Instrument landing system

In a single-frequency precision guidance landing system, the use of a DME interrogator in the aircraft and a DME receiver at the ground installation, each tuned to the same DME channel frequency, to uniquely interrogate a selected ground station and hence identify it by virtue of its replies being synchronous in the aircraft with the interrogations, the interrogations and the replies also being used to obtain range to the ground installation. This technique uses airborne already-installed DME interrogators for selective interrogation of a desired landing installation, thereby to eliminate any need to add additional special purpose equipment to the aircraft to accomplish the desired uniqueness of interrogation and ground installation identification achieved by this invention.

BACKGROUND AND PRIOR ART 
This invention relates to an improved instrument landing system, and more 
particularly relates to improvements in a type of landing system in which 
all installations sequentially radiate guidance pulses on the same 
frequency, the improvements permitting the ground installation of such a 
same-frequency landing system to be uniquely interrogated and hence 
identified by an approaching aircraft. 
In cases where a ground installation of such a single frequency landing 
system is remotely located from other similar installations there is no 
need to be able to uniquely interrogate it and hence identify it to an 
approaching aircraft. However, in impacted geographic locations where 
there are multiple similar landing installations located relatively 
closely together it is necessary to provide means for uniquely 
interrogating and identifying one such same-frequency installation to the 
exclusion of others in the vicinity. 
In conventional landing systems such as the present ILS and MLS systems, 
unique identification and signal exchanges between approaching aircraft 
and a particular ground installation are established by uniquely assigning 
different frequencies out of a band of frequencies to each of the various 
installations, and tuning the airborne units to the frequency of the 
selected installation. In my U.S. Pat. No. 4,429,312 entitled Independent 
Landing Monitoring System, a different type of identification of a 
same-frequency landing installation is discussed in which some of the 
signals transmitted to the aircraft are pulse encoded to identify that 
installation. 
In this invention, ground installation identification is established by a 
unique interrogation technique, and in addition, the invention provides 
improved range data generation by the landing system which improved 
ranging is related to the presently disclosed unique technique of 
interrogation and station identification. 
A very desirable characteristic for a landing system is the capability of 
providing range to touchdown, thereby to provide range data which serves 
three major purposes. A first purpose is providing range data for alerting 
the pilot of his proximity to touchdown. A second purpose is to provide 
means for automatically reducing the gain of the landing installation as 
the aircraft range to touchdown diminishes in order to maintain loop 
stability, often referred to as "course softening". A third purpose in 
providing range data which can be used, together with the elevational 
angular data provided by the landing system, to determine altitude above 
the runway during approach. 
In the conventional ILS system, range to touchdown is generally provided by 
marker beacons on the ground at established distances from touchdown. 
These beacons radiate vertical fan shaped-beams through which the 
approaching aircraft passes. The range information thus acquired in the 
aircraft is used for pilot alerting and for "course softening" purposes. 
In MLS and ILS practice, an alternative more accurate measurement of range 
is provided by conventional TACAN/DME interrogators which are carried by 
almost all aircraft. The airborne TACAN/DME equipment interrogates a DME 
beacon that is co-located with the MLS or ILS ground installation and 
receives therefrom a direct measurement of range using usual DME 
techniques. The DME units are assigned a band of 252 freqency channels in 
the 1000 MHz range, which band is separate from the 100-300 MHz frequency 
band allocated to ILS systems in current use or the 5000-5250 MHz band 
assigned to MLS. 
For some landing applications, a very precise measurement of range is 
required, and for this purpose a Precision DME, usually referred to as 
PDME, is employed. The PDME is similar to the conventional DME, but uses 
faster rise time pulses to obtain higher precision. This PDME system 
imposes on aircraft, which have to use it in order to obtain a required 
very precise measurement of range, the additional burden of having 
installed on board appropriate PDME airborne equipment. Another technique 
for obtaining precision range in a landing system is provided by the 
teaching of my U.S. Pat. No. 4,429,312. Range is measured in that 
disclosure by having the weather radar interrogate the landing system 
ground installation and trigger the transmission of pulsed angular 
guidance signals. These pulsed replies are synchronous with the weather 
radar interrogations and are range tracked in a conventional manner to 
provide precision range in the aircraft. Range measurements of higher 
precision can be obtained by the use of fast rise time pulses. 
