INFDMATID'N ‘RATES IN REEDTED RADAR SYSTEMS
`
`Eugene A. hechler
`The Franklin Institute Laboratories for
`Research and Develth
`Philadelphia, Pennsylvania
`
`Abstract ,
`
`the radar collects. Between the radar and the
`presentation. the information may be transmitted
`by a wide band commnication channel that trans—
`mits all the radar video information or- it maybe
`compressed by analog, digital or manual methods
`so that it can be transmitted over a much narrow—
`er comunications channel. The final choice of
`system components will be decided by the re—
`quirements of the basic problem for resolution,
`0 f-centered displays, etc.
`The basic problem
`is to remote information from various radars
`such as the ASR's at airports and the long range
`radars at other sites to a traffic control center
`such as AOEG. Previous studies indicate that the
`ASR's will be used to cover a circle of 30 miles
`radius up to 10,000 feet.
`
`The long range radars will provide low altitude
`coverage out to 30 miles radius and high altitude
`coverage out to 100 miles range. Figure 1 shows
`a suggested PPI coverage pattern for a long range
`radar in a typical enroute area.
`
`B. Air Traffic — The Data Source
`
`The source of the data for air traffic con—
`trol is the traffic itself. The number of air—
`craft within the control area determines the maxi—
`mum information content of the system. Various
`studies have been made of traffic density in busy
`areas. Predictions of future traffic densities
`are also available. Since this subject has been
`treated elsewhere, we will not treat it here.
`However, we will quote two estimates for the New
`York city area to provide an order of magnitude.
`In VFR weather peak hour aircraft movements are
`between 107 and 163 per hour.
`In 1960 the New
`York Port Authority estimates there will be 210
`
`a E
`
`MBEDDED
`
`Air traffic information collected by long
`range radars is available to the CAA at the ra-
`dar sites. This information will be remoted to
`the air route traffic control center.
`A theo—
`retical study is made here of resolving power
`and the informtion rate inherent in the air
`traffic , radar video signals, displays, and vari-
`ous transmission techniques. Air traffic pro-
`vides the basic irlfomnation and radar _video es-
`tablishes the maxim information rate for the
`system. On the other hand, a display can show
`only a limited amunt of this information. But
`if the display magnifies a small area, it can
`show all the information the radar collects. Be-
`tween the radar and the presentation, the infor—
`mation may be transmitted by a wide band communi—
`cation ohannelthat transmits all the radar video
`information or it my be compressed by analog,
`digital or normal methods so that it can be
`transmitted over a much narrower communication
`channel. The final choice of system components
`will be decided by the requirements of the basic
`problem for -resolution, off—centered displays
`and the needs of the traffic controller.
`
`120
`
`NW
`
`A theoretical study is made here of the in-
`formation rate inherent in the air traffic, ra-
`dar video signals, displays, and various trans-
`mission techniques. Air traffic provides the
`basic information and radar video establishes
`the maximum rate for the system. On the other
`hand, a display can show only a limited amount of
`this infomtion. But if the display magnifies
`a small area, it can show all the information
`
`0,0,0
`0,0,0
`0 ee
`
`SQUARE
`
`Fig. 1 - Suggested PPI coverage pattern for a long range radar.
`
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`

`movements in the peak hour in the New York areal.
`Assuming a radar with a 100 nautical mile range
`and an average aircraft speed of 200 knots, an
`average of 120 aircraft will be visible at any
`one want.
`
`C. hdar — The Data Collector
`
`The target resolution in range and azimuth
`and the maximum number of targets the radar can
`acquire can be determined from these characteris-
`tics. The range resolution is determined by the
`pulse width since Range Resolution (nautical miles)
`_ Pulse width {microsst
`°
`12.2
`Pulse width also determines the information rate
`of the radar set. The information rate of target
`reflections is the reciprocal of the pulse width.
`
`The azimuth resolution of the radar is de-
`termined by the antenna beamidth. This does
`not affect the minimum bandwidth for the system
`but it does affect the number of composite tar-
`
`It also influences
`gets acquired per unit time.
