Space Surveillance Sensors: The Millstone Hill Radar (May 5, 2012)

The Millstone Hill Radar

The Millstone Hill Radar (MHR) is a large (84 foot diameter) L-Band dish tracking radar located, appropriately enough, on Millstone Hill in Westford MA, a suburb of Boston (42.62˚ N, 71.49˚ W).[1]  It is an important contributing sensor in the Space Surveillance Network (SSN) and is used both for near- and deep-space surveillance.  Two other SSN contributing sensors, the Haystack Radar and the Haystack Auxiliary Radar, which are primarily imaging radars, are located at essentially the same site, and all three are operated by Lincoln Laboratories.  The MHR should not be confused with two other radars at the same site and sometimes also referred to as Millstone Hill radars, one with a 220 foot fixed zenith-pointing antenna and the other with a 150 steerable dish antenna.  Both of these radars operate in the UHF Band and are used almost exclusively for scientific (ionospheric) research, although the steerable dish serves as a backup to MHR for space surveillance purposes.


Figure: Radars at the Lincoln Space Surveillance Center.  The Millstone Hill Radar (MHR) and the domes enclosing the Haystack and Haystack Auxiliary Radars are labeled.  The large steerable antenna and the vertical pointing antenna at the center of the picture are part of UHF radars (which share the original Millstone Hill UHF transmitter) used for scientific research. The domed building at lower left houses the Firepond Optical Research Facility.  From Stone and Banner, p. 240.


The original Millstone Hill Radar began operations in the fall of 1957.  This was a UHF radar, operating at about 450 MHz.  On October 5, 1957 it detected the Soviet Sputnik I satellite, just one day after its launch.   On April 11, 1958 became the first radar (at least the first U.S. radar) to track (not just detect) a satellite when it tracked Sputnik II from horizon to horizon.  This original UHF version of the Millstone Hill Radar was the basis for the FPS-49 and FPS-92 missile tracking radars in the BMEWS (Ballistic Missile Early Warning System) early warning system.

 This original radar was upgraded to L-band in 1962, which involved installing a new antenna with the same diameter.   [The old antenna was moved to the BMEWS early warning radar site in Diyarbakir (Pirinclik), Turkey, where it was designated as the FPS-79 and continued to be used for space surveillance until about 1997.]  In 1963, MHR successfully detected a satellite in geostationary orbit (likely the first time this was accomplished by radar), which involved non-coherent integration of about 40,000 pulses over about 45 minutes.[2]

In the 1960s and 1970s MHR was the site of significant work in the development of techniques for coherently integrating large numbers of pulses in order to detect and track objects and in deep space and geosynchronous orbits.  In particular, Lincoln Laboratory developed the Satellite Acquisition and Tracking using Coherent Integration Techniques (SACCIT) software for its Millstone Hill radar.  This technology allowed the detection of geosynchronous satellites (provided they were stable) using coherent integration of large numbers of pulses.  Geostationary satellites as small as 5 m2 could then be detected with an integration gain of 256. [3]  Coherent integration of 1,000 or more pulses is now routinely used by MHR to detect objects as small as 1 m2 in geosynchronous orbits

This deep-space coherent integration processing capability subsequently was installed at the ALTAIR radar in 1982 and at the FPS-85 in the late 1980s.[4]  A multi-pulse processing capability, likely this one or one derived from it, was also installed on Have Stare (now GLOBUS II in Norway), the FPS-79 at Pirinclik in Turkey (no longer operational), TRADEX on Kwajalein, and a at least one dish radar on Ascension Island.  As an example, according to one source from 1993, at Pirinclik, “For a single deep space track the new coherent integration technique typically requires 10-15 dwells with each dwell lasting 80 seconds and consisting and consisting of 2,000 or more pulses.”[5]

MHR is also used to provide real time pointing data for two SSN contributing radars at the Millstone Hill site, the Haystack and Haystack Auxiliary radars, both of which have much narrower beam widths than MHR.

