Another space challenge was the need for continuous
tracking of artificial satellites. The requirements were
rapid acquisition of targets, along with unambiguous and
accurate tracking in range and angle. This section will
describe a development to meet this need which was led by
this author at Reeves Instrument Corp. Many existing
elements were utilised so that intense concentration could
be applied to the new elements to be invented. Systems
that met the needs were produced within the time available.
The space tracking of artificial satellites presented
problems which were very different from those of radar
detection of the moon. The tracking of earth orbiting artificial
satellites where precise position data were required
implied narrow pulses and high pulse repetition rates.
Reeves had made many ground based instrumentation and radar
aircraft control sets which were adaptations of or similar
to the SCR-584 radar. The radar for satellite tracking
required extensive variations from the SCR-584, including
greatly improved range and angle tracking systems and numerous
antenna refinements. Many of the earlier improvements
were adaptable to the new application, and the tracking
range in free space with beacon transponders was ample.
However, a new type ranging system having good accuracy and
operational at high repetition rates without range ambiguities
and interference between transmitted and received
pulses was essential.
The overall system was termed the VER(y) LO(ng) R(ange)
T(racker) 'VERLORT' and in a later version, with precision
direct (torque) motor drives in both antenna axes, the term
became PRE(cision) LO(ng) R(ange) T(racker) 'PRELORT' [12].
Fig.37.12. indicates that \ a line-of-sight range of
22500 to 51000km was readily within the then state of the
art. The planned orbital heights for the satellites to be
tracked were not expected to exceed a few hundred

kilometres, so that line of sight limits rather than
communications range set the specified range requirement.
The original specifications called for 4630km, but reality
later increased this to 9260km to accommodate orbits which
departed substantially from circularity.

Range capability of 3000MHz space tracking system
                                                         A B C D E F
Ground antenna reflector diameter (metres)          3 3 18 18 18 18
Space antenna reflector diameter (metres)     I I I 0.6 3 3
Space receiver sensitivity (dB with respect to JAW) 72 75 88 88 88 88
Space transmitter peak power (kW)                  2 2 10 10 10 500
Ground receiver sensitivity (dB with respect to 1mW)        98 100 107 107 107 107
Marginal system range (Mm)                                              22 51 640 8800 44800 720000
(I = isotropic)
Fig. 37.12 Tracking range for various power and receiver combinations

Fig.37.13 illustrates the two key problems of continuous
tracking of a target whose range transit time is greater
than the interval between pulses. A 320km range time
between pulses (about 410Hz) is illustrated; as the target
proceeds from 40km outward, at 320km range the received
pulse is interfered with by the next transmitted pulse of
the 410Hz pulse train. As the target pulse moves to 360km
the pulse patterns have the same appearance as at 40km.
Automatic selection of the repetition rate as a function
of range avoided transmitter/receiver interference. Cam
operated switches coupled to the ranging system selected an
alternate repetition rate as each range band of pulse interference
was passed, (Fig.37.14.).
A slow speed range indicator was utilised to keep track
of the multi-time-around real range. Fig.37.15. shows the
main range counter coupled to a 320km per revolution ranging
system (about 410Hz p.r.f.). Separate coupling of each
repetition rate ranger to the main range wheel, with appropriate
gear ratios, was achieved, and Fig.37.16. shows 320
and 256km rangers coupled as required. This scheme allows
continuous ranging from the minimum range of a fraction of a

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kilometre to the maximum of 8000km with an accuracy of only
a few metres. However, since the highest repetition rate
used required some 30 revolutions of its ranger to cover the
8000km,means had to be provided to make the initial acquisition
of the target occur on the correct revolution of the
ranger.


Fig. 37.13 Pulses with single range indicating dial


Fig. 37.14 Selected repetition rate to avoid transmitter/receiver interference

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Pulse train coding was employed to overcome ambiguity
resolution. A transmitted pulse was omitted from the
transmitted pulse train once in each time interval corresponding
to the time of pulse travel from the radar to the
target and back (about 0.049s). This corresponded to a
pulse deletion frequency of about 20Hz, see Fig.37.17.
The oscilloscope display was also designed to resolve
range ambiguity. In this display the received pulse gate
periods are individually presented in a vertical sequence
with the top line corresponding to the first received pulse
period after the omitted transmitted pulse, the second line
corresponding to the second received pulse period after the
omitted transmitter pulse, and so on for the 1/20s period
for the 8000km range, Fig.37.18.