`the design of narrow band systems and their
`bandWidt‘hs-
`
`The size of the radar reflection is a func-
`tion of the lilysical size of the target and the
`characteristics of the radar set.
`In general,
`aircraft dimensions are small enough when com-
`pared with pulse width and beamidth to be con-
`sidered point sources for reflections. Since the
`duration of a reflection determines its range
`dimension, a point target will have a minimum
`length equal to the pulse width. The madman
`number of targets along any range sweep is the
`selected range divided by the pulse width.
`
`This same point target reflects as long as
`it is in the rotating beam. mm the azimuth
`dimension of the average radar target will be
`about one beamidth wide. The maximm number of
`targets at the same range is 360' divided by the
`beamwidth.
`
`The total number of radar targets that can
`be resolved in a circle of given range is: Hari-
`mum Ho. Targets -
`360' x Rangsgnautical miles!
`beamwidth 1: pulse width nautical miles .
`
`For the two radar sets being considered,
`the number of resolvable point targets and the
`maximum information rate in targets per second
`are listed in Table II.
`
`While resolution is important, the accuracy
`with which a target is located is also important
`to the traffic controller. For radar sets in
`
`The radar set gathers the data for the sys-
`1With the present air traffic densities,
`tem.
`targets are seldom close enough so that the radar
`imposes a limitation on the information available.
`The exception occurs when aircraft are at or near
`the same slant range and same azimuth but differ-
`ent altitudes. The method of collecting the data
`is fixed. Radars.locats targets by the polar co—
`ordinates of range and azimuth. The characteris-
`tics of the set determine the target resolution
`and information rate and affect the design and
`use of the other components of the system (Table
`I).
`
`121
`
`Table I
`
`RADAR CHARACTERISTICS
`
`Characteristic
`
`Pulse length, ndcrcseconds
`
`Range resolution, nautical miles
`
`Minimum bandwidth, cps
`
`Antewma rotation rate, deg. per second
`
`Azimuth beamidth, degrees
`Maximum range considered, nautical miles
`
`PRF in pulses per second
`
`Mae
`
`0.83
`
`, 0.07
`
`1.2 x 10
`to 180
`
`6
`
`3
`
`30
`1200
`
`Table II.
`
`MAXIMUM REOLVABLE NUMBER OF RADAR wears
`
`Selected Range
`{mutical miles)
`
`103
`
`30
`30
`
`Maximum flesolvabls Number of Point Targets .
`
`lungs
`
`1220
`
`366
`1.1.0
`
`Azimth
`
`Total per
`Rotation
`
`Total
`p_e_r sec.
`
`360
`
`- 360
`120
`
`Ammo
`
`132,000
`
`52.900
`
`MHOOO
`
`13,200
`
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`

`general the relative positions of targets can be
`located to at least one third of the resolution
`in both range and azimuth. The absolute ac-
`curacy of a target's location depends on the
`circuit and components of the radar set and the
`calibration of the whole system. This addition—
`al detail available from the radar information
`is an aid to the controller in vectoring air-
`craft and maintaining radar separations.
`
`10.an
`
`The way in which the radar information is
`displayed determines the information rate which
`is required for the narrow band systems.
`
`There is a circle between the origin of the
`display and the periphery that has a circumfer-
`ence such that each resolvable spot on the cir—
`cumference is equivalent to a beamidth. He can
`call this the minimum circle of resolution. Hith-
`in this circle, sweeps one spot diameter wide
`overlap. The overlap is 1M at the origin and
`zero at the circumference of the minimum circle
`of resolution. The redundancy within this circle
`can be computed as follows: The number of ele—
`ments or resolvable spots on the circumference of
`the circle is defined as - 360' e where 9 is the
`beamwidth of a scan. Elments radius - 360/6 1
`2:. The number of resolvable elements covering
`the area within a circle of this radius
`2
`2
`
`2 _
`
`'13
`
`.360
`
`"(9x23
`
`_ 32
`
`(9)
`
`i
`
`xii-rt
`
`The number of elements "painted" in this area I
`(No. sweeps) (Elements per radius)
`
`.32 _iso__m2 .1.
