{This report has been converted to text using OCR}
81-2289
This paper is declared a work of the U.S. Government and therefore is in
the public domain.
THE
ELECTRONIC TERRAIN MAP
-
A NEW AVIONICS INTEGRATOR -
D.
M. Small, Project Engineer
Avionics
Laboratory
Wright-Patterson
Air Force Base, Ohio
ABSTRACT
This
paper will discuss the concept of a digital Electronic Terrain Map (ETM) and its
associated technologies. Further, this paper discusses the ETM as an integrating
element in a modern avionics system which can enhance an airborne mission
capability against sophisticated threats, while potentially reducing the
constraints imposed by night and adverse weather. An existing brassboard map and
projected implementations of the ETM technology will be described in this paper
with the application being in the context of a low altitude attack
mission.
The
Avionics Laboratory at Wright-Patterson Air Force Base is developing the ETM and
it's associated technologies described in this paper. A discussion of the
features (including typical specifications) of an ETM and the impact of adding
an ETM to an avionics system will be included. The resulting avionics system, by
containing an ETM, may improve the avionic subsystem performance and provide
better utilization of the aircraft in the areas of navigation; terrain
following/avoidance; threat avoidance, analysis, warning and display; terrain
masking; weapon delivery; and route planning.
INTRODUCTION
Currently,
the Air Force has in the inventory paper and film map systems, which were
developed to support the high and level flight environment. These maps were an
effective means of tapping the vast files of information stored in the Defense
Mapping Agency (DMA) data base, when the crew had time to study and interpret
them (in fact, much of their value was actually obtained from pre-flight mission
preparations). Interviews with pilots indicate that paper maps are less useful
for low altitude flights. Film maps with CRT annotation are somewhat better, but
still have a fundamental limitation in that it takes an operator to access any
information. That is, it is not possible to transfer information directly from
the data base to any other avionics system when it is stored on paper or film
maps in what is essentially an analog form.
The
map reading process is a demanding task that can be simplified by using a
digital map subsystem which accesses the information needed and presents it in a
form which can be easily interpreted. At low altitude, and with a line of sight
limited to the next ridge line, it's very difficult to interpret standard paper
maps, which are presented as a vertical projection of a large area. An
electronic map subsystem can generate perspective scenes, which are essentially
computer generated images of the surrounding area, and an electronic map should
be much easier to interpret. In addition, essential information from the map
data base can be placed on the pilots Head Up Display, reducing the need for
head down operations.
The
Air Force is in transition from yesterday's high altitude level flight situation
to tomorrow's low altitude, day and night (unconstrained) operations. Increased
pilot workload and survivability are two of the generally recognized problems
associated with low altitude operations. Pilot workload is always a serious
consideration, and in the case of single seat aircraft, it is now critical. Ways
must be found to reduce the workload in the single place aircraft by integrating
the many sources of information.
Added
to the low altitude requirement is the need for improved survivability and the
ability to penetrate heavily defended areas. In response to this, missions are
now increasingly concerned with low altitude flight, threat avoidance, and the
reduction of emission profiles to counter the very efficient, mobile, and
prolific ground defenses that enemy forces may deploy.
BACKGROUND
Recognizing
the need for a digital map system in 1976, Avionics Laboratory started work on
an all electronic map for aircraft display applications with the in-house
effort, "Electronic Terrain Map System (ETMS)". Follow-on projects in this area
were the "Airborne
Electronic Terrain Map Applications Study", completed in October 1979, and the
design and fabrication of an "Airborne Electronic Terrain Map System (AETMS)"
started in May 1980. The AETMS effort demonstrates the state-of-the-art in
electronic map technology. Hence, the basic exploratory work necessary to
develop a successful map system has already been initiated to provide the Air
Force with this new capability for the tactical and strategic
environment.
Over
the past four years, the Avionics Laboratory has been developing the
technologies required for future ETMs. Typical technologies/attributes, obtained
from the AETMS program, for the ETMs are: large mass memory (probably bubble)
that utilizes DMA data to store up to a quarter million square miles of terrain
and cultural data; high speed processors (70 operations a micro second) to
support the real-time display update requirement; and display generation and
pictorial display hardware and software.
