Space Surveillance Sensors: The PAVE PAWS and BMEWS Radars (April 12, 2012)

PAVE PAWS

Background                                                                                                                            

Two PAVE PAWS (FPS-115) ballistic missile early warning radars are located on Cape Cod, Massachusetts (41.75˚E, 70.54˚W, oriented east), and at Beale Air Force Base in California (39,14˚N, 121.35˚W, oriented west).  A third radar, the BMEWS (Ballistic Missile Early Warning System) radar at Clear, Alaska (64.30˚N, 149.19˚W, oriented north) appears to be essentially identical to the two PAVE PAWS and so is discussed with them. All three radars are collateral sensors in the SSN.   PAVE PAWS radars in Georgia and Texas were deactivated in the mid-1980s and parts from these radars were used in building the Alaska radar. 

The California radar has received upgrades that allowed it to be incorporated into the U.S. ground-based national missile defense system.  These upgrades apparently did not involve hardware changes and are unlikely to have affected its space surveillance capabilities.  The upgrading of the Alaska and Cape Cod radars currently is not scheduled to begin until at least 2016.

 

(PAVE PAWS at Beale AFB, California.  Photo from: http://www.mda.mil/global/images/system/sensors/BealeRADAR3.jpg)

Technical Characteristics

Each radar has two faces, covering a total azimuth of 240˚ per radar, with boresight elevations of 20˚, covering elevations from 3˚ to 85˚.  The PAVE PAWS and the subsequent BMEWS radars (see next section) use very similar transmit/receive modules.  These modules, which operate in the 420-450 MHz band, produce peak powers of about 330-340 W each.[1] 

Each face has a diameter of about 22.1 m and contains 1792 active T/R modules and 885 passive elements.  The nominal peak power per face is usually stated as being 600 kW, with a corresponding average power of 150 kW.  According to a Missile Defense Agency environmental impact statement, the radar’s peak power per face is 582.4 kW, the average power is 145.6 kW, and the antenna effective transmit gain is 37.92 dB (= 6,194), and the beam width is about 2.2˚.[2]  The maximum duty cycle of a face is 25%.[3]

Search pulse lengths are 0.3, 5 (typical) and 8 ms with a chirp bandwidth of 0.1 MHz.[4]  Track pulse length are between 0.25 and 16 ms. (0.25, 0.5, 1, 2, 4, 8, and 16 ms.) with a 1 MHz bandwidth.

Operations

The PAVE PAWS radars appear to contribute to the SSN primarily or exclusively through detections (with follow-up tracking) made with radar fences while carrying out their primary missile warning mission.

While a PAVE PAWS face can operate at a duty cycle up to 25%, typically each face operates at an 18% duty cycle, and operating one face at 25% requires reducing the other face’s duty cycle to 11%.[5]  In normal operation, a duty factor of 11% per face is dedicated to a 120˚ wide surveillance fence, at elevation angles between 3˚ and 10˚, and the remaining 7% duty factor is used for tracking.  During periods of very high tracking requirements, the total duty cycle of one face can be increased by 7% to 25%.  However, the duty cycle of the other face must be reduced correspondingly (that is, down to as low as 11% total).

PAVE PAWS Performance Claims

The PAVE PAWS radars are about ten times less sensitive than the FPS-85.[6]    The radars are 5-10 dB less sensitive than the FPS-85 (it is unclear if this statement applies to the PAVE PAWS or BMEWS radar, or both)[7]

PAVE PAWS is designed to detect and track a 10 m2 target at a range of 3,000 nmi.[8] At its maximum range of 3,500 miles it can detect objects as small as a Volkswagen.[9]

 

 BMEWS

Background

The two BMEWS (Ballistic Missile Early Warning System) are collateral sensors in the SSN’s network (as noted above, a third radar in Clear, Alaska is officially classified as a BMEWS radar, although its characteristics are same as a PAVE PAWS radar), and are essentially larger versions of the PAVE PAWS.  Located at Fylingdales in Britain (54.36˚N, 0.67˚W) and Thule in Greenland (76.57˚N, 68.32 W˚), these radars’ primary mission is early warning of ballistic missile attack.  The radar in Greenland has two faces, providing 240˚ degree azimuthal coverage, while the radar in Britain has three faces, providing 360˚ coverage.  Both of these radars have received upgrades that allowed them to be incorporated into the United States’ ground-based national missile defense (GMD) system.