`1:21:
`e "e
`e
`1:27:
`
`The redundancy -
`"Painted" smte - Resolvable smte
`"Painted" Spots
`
`m2 $-3fl2l
`e
`3‘21:
`9
`1:
`
`(m2 l
`
`x21:
`
`A rule of thumb states that 150 spots can
`be resolved on the radius of a PPI display on a
`cathode ray tubez. With increasing tube size
`the spot size also increases and-resolution
`stays about the same. However, with more modern
`gun constructionJIhL additional resolution be-
`Ween 300 and W spots/radius can be expected.
`If the long range radar set has a resolution of
`.08 nautical mile and the scale chosen for a
`centered display on a 30" tube is 2 nautical
`miles per inch,
`than the maidmum sise desired for
`a. resolvable spot is 1/25 inch and the maximum
`number of spots per radius is 390. Since an AS]!
`radar has a. pulse length of .83 microseconds, its
`resolution is .07 nautical miles.
`A resolution
`of 1,30 spots per radius is required from the 30"
`display to use the maximum range resolution of
`the radar set. This spot sise is 1/29 inch in
`diameter.
`than off-centered displays are used,
`the range resolution of the display does not
`change if the range scale is not changed.
`
`
`
`9
`1
`
`1
`
`Asimth resolution for a PPI display varies
`directly with the radial distance from the center
`of the tube.
`0n the basis of 150 spots per ra-
`dius, there are 91.2 resolvable spots around the
`periphery.
`So at the periphery of the scope the
`azimuth resolution is .38'. On a scope having a
`resolution of 1.00 spots per radius, the resolu-
`tion at the periphery is .lh'. These resolutions
`are better than one beamidth for both the short
`and long range radars. As mentioned in section C,
`the azimuth accuracy of a search radar set is
`generally accepted to be 1/3 of the beamidth of
`the antenna pattern. Whether the resolution of
`the system should be good enough to present this
`accuracy needs to be determined for short range
`targets.
`In general it is better than this for
`long range targets. The accuracy with which a
`target must be located probably will need to be
`decided by operational experience.
`
`The azinIuth resolution must" be considered
`when using off-centered displays. Then the azi-
`muth scale is magnified.‘ A display 60 miles in
`diameter with the center offset 60 miles sub-
`tends an angle of approximately 53'.
`A scope
`having a resolution of LCD spots per diameter has
`a resolution of 7.5 spots per degree or 0.1!. de-
`gree between adjacent spots at the center of the
`scope. Thus the limit on azimuth resolution is
`not the off—centsred display, but rather the asi-
`muth accuracy of the radar.
`
`.LII..l-:‘
`1
`2m'5m
`2
`
`The display paints twice as many spots as are
`actually needed. This redundancy is inherent in
`the way the radar set gathers information. Thus
`there is no need to eliminate it in a normal PPI
`presentation of radar returns. However, the
`bandwidth required for narrow band relay systems
`can be reduced if this redundancy is elindnated.
`How much reduction depends on the particular
`scanning system used, such as TV or spiral to
`form either an embedded or a square pattern of
`spots,- Figure 1. Redundancy in the scanning
`pattern is considered further in section F.
`
`E. Wide Band Relay For Data Transmission5
`
`To transmit the radar video or raw beacon
`video information without distortion of the sig—
`nals requires a very wide video bandwidth. This
`bandwidth should transmit at least the fundamen—
`tal frequency corresponding to the reciprocal of
`the pulse width. Thus the minimum bandwidth re-
`quired is 1.2 141; for the ASR radar or 1.0 MC for
`the long range radar. Beacon pulses are only
`0.35 microseconds wide and require a minimum
`video bandwidth of 2.86 KC.
