capabilities of two-dimensional pilot guidance from RNAV computations in BAZ coverage and storage of this additional data for post-flight analysis.
MLS evaluation takes many forms. A typical application is an MLS approach procedure where the aircraft is guided from a position out of the airport region to a landing, based on received MLS azimuth and elevation angle information and DME/(N or P) range information. Throughout the approach procedure the aircraft guidance information is recorded by a data collection system. The data collection system also records the position information transmitted by a tracking system independent of the MLS. The comparison of these two sets of position data gives an indication of the stability and accuracy of the MLS equipment. Evidence of noise and interference is evaluated and correlated with site geometry and obstacle location. The evaluation process begins in the aircraft with simple plots of the data, to detect obvious navigational-aid or data collection problems. Post-flight analysis reveals more information about the guidance system.
MLS data are transmitted over the ARINC-429 interface. MLS functions (azimuth, elevation, back-azimuth, and DME) are configured to transmit at given rates to provide the most accurate position information to approaching aircraft. The MLS azimuth function has a typical update rate of 39/second; the elevation signal also scans at a rate of 39/second. There is an azimuth rate of 13/second for 60º coverage. Back-azimuth update rates are published at 6.5/second. Operation of the MLS system with published accuracy requires that these function rates be available.