Both of the above ways for identifying ground station installations, i.e. 
frequency selection or pulse group encoding, require additional adjustable 
cockpit controls for either tuning to the frequency of the ground 
installation, or for selecting the decodement of the signals radiated from 
that ground station. The measurement of range by means of marker beacons 
or DME equipment requires the installation of appropriate marker beacons 
or DME beacons with the landing system ground installation. The 
measurement of very precise range requires the addition of specialized 
PDME equipment, both air and ground. While the use of the weather radar to 
provide precision range, as taught in my U.S. Pat. No. 4,429,312, 
eliminates the need for added PDME equipment, not all aircraft carry a 
weather radar. Thus all conventional landing systems therefore have tended 
to require either added airborne equipment, or cockpit controls, or both, 
in order to achieve unique communication with a selected ground 
installation. 
Considered broadly, a landing system does not inherently require the use of 
multiple different frequencies since operation at all installation sites 
is usually performed on a single frequency. Single-frequency operation is 
an advantage because if the actual landing guidance system can always 
operate on the same frequency for different sites, great simplification in 
terms of airborne equipment complexity is made possible since the receiver 
can be fixed-frequency. A technique to achieve both station selection and 
ranging data in a fixed frequency landing system, using only airborne 
equipment which is already installed in IFR aircraft, would eliminate the 
need to install in the aircraft any additional channel selection switches 
or decoder control switches, and hence would result in much simpler and 
lower cost airborne landing system installations. 
THE INVENTION 
This invention relates to an improved landing system in which the 
ground-installation responses to interrogations by an approaching aircraft 
comprise groups of guidance pulses which are normally radiated on the same 
frequency for all ground installations. Since IFR equipped aircraft always 
include a DME interrogator, the teaching of this invention is to use the 
DME interrogator already installed in the aircraft to interrogate a DME 
receiver at the selected ground installation, with the output for the DME 
ground receiver triggering the radiation of at least some of the guidance 
pulse responses. Since there are 252 DME channels, therefore, in any 
particular geographic location, there are always a number of unused 
channels available, and one such available channel can be uniquely 
assigned to each landing system installation for the purpose of 
identifying it by unique interrogation thereof. 
In practice, the pilot of an aircraft will have available to him from 
published navigational information the correct channel to select on his 
DME interrogator to trigger the landing installation he intends to 
approach. His DME interrogator will then periodically interrogate a 
TACAN/DME receiver at that ground installation, which receiver will be 
wired to trigger the ground installation timing and switching circuitry, 
which then appropriately drives the pulse transmitter of the selected 
ground installation to begin the radiation of precision guidance signals. 
Thus, by selecting different DME channels, the pilot can uniquely select 
different landing system installations to guide his approach. 
In the aircraft, besides the DME interrogator, there will be a receiver 
fixed-tuned to receive the ground installation's pulse responses, and a 
programmed processor which is provided with a timing signal to indicate to 
it when the airborne DME interrogator transmitted an interrogation to the 
ground installation to trigger its guidance signal response. The airborne 
processor is therefore able to identify the signals of interest from the 
selected ground installation, i.e. the installation being interrogated by 
that aircraft, in contrast to other same-frequency signals resulting from 
other aircraft interrogations or from other ground installations. This 
identification results from the fact that the signals of interest are 
synchronous with the aircraft's own airborne interrogations, and hence can 
be range tracked, whereby the aircraft uses only response pulses within 
its range tracking gate. In this system, the ground installation 
interrogating signal from the aircraft will be transmitted on an 
appropriate one of the 252 standard DME channels, but the pulse responses 
from the ground installation will be in a different frequency band, i.e. 
at a particular single frequency used by all landing systems of this type. 
Thus this system uses cross-band interrogation and response frequencies. 
Range to touchdown is obtained based upon the time lapse between the DME 
interrogation signal to the ground installation, and the time of reception 
of the ground installation's pulsed response. 
The basic identification technique therefore involves range tracking and 
processing only responses that are synchronous with that aircraft's own 
interrogations of the selected ground station, and in this respect is 
similar to conventional DME functioning in which non-synchronous received 
signals are ignored. It is therefore possible for plural aircraft to 
simultaneously use a single selected landing station in the presence of 
other same-frequency stations, and to obtain both precision angular 
guidance and range without mutual interference since their interrogations 
will bear random relationship to each other. A basic difference however is 
that this system uses cross-band transmissions, i.e. interrogations in a 
DME channel, and responses in a different frequency band more suitable to 
the radiation of precision landing guidance beams. The particular ground 
station interrogated will be determined by selection of the particular DME 
channel which has been assigned to it. 
