The development and operational use of the E-3A AWACS, E-8C JSTARS and RC-135 Rivet Joint aircraft has given the USA a unique and comprehensive C4ISTAR capability, allowing commanders to quickly identify potential threats as well as controlling and directing their forces in the most efficient manner. However, technology has matured considerably since the design of these systems, particularly in the size and performance of Ground Moving Target Identification (GMTI) and Synthetic Aperture Radar (SAR) technologies, as demonstrated by their use of the Global Hawk UAV. However, both a conventional aircraft and a UAV take time to deploy to a specific theatre, require significant logistical support and have endurance limitations - weakness that any replacement system would need to address. Consequently, in 2001 when the Secretary of Defence to the Air Force was considering the possible options to replace these systems, it was decided to instigate the development of a programme to investigate the deployment of a Space Based Radar (SBR) system that could undertake all the functions of these 3 aircraft from the lofty reaches of earth orbit.
The planned Space Radar (SR) will be the culmination of the concept analysis and technology risk reduction program. A decision will be made on what type of satellite system to develop. The three candidates include a Mono-static SBR, a Bi-static SBR with Geosynchronous (GEO) transmitters and Medium Earth Orbit (MEO) receivers, or a Bi-static SBR with Geosynchronous transmitters and Unmanned Aerial Vehicles (UAVs) equipped as receivers. Potentially an SBR system could provide world-wide coverage with little deployment time and at far less cost than the current systems. However, global events have moved on since 2001 and now, rather than attempt to develop a world-wide system, the focus for development has shifted towards theatre support of deployed forces.
The nominal airborne warning requirements for the SR includes:
The nominal JSTARS requirements for the SR includes:
Unsurprisingly, the nominal Rivet Joint requirements have not been made public. However, as whole generations of ELINT satellites have been in orbit for many years, and the ability of low and medium earth orbit, and in particular geosynchronous orbit satellites to undertake this functions is well known, the SBR Rivet Joint requirement is probably just an enhancement of systems already in place.
The benefits of an SR system have been understood for some time. Back in 1995 the Mono-static SR system was developed by SMC, ACC and AFSPC during a Space Sensor Study. Two designs consisted of either 24-30 radar equipped satellites at 1400nm or 9-10 satellites operating at 5600nm from earth. The radar’s would need to be capable of operating at appropriate frequencies for either the AWACS or JSTARS role and data-linked to ground stations for command and control. These systems would have a theoretical field of view over the entire earth, apart from near the poles. However, in practice the desired Area of Operations (AOR) would be up to six 350nm areas, designated by the battlefield commander, that would roughly correspond to the current AWACS coverage for two theatres of operation. The AORs would receive continuous coverage – a 10 second revisit rate and the AORs could be quickly changed by re-focusing the phased array antenna feeds on the radar satellites.
The Bi-static Geosynchronous/Medium Earth Orbit (GEO/MEO) Space Radar concept has been developed by ESC. The proposed system consists of three to four radar transmitter satellites at geosynchronous orbit, combined with 24-36 receiver satellites at MEO. The theoretical coverage and control methods would be similar to those for a monostatic system. However, due to the "fixed" locations of the GEO transmitters, the size of the antennas, and the revisit rates required, one transmit satellite would be dedicated to each theater of operation. Orbital positions would be adjusted if necessary to provide appropriate instantaneous fields of regard. The use of bi-statics requires fewer transmitters than the mono-static constellations, and would also allow for a lighter fleet of receiver satellites at MEO. However, the bi-static approach would also result in a significant increase in signal processing to match filter the target return signals received at the MEO satellites, and polar coverage is not possible. Thus the tradeoff between mono- and bi-static concepts is weight vs. signal processing power and flexibility.
A Bi-static GEO Space Radar would include a constellation of 3-4 GEO transmitters equipped with an L-band radar for airborne warning and control missions which would require a 100 m dish that would weigh 30,000 lbs and require 20+ kw of power. A Joint STARS-like radar in geosynchronous orbit would function in the S-band and would require a 25 m dish which would weigh 6,000 lbs and require 2 kw+ of power. The MEO receivers would include a constellation of 24-36 receivers at an altitude of 1600 km. For AWACS-like missions, the MEO satellites would require a 35 x 35 m array weighing 10,000 lbs. A Joint-STARS-like mission would require a 10 x 10 m receive array that would weigh 4,000 lbs.
