Technical Report
Human Machine Interface for Radar Systems
November 14, 2002
Revision No. 3
Prepared by:
Lav Varshney
Syracuse Research Corporation
6225 Running Ridge Road
North Syracuse, NY 13212-2509
Introduction
The purposes of radar systems are to detect and locate targets. The information that is
gathered by the radar system must be communicated to the human user of the system for
analysis, interpretation, and decision-making. The user must also communicate to the radar
system in order to issue commands, change settings, and perform other operations. The need to
communicate information necessitates an effective Human-Machine Interface (HMI).
HMI
design for radar systems takes human factors into account, but is often constrained by
technological limitations that prevent optimal interfaces from being implemented. The main
focus of this paper will be on the machine-to-man communication path. This paper will discuss
some commonly used radar system displays, some of the main principles in Human Factors
Engineering (HFE), and the application of these principles in the development of modern radar
system interfaces.
Radar Information
Radar systems can determine many different characteristics of targets. The primary
information that is produced by radar systems is target location. A radar display must be able to
communicate the target’s position effectively in three-dimensional space. Three coordinates are
required to specify the location of a target in three-dimensional space. Generally, spherical
coordinates are used, where range and angles of azimuth and elevation determine position.
Range is defined as the radial distance from the radar to the target. Range always has a positive
value. Azimuth angle, or bearing, is the angle from the radar to the target measured from a fixed
north point along the arc of the horizon. The azimuth angle ranges from north (0°), through east
(90°), south (180°), and west (270°), up to north again (360°). Elevation angle is determined by
first finding the point on the horizon with the same azimuth angle as the target. The angle from
that point to the target is the elevation angle. The elevation angle coordinate ranges from the
horizon (0°) to the zenith (90°); sometimes the range is extended down to the nadir (-90°).
In addition to the fundamental operation of locating targets, other information can also be
determined by radar systems. Radial velocity can be determined using the Doppler effect.
Targets can be classified according to velocity, size, and other factors. Targets can be tracked
using probabilistic and signal processing techniques.
This extra information, if determined,
should also be presented to the user by the radar system in an easy to use fashion.
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Analog Radar System Displays
The cathode ray tube (CRT) has historically been the main device used for radar displays.
A CRT consists of an electron gun that projects a narrow beam of electrons along the axis of the
tube, a means of deflecting the beam, and a fluorescent screen to indicate the deflection of the
beam. There are two basic kinds of CRT displays, deflection-modulated CRT and intensitymodulated CRT. In deflection-modulated CRT, deflection of the electron beam indicates targets,
whereas in intensity-modulated CRT, target indication is performed by intensifying the electron
beam so that a luminous spot is presented on the face of the CRT. In general, the circuitry for
deflection-modulated CRT is simpler and less susceptible to noise or interference. Intensitymodulated displays have the advantage of presenting data in an easy to interpret format. A
number of different CRT screens are used for radar applications, differing in decay time and
persistence. Persistence is the amount of time that a blip, the indication of a target, remains on
the screen. Different phosphors are used to provide differing persistence and colors. The decay
of the blip on the CRT should be long enough to allow the operator to see all target detections,
but not so long that it interferes with information from the next scan. Color CRTs are often used
to provide classification information.
The three most common types of CRT displays are the A-scope, the Range-Height
Indicator (RHI), and the Plan Position Indicator (PPI). The A-scope is a defection-modulated
CRT display in which the vertical deflection is proportional to target echo amplitude and the
horizontal coordinate is proportional to range. Such a display is normally used in weapons
control radar systems. The azimuth and elevation angles are presented as dial or digital readouts
that correspond to the actual physical position of the antenna. The ranges of individual targets
on an A-scope are usually determined by using a movable range gate or step that is superimposed
on the sweep. The A-scope is shown in Figure 1.
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Figure 1: A-Scope
The RHI is an intensity-modulated display with height (altitude) as the vertical axis and
range as the horizontal axis. This display is often used with height finding search radars to
obtain altitude information. The sweep originates in the lower left side of the scope. It moves
across the scope, to the right, at an angle that is the same as the angle of transmission of the
height-finding radar. The bottom horizontal line indicates the line-of-sight to the horizon. The
area directly overhead is straight up the left side of the scope. Target echoes are displayed on the
RHI as blips. The operator determines altitude by adjusting a movable height line to the point
where it bisects the center of the blip. Target height is then read directly from an altitude dial or
digital readout. Vertical range markers are also provided to estimate target range. The RHI is
shown in Figure 2.