`
`In addition to the video information, the
`relay link must provide channels to transmit ra-
`dar and beacon trigger pulses, radar range marl-cs
`and asinmth information. By using a microwave
`
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`

`the optical system. Hhile a slot size small
`enough to provide a range resolution of 150
`spots per range weep has beerubuilt, production
`models use a range resolution of only 100 spots
`par radius. This is more satisfactory and
`j
`trouble free. These resolutions are obtained
`with a cathode ray tube which is 3" in diameter
`and on which the circular display is 2" in die-
`meter. Asimuth resolution is limited by the
`scanning rate of the optical system which reads
`the J-scope. The maximum speed at which the
`present scanning head can rotate is limited to
`36°CBPHor60RPSby-the largebsaringused.
`Thus the mafimm informtion rate of production
`Ray-fax units is 60 x 100 or 60C!) bits per second.
`Other Rayfax units having a slower seaming rate
`as low as 1.2 x 103 bits per second.‘
`
`relay system, all the information inherent in
`the radar and beacon signals is transmitted to
`the air traffic control center. No information
`is lost through encoding and decoding systems
`for the compression of the data. This allows
`radar data to be shown on magnified displays
`such as' off-centered PPI scopes without any loss
`in range or aeimuth resolution over the original
`radar picture.- It also allows the beacon rs-
`turns to be decoded most conveniently at the
`traffic control center.
`
`Hhere coding is by aircraft altitude, for
`example, decoding must be done at the center un-
`less complicated switching systems, multiple de-
`coders at the receiver, and multiple narrow baud
`relay links are furnished. This decoding re-
`quth is the most important reason for using
`a wide band microwave relay system.
`
`P. Harrow End §Etsms For Data Transmission
`
`A narrow band system is predicated on the
`basis that the radar picture contains less in-
`formation than the radar video signal and possi—
`bly nore information than the user needs. There-
`are two general types of narrow band systems:
`mmely, analog and digital. One such as the
`Bayfax is an analog system. Here a scanning
`system reads the composite radar picture rather
`than the individual pulses and transmits this
`information to the remote location where it is
`presented in time synchroniem with the scanning
`system. The other is a digital system which 1o-
`cates a target on a coordinate system by its nu-
`merical coordinates, transmits these numbers to
`the remote location, and locates the target on a
`similar coordinate system.
`‘The requirements for
`a digital system will be discussed in the next
`section.
`'
`
`Raga): Men
`
`The Bayfax system manufactured by Hallsr,
`Raymund a Brown. State College, Pa. is discussed
`below. A detailed description of the Rayfax
`system will be found in refersncu 6, 7, B and 9.
`The Rayfax- system limits the range and asi-
`muth resolution as follows. Range resolution is
`determined by the diameter of the circular trace
`on an intensity modulated J—scope and the width
`of the seaming slot. The minimum slot sine is
`determined by the light gathering qualities of
`
`123
`
`l-hllsr. Rsymnd a Brown advise that some
`simple modifications would allow the resolution
`and picture stability to be increased.
`A new
`bearing arrangement was designed for the scan-
`ning head which would allow it to rotate at
`speeds up to 167 revolutions per second with in-
`creased reliability and azimuth resolution. The
`ultimate degree of azimuth resolution would al-
`low the Rayfax presentation to mtch the accu-
`racy of the radar. For search radars this is
`about 1/3 beamidth. Tablet-(I shuts the char-
`acteristics of the radar which'are used to con-
`pute the scanning rates needed to provide this
`ultimate resolution for the Rsyfax.
`The tabulation also shows that the munber
`of Mar pulses per optical scans is 7 for the
`ASEradarandmflnyorthelongrangsr-adar.
`it once the question arises as to how many hits
`are required 'on a target to provide a sufficient
`signal for the optical system. Heller, Rsyh
`mend a Brown made some tests using a signal
`generator. Results showed that a single hit
`equivalent tons radar return from an aircraft
`at .about 50 miles range would provide a signal
`output.
