An in-flight hose-and-drogue refueling system has the capability of steering the drogue using four thrusters which are energized by pressurized air. The system measures the drogue's 3-dimensional position relative to either the refueling aircraft or the probe on the receiver aircraft. A control system utilizes the measurements of the drogue's 3-dimensional position to activate the thrusters so as to either minimize the excursions of the drogue due to turbulence, thus enabling easier hook-up, or alternatively, to track the receiver's probe and automatically guide and contact the drogue to the probe.

FIELD OF THE INVENTION 
The present invention relates to automatic hook-up of a hose-and-drogue 
aerial refueling apparatus to a receiver aircraft probe and, more 
particularly, to apparatus and methods for in-flight hose-and-drogue 
refueling using electrooptical technology. 
BACKGROUND OF THE INVENTION 
The usefulness of air refueling became apparent to the military almost as 
soon as they started using aircraft. The main advantage of air refueling 
is obvious: it enables aircraft to stay airborne longer. Since most 
aircraft are incapable of taking off with maximum fuel and full payload, 
without in-flight refueling there is always a balance to be struck between 
range, payload, and fuel. Air refueling is more than just a range 
stretcher: it allows one to carry out missions with a smaller number of 
sorties, or alternatively, fewer aircraft. 
Currently, two major systems are used in mid-air refueling. One is the 
"flying boom" method in which the tanker has a tail boom equipped with 
aerodynamic control surfaces, which are commanded by the boom operator. 
The operator, who is located at the rear section of the tanker aircraft, 
utilizes a steering device to guide the boom into a hatch or receptacle in 
the receiving aircraft's upper structure. 
The second common method is the hose-and-drogue system, in which a drogue 
attached to the fuel hose is extended from the refueling aircraft's belly 
or wings. The receiver aircraft is equipped with a fixed or retractable 
probe. In the latter method the receiving aircraft's pilot flies the 
aircraft "into" the drogue. 
The advantages of the hose/drogue system are the following: (a) up to three 
receivers can take fuel simultaneously; (b) if one hose/drogue unit (HDU) 
becomes unserviceable, the tanker still can offload its fuel; (c) the HDU 
is inherently safer than the heavy, rigid boom, which is restricted in its 
movements; (d) it is easier to install on non-purpose-built aircraft; and 
(e) it is compatible with most receivers, e.g., fixed wings, as well as 
rotorcraft. 
There are, however, two disadvantages to the hose and drogue system: it has 
lower fuel transfer rates than the boom system, and the drogue is 
uncontrollable and is susceptible to winds and gusts. In bad weather 
conditions and particularly in low level refueling situations the hookup 
process is very difficult and demands excessive receiving aircraft pilot 
maneuvers. Since the aircraft &o be refueled is likely to be already low 
on fuel, excessive maneuvering may result in the necessity to abandon the 
aircraft. 
U.S. Pat. No. 4,763,861 to Newman, for "Microwave Rendezvous System for 
Aerial Refueling," relates to a microwave rendezvous system for use on a 
tanker aircraft for aerial refueling of a receiver aircraft. The system 
comprises a microwave transmitter signaling directional data towards the 
receiver aircraft, providing a larger rendezvous envelope in space between 
the tanker and receiver aircraft, and thereby requiring less tedious 
navigation and attention during a refueling operation. 
U.S. Pat. No. 4,763,125 to Newman, for "Light Array for Providing Passive 
Rendezvous Guidance between Closing Aircraft Spacecraft and the Like," 
relates to a light array disposed along the ventral centerline of a lead 
aircraft to provide passive rendezvous guidance to a closing aircraft. 
U.S. Pat. No. 4,633,376 to Newman, for "Advanced Fuel Receptacle Lighting 
System for Aerial Refueling," relates to a lighting system mounted on a 
tanker aircraft for enhancing a boom operator's visual view of a fuel 
receptacle area on the receiver aircraft during nighttime operations. 