Figure 2. RHI Display
The PPI, also called the P-Scope, is by far the most commonly used radar display. It is an
intensity-modulated circular display on which echo signals are shown in plan position with range
and azimuth angle displayed in polar coordinates. The origin of the polar coordinates is at the
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location of the radar, and is normally located at the center of the display. The PPI uses a radial
sweep pivoting around the center. The result is a map-like display of the area covered by the
radar beam. Azimuth angle to the target is indicated by the target's angular position in relation to
a line extending vertically from the sweep origin to the top of the scope. The top of the scope is
usually true north. Figure 3 shows the PPI display.
Figure 3. PPI Display
Digital Radar System Displays
Synthetic video displays, which use digital computers to generate graphics, have
superseded plain CRT displays within the past few decades. At first, these synthetic displays
merely copied the existing CRT display format. The advantages gained included better visibility
in bright ambient light conditions, and greater refresh rates to facilitate target tracking. The use
of a computer to generate the graphics and control the display offers flexibility in such things as
range scales, off center display, enlargement of selected areas, physical map outlines, and grid
displays. More recent displays have incorporated many of these features, to enhance usability.
As an example, the PPI display from the AN/PPS-5 battlefield surveillance radar is shown in
Figure 4. This display allows the user to change many of the flexible parameters mentioned.
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Figure 4. AN/PPS-5 PPI Display
HFE strategies have also been incorporated into many radar displays, as technical
advances in digital radar displays have allowed.
Human Factors Engineering
HFE deals with knowledge concerning the characteristics of human beings that are
applicable to the design of systems and devices of all kinds. The goal is to achieve compatibility
in the design of interactive systems of people, machines, and environments to ensure their
effectiveness, safety, and ease of performance. There are many general principles in HMI design
that have been defined by HFE research.
There should always be feedback to the user from the machine when a command is
issued or a button pushed. For mechanical devices this may be the sound of a motor starting, for
electronic devices it may be a light turning on, and for a software program it may be a radio
button depressing.
If a device gives no feedback, the user will generally assume that the
command has not been received and will try to reissue the command.
A machine should have predictable behavior for given input commands. For example, a
single button should not perform different actions in different situations. In addition, a machine
should have some transparency, so that the user can easily ascertain what is going on beneath the
surface. A mechanical device may be designed so that the movements of important parts are
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visible. In electronic devices there may not be any mechanical parts to show, so the state of
internal actions must be visualized with text or drawings. For example, a text field showing the
transmission frequency in a frequency agile radar would allow the user to know the state of the
frequency synthesizer.
Another guiding principle in HMI design is not to interrupt the user.
violation of this design principle occurs with pop-up windows.
A common
Pop-up windows break a
computer's predictability because the unexpected occurrence of the message box radically
changes what the keys do; violates the principle that the user should be in control; and breaks the
user's train of thought, which can be quite annoying.
Machine design should incorporate some tolerance of errors on the part of users. For
example, if a radar operator enters a frequency out of range for a radar, the radar might go to the
nearest allowable frequency, or perhaps stay at the frequency it was at before the change
command was issued.
HFE also teaches that computer screens should be bright enough that they are easy to
read, but not so bright that they irritate the eyes. The refresh rate should be sufficient so that
there is no flickering. A screen should be placed so that there isn't too much light from behind,
to avoid glare and reflections. Dark text on a light background allows ease in distinguishing
small details, but a very bright background is fatiguing to the eyes. Light text on a dark
background is more comfortable to read.
Alarms are often built into HMIs to inform the user of an unusual event or danger. An
alarm has no value if it sounds so often that people lose confidence in it, hence the false alarm
rate should be kept at a minimum. If audio-based, the volume of the alarm should be high
enough that everybody can hear it, but never so high that it is painful, causes panic, or prevents
talking. Surveillance staffs that have nothing to do but wait for alarms get dull and demoralized
if they have nothing to do. So dull, in fact, that they fail to react promptly and adequately when
an alarm finally occurs. Their job must be organized in such a way that they have plenty of other
work to do when there are no alarms, and should participate in regular drills to rehearse what to
do when an alarm does occur.