The precision of the ranging technique according to this invention is 
improved by the fact that the signals radiated as a reply to each 
interrogation and used for ranging in the aircraft, can utilize fast rise 
time pulses, in contrast to the slow rise times of conventional DME 
replies. The use of fast rise time pulse replies is aided by the use of 
only one frequency channel for their transmission, which single channel 
can therefore be made wider to accommodate faster rise times. 
It should further be noted that in a system as disclosed herein using DME 
interrogators to trigger replies from the precision guidance ground 
installation, such ground installations can still also transmit randomly 
timed precision guidance paired signals which are initiated by a local 
squitter generator for use by aircraft which do not include airborne 
interrogators. Such ground installations could be used in areas not 
including other similar possibly interfering systems in the immediate 
geographic vicinity. Squitter operation would be analogous to usual 
TACAN/DME operation, and is of the general type which is described as an 
alternative embodiment in my U.S. Pat. No. 4,429,312. Squitter initiated 
precision guidance sequences would include also an omni radiated signal 
which would still be encoded to identify the ground installation. It 
should be noted that, in general, only signals from the selectively 
interrogated ground stations will fall inside the range tracking gate and 
will be processed. Occasionally, however, same-frequency signals from 
other ground installations in the vicinity can fall within the tracking 
gate and hence, if processed, would tend to generate guidance errors since 
they are actually providing guidance to another location. There are well 
known techniques to minimize this problem, i.e. averaging, and 
"wild-point" elimination. 
OBJECTS AND ADVANTAGES OF THE INVENTION 
It is a principal object of this invention to provide in a single-frequency 
precision guidance landing system the capability of uniquely interrogating 
a selected ground station and hence identifying it by virtue of its 
replies being synchronous in the aircraft with the interrogations, such 
technique using already-existing airborne DME interrogators to uniquely 
interrogate each different landing installation via different DME channels 
assigned thereto, thereby eliminating any need to add additional special 
purpose equipment to the aircraft to achieve such uniqueness of 
interrogation and identification. 
It is a corollary object of the invention to reduce the complexity of 
airborne equipment and the cost of initial installation and maintenance by 
utilizing what is already included aboard all IFR capability aircraft to 
achieve unique interrogations of precision guidance landing installations, 
and to use the combination of the airborne DME interrogator, a fixed 
frequency receiver and landing guidance processor in the aircraft to range 
to the ground station and process only range-tracked responses for 
guidance purposes. At present, there is no wired interconnection and 
cooperation between DME equipment and landing system guidance 
installations, either ground based or airborne. This invention proposes 
their interconnection to achieve reduction of complexity of the landing 
system installations and accompanying economies and weight reduction, 
while providing selection by frequency of particular ground based 
precision landing installations with accompanying positive identification 
thereof and ranging thereto. 
Other objects and advantages of the invention will become apparent during 
the following discussion of the drawings.

DESCRIPTION OF PREFERRED EMBODIMENT 
Although this invention provides techniques which are applicable to many 
different landing systems, and therefore are not to be limited to 
improvements to the precision landing system of the type shown and 
described in my U.S. Pat. No. 4,429,312, supra, a preferred embodiment of 
the present improved system will be illustrated and described with 
reference to the landing system of that patent. 
As shown in the patent, and described in columns 8 and 9 thereof, the 
patented system provides a ground based precision landing guidance 
installation which radiates localizer and glide slope guidance beams from 
separate antennas which are directed along the approach path toward a 
landing aircraft, and which are received in the aircraft and processed to 
recover signals which provide landing indications to the pilot. In the 
system shown in U.S. Pat. No. 4,429,312 the ground based system can either 
be triggered to respond to interrogations from the approaching aircraft, 
transmitted by its weather radar, or can be free running and simply 
received and used by an approaching aircraft which does not have a weather 
radar. The former type of triggered system is the system to which the 
present improvements are directed, and therefore the free running mode of 
operation of the landing system will not be further discussed. 
FIG. 1 shows the basic precision landing guidance system of U.S. Pat. No. 
