Space Surveillance Sensors: The ALTAIR Radar (May 11, 2012)

pALTAIR (ARPA Long-Range Tracking and Instrumentation Radar) is a large steerable dish-array radar at the U.S. ballistic missile test range on Kwajalein in the Pacific Ocean.[1]  Operating at both VHF and UHF frequencies, it is an important collateral sensor in the U.S. Space Surveillance Network, particularly for detecting and tracking newly-launched satellites and for tracking objects in deep-space and geosynchronous orbits.  Together with the Millstone Hill and GLOBUS II radars, ALTAIR provides complete coverage of the geosynchronous belt.


The four SSN radars at Kwajalein.  The ALTAIR antenna is the large dish at upper center, viewed partially against the lagoon.  The antenna for TRADEX, which backs up ALTAIR in the Space Surveillance Network (SSN), is the dish antenna on top of the building near the center of the picture.  The antenna for the ALCOR imaging radar is in the dome at lower left, and the antenna for the MMW imaging radar is in the dome in the center of the picture between the ALTAIR and TRADEX antennas.  (Photograph from

ALTAIR originally was built to study the potential capabilities of Soviet early warning and missile defense radars against U.S. intercontinental ballistic missiles and warheads.  In the early 1960s, the Soviet Union began deploying large VHF (Hen House) and UHF (Dog House) radars that were believed to be for uses such as missile early warning, missile defense, and space surveillance.  Thus ALTAIR was designed to operate at these frequencies.

ALTAIR became operational in 1969.  Although dual-frequency, initially ALTAIR could only track at VHF.  However, in 1973 ALTAIR was given a UHF-tracking capability as part of a program in which it was modified in order to simulate the Perimeter Acquisition Radar (PAR, now the PARCS radar, see post of April 12, 2012) of the Safeguard anti-ballistic missile system (Project SIMPAR).  These modifications enabled ALTAIR to track simultaneously in both VHF and UHF.

ALTAIR began supporting U.S. Space Command in 1982.  As part of the upgrades it received at that time, a deep space tracking capability (at UHF only) was installed.[2] (This deep-space radar tracking capability is described briefly in the May 5, 2012 post on the Millstone Hill radar.)

In 1984, ALTAIR’s original UHF Klystron transmitter was replaced with a traveling-wave tube (TWT) transmitter.  This new transmitter used 24 TWTs in three banks of eight, giving a combined peak power of 4.5 MW and a peak duty cycle of 5%.[3]  (This transmitter has originally been installed on the USNS Arnold missile range instrumentation ship, and became available when that ship was decommissioned).  In about 1993, additional transmitters became available after the USNS Vandenberg was decommissioned.  The ALTAIR transmitter was then upgraded to 32 TWTs (4 x 8) with a peak power of 6.4 MW.

Currently ALTAIR uses 32 L-4920 UHF TWTs, which are the same TWTs used by the PARCS radar.[4]

Technical Characteristics

 ALTAIR’s antenna is 150 foot (45.7 m) diameter paraboloid, which produce a beamwidth of 1.1˚ at UHF (2.8˚ at VHF).  The antenna is a capable of rotating at an angular rate of 10 degrees/second (the rotating portion of the antenna weighs nearly one million pounds [corrected 06/05/2017 –see comment below]).

ALTAIR operates both in VHF (253-163 MHz) and UHF (414-440 MHz, center frequency frequently cited as 422 MHz).  Its VHF capability, with peak power of 7 MW, is used for search and acquisition of new foreign launches.[5] Its maximum bandwidth at VHF is 7 MHz.  Its maximum pulse repetition frequency in both frequency bands is 2,976 Hz.[6]

ALTAIR’s UHF capability is used both for near-earth and deep-space tracking.  At UHF, ALTAIR’s peak and average powers are in the range of 5 MW and 120-250 kW, respectively.[7] Its maximum duty cycle at UHF is 5%.

ALTAIR originally had two deep-space UHF waveforms, a 1 ms pulse length with 50 kHz bandwidth (range resolution = 3 km) for search and a 0.6 ms pulse with a 250 kHz bandwidth (range resolution = 600 m) for tracking.  In the mid-1990s, new deep-space tracking waveforms were added (1 ms with 250 kHz bandwidth; 1 ms with 1 MHz bandwidth; 0.6 ms with 4 MHz bandwidth), providing a deep-space resolution as low as 37.5 m.[8] This improved resolution was described as being primarily useful for tracking the injection of new space launches into transfer or deep space orbits and for separating objects in geostationary orbit clusters.

According to a 1996 paper, waveforms with up to an 18 MHz bandwidth were available for near-Earth tracking, but such resolution had not yet been considered necessary for deep space tracking.[9]  A bandwidth of 18 MHz could in principle provide a range resolution as low as about 8.4 m; however, a U.S. Army fact sheet gives ALTAIR’s UHF resolution as 15 m.[10]  ALTAIR’s range accuracy is better than 5 meters for near-earth tracks, and about 25 meters for deep space tracks.[11]

 Operations and Capabilities

ALTAIR is used primarily for tests of ballistic missiles and ballistic missile defenses and for space surveillance. It has also been used for meteor and ionospheric research.

ALTAIR is the most heavily used radar at Kwajalein.  As of 2002, it spent 128 hours per week on space surveillance, and carried out more than 35,000 deep space tracks and 2,500 high-priority near-earth tracks per year.[12]    [As of 2004, all four SSN radars at Kwajalein contributed 138 hours of space tracking and over 45,000 tracks annually.[13]] The other 40 hours per week were used for preparing for and supporting ballistic missile and ballistic missile defense tests. At all times, ALTAIR is subject to 15-minute recall for top category SSN tasking, particularly new satellite launches.