WHAT
IS NEEDED
The
delivery of the AETMS completes the first step in developing operational ETM
hardware for future avionics systems. Experiments with AETMS, by demonstrating
the feasibility of providing on-board storage and retrieval of digitized
cartographic data, should establish the needs of future ETMs. Also, these
experiments, by exploring the needs of future avionics systems, will establish
the advantages of using ETMs. The needs of future avionics systems, that the
ETMs may be required to simultaneously fulfill, are: real-time updates,
displaying out-the-window true perspective and contour scenes and providing
digital values. Future ETMs should have the capability of accepting extensive
real-time updates (i.e. route planning and weapon delivery). Future ETMs should
provide terrain data for out-the-window scenes with variable shading, sun angle
and scales. These scenes will not be restricted to one of DMA's standard map
scales. Future ETMs should provide digital values of elevation for terrain
following, threat avoidance and scene correlation
applications.
Future
aircraft avionics suites should be constructed in a form suitable for laboratory
and flight testing so that the concepts proposed can be fully evaluated. An ETM
subsystem should be developed including the flyable map hardware, and associated
software, and the data base necessary for laboratory and flight tests. Also,
support hardware and software necessary to support the ETM subsystem and its
data base will be required. The design of the AETMS brassboard provides an
architecture which may be used as a baseline for comparative purposes.
Additionally, results of on-going in-house efforts in the Avionics Laboratory
may be used to establish performance guidelines. The planned Avionics
Laboratory's map simulation capability may also be used to evaluate proposed
algorithms. Future ETMs should include efforts for hardware, software, data
base, and support. The data base and support equipment tasks should include the
development of software, equipment and procedures to convert the DMA data and
allow for data base manipulation to meet the scenario requirements. An interface
should be developed which supports the exchange of terrain and cultural data
with other members of the avionics suite at the rates necessary to support
simultaneous operations of all avionics subsystems. Special purpose processing
equipment necessary to support non-display related applications of the ETM
should be developed. The resultant avionics suite should be capable of being
installed, on a pallet, for flight testing in a bomber, fighter or cargo
aircraft.
The
ETM should be integrated into an aircraft avionics system suite. Advanced
concepts for threat avoidance, navigation, TF/TA and weapon delivery subsystems
using the ETM should be incorporated into the design of future avionics
suites.
FUTURE
AIRCRAFT SYSTEM
The
purpose of adding an ETM subsystem to a future avionics suite is to provide map
data and displays that can be interfaced with other subsystems to improve the
performance of the terrain following/terrain avoidance (TF/TA), threat avoidance
and navigation avionics subsystems. The requirement for the simultaneous
exchange of processed map data by three or four avionics subsystems will be the
most difficult objective and important feature of the ETM. Development and
incorporation of the advanced ETM concepts and technologies will be required to
augment future threat avoidance, navigation, TF/TA, and weapon delivery avionics
subsystems. Applications/examples of using these ETM concepts and/or
technologies and the utilization of an ETM subsystem as a source of information
follows.
TF/TA
The
first example will be the automatic TF/TA avionics subsystem. Our existing
automatic TF subsystems operate using only active sensors as sources of terrain
profile information
(i.e. radar). This makes the subsystem totally dependent on the limitations of
this single information source. In case of radar, range is limited to line of
sight. Absolutely no information is available beyond line of sight. This forces
the TF subsystem to provide unnecessarily large clearances over ridges to avoid
the following peak which may or may not be imminent. Further, the TF subsystem
must radiate on an almost continuous basis to provide a continuous terrain
profile. Consequently detection and jamming are TF subsystem vulnerabilities. A
digital terrain map could provide a second source of information to the TF
flight command processing subsystem and the use of the map could serve as a
backup in case of radar failures or jamming. The ETM could provide information
concerning beyond line of sight conditions, enlarge the total field of view
scanned for turning, and avoid the reduction of the duty cycle of the radar
emission. In fact, this ability to scan the terrain to the side without turning
and looking beyond the line of sight makes it possible for the first time to
consider true automation of the TA function. Because of limitations in the
existing DMA data base, the approach should be cautious and an active sensor
will be needed to make absolute clearance measurements. None the less, the
application of stored data, to the TF/TA problem can potentially have tremendous
impact on Air Force capabilities in the low altitude flight
mission.