4,429,312, which includes for lateral aircraft guidance two directive 
antennas 21 and 22 having precision guidance localizer antenna beam 
patterns 23 and 24, marked B and C. The ground installation also includes 
a non-directive antenna 5 delivering an omni-pattern 25. These antennas 5, 
21 and 22 are connected by a switch 26 and cable 27 to a radar beacon 6, 
which conventionally includes a transmitter 32 and a receiver 30, and 
which includes timing and switching circuitry 29 which controls the switch 
26 and initiates the outputs of the transmitter 32. For vertical 
glideslope guidance, the ground installation further includes two 
directive antennas 33 and 34 for radiating paired precision glideslope 
guidance beams 33a and 34a, marked D and E, which antennas are likewise 
connected to the transmitter 32 through the antenna switch 26. The paired 
beam patterns of the antennas 23 and 24 for lateral guidance overlap so 
that they provide equal intensity signals along the extended centerline CL 
of the runway. Thus, if the signal intensities of both antennas are equal 
as received in the airborne vehicle, it must be laterally located over the 
centerline of the runway. Likewise, the directive antenna patterns of two 
paired glideslope antennas 33 and 34 are aligned and partially overlapped 
respectively above and below a predetermined glideslope (usually 
3.degree.), so that for aircraft approaching precisely along the 
glideslope, the signal intensities received in the aircraft from these 
paired antennas 33 and 34 will be equal. Thus for an on-course approach, 
all four guidance signal intensities received in the aircraft will be 
equal. However, deviation above or below, or to the right or left of the 
desired approach course, will cause an unbalance in the paired signals 
received at the receiver, indicating to the pilot the direction in which 
the aircraft has deviated from the desired course. This operation is 
thoroughly described in U.S. Pat. No. 4,429,312. 
The airborne installation of the system according to that patent is shown 
to the right in FIG. 1, and includes a weather radar which transmits and 
receives through the antenna 3. The radar conventionally includes a radar 
transmitter 1, a radar receiver 7, a beacon receiver 8, a switch 10, and a 
conventional radar indicator 11. The switch 10 is used to connect the 
radar indicator 11 to either the radar receiver 7 to display conventional 
radar echoes, or to the beacon receiver 8 to display beacon returns, all 
as well known in the art. The beacon receiver 8 is also connected to a 
range gate and navigation processor 15 which provides range data to a 
range readout 18 and to a course deviation indicator 20 connected thereto. 
The airborne radar transmitter 1 is used to trigger a response sequence 
from the ground installation by transmitting a trigger signal T thereto 
which is received by the omni antenna 5 and delivered through the ground 
receiver 30 to the timing and switching circuitry 29 which then initiates 
a response sequence from the ground installation. This response sequence 
includes multiple successively delivered transmissions. First, the timing 
and switching circuitry 29 delivers through the omni antenna 5 a coded 
pulse group reference signal A from the transmitter which identifies the 
ground installation, and also provides range information in the aircraft 
as well as a signal whose strength is used to set the gain of the aircraft 
receiver so as to keep the airborne receiver operating within a linear 
portion of its response characteristic. After a fixed delay determined by 
the timing and switching circuitry, the switch 26 then steps sequentially 
to connect the transmitter 32 in turn to each of the four directive 
antennas to deliver responses R including right and left paired localizer 
pulses, and to deliver up and down paired glideslope pulses. These pulses 
are delivered one at a time with suitable delays between them. Adjustable 
attenuators 44 serve to balance the antenna drives so that the guidance 
signals are all of equal amplitude when the aircraft is exactly on course 
for landing, as explained in U.S. Pat. No. 4,429,312. The sequence of 
these four guidance signals is predetermined and fixed so that the 
aircraft can identify the signals by their order in the succession. 
The pulses radiated in these precision guidance beams B, C, D and E in FIG. 
1, plus the reference signal group A from the omni antenna, are received 
at the airborne antenna 3, and delivered by the beacon receiver 8 to the 
processor 15 in the aircraft. The processor 15 is programmed to use the 
reference signal A to determine range and to display it at the range 
readout 18, and to use the precision landing signals B, C, D and E to 
create and deliver to the course deviation indicator 20 output signals 
which show the position of the aircraft with respect to the desired 
approach path. These techniques define a type of prior art system on which 
the present invention seeks to improve. 
The precision guidance system shown in FIG. 1 is generally satisfactory 
when the aircraft has a weather radar to interrogate the ground 
installation, and a decoding circuit has been added to the radar together 
with an appropriate code selector switch for station selection in the 
cockpit. However, not all aircraft have weather radars to interrogate the 
ground installation, and in addition, it is often not desirable to add a 
code selector switch in an already overcrowded cockpit, as in fighter 
aircraft, for example. In addition, where there are several airfields in 
close geographic proximity, and/or where there are several landing 
installations of this type at the same airport, the same-frequency signals 
from all such landing systems can arrive at the aircraft synchronously and 
hence they can not be adequately separated for unique range tracking and 
guidance generation purposes. This is basically the same problem that 
plagues the conventional radar/beacon system (ATCRBS) used by the FAA for 
air traffic control purposes. It is called "garbling". The weather radar 
technique of U.S. Pat. No. 4,429,312, with associated identifying codes, 
is thus very suitable for use at isolated remote sites, such as offshore 
oil rigs, but not suitable for areas with many same-frequency landing 
systems in close proximity. The problem comes basically from the fact that 
these systems, and the airborne radars all use a common frequency, and 
there is no way to trigger one particular installation uniquely. There is 
therefore always the risk of undesirably triggering a nearby installation 
with the result that confusing responses to the aircraft from both 
locations will be synchronously received in that aircraft. 