`,Since multiple returns are integrat-
`edl three hits were believed to be adequate
`and ten hits would be a good margin of safety
`to provide a Rayfax signal. The resolution
`and seaming speed necessary to provide 10
`pulses per optical scan are shown below.
`Characteristic
`Q mw
`120 FPS
`35 RPS
`Scanning Speed
`Asimuth Resolution
`1.5‘
`1‘
`
`Table III
`
`RADAR WOMSTIGS NOTED IN MAI REIGN
`
`Radar Set
`
`M 3
`
`6
`
`Chara tsristic
`
`Rotation rate in dog. per sec.
`
`1/3 beanuidth in deg.
`
`Hatching scanning rate in rev. per sec.
`Pulse repetition rate, in pulses per sec.
`
`Pulses per optical scan
`
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`Theoretically it seems that maximum scanning
`rates provided by Rayfax designs may be ade-
`quate but this must be confirmed by experimental
`tests.
`
`The maximum information rate for the modi—
`fied system can be calculated.
`If the sweep
`speed is 120 sweeps per second and the range re—
`solution is 300 spots per radius, the information
`rate is 36 x 103 bits/sec.
`
`While the present mechanical scanning ap-
`proach is simple, direct and practical, future
`designs might employ an electronic scanning sys-
`tem.
`A tube with separate reading and writing
`guns could perfonm the same functions now per-
`formed mechanically. Hbre0ver, scanning speed
`and range resolution would not be limited by the
`mechanical design. Electronic reading speed
`could be as high as the writing speed, and the
`range resolution need be limited only by spot
`size. Future requirements will have to deterndne
`the need for such a design.
`
`In the final analysis, the way in which the
`Rayfaxtinfommation is used and displayed will de-
`termine the bandwidth requirements for the system.
`For example, the amount of range infonnation re-
`quired from the Rayfax to present a centered dis—
`play is only half that required for an equivalent
`display off—est by one radius and only one-third
`
`When a scanning rate is chosen, several
`other design parametsre must be matched to it.
`The antenna rotation rate, beamwidth, persis-
`tence of the phosphor and pulse repetition rate
`are all related to the optical scanning rate.
`The scanning time can be as long as the storage
`time of the phosphor.
`If it is shorter than
`this, targets will be smeared in the direction
`of rotation. Conversely the longest scanning
`rotation time should be no longer than the tarb
`get illumination time so that no targets are
`missed. This includes the time required for the
`radar antenna to rotate one beamwidth plus the
`phosphor decay time. Greater range resolution
`can be obtained by increasing the diameter of
`the J-ecope display while the slot size in the
`scanning head remains the same.
`The increased
`range resolution must be obtained by increasing
`the length of the display rather than decreasing
`the width of the slot because the minimum width
`is limited by the amount of light required by
`the optical system. By increasing the size of
`the J—scope to a 6" display on a 7" cathode ray
`tube,
`the range resolution could be increased 3
`times. This would allow 3 x 100 or 300 spots to
`be resolved in range. On presently available
`30" cathode ray tubes this is comparable to the
`maximum resolution obtainable on a centered PPI
`display and is about one-half the resolution ob-
`tainable on a display off-centered by one radius.
`If more resolution is desired for displays off-
`centered more than one radius, a delayed sweep
`might be designed so that the delay corresponded
`to the additional off-centering. Thus the dis-
`played portion of the sweep would still have
`about one—half the resolution of the cathode ray
`tube.
`
`pends on the coordinate system used. Various co-
`
`that for one off-set by one diameter provided
`range delay is not used. Azimuth resolution on
`the other hand is about the same as that of the
`radar and is not as likely to be influenced by
`the magnification of the sector displayed. It de-
`pends on the rotation rate and antenna beamwidth
`of the radar, where azimuth readuout rate -
`-
`Emmi—to
`. For the ASR, the azimuth rate is
`Be
`ldth
`60 beamwidthe per second, while for the long
`range radar the rate is only 36.