U.S. Pats. Nos. 4,519,560 and 4,231,536 to Ishimitsu et al. both relate to 
a ruddevator assembly comprising a pair of airfoil configurations mounted 
on a boom, whose movements are controlled by creating aerodynamic forces 
in the vertical and horizontal directions by deflecting the airfoils with 
respect to the flow. 
U.S. Pat. No. 4,288,845 to Fisness et al. for "Aerial Refueling Receptacle 
Floodlights-Spoiler and Fuselage, Nose Mounted," relates to a floodlight 
illuminating system for use at night in combination with an aircraft 
having receptacle surfaces mounted on its nose section. 
U.S. Pat. No. 4,158,885 to Neuberger, for "Guidance-Light Display Apparatus 
and Method for In-Flight Link-up of Two Aircraft," relates to a light 
system for mounting on a lead aircraft so as to be visible to a pilot of a 
trailing aircraft for guiding such pilot in flying the trailing aircraft 
into predetermined in-flight link-up position with respect to the lead 
aircraft. The apparatus comprises an array of guidance lights arranged on 
the body of the lead aircraft and sensor means for producing an electrical 
signal representing the instantaneous position and velocity of the 
trailing aircraft relative to the lead aircraft. 
U.S. Pat. No. 4,072,283 to Weiland, for "Aerial Refueling Boom 
Articulation," relates to a flying refueling boom for an aerial tanker 
airplane, with a mechanism for moving the boom about different axes. The 
motion of the boom is obtained by deflecting a pair of aerodynamic 
surfaces mounted on it. The motion of the aerodynamic surfaces is 
controlled using a cable system that allows their deflection with respect 
to the air stream. 
U.S. Pat. No. 4,025,193 to Pond et al., for "Apparatus Suitable for Use in 
Orienting Aircraft In-Flight for Refueling or Other Purposes," and U.S. 
Pat. No. 3,917,196 to Pond et al., for "Apparatus Suitable for Use in 
Orienting Aircraft Flight for Refueling or Other Purposes," both relate to 
a system capable of measuring both receiver aircraft and refueling boom 
locations relative to the tanker. This information is used either to 
automatically orient the two aircraft and the boom and nozzle or to 
generate displays suitable for both pilots in order to ease the refueling 
process. 
U.S. Pat. No. 3,285,544 to Chope et al. for "Mid-Air Refueling System," 
relates to a system in which a nuclear radiation source is mounted on the 
probe of the receiving aircraft and a directional sensitive detector is 
mounted on the tanker, enabling the measurement of the relative elevation 
and azimuth of the drogue to the probe. The above measurements are used 
mainly to generate displays for the pilots of the aircraft for easier 
engagement, although the possibility of providing drogue-steering 
capability by means of an aerodynamic rigid rudder and stabilizer attached 
to the drogue is mentioned. 
SUMMARY OF THE INVENTION 
The present invention overcomes the problem of drogue instability by 
providing a system with means for both drogue steering and drogue motion 
measurement, thus enabling either stabilization of the drogue or, 
alternatively, fully automatic hookup. 
Specifically, in accordance with the present invention, a plurality of 
miniature, pressurized gas thrusters are mounted, preferably equally 
spaced, on the perimeter of the drogue in such way that activation of any 
thruster generates a force in a direction perpendicular to the fuel outlet 
nozzle. In principle, four thrusters are sufficient. Two are required to 
control the drogue in up-and-down motion and two in the sideways motion. 
The position of the drogue is measured by electrooptical position-sensing 
devices. 
The measurement system consists of a plurality of light sources (LS) such 
as Light Emitting Diodes (LEDs) or Laser Diodes (LDs) mounted at a certain 
distance from the end of the receiving fuel probe, a plurality of sensors 
each comprising a lens and a position-measuring photodetector that is 
mounted on the perimeter of the drogue with its sensitive area directed 
toward the receiving aircraft, and a processing electronic circuit for 
calculating the position of the light-source assembly relative to the 
detector assembly and computing the control commands to the thrusters. 
In accordance with one preferred form of the invention, the measured 
position of the drogue relative to the probe of the receiving aircraft is 
used to determine the control commands to the thrusters so that the drogue 
will track the position of the probe in order to achieve automatic hookup. 