One of the long-term goals in HMI design has been to take the natural means that humans
employ to communicate, and incorporate them into the interface design. Almost any natural
human communication involves multiple, concurrent modes of communications. For example in
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a conversation, the words themselves are not the only form of communication. The tone of a
person’s voice, the expression on a person’s face, and the movement of a person’s arms all
contribute to the communication. There are five main human senses: vision, hearing, touch,
smell, and taste. The main human actuators are the body, hand, face, and voice. To approach the
goal of natural man-machine communications, communication channels must be formed through
many of the human senses and actuators. In terms of military radar design, this is often very
difficult, due to the adverse conditions present. Combination video and audio displays are the
one use of multisensory interaction that is common. Targets are presented as audio tones
corresponding to Doppler frequency, and targets are shown on video displays such as PPI.
Advanced Radar System Displays
As computer workstations continue to increase in capability of displaying more
information, the necessity for simple intuitive Graphical User Interfaces (GUI) for computer
monitors becomes paramount. Many display design enhancements are in response to the fact
that an overwhelming quantity of data must be assimilated by personnel making crucial life and
death decisions within a short time.
Touch screens can reduce HMI complexity and minimize response time for human
interaction. Although touch screens have these advantages, humans tire when required to touch a
screen for hours, so a combination of touch screen and mouse is considered optimal. The use of
color coding to alert operators of events has been shown to reduce response time. An example of
this for radar systems would be to change background color when the system becomes not ready,
so that it becomes readily apparent. Artificially Intelligent decision aids have also proved useful.
These aids would help the user decide a course of action for specific events.
Software-based displays have also facilitated the incorporation of various other useful
information elements. An example is geographical information in the form of map overlays and
cursor location.
Another example is the display of tracking algorithm-determined tracks.
Symbolic representation, based on target classification is often used to present targets. Figure 5
shows a bistatic radar PPI display with map overlay, and tracking.
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Figure 5: PPI Display with Map Overlay and Tracking for Bistatic Radar
Conclusion
The introduction of PPI during World War II gave the Allies a large advantage in radar
ability over the Axis powers and contributed to the Allied victory. This historical fact clearly
demonstrates the importance of display systems for radar systems, since the advantage was
provided by a better display system, not better detection capability.
As technological
developments allow, steps will be taken to improve radar displays. The interface between the
radar system and the operator will greatly improve as HFE techniques are applied. Modern
displays have started to incorporate these designs, and will possibly be the advantage in future
military conflicts.
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List of Acronyms
CRT
Cathode Ray Tube
GUI
Graphical User Interface
HFE
Human Factors Engineering
HMI
Human Machine Interface
PPI
Plan Position Indicator
RHI
Range Height Indicator
Figures
Figures 1, 2, 3: Wolff, Christian, “Radar Indicators,” 2001. [Online]. Available:
http://mitglied.lycos.de/radargrundlagen/scopes/sg-en.html.
Figure 4: AN/PPS-5D Radar Set Technical Manual
Figure 5: Thomas, Daniel, Signal/Data Processing. Radar 101 Lecture Series. Syracuse
Research Corporation, Syracuse. 6 Nov. 2001.
References
[1] “About the HFES,” Human Factors and Ergonomics Society, 1995. [Online]. Available:
http://www.hfes.org/About/Menu.html.
[2] Fog, Agner, “Man-Machine Interface: Compendium on User-friendly Design,” Copenhagen
Engineering College, 2000. [Online]. Available: http://www.eit.ihk-edu.dk/subjects/mmi/.
[3] Hugill, Peter J., Global Communications Since 1844: Geopolitics and Technology. Baltimore,
Maryland: Johns Hopkins University Press, 1999.
[4] Radar Principles, United States Navy Electrical Engineering Training Series. [Online].
Available: http://www.tpub.com/neets/book18/index.htm.
[5] Reintjes, J. Francis and Godfrey T. Coate, Principles of Radar. New York: McGraw-Hill,
1952.
[6] Sharma, Rajeev, V.I. Pavlovic, T.S. Huang, “Toward Multimodal Human-Computer
Interface,” Proceedings of the IEEE, vol. 86, no. 5, pp. 853-969, May 1998.
[7] Skolnik, Merrill I., Introduction to Radar Systems. New York: McGraw-Hill, 1980.
[8] Steinberg, R.K., K.M. Pathak, “Human-Engineered Real-Time Displays for U.S. Army
Missions,” Proceedings of the Society for Information Display International Symposium, May
1994.
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