Space Surveillance Sensors: Millimeter Wave (MMW) Radar (June 19, 2012)

Millimeter Wave (MMW) Radar

The Millimeter Wave (MMW) radar is a Ka-band (35 GHz) and possibly W-Band (95 GHz) imaging radar at the Kwajalein Atoll on the Pacific. It is a collateral sensor in the Space Surveillance Network (SSN).  It has recently been upgraded to a 4 GHz bandwidth, giving it a range resolution of about six centimeters.  It is currently the highest resolution imaging radar in the SSN (although it will be surpassed by the W-band upgrade to the Haystack radar when this becomes operational, likely in 2013).   

 

Photograph from: http://www.ll.mit.edu/publications/MITLL_2011_annual_report.pdf

 

Background

The MMW was built at Kwajalein to improve on the imaging capabilities of ALCOR and to collect millimeter wavelength signature data on ballistic missiles.[1]  It became operational in 1983 at Ka-band (35 GHz, corresponding to a wavelength of 8.7 mm) and in 1985 at W-band (95.5 GHz, corresponding to a wavelength of 3.14 cm).  These frequencies are used because they fall in atmospheric transmission windows.

It has a paraboloidal antenna with a diameter of 45 feet (13.7 m), which produces beam widths of 0.042˚ at Ka -band and 0.014˚ at W-band.  Its metric accuracy is less than or equal to 50 μrad.

When it first became operational in 1983-1985, the MMW had peak powers of about 25 kW at Ka-Band and 5 kW at W-band.

At both frequencies, the radar initially had a 1 GHz bandwidth using linear frequency modulation waveforms, giving a range resolution of about 0.25 m.  All of the radar’s waveforms (narrowband and wideband) used a 50 μs pulse at a PRF of 2,000.

Initially, at Ka-band the MMW was capable of achieving a S/N = 17 dB (= 50) against a 1 m2 target at a range of 1,000 km (and at a 30˚ clear-weather elevation) with a single 50 μs pulse.  This provided a useful capability to collect data on reentry vehicles (RVs) reentering near Kwajalein, but only limited capabilities against satellites and RVs in midcourse.  However, the equivalent S/N at W-Band was only about -3 dB (= 0.5), adequate only for some reentry measurements and of little if any use against satellites.

 

Ka-Band Upgrade

Starting in 1988, a number of upgrades were made to the radar’s 35 GHz capability.  As part of this process, its W-band capability was at least temporarily eliminated. 

The upgrades to the 35 GHz capability, made over a number of years, included enhanced pulse-pulse-compression and integration capabilities, a new beam waveguide system that significantly reduced losses, and more a more powerful transmitter, including a second traveling-wave tube.  The new travelling wave tubes each had a peak power of about 50-60 kW.  These changes reportedly increased the MMW’s single-pulse detection range to over 2,000 km, and its tracking range (multiple pulses) to about 2,500 km.

In addition, the MMW’s bandwidth at Ka was doubled to 2 GHZ, giving a range resolution of about 0.12 m. 

W-Band Upgrade

As of 2000, modifications to restore and upgrade the MMW radar’s 95 GHz capability had been underway for some time.  These upgrades included a higher transmit power and lower receiver noise and system losses.  These improvements were intended to increase its sensitivity by almost two orders of magnitude.  This upgrade would have given the MMW a W-band sensitivity similar to its original capability at Ka-band.  However, it is unclear (to me) if these upgrades were ever completed or if the MMW radar’s W-band capability was ever restored.[2]

4 GHz Upgrade

A 2001 Naval Research Laboratory report stated that upgrading the MMW radar “by equipping it with higher-power, broader-band amplifiers for increased S/N on target as well as increased radar bandwidth, is needed for TBMD and National Missile Defense (NMD) evaluation.  In Ka-band, the desired output power is 100 kw peak, 20% duty factor, with 4 GHz instantaneous bandwidth.”[3]

In 2005, a 4 GHz upgrade program for the MMW was begun.[4]  This upgrade involved a new wider-bandwidth transmitter tube, a more sensitive receiver, improved radio-frequency hardware and a more capable signal processor, and reportedly produced a doubling of the radar’s tracking range.  It doubled both the radar’s pulse length (to 100 μs) and duty factor (to 20%, corresponding to 2,000 pulses per second).   The upgrade doubled the radar’s bandwidth to 4 GHz, giving a range resolution of about 6 cm.  The imaging range window (which gives the maximum size object that could be imaged) was increased from 37.5 m to 63 m.

The upgraded radar became operational in March 2011.  The discussions of the upgrade do not refer to which frequency band(s) was involved, but it seems certain to apply to at least the Ka-Band, still leaving it unclear (to me) if the MMW Radar currently has a W-band capability.