In such an arrangement the pilot of the receiver aircraft is required to 
approach the vicinity of the tanker aircraft; once the two aircraft have 
closed to a certain distance, the automatic control system of the drogue 
is activated and guides the drogue until contact with the probe has been 
achieved. 
In accordance with an alternative form of the invention, measurements are 
made of the drogue position relative to the tanker aircraft, either belly 
or wings, and are used to control the motion of the drogue so as to 
stabilize or, equivalently, to minimize the motion of the drogue relative 
to the tanker aircraft. In such an arrangement, the pilot of the receiver 
aircraft is required to track only the tanker and not the drogue, which is 
much more susceptible to wind gusts and turbulence.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 serves to illustrate the principle of the invention. It shows the 
drogue 10 having four thrusters 20 and 30 mounted on the perimeter of the 
drogue with their nozzles directed perpendicular to the axis of symmetry 
of the drogue. Thrusters 20 are used to generate forces in the up-down 
directions while thrusters 30 are used to generate forces in the 
left-right directions. 
In accordance with the invention, the thrusters are energized by 
pressurized air supplied from a reservoir tank 92 located in the tanker 
belly (see FIG. 5) via supply lines 40. The reservoir tank volume is very 
large compared to the airflow through the thrusters during operation. 
Thus, the pressure in the tank and supply lines is for all practical 
purposes constant; therefore, no pressure build-up occurs during 
activation of the thrusters, and their reaction to commands is almost 
instantaneous. In cases where the refueling source is a wing-mounted pod 
in which there will not be enough room for such a reservoir, a 
propeller-driven compressor, which is employed in such systems for 
generation of the air pressure that is needed for the fuel flow, can be 
used to generate such air supply. Each thruster is controlled 
independently by electrically activating a solenoid-operated valve in each 
thruster. If desired, any type of nonflammable gas can be used instead of 
the pressurized air. 
Also mounted on the perimeter of the drogue are electrooptical sensors 50 
and 60, each comprising a miniature lens 52, 54 and a position-measuring 
photodetector 51, 53 (see FIG. 2). Each one of the sensors detects a light 
source 70 mounted on the receiver aircraft probe at a certain distance 
from the end of the probe and measures the two-dimensional position of the 
image of the light source from the center of the sensor. The light source 
70 consists of a single miniature light source or a ring of several 
miniature light sources (LS) such as LEDs or LDs distributed over the 
perimeter of the probe for better angular coverage. The light source can 
be pulsed for better identification by the system and concealment from a 
foe. 
It may be desirable to place a second ring of light sources 80 at some 
distance behind the first one 70 in order to establish an axis relative to 
which rotation of the drogue can be measured and, if desired, altered. In 
order to discriminate between the two light-source assemblies, it is 
possible to pulse the LS assemblies at two different frequencies so that 
the corresponding signals can be demultiplexed; the detectors will provide 
a simultaneous measurement of both light sources, which is sufficient to 
deduce the complete spatial position and orientation of the probe relative 
to the drogue. 
The proposed arrangement enables the measurement of the lateral and 
vertical displacements of the drogue relative to the probe as well as 
their relative distance, since it provides a stereoscopic image of the 
light source on the probe, as illustrated in FIG. 2. Shown in this figure 
are two beams of light 71 and 72 radiating from &he LED 70. These beams 
are collected by the lenses 52 and 54 and are projected onto the two 
position-sensing devices (PSDs) 51 and 53 at distances Y.sub.1 and Y.sub.2 
from the centers of the two detectors, respectively. These distances are 
proportional to the distance R of the LED from the sensor and to the 
displacement S of the LED from the center of the sensor. Two sensors 
(e.g., the pair 50 or pair 60 in FIG. 1) are, in principle, sufficient for 
the determination of the range and the lateral and vertical displacements. 
However, four sensors are proposed for robustness, though three sensors 
are the minimum necessary for redundancy. Four sensors have the advantage 
of easier implementation because of symmetries. 