Although ALTAIR’s primary space surveillance tasking is for deep space objects, it also tracks near-earth objects.  For example, during FY1998, ALTAIR produced 3,462 near earth tracks and 17,753 deep space tracks, and was tasked by Space Command to provide support on 76 out of the total of the world’s 86 space launches. [14]

ALTAIR is also used to provide the ALCOR and MMW imaging radars with pointing information.

ALTAIR has tracked an artificial satellite at a range of 113,100 km (the Galileo satellite on its second earth flyby on its way to Jupiter).[15]

In UHF, ALTAIR has a sensitivity (S/N ratio at 1,000 km for a 1 m2 target with a single 1 ms pulse) of about S = 49 dB = 80,000.  According to a 2000 source, it can achieve a S/N = 38 dB on a 1 m2 target at a range of 1,000 km with an 80 μs pulse (equivalent to S = 49 dB).[16]  During a 1994 space debris measurement campaign, ALTAIR could produce S/N of about 9 on a 0.0001 m2 target at a range of 1,000 km with a single 1 ms pulse (equivalent to S =49 dB).[17]  According to a 1989 source, ALTAIR could produce

S/N of 45 dB on 1.0 m2 target at 1,000 km with a 1.0 ms pulse (equivalent to S =25 dB).[18]

[1] The main sources of 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; A. Gerber, G. Hogan, M. Corbin, J. Corrado, J. Mathwig, H. Fitzpatrick, S. Murphy, M. Schlueter, J.B. Sherill, and T. White, “Recent Upgrades at the ALTAIR Radar for Improved Space Surveillance Support,” Proceedings of the 1996 Space Surveillance Workshop, April 1996, pp. 81-92.; Gary Duff, “Kwajalein Missile Range: ALTAIR Radar Contributions to the Space Surveillance Network,” Proceedings of the 1999 Space Control Conference, pp. 11-15; and 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.

[2] Stone and Banner, p. 236.

[3] S. Chapman, A. Gerber, G. Hogan, S. Hunt, R. Anderson, J. Conrad, D. Sponseller, and M. Schleueter, Recent Improvements at the ALTAIR Radar, Proceedings of the 1994 Space Surveillance Workshop, Lincoln Laboratory,  pp. 229-239. 

[4] Chris Wheeland, Michael Boyle, Marc Barsanti, and Richard True, “Magnetic Interaction between Traveling Wave Tubes and its Effect on Performance and Reliability,” 2010 IEEE International Vacuum Electronics Conference, IVEC 2010, pp. 85-86 (May 18-20, 2010)

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

[6] Roth, et. al.

[7] Example of reports on ALTAIR’s power output at UHF include:

1989: UHF peak power is 5 MW and average power is 250 kW, with pulse lengths from 0.1 to 1,000 us. (Roth, et. al.)

2000:  ALTAIR has a UHF peak power of 5 MW, an average power of 120 kW, and can emit 0.08 ms pulses at a pulse repetition frequency of 300 Hz (Banner and Stone).

2006: “nearly full use of the available average power of the UHF transmitter was 4 MW peak power, with a 5% duty cycle (corresponding to an average power of 200 kW))…”  Pulse lengths of 0.4 ms at a pulse repetition frequency of 120 Hz were used. (Erhan Kudeki, Marco Milla, Martin Friedrich, Gerald Lehmacher, and Dale Sponseller, “ALTAIR Incoherent Scatter Observations of the Equatorial Daytime Ionosphere,” Geophysical Research Letters, Vol. 13, pp. L08108-L08112.)

2010: UHF peak power of 5 MW.  (Gene Stansbery, Paul Kervin, and Mark Mulrooney, “Plans for the Meter Class Autonomous Telescope and Potential Coordinated Measurements With Kwajalein Radars,” 8th US-Russian Space Surveillance Workshop, April 18-23, 2010. Available at:

[8] Gerber, et. al. (1996).

[9] Gerber, et. al. (1996).

[10] U.S. Army Space and Missile Defense Command, Factsheet, “ARPA Long-Range Tracking and Instrumentation Radar,” available at:

[11] Duff, p. 12.

[12] Philip A. Ingerwersen, William W. Camp, and Alan J. Fenn, “Radar Technology for Ballistic Missile Defense,” Lincoln Laboratory Journal, Vol. 13, No. 1 (2002), pp. 109-147 (p. 125).

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

[14] Duff,  p. 12.

[15] Scott M. Wacker, S.M. Hunt, and M.J. Lewis, “The U.S. Space Surveillance Network’s Tracking of Artificial Satellites Orbiting with Hyperbolic Trajectories,” Proceedings of the 1993 Space Surveillance Workshop,  Lincoln Laboratory, March 30-April 1, 1993, v. 2, p. 41.

[16] Stone and Banner, p.237

[17] A. Gerber, G. Duff, and D. Izatt, “Kwajalein Missile Range Contribution to the 1994 Debris Campaign,” Proceedings of the 1995 Space Surveillance Workshop, Lincoln Laboratory, March 1995, pp. 111-119.

[18] Roth, et. al.

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1 Comment

  1. Mike Minardi

     /  June 2, 2017

    You said the antenna weighs 1 million tons I think you mean 1 million pounds. The antenna is actually 800,000 lbs. At least it was when I worked there in the late ’90’s.


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