Technical Characteristics

Each radar face is bore-sited at 20˚ above the horizon, and can provide elevation coverage from 3˚ to 85˚.  Each octagonal BMEWS radar face has a diameter of about 25.6 m and contains 2,560 active transmit/receive modules (the total number of elements, including passive ones, is 3584).  Each face has a peak power of 850 kw and an average power of 255 kw (corresponding to a duty factor of 0.3).  The radar’s beam width is about 2.0˚

The BMEWS transmit/receive modules have a maximum duty factor of 30% and can produce pulses with lengths between 0.25 and 16 msec. The BMEWS radars use pulse lengths of 0.3 and 6 (typical) msec. in search and pulse lengths between 1 and 16 msec. in track.[10]  Search bandwidth is 0.3 MHz (Britain) and 0.6 MHz (Greenland) and track bandwidths are 5-10 MHz respectively.  The radars operate using a 54 msec. resource periods.[11]

BMEWS Performance Claims

3,000 nmi range coverage.[12] 


[1] Donald J. Hoft, “Solid State Transmit/Receive Module for the PAVE PAWS Phased Array Radar,’’ in Edward D. Ostroff, Michael Borkowski, Harry Thomas, and James Curtis, Solid State Radar Transmitters (Artech; Dedham, Mass., 1985) pp. 246-249 (reprinted from Microwave Journal, October 1978);  Michael T. Borkowski, “Solid State Transmitters,” in Merrill Skolnik, ed., Radar Handbook, 2nd. Ed. (McGraw-Hill: New York, 1990), pp. 5-3, 5-8, 5-25, and 5-26.

[2] U.S. Missile Defense Agency (MDA), Environmental Impact Statement, National Missile Defense System, Appendix H, Upgraded Early Warning Radars Analysis, July 1, 2000, p. H-1-7. Available at:   (http://www.mda.mil/global/documents/pdf/env_gmd_eis_append_h.pdf)

[3] MDA, “Environmental Impact Statement,”  p. H-1-8.

[4] Marvin N. Cohen, “Pulse Compression in Radar Systems,” in Jerry L. Eaves and Edward K. Reedy, eds. Principles of Modern Radar (Van Nostrand Reinhold: New York, 1987), p. 475.; F. Shackford, Technical Seminar, MIT Defense and Arms Control Studies Program.

[5] MDA, “Environmental Impact Statement,” pp. H-1-8, H-1-9.

[6] R. Sridharan and Antonio F. Pensa, “U.S. Space Surveillance Network Capabilities,” in C. Bruce Johnson, Timothy D. Maclay and Firooz A. Allahdadi, Image Intensifiers and Applications; and Characteristics and Consequences of Space Debris and Near Earth Objects, Proceedings of SPIE, Vol. 3434, July 1988, pp. 88-100.

[7] USAF Scientific Advisory Board, p. 9.

[8] “Pave Paws, BMEWS Radar Site Updates Will Broaden Missile Threat Coverage,” Aviation Week and Space Technology, December 9, 1985, pp. 52, 54.

[9] Ross Kerber, “Making (Radar) Waves, Boston Globe, July 2, 2001, p. C1.

[10] Cohen, “Pulse Compression in Radar Systems,”; F. Shackford, Technical Seminar.

[11] Shackford, Technical Seminar.  It is unclear if this applies to BMEWs or PAVE PAWS or both, but is consistent with the 16 msec. maximum pulse length and a 30% duty factor of BMEWS.

[12] Shackford, Technical Seminar.

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