The concept of the measurement system is depicted in FIG. 1, showing two 
pairs (50 and 60) of sensors mounted along the drogue periphery and light 
sources mounted on the probe. In each pair of sensors, each of the sensors 
contains a lens and a position-sensing device (PSD). The lens forms a spot 
of light on the surface of the PSD, corresponding to the image of the 
light source (FIG. 2), and the PSD 82 (FIG. 3) outputs four photocurrents 
I.sub.x1 83, I.sub.x2 84, I.sub.y1 85, and I.sub.y2 86 proportional to the 
two-dimensional distance of the centroid of the light spot from the edges 
of the device (FIG. 2). The coordinates X,Y of the center of the spot 
relative to the center of the detector are then computed by an electronic 
circuit that is composed of two amplifiers, one for subtraction 87 and one 
for summation 88, and a divider, thus yielding with the normalized 
differences of the appropriate pairs of current: 
EQU X=(I.sub.x1 -I.sub.x2)/(I.sub.x1 +I.sub.x2) 
EQU Y=(I.sub.y1 -I.sub.y2)/(I.sub.y1 +I.sub.y2) 
The vertical displacement S of the LEDs from the center of the drogue is 
obtained by: 
##EQU1## 
where Y.sub.1 and Y.sub.2 are the displacements of the center of the spot 
on the lower and upper detectors, respectively. 
Similarly, the lateral displacement (not shown in FIG. 2) is obtained by 
replacing Y.sub.1 and Y.sub.2 of equation (2) by X.sub.1 and X.sub.2, 
respectively. 
The distance of the probe from the drogue is obtained by: 
##EQU2## 
where f is the focal length Of the lens 52 (or 54) to the PSD 51 (or 53). 
In a similar manner, the values of R, S and the lateral displacement can be 
obtained also from the second pair of sensors. 
In principle, two sensors mounted on the drogue are sufficient to give both 
the off-axis displacement of the probe and the probe's distance from the 
drogue with sufficient accuracy. Three or four sensors may be mounted in 
an actual system in order to provide redundancy in failure situations and, 
thus, increase system robustness. 
If the direction of the probe's axis (rather than only the position of its 
tip) is also required, a second light source 80 (another ring of infrared 
LEDs) may be mounted at some distance from the first one 70. These LEDs 80 
will be pulsed at a different frequency from the first ones 70 so that the 
corresponding photocurrents can be demultiplexed and the detectors will 
provide a simultaneous measurement of both light sources, and that is 
sufficient to deduce the complete spatial position and orientation of the 
probe relative to the drogue. 
The measurements of the electrooptical sensors are used to determine which 
thruster on the drogue is to be activated at any instant, the activation 
time and duration are chosen such that the relative distance and speed 
between the probe and the drogue are minimized and the relative 
displacement between the probe and drogue is kept smaller than the inner 
radius of the drogue opening. 
The signal S is used by the controller to activate the thrusters (FIG. 4). 
The controller logic determines which thruster is to be activated at a 
given instant (upper or lower) and the duration of the activation. The 
same logic applies for the lateral sensors and thrusters. The block 
diagram of the complete control system is shown in FIG. 4. It shows the 
pair of sensors: upper and lower 50, which detect the relative position 
(E) between the probe (h.sub.p) and the drogue (h.sub.d) and outputs two 
signals Y.sub.1 and Y.sub.2, which are processed by a digital or analog 
computer 62 in which Eqs. (2) and (3) are implemented, yielding the 
signals S and R, where S and R are, respectively, the vertical and range 
displacements between the probe and drogue. A control scheme 63, also 
implemented in the computer, utilizes the signals S and R to determine 
which thruster to activate (upper or lower), the activation instant, and 
the duration of the thrust pulse. 
A similar control scheme which utilizes the two signals X.sub.1 and X.sub.2 
is implemented to control the lateral pair 30 of thrusters. The thrusts 
generated by the thrusters 20 and 30 affect, respectively, the vertical 
and lateral motions of the drogue 10. 
