Space Surveillance Sensors: The FPS-85 Radar (April 12, 2012)


The FPS-85 has been described as the workhorse of the SSN, and is the largest, most sensitive and most important (for space surveillance purposes) of the SSN’s LPARs.  It is one of the three dedicated radar sensors in the SSN (the others are the Air Force Fence and the GLOBUS II dish antenna radar in Norway).  The radar is located in Eglin, Florida (and thus sometimes referred to as the Eglin Radar) at about 30.6° N (30.57N, 86.22 E) and points directly south. 

The FPS-85 was the world’s first large phased-array radar.   Its construction began in 1962, but it was destroyed by fire in 1965 before becoming fully operational.  It was rebuilt and began operations in 1969.  The radar was originally intended only for space surveillance, but in 1975 it was also assigned a submarine-launched ballistic missile warning mission.  In 1987, corresponding to the activation of two south-facing PAVE PAWS early warning radars in Georgia and Texas (both since deactivated), it returned to full-time space surveillance. 

The FPS-85 has separate receive and transmit antennas contained within the same building, with boresites pointing due south at a 45º elevations.  It can scan ±60˚ in azimuth and from the horizon to 15˚ beyond zenith in elevation.[1]  It was built with two separate antennas because at the time of its construction it was less costly to do this than building a single transmit/receive antenna.  This choice also facilitated simultaneously obtaining multiple narrow receive beams for more precise tracking and a broader transmit beam more suitable for surveillance.

(photo source:

Technical Characteristics

The transmitter is a 72×72 rectangular array, with 5,184 transmit modules with 0.55 λ spacing.[2]  Its center frequency is 442 MHz, with a 10 MHz bandwidth.[3]  Its wavelength is thus about 0.68 m, and the antenna’s diameter is about 26.9 m with an area of 724 m2.  The transmit antenna is uniformly illuminated and has a 1.4 degree beam width (for comparison, λ/D = 0.68/27 = 1.44 degrees).

Each of the 5,184 transmitter element is rated for a peak power of 10 kW and a 0.5% duty cycle.[4] These give an array peak power of 52 MW and an average power of 260 kW.  However, according to a 1994 paper the average power of individual elements individual elements varied from 2.5 to 10 W, with an average of about 6 W.[5]  This is consistent with a number of sources that give the radar’s peak power as about 30-35 MW.[6]  Assuming a peak power of 35 MW, the radar’s average power would then be about 175 kW.

The receive antenna is a tapered array with a diameter of 58 m containing 19,500 crossed dipole elements on a square grid, forming a circular aperture 152 elements in diameter.[7]  There are 4660 active receive modules.  Its receive beam width is 0.8º (in comparison, 0.68/58 = 0.0117 = 0.67 degrees).[8]   It receives using a 3×3 cluster of receive beams, with a 0.4º spacing, giving a 1 db crossover and thus a low beam-shape loss.[9]  The combined beam width is therefore 0.4+0.8+0.4 = 1.6º.  All nine receive beams are used in search, but only five in track.

The FPS-85 operates in time blocks called resource periods, each of which is 50 ms long.  During a resource period, the radar can transmit up to eight pulses for a maximum total transmit time of 250 μs (corresponding to duty cycle of 0.5%).[10] 

Pulse lengths are 1, 5, 10, 25, 125, and 250 μs.[11]  Its maximum bandwidth is 10 MHz.  The pulse compression (used to obtain greater better range resolution) ratio may be as large as 1,600.[12]  In its long-range surveillance mode, it emits a single 250 μs frequency-modulated compressed pulse every 50 ms. 


The FPS-85 initially conducted surveillance using several different radar fences.[13]   A 1994 software upgrade left the FPS-85 with only relatively low-elevation radar fences, as the software needed for a higher-elevation fence intended for detecting lower RCS space objects was not funded.[14] 

Figure 1 below shows a fence described as the existing SPACETRACK fence in a 1996 paper.[15] This fence begins at an azimuth of 142˚ at an elevation of 15˚, reaches an elevation of 23˚ at 180˚ degrees azimuth (along the boresite direction) and continues on to an azimuth of 218˚ at an elevation angle of 15˚. This total azimuth extent of 76˚ used 76 beam positions.  An additional 24 beam positions were used to extend coverage directly down to an elevation angle of 3˚ at each end of the fence.   A 2004 report describes a previously existing fence, the S-1 fence, as scanning between the same endpoints, but reaching a maximum elevation angle of 25˚, as shown in figure 2 below.[16]  Another source describes a low-elevation fence (which was not leak-proof for all altitudes and elevations) with a peak elevation of about 23˚ extending 120˚ in azimuth, dipping down to the horizon at its ends.[17] 