THREAT
AVOIDANCE
The
second example will be the threat avoidance avionics subsystem. The whole
purpose of low altitude missions is to reduce the probability of detection and
attrition. If the threat avoidance problem is solved without regard to the
location and lethal range of threats, the resultant path may place the aircraft
in greater jeopardy than before. Terrain masking and launch dynamics limitations
must be exploited to the fullest. Careful selection of the aircraft's routes to
the target may be done by the crew or automatically. In either case, a digital
map is required to provide the terrain information and the position of the
threats identified by the avionics system. Pre-mission planning can provide a
starting point for this analysis, but the dynamics of the threat assessment
makes it essential that the crew be able to redefine the mission as new
information is received from command and control functions or via the aircraft's
own suite of threat defense sensors.
NAVIGATION
The
third example will be the navigation avionics subsystem. With the addition of a
correlator to the avionic suite and using the on-board sensors together with the
ETM, navigation can be accomplished. Also, by displaying the ridge lines derived
from stored terrain data on the head up display, passive navigation is possible.
Hence, the ETM could also improve the utilization of the navigation
subsystem.
TYPICAL
ETM SPECIFICATIONS
Typical
ETM specifications for interfacing/connecting the ETM subsystem to the other
avionics subsystems, including both display and non-display map uses, will be
discussed. The specifications for a future ETM may be divided into the following
functions:
REGIONAL
MEMORY
The
Regional Memory (RM) is the portion of the ETM memory where the terrain and
cultural data is stored. The RM should have the following
characteristics:
1. Storage of up to 400 meg
bits.
2. Storage of up to 500 mi. sq. of terrain
and cultural data.
3. Modularity; that is, memory
configurations packages ranging from 50000 to 400 meg
bits.
4.
Variable format; that is, the word size memory packing should be
variable.
5.
The RM should contain both terrain and cultural (cartographic)
data.
6.
The RM should be capable of random access.
7.
The RM should be partitioned to allow for the accessing of detail data; that is,
"zoom".
8.
The RM should be capable of simultaneous read and write
operations.
DATA
PROCESSOR
The
Data Processor (DP) calculates the elevation values and controls the ETM
operations. The general purpose low-speed data processing and control operations
should be performed by an Embedded Avionics Processor. The DP may perform the
following functions:
1.
Control both inputting and outputting of messages.
2.
Calculate the elevation values.
3.
Control the simultaneous data exchanged with the other avionics subsystems. The
ETM will be required to provide simultaneous terrain and cultural data for up to
four different subsystems. Each avionics subsystem (TF/TA, threat analysis,
navigation, weapon delivery, display, etc) will require individual scales and
rates.
4.
The DP should be dynamically programmable; that is, where appropriate, use one
microprocessor to perform many functions.
5.
The processing speed required for the calculation of the elevation values is
aircraft dependent. However, for the simultaneous operation, the elevation
values calculated at 1 million elevation points per second or faster may be
required.
6.
The DP should contain separate memory from the RM. This separate memory may
contain the main program, subprograms (for the programmable microprocessors),
special features and other data.
DISPLAY
GENERATOR
The
ETM may be connected to a Display Generator (DG). If so, the characteristics
required for the DG are:
1.
Special Displays.
a. Threat Display.
b.
TF/TA Display.
c.
Navigation Display.
d.
Weapon Delivery Display.
e.
Approach Plates.
2.
Display formats.
a.
Plan view.
b.
True Perspective.
3.
Display Types.
a.
Raster (standard TV).
b.
Stroke (heads up display - HUD).
SUPPORT
EQUIPMENT
The
support equipment (SE) required for the ETM should be used for development,
testing, demonstration, and maintenance of the ETM. The SE should perform the
following functions.
1.
Stand alone operation and testing.
2.
Navigation Simulation to exercise the ETM subsystem.
3.
Data base formation (loading and reformatting DMA data).
SUMMARY
It
is now possible to integrate external data from Intelligence, C3
sources, threat and target sensors to optimize total situation information
presented to the pilot, while simultaneously providing timely cultural and
terrain (cartographic) data to other on-board systems. The avionics system of
the future, by containing an advanced version of the ETM, may potentially
improve avionic subsystem performance and provide better utilization of the
aircraft in the areas of navigation; terrain following/avoidance; threat
detection, warning and display; terrain masking; weapon delivery; and route
planning. Potential ETM applications are the integration of map data base with
(1) radar and inertial data for terrain correlation and navigation; (2) GPS and
inertial navigation data for completely passive terrain following; and (3)
navigation data, threat sensor intelligence and C3 data for threat
avoidance with terrain following. Each application should draw upon the ETM
subsystem as a source of information - A New Avionics
Integrator.