A 2010 paper gives the MMW’s peak power at 35 GHz as 25 kW.[5]

Operations

Primary uses of the MMW Radar include high-accuracy tracking, high-resolution reentry vehicle (RV)  and wake measurements, RV and satellite imaging, miss distance estimates and real-time discrimination.  In 1987, as part of the Kwajalein Discrimination System program, the MMW became capable of real-time imaging of RVs.  As of about 1996, the MMW imaged about 100 satellites annually.[6]

MMWs capabilities were extensively used in developing techniques for measuring motion differences between objects with different mass distribution and other potential means for discriminating between warheads and decoys. 

As of 2004, together with ALCOR (the other imaging radar at Kwajalein (see post of May 17, 2012) the MMW provided up to 300 satellite images sets annually.[7]  


[1] The primary technical sources for this discussion are: K.R. Roth, M.E. Austin, D.J. Frediani, G.H. Knittle, A.V. Mrstik, “The Kiernan Reentry Measurements System on Kwajalein Atoll,” MIT Lincoln Laboratory Journal, Vol. 2. No. 2 (1989), pp. 247-275;  M. D. Abouzahra and R. K. Avent, “The 100-kW Millimeter-Wave Radar at the Kwajalein Atoll,” IEEE Antennas and Propagation Magazine, Vol. 36, No. 24 (April 1994), pp. 7-19; J. C. McHarg and W. D. Fitzgerald, “A Sensitivity Upgrade for the Millimeter Wave Radar at Kwajalein,” Proceedings of the 1994 Space Surveillance Workshop, Vol. 1, Lincoln Laboratory, April 5-7, 1994, pp. 217-227; J.C. McHarg and R. F. Lucey, Jr., “95 GHz Sensitivity Improvements at the Kwajalein Missile Range,” Proceedings of the 1995 Space Surveillance Workshop, Vol. 1, Lincoln Laboratory, March 28-30, 1995, pp. 159-677; J.C. McHarg, M.D. Abouzahra, and R.F. Lucey, “95 GHz Millimeter Wave Radar”  Millimeter and Submillimeter Waves III, Proceedings of the SPIE, Vol. 2842 (August 5-7, 1996), pp. 494-500; 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. 267-279; Philip A. Ingerwesen, William W. Camp, and Alan J. Fenn, “Radar Technology for Ballistic Missile Defense,” Lincoln Laboratory Journal, Vol. 13, No. 1 (2002), pp. 109-147.

[2] Some post-2000 sources show a W-band capability, including “Millimeter Wave Radar,” website of Ronald Reagan Ballistic Missile Defense Test Site at Kwajalein Atoll (undated) at www.smdc.army.mil/kwaj/rangeinst/MMW.html.  Susan E. Andrews, Peter Yoho, Gerald P. Banner, Thomas Sangiolo, “Radar Open System Architecture for Lincoln Space Surveillance Activities,” 8th U.S.-Russian Space Surveillance Workshop, Maui, Hawaii, April 18-23 (available at: http://www.amostech.com/ssw/presentations/Session3/S3-4Andrews.pdf) has figure for the radars at Kwajalein showing the MMW radar with a W-Band capability

Other sources list only a 35 GHZ capability, including: Michael C. Schexnayder, “Technology Development and Transition: Kwajalein Complex Makes Unique Contributions,” Army Space Journal, Fall 2004, pp. 6-8 (available at: http://www.smdc-armyforces.army.mil/Pic_Archive/ASJ_PDFs/ASJ_VOL_3_NO_2_3_RDA_COLUMN.pdf) and  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, Maui, Hawaii, April 18-23, 2010 (available at: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110007079_2010014487.pdf).  It also is possible that these differences reflect a W-band capability that is used only or mostly for missile defense purposes and not space surveillance.

[3] G. M. Borsuk, Robert Kemerley, Bruce Wallace, Bobby Junker, Max Yoder, “DoD Investment Strategy for Vacuum Electronics R&D and Investment Balance for RF Vacuum Electronics and Solid State R&D,” Naval Research Laboratory NRL/MR/6800–01-8570, April 8, 2001.  Available at: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA393327.   This report does not specifically discuss a W-Band upgrade for the MMW radar, but does refer to such an upgrade for the Haystack radar.

[4] “Millimeter Wave Radar Upgrade,” 2011 Annual Report, MIT Lincoln Laboratory, p. 23.  Available at: http://www.ll.mit.edu/publications/MITLL_2011_annual_report.pdf; Lincoln Laboratory, “MMW (Millimeter-Wave) Radar 4 GHz Upgrade,” July 2011. Available at: www.ll.mit.edu/60thAnniversary/timeline.html.

[5] Stansbery, “Plans for the Meter Class Autonomous Telescope.”

[6] “Chapter 6: Ballistic Missile Defense” (pp. 91-92) in Eva C. Freeman, ed., MIT Lincoln Laboratory: Technology in the National Interest (Lexington, Mass., Lincoln Laboratory, 1995.

[7] Schexnayder, “Technology Development and Transition.”

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