`
`Not all the range and azimuth information
`from the radar set may be required in the dis-I
`play.
`Just how much detail is required to follow
`an aircraft or to resolve nearby aircraft from
`each other must be determined by operational
`tests. However, if a fixed bandwidth is avail-
`able for transmitting the Hayfax signal, there
`may be an optimum compromiss between range and
`azimuth information. The total information con—
`tent is the product of these two items; again any
`compromise should be determined operationally.
`
`When decoded beacon returns are transmitted
`over a Rayfax system, three codes may be required
`They are the single blip, the double blip, and
`the bloomer.
`If the distance between the two
`spots in the double blip reply can be adjusted
`easily, the setting where the Rayfax encoder will
`resolve these spots can be determined. The spot
`size required to transmit a bloomsr can also be
`determined. Whether or not these spacings and
`sizes are suitable for the controller can then be
`determined by operational test. Where multiple
`beacon-interrogations occur, it may be possible
`to eliminate the "fruit" by the proper setting of
`the Rayfax encoder. Where only the strong beacon
`returns are displayed there may be considerable
`latitude in the adjustment.
`If mixed radar and
`beacon returns are displayed, some method of set—
`ting the level must be devised so that weak radar
`targets are not eliminated. Separate encoders
`for radar returns and beacon returns may be the
`required solution. These two signals could then
`be mixed and transmitted as a single signal.
`If
`synchronizing problems occur in the mechanical
`system, an electronic scanning system might solve
`the problems.
`
`One trouble encountered in the Rayfax sys—
`tem is the stability of the picture. Synchroniza—
`tion is established mechanically at north on
`every rotation. This tends to allow the presen—
`tation to shift a few degrees in azimuth. By eli—
`minating this mechanical sync and depending on
`ths electrical sync. the shift should be elimi—
`nated after the initial registration of the pic-
`ture.
`A north strobe would still be provided to
`check registration.
`
`The transmission of raw beacon returns will
`require different techniques. This problem will
`not be dealt with here.
`
`A Digital §xstem For Narrow Band Transmission
`
`The basic design of a digital system de—
`
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`

`ordinate systems such as Cartesian and polar may
`be used to locate the position of a target. The
`accuracy with which the position can be deter—
`mined should equal the accuracy of the radar set.
`Targets should be encoded as rapidly as they are
`acquired. However, target distribution is random
`and some type of storage should be used so that
`target information may be transmitted at an aver-
`age rate per radar rotation instead of at the in-
`stantaneous rate which can be very rapid. Other
`requirements for a digital system are to elimi-
`nate or minimize the transmission of noise tar-
`gets and eliminate the transmission of ground re—
`turn.
`Some type of level setting device should
`be used to minimize noise targets. The ground
`clutter pattern is fixed and may be minimized by
`RH circuits. Another less suitable method would
`be to mask the area of ground clutter so that it
`was not stored or transmitted. Beacon targets
`without radar to present a cleaner picture since
`ground return is eliminated. However, false in-
`terrogations present "fruit" which needs to be
`distinguished from actual target returns.
`Some
`type of integrating method might do this satis-
`factorily.
`
`author.
`
`The accuracy with which target coordinates
`should be determined is computed here for a spe-
`cific example. The long range radar set is used
`to gather information and the digital system deta-
`mines the polar coordinates of the target. Since
`this is the normal coordinate system used by ra—
`dar sets, no transformation is required. For a
`100 nautical mile range a radar determines the
`position of a target to .08 nautical mile and the
`digital encoder should have an accuracy of 1 part
`in 1250. The angular beamwidth of the long range
`radar is 1' and its accuracy is 1/3 beamwidth.
`Thus the digital encoder for azimuth should have
`an accuracy of 1 part in 1080.
`
`Using a binary coding system, ten binary di—
`gits provide an accuracy of 1 part in 1021. which
`is close enough to the above figures. For each
`target two code words of 10 digits each are re-
`quired. For 100 targets in the area scanned by
`the radar at a scanning rate of 1/10 revolution
`per second, the average amount of information
`transmitted per second would be 20 x 100 x 1/10
`- 200 bits per second.