FIG. 2 shows the system according to the present invention which improves 
over the prior art system shown in FIG. 1. As pointed out above in this 
specification, at any particular geographic location, there are always far 
more of the 252 TACAN/DME channels available for use than are actually in 
use. Moreover, almost all aircraft already have either TACAN or DME 
capability on board, while many light aircraft and military aircraft may 
not have weather radar on board, as would be required in the prior art 
system shown in FIG. 1. 
This invention proposes to trigger the response of a selected ground 
installation by using the TACAN or DME interrogator 50 already on board 
the aircraft. The airborne DME interrogator is free running so that it 
will repeatedly trigger the ground installation using a channel which is 
not otherwise in use in that geographic location. In order to implement 
this invention, the ground installation must be provided with a TACAN/DME 
receiver 60 tuned by a conventional tuning control 62 to that channel 
which is uniquely assigned to it. The TACAN/DME receiver 60 is operative 
to trigger the transmitter 32 through timing and switching circuitry 29. 
The TACAN/DME receiver 60 has its own antenna 64 which is appropriate for 
receiving signals in the 1000 MHz range used by TACAN/DME, and the 
receiver outputs trigger signals for each received interrogation via wire 
66, corresponding in fuction with the triggering wire 31 in FIG. 1, to 
drive the timing and switching circuitry 29. As is the case in FIG. 1, the 
timing and switching circuitry 29 sets the switch 26 to the correct 
position, provides delays, and drives the transmitter to deliver the omni 
encoded reference signal A followed by the two sets of paired directive 
signals B and C, and D and E. 
In the aircraft, the DME unit 50 is tunable by the control 52 to whatever 
channel the pilot selects in order to trigger the desired ground 
installation. The TACAN/DME interrogator 50 will then continue to 
periodically trigger the ground installation so that signals returned from 
the ground installation will be synchronous with respect to the 
interrogations from that particular aircraft, and thus identifiable as the 
returns of interest in that aircraft as distinguished from same-frequency 
non-synchronous returns from the same or other nearby ground installations 
in the vicinity. The airborne installation will also include an airborne 
receiver 8 corresponding to that in FIG. 1 and tuned to receive the omni 
reference signal A and the paired directive signals B and C, and D and E 
from the ground installation. The TACAN/DME interrogator 50 is connected 
to deliver a timing signal via wire 13 to the navigation processor 15 to 
indicate when its interrogation signal was sent out. The processor 15 uses 
this timing signal and the reception of the omni reference signal A from 
the ground installation to determine range to the ground installation and 
display it at the range readout 18. In addition, the processor includes a 
range gate for tracking all of the ground installation response signals, 
including the omni signals A and the paired directive signals B and C, and 
D and E from the ground installation. The directive signals are processed 
to give precision guidance to the pilot using the visual course deviation 
indicator display 20 which is the same as in FIG. 1. Infrequently, 
however, other same-frequency signals from the selected landing 
installation or other landing installations in the vicinity, will fall 
within the range gate, just as they do in conventional DME range tracking. 
The effect of these same-frequency signals will be minor, if averaged with 
the desired signals from the selected ground installation, since they 
occur relatively infrequently. It is possible however to further minimize 
even this minor effect by storing the values of all received signals that 
fall within the range gate in computer memory, and by utilizing for 
guidance purposes only those stored signals that fall within prescribed 
limits of the running average of all signals. This is termed "wild-point" 
editing. Thus on the ground, the radar receiver 30 of FIG. 1 has been 
replaced by a TACAN/DME receiver 60, and in the air, the radar transmitter 
1 and receiver 7 have been replaced by a TACAN/DME interrogator 50, with 
the result that one ground installation at a time can be uniquely 
interrogated via its own assigned frequency channel. 
This invention is not to be limited to the embodiments shown and described, 
because changes may be made within the scope of the following claims.