Fig. 37.20 Schematic of dual servo range electro-mechanical unit for fast target acquisition

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Fig.37.19 shows the operational use of the ambiguity
resolution display. A target at 920km is shown in 'A'
with a 409.5Hz p.r.f. and in 'B' with a 511.9Hz p.r.f. The
operator can readily identify the line with the missing pulse
and slew the range unit to move a vertical marker cursor to
that line as shown in the figure. This synchronizes the
range unit to track the target continuously over the whole
range without ambiguity and without pulse interference
between transmitted and received pulses.
The operation of the above ambiguity resolving synchronization
implies a damagingly fast slew for a mechanical range
unit if all parts of it down to the precision range phase
shifter were turned. To avoid this, the precision part of
the range system drives the lower speed elements through a
synchro coupled servo system rather than by a direct shaft
coupling. This allows only the second servo unit to be
slewed to move the position cf the ambiguity resolving
cursor described above, see Fig.37.20. When the cursor is
properly positioned, the synchro control of the low speed
servo is resumed, and the shafts of the two servo subsystems
are immediately realigned exactly. Although the
operation sounds complicated, it operated very reliably and
operators rapidly became skilled in using it; establishment
of target tracking was easily accomplished as the satellite
came into line of sight range. Today's similar systems are
all-digital and do the ambiguity resolution automatically.
The repetition rates used required a simple integral
relationship and had to provide ample clearance between the
transmitted pulses and the received beacon returns as transmit/
receive interference approached on a p.r.f. being used.
Using these criteria it was found that three p.r.f.s. were
required for a 4000km range and four for 8000km range, (see
Fig.37.21.).


Fig. 37.21 Pulse-to-pulse ranges versus repetition rates

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These radars were used for years on the USAF Discoverer
satellite programme and later in the NASA first manned space
programme, Project Mercury, Fig.37.22. is a plot of tracking
data on one of the first Discoverer flights as the satellite
is followed by a VERLORT radar at one of the tracking
stations. Some later models (or modifications of the
original VERLORTs), the PRELORTs used a precision direct
drive Reeves pedestal, a larger antenna reflector, and a high
power wideband polarisation controlled feed to accommodate
other tracking/control signals for a newer system as well
as VERLORT and PRELORT radar signals.


Fig. 37.22 Plot of data acquired on early flight of Discoverer satellite

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BIBLIOGRAPHY

1. Norton, K.A., and Omberg, A.C.: 'The maximum range of
a radar set', Proc. IRE, 35, January 1947

2. Stodola, E.K.: 'Summary of investigation of Doppler
effect at Camp Evans, Signals Laboratory (CESL),
17th December 1943', CESL Technical memorandum no. 21
(Available from Modern Military Field Branch, Military
Archives Division, US Archives Office, 4205 Suitland
Road, Suitland MD 24049)

3. McMullen, C.G.: 'Experimental investigation of ground
clutter problem, 29th May 1944', CESL Technical
memorandum no. 90 (Available ibid.)

4. Anon.: 'Coherent pulse system SCR 270-271 console
BC-1386 and accessories, 10th March 1945', CESL
Technical memorandum 168E. (Available ibid.)

5. Stodola, E.K.: 'World War II army coherent pulse search
radar for moving target detection', supplement to the
Record of the IEEE International Radar Conference, 1985.
(Available from the IEEE Aerospace and Electronics
Systems Society)

6. Bailey, A.E.: 'MTI research at ARDE 1943-1945', supplement
to the Record of the IEEE International Radar
Conference. (Available ibid.)

7. Stodola, E.K.: 'Moving target pulse echo measuring
system', US patent no. 2 614 250

8. McMullen, C.G.: 'Moving target indicator for radioobject
locating system', US patent no. 2 679 042

9. Bailey, A.E.: 'The use of AB scope presentation for
the detection of moving ground vehicles', RRDE
memorandum no. 100, 28th June 1945

10. Dewitt, J.H., and Stodola, E.K.: 'Detection of radio
signals reflected from the moon', Proc. IRE, March
1949

11. Clark, T.: 'How Diana touched the moon', IEEE Spectrum,
May 1980

12. Stodola, E.K.: 'Long range radar ranging system', US
patent no. 3 147 476 (Also UK patent no. 932 294,
and Canada patent nos. 849 721, 849 722 and 859 322)