One of the most significant deficiencies of the current Ground-Based Midcourse Defense (GMD) national missile defense system is its lack of radars that can even attempt to gather data for discrimination purposes. The core ground-based sensors of the GMD system are four (eventually six) upgraded early warning radars (UEWRs), as shown on the map below (click on it for a larger image). However, because of their low operating frequencies and correspondingly small bandwidths, these radars have essentially no discrimination capabilities.
Figure 1. Current GMD radars. The base map is from the 2012 Ballistic Missile Defense Review, showing theoretical coverage of the GMD system against Iran. Aegis radars are not shown (the TPY-2s in Israel and Qatar are not formally part of the GMD system).
These UEWRs are supplemented by forward-deployed TPY-2 and Aegis radars and by a single large Sea-Based X-Band (SBX) radar. However these radars have significant limitations. The forward-based radars have only limited ranges against warhead targets and are primarily useful for tracking missile targets in the early phases of their flights. The SBX, while a large and powerful radar, was built primarily for test purposes and lacks the reliability and hardening of an operational system and has only a limited electronic scan field of view. Last year the SBX’s operating budget was drastically slashed and it was placed in a limited operations and testing status. Moreover, there is only one SBX, and thus it cannot cover the entire country.
In the last year, there has been a series of calls for building and deploying new X-band radars intended to provide discrimination support to the GMD system. The September 2012 National Academy of Sciences Report called for five new “stacked” TPY-2 radars, each using two TPY-2 antennas stacked one on top of the other, to be built and deployed alongside existing upgraded early warning radars. A February 2013 Missile Defense Agency Report to Congress, on the other hand, argued that other radar concepts, “such as a single phased array radar or X-band dish radars” could provide greater capability at a lower cost than the NAS’s proposed stacked TPY-2 radars. In May 2013 Congressional testimony, Lt. General Richard Formica (Commander of the U.S. Army Space and Missile Defense Command/Army Forces Strategic Command and Commander of the Joint Functional Component Command for Integrated Missile Defense) stated that an X-band radar would “certainly” be part of any east coast GMD deployment. In early June 2013, former MDA Director Trey Obering called for upgrading the X-band Ground-Based Radar Prototype (GBR-P) currently located on Kwajalein Atoll and moving it to the U.S. East Coast.
Table 1 below compares various actual, proposed or hypothetical X-band radars. With the exception of the last radar, these radars are all members of the Raytheon’s “family”of X-band phased array radars that utilize essentially the same basic transmit/receive module technology, which facilitates comparisons between them. These are the current TPY-2 forward-based and THAAD radars, the stacked TPY-2 proposed in the NAS Report, the current SBX, the never-built GBR proposed by the Clinton Administration, the current GPR-P, and three possible upgrades to the GBR-P, and a hypothetical new radar with a detection range similar to the SBX but with a larger electronic field of view. The last radar in the table is an existing X-band dish radar.
The Raytheon X-band “family” can be divided into two main branches. One branch consists of the current TPY-2 radars. These radars are used both as the fire control radar for the THAAD theater ballistic missile defense system and as forward-based radars (currently in Japan, Israel, Turkey and Qatar as shown on the map above). These radars are designed to be air-transportable and are thus relatively small (9.2 m2 antenna area). They are intended to be able to carry out a wide range of radar missions, including surveillance, tracking, discrimination and fire control. The NAS proposed “stacked” TPY-2 radars would use two TPY-2 antennas, one on top of the other.
The other branch consists of radars with much larger antennas. The 3+3 national missile defense plan developed by the Clinton Administration envisioned deploying up to nine very large (antenna area = 384 m2) X-band Ground-Based Radars (GBRs) alongside existing early warning radars and at other locations (such as Hawaii). However, none of these GBRs were ever built as the George W. Bush Administration instead chose to build only the single, somewhat smaller SBX. A significantly smaller prototype GBR (the GBR-P) radar was built as Kwajalein and used in the early intercept tests of the GMD system. Under the Bush Administration’s now-cancelled European missile the GBR-P would have been moved to the Czech Republic, where it would have been known as the European Midcourse Radar. Instead, it remains in caretaker status at Kwajalein. A drawback of the larger X-band radars is that they use a much larger module spacing than the TPY-2 radars, resulting in very electronic limited scan angles (11-12 degrees, compared to about 60 degrees for the TPY-2s). For target separations greater than this, these radars would need to mechanically rotate and/or tilt, a relatively very slow process compared to electronic beam positioning. These larger radars are specialized for precision tracking and discrimination.