`If all 100 targets were
`transmitted in one second, the rate at which in-
`formation would be required would be 20 x 100 -
`2000 bits per second. Potentialities for saving.
`bandwidth are very great. At present the writer
`does not know of a digital coding system which is
`available for this application. Components may
`be available and quickly adaptable to the problem
`
`Since a storage device should be used to de-
`bunch targets, this same device could be used to
`transform the target coordinates from a polar to
`a Cartesian system. The diameter of the radar
`sweep determines the maximum coordinate. To min-
`tain the same range resolution along a diameter
`as is available with the polar coordinate system,
`the digital encoder must resolvs 1 part in 2000.
`This requires‘ll binary digits for each x and y
`coordinate.
`The extra digit is required because
`the Cartesian system has greater resolution in
`
`the outer section of the circle than the polar
`system. The Cartesian system provides a resolu—
`tion of 2000 x 2000 x u/h - 3.11+ x 106 spots
`while the polar system provides (1000 x 1000) -
`(160/2 1 1000) - .92 x 106 spots. The quantity
`160/2 x 1000 is the number of redundant spots in
`the polar coordinate system within the minimum
`circle of resolution. This circle has a radius
`of only 160 spots.
`If the Cartesian system used
`10 binary digits for each x and y coordinate, it
`would provide 1000 x 1000 x u/h = .79 x 106 spots.
`Thus the display would sacrifice some over-all
`resolution and some resolution in the center of
`the display in exchange for more resolution in
`the outer portions of the display. Because the
`information in the corners of the Cartesian sys-
`tem is not used in the circular radar display a
`digital system using Cartesian coordinates does
`not provide as much useful information in the
`same bandwidth as a digital system using polar
`coordinates. However, if square displays were
`used, a Cartesian coordinate system would provide
`the most- information for a given bandwidth.
`
`One more item of interest is the transmis-
`sion of beacon codes.
`If 1021. different beacon
`codes were available, 10 binary digits could be
`used to transmit all this information. Adding
`this code group to the above groups for range and
`azimuth coordinates of the polar system increases
`the number of bits per target to 30. The above
`figures on information rate then become an aver—
`age of 300 bits per second and a maximum rate of
`3000 bits per second.
`
`If the ASR set is used to gather information
`over a 30 nautical mile range and has a resolu-
`tion of .07 miles, the range resolution is about
`1 part in 1.30.
`In azimuth the beamidth is 3'
`and with the radar accuracy of 1/3 bemidth, an
`azimuth resolution of 1 part in 360 should be
`plentiful. Nine binary digits providing 512 bits
`of information will be required for each coordi-
`nate to specify the location of the target. Thus,
`18 binary digits are required per target. For 80
`targets on the scope and a scan rate of 180" per
`second the average rate would be 80/2 3: 18 - 720
`bits per second.
`If data on 100 targets were
`transmitted in one second, 1800 bits per second
`would be the information rate.
`
`Other Narrow Band Systems
`
`Manual Systems
`
`Two other systems can be used for narrOw
`band transmission. They are the voice telling
`system and the Radar Target Synchro Repeater.
`
`A voics telling system requires a grid sys-
`tem on the FPI at the sending position and also
`on the presentation at the receiving position.
`Two men are required. The one at the sending end
`determines the coordinates of a target and calls
`them off to the mm at the receiving end. The
`receiver then plots them on his display. The ef-
`ficiency of this system and the information rates
`that can be maintained are not known to the
`
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`

`

`A Radar Target Synchro Repeater was de-
`veloped at The Franklin Institute laboratories.
`Briefly the system requires a 30" display at the
`transmitting end. The operator at this end posi-
`tions a lurker over each target in turn and
`presses a button. Through a synchro repeater
`system, this operation places a-target pip in a
`corresponding position on a phoephorescent screen
`at the receiving end. Sines two synchro systems
`are used,
`three 1.00 cycle voltages are required
`to transmit the information. These voltages
`could be transmitted over a 7'00 cycle system.