Figure 1: FPS-85 Fence (plus model fence, from Burnham and Sridharan)

In 1999, the radar’s software was upgraded to provide a higher-elevation fence.  This new “debris” fence was scanned at a maximum of about 35˚ in elevation and ±25˚ degrees in azimuth from the radar’s due south azimuth boresite, and is also shown in Figure 2. 

Following tests in May 2000, this new debris fence was apparently put into operation, although at the time it was expected that it would be operated in a “background” mode, that is, only when time was left over from its normal space surveillance tasks.[18]  However, another source indicates that the FPS-85 was using a 35˚ elevation fence (with a length of ±40˚) for detecting low-earth orbit objects as early as 1997.[19]

By integrating large numbers of pulses, the FPS-85 is capable of tracking previously detected objects at least out to geosynchronous orbit range.  It is the only phased-array radar in  the U.S. Space Surveillance Network capable of tracking objects in geosynchronous orbit (the next two largest phased arrays are not oriented so as to be able to view geosynchronous orbit).  The FPS-85 assumed a deep space role in November 1988 after receiving a range-extension upgrade enabling integration of many pulses.[20]


Figure 2: FPS-85 Fences (from Setticerri, et. al.)


FPS-85 Performance Claims                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 

The FPS-85 is the most sensitive of the U.S. LPARs.  (The sensitivity S of a radar is signal-to noise ratio obtained under a specified set of conditions, for example, a bore-sighted target with an RCS of 1.0 m2 at a range of 1,000 km with a single 1.0 ms pulse).  Published accounts indicate that the FPS-85 has a sensitivity of 100,000, with a target range = 1,000 km, target σ = 1.0 m2, and pulse length = 0.25 ms (the longest pulse length the radar can produce).[21] 

A 1996 paper that cites a sensitivity of 100,000 states that greater sensitivity in actually achieved, citing single pulse detection ranges (with no S/N specified) of 2,500, 4,000 and 7,200 km for targets with radar cross sections of 0.01, 0.2 and 1.0 m2, respectively (for S/N = 20, this would give S = 78,000, 51,000, and 54,000 respectively).[22]  A 1997 Air Force Scientific Board Report states the FPS-85 can detect a 1.0 m2 target with a single pulse at 7,500 km (which would give a sensitivity of 63,000 if the required S/N = 20).[23]  Another source indicates a sensitivity of about 39,000-45,000.[24]

The FPS-85 can track objects with RCSs smaller than -30 dBsm (corresponding to about a 9 cm diameter).[25]  It can detect objects with RCS of 0.04 m2 (15 cm) in geostationary transfer orbits and as low as 0.0001 m2 (6 cm) in low earth orbits.[26]  By integrating large numbers of pulses, it can detect 1.0 m2 objects in geosynchronous orbits.[27] 

The FPS-85 accounted for over 40% of the SSN’s near-earth tracks (excluding Air Force Fence detections) as of the end of 2000.[28] 

[1] Montgomery D. Grimes, J. Mark Major, and Thomas J. Warnagiris, “Pulsed Power Management and Design Philosophy in the T-1028A/FPS-85 Radar Transmitter,” Intense Beams and Applications: Lasers, Ions and Microwaves, Proceedings SPIE 2119 (1994), pp. 183-187.

[2] J. Emory Reed, “The AN/FPS-85 Radar System,” Proceedings of the IEEE, Vol. 57, No. 3 (March 1969), pp. 324-335.

[3] Reed, “The AN/FPS-85 Radar System,” p. 330.

[4] Benjamin M. Elson, “U.S. Space Tracking Capability to Double,” Aviation Week and Space Technology, January 1, 1968, pp. 64-67);  Reed (p. 330).