An extensive renovation of the radar antenna’s mounting and control system, intended to make the radar easier to maintain and reduce downtime, was completed in 2008.[6]

Technical Characteristics

The Millstone Hill Radar operates in L-band at a frequency of 1.295 GHz. This corresponds to a wavelength of 23.3 cm. 

It has a peak power of 3 MW and an average power of 120 kW.   It typically emits 40 one ms pulses per second (although it can produce shorter pulses), which gives a duty cycle of 0.4 and is consistent with its peak and average powers.  

 MHR has an 84 foot (25.6 m) parabolic dish mounted on top of a 26 m high pedestal.  This gives a beam width of 0.6˚ (for comparison λ/D = 0.232/25.6 = 0.0091 = 0.52˚). 

Operations and Capabilities

 MHR is used for both near-earth and deep space (including geostationary orbits) tracking.  Together with two other large dish radars, ALTAIR at Kwajalein in the Pacific and the GLOBUS II in Norway, MHR provides 360 degree coverage of the geostationary orbital band.

According to a recent description MHR spends 80 hours per week on space surveillance.[7]

As of 2000, MHR’s space surveillance priorities “in approximate order of priority, include new NFL [new foreign launch] acquisition, deep-space catalog maintenance, initial orbit determination, and object identification for orbit and for signature correlation and uncorrelated object resolution.”[8]

“At Millstone the mean radar cross section, polarization and Doppler spread (components of the signature) are combined with the orbital characteristics to determine the satellite class, and in many cases the actual satellite.”[9]

MHR is capable of achieving tracking accuracies (1 σ) of 0.01˚ in both azimuth and elevation angle.  It has a range accuracy of 5 m and a range-rate accuracy of 0.005 m/s.[10]

As of 2000, MHR reportedly had a sensitivity of S = 50 db.[11]  That is, it can achieve a S/N = 100.000 at a range of 1,000 km against a 1 m2 target with a single 1 ms pulse.  A 1998 paper gives a similar (although slightly lower) S/N = -16 dB = 0.025 for a single pulse against a 1 m2 target at a range of 40,000 km.[12]  This corresponds to S/N = 64,000 at 1,000 km.

[1] Unless otherwise cited, information is from Melvin F. Stone and Gerald P. Banner, “Radars for the Detection and Tracking of Ballistic Missiles, Satellites, and Planets,” Lincoln Laboratory  Journal,  Vol. 12, No. 2 (2000), pp. 217-24; and Eva C. Freeman, ed., MIT Lincoln Laboratory Technology in the National Interest (Lexington, Mass.: Lincoln Laboratory,  1995), “Chapter 8: Space Surveillance,” pp. 111-128.

[2] Stone and Banner, p. 234.

[3] Freeman, page 115.  Ultimately, coherent integration of 60,000 pulses, with an overall gain of 47.3 dB (0.5 dB less than perfect integration) was demonstrated.  This required correcting for the mismatch between the actual and predicted target line-of-sight acceleration.

[4] Stone and Banner, p. 236.

[5] Nicholas L. Johnson, “U.S. Space Surveillance, Advances in Space Research, Vol. 13, No. 8, pp. 5 – 20.

[6] Gregory P. Hamill, “MIT’s Lincoln Lab Upgrades Sputnik Era Antenna, Space Daily, September 8, 2008; “A Big Eye Sees Small Things,” Lab Notes, Lincoln Laboratory, August 2008 (available at: 

[7] Edward P. Chatters IV. and Brian J. Crothers, “Space Surveillance Network,” Chapter 19 of U.S. Air University, Space Handbook, AU-18, 2nd Ed.. Available at:

[8] Freeman, p. 115.

[9] Freeman, p. 116.

[10] Banner and Stone, p. 237, 240.

[11] Banner and Stone, p. 236.

[12] 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.

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