`Tests with the Radar Target Synchro Repeater show
`that‘ an operator can plot 20 targets every 30
`seconds for extended periods of time.
`
`'mis system is superior to a voice telling
`system since there is no need for a "sender" to
`tmnslate radar positions into verbal coordinates
`nor for the "receiver" to re-translate these ver-
`bal coordinates back into a plot. The system re-
`quires a man at ths sending end, but this man
`performs a useful service as a filter by elimi—
`nating clutter, noise, and "fruit" from the pic-
`ture at the receiving end.
`
`This apparatus should be useful as an in-
`terim method for remoting radar data. However,
`in the long run, manual methods should be sup—
`planted by automatic methods.
`
`G. Suntan
`
`Table V relates resolution to time and shows
`the information rate needed for each component of
`the system. The original data to be remoted to
`the air traffic controller are the positions of
`the aircraft in the area surrounding the radar
`set. Because these aircraft are limited in num—
`ber, their positions lend themselves to selective
`transmission. The systems‘coneidsred do not all
`take advantage of this fact. The wide band mi-
`crowave radar relay transmits all the information
`collected by the radar and is not at all selec-
`
`The resolution of the components of various
`radar relay systems are compared in Table IV.
`Some of the components have a fixed resolution
`regardless of the area covered. These are the
`aircraft themselves, the radar, the wide band re-
`lay and the narrow band digital relay. Other
`components have a variable resolution depending
`on the area ,they cover. These are the display,
`the Rayfax unit and the target synchro-repeater
`unit. Because the radar gathers the data, the
`other components should not be designed with a
`resolution which exceeds its accuracy. However,
`they can all be designed to match this accuracy.
`The components with fixed resolution match it
`over the total area covered while the ones with
`variable resolution match it over limited areas.
`
`135
`
`Table IV
`
`HESOIIJTION 0F RADARS AND RADAR RELAY SYSTEMS
`
`System
`
`Air traffic
`
`Radar data
`
`a. Long range radar
`(100 nautical mile range)
`
`b. ASE-2 (30 nautical
`mile range)
`
`PPI display (30" C.R.T.)
`a. Centered
`
`Resolution
`
`Azimuth
`
`Range
`
`Pin point
`
`Pin point
`
`.08 nautical miles,
`1220 targets
`
`.07 nautical miles,
`1.1.0 targets
`
`1 deg., 360 targets
`
`3 deg., 120 targets
`
`1/h00 radius scale
`
`0.11; deg.* at circumference
`
`b. Off-centered one radius
`
`1/800 diameter scale
`
`0.07 dsgfl‘ at maxirmm range
`displayed
`
`Wide Band Relay
`
`Rayfax relay system
`
`Same as radar
`
`Same as radar
`
`a. Centered display
`13. Off—centered one radius
`
`1/300 radius scale
`
`0.3 to 3 deg.“-
`
`1/300 diameter scale
`
`0.3 to 3 deg.”
`
`Digital Relay system
`(at 20 bits per target)
`
`1/1021. radius scale
`
`0-33 deg.
`
`Radar synchro target repeater
`
`1/60 radius scale
`
`1/ " diameter spot
`
`* Resolution decreases as origin of display is approached
`
`H Depends on radar resolution and Rayi'ax scan rate.
`
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`EFOMTIW RATE OF BADABS AND RADAR RELAY SISTDB
`
`Table V
`
`Information Rate
`
`l63tomeerhaur
`
`M
`
`Air traffic
`
`100 mile range
`
`Radar data
`(£14,000 target positions/sec.)
`a. Long range radar (100 nautical miles) 106 bits/sec.
`1.2 J: 106 bits/sec.
`(26,1.50 target positions/sec.)
`b. ASE—2 (30 nautical miles)
`
`Maximum data on CRT display (Aesume the
`display contains approx. 600,000 resolv-
`abl