[5] Montgomery D. Grimes, J. Mark Major, and  Thomas J. Warnagiris, “Peak Power Tailoring and Phase Nulling of the AN/FPS-85 Radar,” in Howard E. Brandt, ed., Intense Microwave Pulses II, Proceedings of the SPIE, Vol. 2154 (May 1994), pp. 241-246;

[6] W.F. Burnham and R. Sridharan, “An Eglin Fence for the Detection of Low Inclination/High Eccentricity Satellites,” in K.P. Schwan, ed., Proceedings of the 1996 Space Surveillance Workshop (Lexington, Mass.; Lincoln Laboratories, 1996), pp. 45-55; J. Mark  Major, “Upgrading the Nation’s Largest Space Surveillance Radar,” Technology Today, September 1994, available at; Burnham and Sridharan; Eli Brookner, Radar Technology (Dedham, Mass.: Artech, 1977), p. 25

[7] Reed, “The AN/FPS-85 Radar System,” p. 331. “Tapered” means that the receive array is not fully populated with active modules, with the average density of active modules decreasing as one moves away from the center of the array.

[8] Eli Brookner, Radar Technology (Dedham, Mass.: Artech, 1985), p. 25.

[9] Reed, “The AN/FPS-85 Radar System,” p. 331.   A beam-shape loss occurs when a target is off-center in a beam, since the power density in the beam decreases as one moves away from its center.

[10] Thomas J. Settecerri, Alan D. Skillicorn, and Paul C. Spikes, “Analysis of the Eglin Radar Debris Fence,” Acta Astronautica, Vol. 54, No. 3 (February 2004), pp. 203-213.

[11] Reed, “The AN/FPS-85 Radar System,” pp. 330. Brookner (Brookner, Radar Technology, p. 25) lists several additional intermediate pulse lengths.

[12] Brookner, Radar Technology, p. 25. This source lists eight different pulse compression ratios, but does not directly associate them with specific pulse lengths.

[13] In search mode, “Either of three fence combinations can be selected with no interruption to service.”  Reed, “The AN/FPS-85 Radar System,” p. 325.

[14] Setticerri, et. al., “Analysis of the Eglin Radar Debris Fence.”

[15] Burnham and Sridharan, “An Eglin Fence,” p. 50.

[16] Setticerri, et. al., “Analysis of the Eglin Radar Debris Fence.”

[17] 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.

[18] SPECULATION:There is an increase of about one thousand objects in the analyst catalog (but not the public catalog) in mid-2002 that does not appear to be readily attributable to any other increase in SSN sensor capability and may represent implementation of the FPS-85 debris fence on a routine basis.

[19] United States Air Force Scientific Advisory Board, Report on Space Surveillance, Asteroids and Comets, and Space Debris, Volume I: Space Surveillance, SAB-TR-96-04, June 1997 (available at, p. 9.

[20] Nicholas L. Johnson, “U.S. Space Surveillance,” Advances in Space Research, pp. 8(5)-8(20).

[21]Sridharan and Pensa, “U.S. Space Surveillance Network Capabilities,” p. 92;  P. Chorman, “COBRA DANE Space Surveillance Capabilities,” in S. E. Andrews, ed., Proceedings of the 2000 Space Control Conference, Lincoln Laboratory, Lexington, Mass., April 11-13, 2000,  pp. 159-168.

[22] Burnham and Sridharan, “An Eglin Fence,” p. 48.

[23] United States Air Force Scientific Advisory Board, Report on Space Surveillance, Asteroids and Comets, and Space Debris, Volume I: Space Surveillance, SAB-TR-96-04, June 1997 (available at, p. 10.

[24] A chart of minimum detectable RCS vs. range for S/N = 13 dB from a 1990 seminar.  Antonio F. Pensa, “Space Surveillance: 1960 to 2000,” Technical Seminar, MIT Defense and Arms Control Studies Program, Spring 1990.

[25] Johnson, “U.S. Space Surveillance,” p. (8)9.

[26] S.A. Chamberlain and T.A. Slauenwhite, “United States Space Command Space Surveillance Network Overview,” Proceedings of  the First European Space Debris Conference, Darmstadt, Germany, April 5-7, 1993.

[27] USAF Scientific Advisory Board, Report on Space Surveillance, p. 9.

[28] This percentage likely decreased somewhat with the return of the Cobra Dane radar to full power operation in 2004. At this time, the FPS-85 also accounted for 22.3% of “deep space” tracks (in the context deep space means an orbital period greater than 225 minutes but excluding geosynchronous orbits).  Gene H. McCall (Chief Scientist, U.S. Air Force), “Space Surveillance,” briefing slides, January 23, 2001. Available at:

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