The signal R is used to obtain the range between the probe and the drogue. 
The range information is utilized in deciding when to activate the 
stabilization process in order to prevent expenditure of pressurized air 
when the receiving aircraft is far from the fueling tanker. 
In the preferred configuration shown in FIG. 1, the PSDs 50 and 60 are 
PIN-DL10 of United Detector Technology, having an effective sensing area 
of 10.times.10 mm.sup.2. Each PSD is equipped with a collecting lens 
having a focal length of 10 mm. The light sources 70, which are mounted on 
the probe of the receiving aircraft, are either four SDL-5400 laser diodes 
having 50 MW power, at a wavelength of approximately 830 nanometers, and 
operating in continuous wave (CW) mode, or alternatively, four Light 
Emitting Diodes (LED) type Philips CQY90A having radiation power of 21 MW 
at a wavelength of 930 nanometers. 
In an alternative configuration, the PSDs are substituted with 
Four-Quadrant type photodetectors. These are Centronic QD 100-3 detectors 
having an active area with a diameter of 11 mm. In this arrangement, the 
collecting lens is defocused to ensure a light spot large enough to be 
sensed by all four quadrants simultaneously. 
In another alternative arrangement, the photodetectors are chosen to be CCD 
Microcameras type Panasonic GP-MS112, which have a small head size (2/3 
inch in diameter and 1 7/16 inches long) and resolution of 682 H.times.492 
V pixels. The cameras are connected by a cable to their control unit, 
which is placed in the aircraft's belly. The images of each pair of 
corresponding cameras (lateral or longitudinal) are merged using video 
mixers, and the resulting image is digitized at a rate of 30Hz and stored 
in a frame grabber (PcVision Plus of Imaging Technology, Inc.). A micro 
computer (PC AT type computer) is used to calculate, from the digitized 
images, the deviations of the probe from the drogue and the required 
control commands to the thrusters in order to achieve the desired 
automatic hook-up. 
The operational procedure using the proposed system is as follows: the 
pilot of the receiving aircraft approaches the tanker using a separate 
navigation system such as the Global Positioning Satellite system (GPS) or 
Inertial Navigation System (INS). Once the rendezvous has been made, and 
the distance between the two aircraft is on the order of 10 meters or 
less, the automatic control system of the drogue is activated by 
commanding the thrusters so as to cause the drogue to track the motion of 
the receiving probe and to compensate for the drogue's own motion due to 
turbulence and wind gust. Thus, the pilot of the receiving aircraft is not 
required to track the drogue; he simply flies the aircraft straight ahead. 
In an alternative configuration illustrated in FIG. 5, the LS are mounted 
on the fuselage 90 or wing of the refueling tanker or aircraft, and the 
sensors assembly is mounted on the rear side of the drogue. In this 
arrangement, the drogue is stabilized relative to the refueling tanker or 
aircraft. This method might be preferable for multiple aircraft refueling, 
since it guarantees adequate separation between the fuel-receiving 
aircraft, thus minimizing hazardous risks arising from the proximity of 
the aircraft. Also, this configuration might be suitable for refueling 
aircraft that have not been equipped with LS on their probes. 
Although, in all the preferred embodiments of the invention described 
above, the electrooptical sensor utilizes a position-sensing device, these 
designs can be implemented equivalently with a different area sensor such 
as a four-quadrant sensor or a charge coupled device (CCD) sensor. 
Furthermore, while the preferred embodiments utilize four thrusters to 
generate thrust force to the drogue, it should be understood that a 
minimum of three such thrusters are necessary to generate forces in all 
directions. More than four thrusters could also be used. 
While preferred embodiments of the invention have been illustrated and 
described, it will be appreciated by those skilled in the art, and others, 
that various changes can be made therein without departing from the spirit 
and scope of the invention. Some of these modifications have briefly been 
discussed above. Other modifications include different implementations of 
the electronic subsystems for interpreting the measurement information, 
different implementations of the mechanical subsystems for operating the 
thrusters, and extra and/or different locations of thrusters on the drogue 
.