Since the radars in the table all (except for the last one) use the same basic Raytheon transmit/receive (T/R) modules, we can directly compare them. All else being equal, the power delivered in a single beam dwell will be proportional to the product of the radar’s average power, antenna area, and gain (which will be proportional to the antenna area). The relative P-A-G for each radar is shown in column five of Table 1 with the SBX taken as the baseline. The maximum tracking range will then be proportional to the fourth root of the P-A-G, and this relative range in shown in column six, again with the SBX as the baseline.
Of course, the P-A-G can be viewed in terms other than just range. If, for example, radar A has a P-A-G ten times greater than radar B it could, at the same range, track ten times as many targets, or obtain a S/N ratio ten times larger on a target for improved discrimination or tracking accuracy, or detect a target with a radar cross section ten times smaller than radar B.
As noted above, except for the TPY-2 and stacked TPY-2 (and the dish radar), all these radars have relatively large module spacings, which limits their maximum electronic scan due to the creation of grating lobes (additional main beams). The maximum scan angle in column seven of Table 1 for the larger X-band radars is based on a maximum radar frequency of 10 GHz (for example a 1 GHz bandwidth radar centered at 9.5 GHz in wide band mode). If the frequency is lower or only narrowband operation is considered, the scan angles will be slightly larger (for example at 9.5 GHz the GBR-P would be ±12 °).
The GBR-P was designed to be upgradable. Three different upgrades to the GBR-P are considered in Table 1:
GBR-P Upgrade #1: The older T/R modules in the GBR-P (assumed to be first generation, with average power of 1.2 W) are replaced with current T/R modules (assumed to be third-generation, same as in the TPY-2, with an average power of 3.2 W).
GBR-P Upgrade #2: The outer edges of the current GBR-P antenna face are not populated with modules. This upgrade fills the entire face (increasing the antenna area from 105 m2 to 123 m2) with current T/R modules.
GBR-P Upgrade #3: Another upgrade option that has been proposed would fill the entire GBR-P antenna face, but with the module spacing cut in half, thus increasing the number of modules by a factor of four compared to upgrade #2. This change also doubles the extent of the radar’s electronic scan. While this would give a range nearly equal to that of the SBX, it would be a very extensive (and likely very expensive) upgrade.
The next to last radar in the table (60° SBX Equivalent) is purely hypothetical radar that uses the same module spacing as in the TPY-2 radars (and thus has a full ±60° horizontal electronic scan), but with an antenna size scaled up to give a detection range equal to the SBX. The large number of modules (over 200,000) required for such a radar would probably rule out such an approach, as this is equal to the total number of modules in all eight of the TPY-2s the United States has built so far.
As noted above, a February 2013 MDA report stated that X-band dish (non-phased arrays) radars could provide a more robust and inexpensive capability than the NAS’s proposed Stacked TPY-2 radars for the GMD system. The last radar in the table is the Have Stare X-band dish radar which was at one time deployed at Vandenberg Air Force base in California. It was subsequently moved to Vardo, Norway, where, renamed the GLOBUS II, it operates as part of the U.S. Space Surveillance Network. The “emergency response” NMD system proposed by the Air Force in the mid-1990s would have used the Have Stare as its primary precision tracking and discrimination radar. Since its antenna is mechanically-steered, the number of targets such a radar can handle simultaneously will be quite limited compared to compare to the other radars in the table.
Here’s the table (click on it for a larger image):
Table 1. Comparison of X-Band Radars
 Missile Defense Agency, “Stacked AN/TPY-2 Array Concept, Report to Congress, February 13, 2013. See Jordana Mishory, “DoD Advises Against ‘Stacked’ An/TPY-2 Radars to Boost Missile Defense,” Inside Missile Defense, May 1, 2013.
 General Formica: “Back to my point on sensoring and assessment and discrimination capability, an X-band radar, frankly, anywhere east would greatly benefit the threat that I and we in the agency see coming. And certainly that would be part of it.” Hearing of the Strategic Forces Subcommittee of the Senate Armed Services Committee, May 9, 2013.
 Military Electronics Briefing, “BMD X-Band Radars & BMD C4I,” Teal Group Corporation, July, 2007.
 The average power in the table for the HAVE STARE assumes it has the same maximum duty cycle (35%) as the larger Haystack X-band dish radar.