Space Surveillance Sensors: ALCOR Radar (May 17, 2012)

ALCOR

ALCOR (ARPA Lincoln C-Band Observables Radar) was the world’s first high-power long-range wide-band radar.[1]  It played a key role in the development and early implementation of range-Doppler techniques for imaging satellites, producing images with a resolution of 0.5 meters (about 20 inches). ALCOR began operations at Kwajalein in late 1969.  Although it has since been exceeded in both range and resolution capabilities by the Millimeter Wave (MMW) Radar at the same Kwajalein site (and by other wide-band radars at other sites), it remains an important collateral sensor in the Space Surveillance Network.[2]  

 

 The ALCOR antenna viewed from behind inside its radome at Kwajalein. (Photograph from Camp, et.al.,  Lincoln Laboratory Journal)

Background

 Shortly after becoming operational, in 1970 ALCOR was used to measure the dimensions of the upper stage of the Chinese rocket booster which remained in orbit after launching China’s first satellite (in order to assess its potential use as part of an ICBM).[3]  In 1972, ALCOR imaged the USSR’s Salyut space station, revealing details that could not be seen in photographs of the satellite released by the Soviets.  In 1973 it imaged the Skylab space station showing that one solar panel was missing, the other was only partially deployed and that its sun and micrometeorite shield was missing.  This information facilitated the eventually successful repair and occupation of the space station.

 

Simulated ALCOR Inverse Synthetic Aperture (ISAR) radar image (lower picture) of the US Skylab space station (upper picture).  Actual U.S. government ISAR images of satellites are classified. (Pictures from Freeman,  p. 117.)

 

Technical Characteristics and Operations

Except where noted, the information below is primarily from 2000 or earlier and so may have changed somewhat.

ALCOR has a 3 MW peak power and an average power of 6 kW.   ALCOR’s maximum pulse repetition frequency is 323 Hz and its maximum duty cycle is 0.0032.  A 2010 source cites a peak power of 2.25 MW.[4]

It has a 40 foot (12.2 m) diameter antenna enclosed in a 68 foot radome.  ALCOR’s beamwidth is 5.2 mrad. = 0.3 degrees and its antenna gain is 55 dB (= 32,000).

ALCOR’s wideband waveform is a 10.5 μs long pulse, linearly swept through 512 MHz.  ALCOR also has a narrowband 10.5 μs pulse (6 MHz bandwidth) for acquisition and tracking.  It is also capable of producing a doublet pulse of two 10.5 μs pulses (either narrowband or wideband) separated by 10.5 μs.[5]  It also has 0.4-0.8 μs length beacon interrogation waveforms and is the only one of Kwajalein’s large radars capable of tracking a beacon.

ALCOR has a maximum bandwidth of 512 MHz at a center frequency of 5.7 GHz, corresponding to wavelength of 5.3 cm.  Weighting of the signal to reduce range sidelobes results in a range resolution of 0.5 meters, somewhat reduced relative to the theoretical minimum of about c/2β = 0.3 m.  It can obtain a cross-range resolution equal to its 0.5 m range resolution with a target rotation of 3.0 degrees during the time it is observed.  [For comparison, the theoretical expression for the rotation angle θ gives θ = λ/2L = 0.053m/(2*0.5m) = 0.53 = 3.0˚, where L is the cross range resolution.]

Its metric accuracies are 0.4 ± 0.015 m in range, 100 ± 135 μrad in azimuth, and 60 ± 140 μrad in elevation.

It can achieve a S/N of about 24 db (= 250) with a single pulse on a 1 m2 target at 1,000 km.  As of the mid-1990s, ALCOR was capable of coherently integrating 256 pulses (with an integration gain of 19 dB = 80).

Prior to 1983, the U.S. Aerospace Defense Command apparently did not allow radar images of satellites to be produced outside the United States, thus the ALCOR images were not processed at Kwajalein.[6]  In 1983, a capability to produce near real-time images, originally developed at the Haystack and HAX radars in Massachusetts, was installed at ALCOR.  As of 1989, ALCOR routinely imaged about 60 satellites a year.  As of about 1995, ALCOR was imaging about 200 satellites per year for U.S. space Command (and the MMW radar at Kwajalein another 100).[7]  As of 2004, ALCOR and the MMW radar at Kwajalein together produced about 300 image sets of satellites per year.[8] 


[1] The primary sources on information used here are: K.R. Roth, M.E. Austin, D.J. Frediani, G.H. Knittel, and A.V. Mrstik, “The Kiernan Reentry Measurements System on Kwajalein Atoll, The Lincoln Laboratory Journal, Vol. 2, No. 2 (1989),  pp. 247-276;  K. J. Witt and R. K. Avent, “ALCOR Sensitivity and Detection Enhancements,” Proceedings of the 1995 Space Surveillance Workshop, Lincoln Laboratory, March 28-30, 1995; R.K. Avent, J.D. Shelton, and P. Brown, “The ALCOR C-Band Imaging Radar,” IEEE Antennas and Propagation Magazine, Vol. 38, No. 3 (June 1996), pp. 16-27; William W. Camp, Joseph T. Mayhan, and Robert M. O’Donnell, “Wideband Radar for Ballistic Missile Defense and Range-Doppler Imaging of Satellites,” Lincoln Laboratory Journal, Vol. 12, No. 2 (2000), pp. 267-280; U.S. Army Space and Missile Defense Command,  “ARPA Lincoln C-Band Observables Radar (ALCOR),” Factsheet  (http://www.smdc.army.mil/kwaj/RangeInst/ALTAIR.html).

[2] In particular, ALCOR is much less affected by rain than the higher frequency MMW radar and is also useful for viewing satellites at low elevation angles where atmospheric effects are greater…

[3] Information in this paragraph from Chapter 8: “Space Surveillance,” in Eva C. Freeman, ed., MIT Lincoln Laboratory: Technology in the National Interest (Lexington, Mass.: Lincoln Laboratory, 1995).

[4]Gene Stansbery, Paul Kervin, and Mark Mulrooney, “Plans for the Meter Class Autonomous Telescope and Potential Coordinated Measurements with Kwajalein Radars,”  8th U.S.-Russian Space Surveillance Workshop, April 18-23, 2010.  Available at: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110007079_2010014487.pdf.

[5] The wideband doublet was used to simultaneously collect data on both a reentering target and its ionized wake.  The narrowband doublet was created in order to map booster fragmentation in support of theater missile defense programs.  Avent, Shelton and Brown, p. 18.

[6] Freeman, p. 118.

[7] Freeman, pp. 91-92.

[8] Michael C. Schexnayder, “Technology Development and Transition: Kwajalein Complex Makes Unique Contributions,” Army Space Journal, Fall 2004, pp. 6-8.

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