The SR system is being designed to complement, rather than compete with, the Global Hawk and other wide-bodied ISR aircraft under development. However, as the technology matures and higher powered radars and more sensitive receivers become available, there is little doubt that the capabilities of this system could soon encompass a number of ISR roles currently covered by fixed-wing aircraft. However, it has become apparent that, although the SR was conceived to replace the AWACS, that task is currently too technically challenging at the moment and this capability is unlikely to be developed until around 2015-2020.
Currently, the main objective of the SR programme is to field, by 2010, a space borne GMTI capability for theatre commanders enabling moving targets to be tracked by day or night, in any weather, from orbit. Notional funding profiles suggest an SR could cost some $700-800m per year by 2008 and it will be interesting to see how this capability is developed over the next few years. Nevertheless, although much of the necessary technology is well known, particularly active electronically scanned arrays and synthetic aperture radar, it will be interesting to see how long it takes for the SBR system to become fully effective. Provided sufficient funds are forthcoming, it is possible that by the end of the decade fused data from AWACS, JSTARS, UAVs and an SBR system could be piped directly into the cockpit of an aircraft en-route to a target, giving US forces an even greater advantage over any potential adversary.
In 2005 the Space Based Radar programme was re-named Space Radar and hit more problems, with the planned on-orbit demonstration cancelled, whilst Congressional and intelligence officials continue to disagree over the efficacy of the proposed demonstration. Congress also voiced great concerns over the high cost of the programme, as well as highlighting significant differences between the military and intelligence communities regarding the systems actual use. To try and resolve these problems an inter-agency panel was formed to sort out the problems and this panel is concerned that there is no gap in satellite coverage between the end of the Future Imagery Architecture (FIA) and the coverage provided by the Space Radar programme. However, this would mean that the first Space Radar satellite would have to be launched in 2015, which also means that a decision to start the System Development and Demonstration (SDD) phase must be made in 2007. Then in Jan 05 the Air Force Secretary, Peter Teets, muddied the waters by restructuring the programme and adding a demonstration of two one-quarter scale on orbit satellites in 2008 costing between $200 – 400 million, using funds earmarked for the SDD, pushing the launch of the first operational satellite back to 2017-8. Unfortunately the idea of an on-orbit demonstration is not supported by either industry contractors or the intelligence community, almost guaranteeing that the idea will be killed by Congress, whilst still allowing the basic programme to survive. Nevertheless, this is yet another disruption to this vital programme and, added to the many people who doubt whether the technology can be made to work in the time available, will only cause the spotlight to refocus once again on this ambitious project.
One of the major stumbling blocks in the differing requirements of the intelligence and military communities is whether the Space Radar system should have a MTI capability. A SAR satellite sufficient for the requirements of the intelligence community would probably only require a relatively small antenna of around 40 square meters, however, the MTI capability sought by the military, sufficently sensitive to track ground vehicles travelling at 30 kilometeres per hour, would probably require an antenna as large as 100 square meters at vastly increased cost. Another area of serious concern is the total estimated cost of the proposed Space Radar system. Estimates for a nine satellite sustem with 40 square meter antennas range bwteen $35 - $50 billion, but given the inability of recent military space systems to stay within budget and the complex immauture technologies involved, there would be every chance the final cost would be more than double this figure.
However, despite the urgent need for the capabilities that would be provided by Space Radar, the doubts about the technology planned for the system and the potential costs involved gathered. Finally, in March 2008 the NRO notified Northrop Grumman that they were terminating the Space Radar programme. In the end this was not a difficult decision to make as the projected costs were becoming too high and the NRO and Air Force had failed to agree on system requirements and control.
In an attempt to try and still try and leverage some capability in this area, the US Air Force is now attemting to see if it can benefit from paying for access to data from international and commercial radar satellites already in orbit. Currently in orbit are the Canadian Radarsat-2, the German SAR-Lupe constellation and the Israeli TecSAR satellite and funding has been requested from "all radar data providers and space radar system developers" to allow access to commence as early as 2009. Radarsat-2 was launched in Dec 2007, the last of the five SAR-Lupe satellites was deployed in Jul 2008 and TecSAR was launched in Jan 2008, so whilst these modern systems would not have many of the bells and whistles that Space Radar would have provided, drawing data from all of them would nevertheless still go some way to providing the capability sought by the US Air Force. The first step in this process will be a demonstration period when the performance, capability and utility of the radar data available from the current systems can be evaluated and the necessary operational concepts agreed.
At this stage it's difficult to estimate when this commercial capability could be available, but it makes sense to use the technology currently available to provide some capability, whilst allowing time for the advanced technologies necessary for the more ambitious Space Radar to reach maturity.
Updated Sep 2008