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WSMR and PMRF represent two excellent locations for evaluating AEW concepts. We deployed RSTER to North Oscura Peak (NOP) on WSMR in early 1993. NOP is at the northeast corner of WSMR, 8000 feet above sea level and approximately 3500 feet above the desert floor. The site offers virtually 360 degree visibility and line-of-site to a variety of terrain types including desert, bare and wooded hills, mountains, lava flows and small suburban areas. Very large clutter returns, sometimes exceeding 60 dBsm in a detection cell, were observed. As a result, interesting and challenging data has been collected to support STAP studies.
The sensor was deployed to Makaha Ridge at PMRF in October 1994. The ridge sits 1500 feet above sea level with a precipitous cliff and an unobstructed view of Niihau, a neighboring island, and the sea below. The site is well suited to address issues related to over-the-water AEW. A variety of targets including surface vessels and low-flying drones are available for Mountaintop tests.
A major challenge of the Mountaintop program is to provide a meaningful emulation of the airborne surveillance environment. Central to this effort is the Inverse Displaced Phase Center Array (IDPCA) designed to produce, from a fixed site, clutter returns with the same spatial and temporal characteristics as observed from an airborne surveillance platform. The concept behind the IDPCA is easily understood if one recognizes that the observed clutter profile in azimuth-Doppler space is due to the motion of the aperture's phase center. To effect the emulation, one can either move an antenna or deploy several antennas, with displaced phase centers, and move between them. The IDPCA employs the latter approach, using multiple patch antennas, transmitting sequentially (PRI to PRI) along a line of them. Apparent motion is along the length of the array. The antennas are sized to emulate returns observed through a surveillance sensor's antenna sidelobes. The clutter returns just span the Doppler space.
The IDPCA is a transmit-only device. Clutter returns are received through the larger RSTER-90 antenna. The IDPCA supports a 16-pulse coherent processing interval (CPI), transmitting either out of one or three columns simultaneously depending on the mode of operation selected. The IDPCA has proven effective for emulating the clutter environment of the AEW platform.
Although the radar system is installed at a fixed ground site, radar platform motion is emulated using the Inverse Displaced Phase Center array (IDPCA). This device, located near the RSTER receive array, transmits a sequence of coherent radar pulses from sequential subarrays (consisting of one or three elements) of a uniform linear horizontal array with a subarray phase center spacing of 12.2 inches. Thus the radar transmitter appears to move horizontally along the IDPCA array axis at a rate of 12.2 inches per PRI seconds, where PRI is the pulse repetition interval. Note that the maximum ground clutter Doppler is (12.2 in)/(PRI*wavelength); this is half of the value for a fully airborne radar because in the Mountaintop system the receiver is stationary, only the transmit phase center moves.
The IDPCA can be operated in a tracking mode where it applies successive phase shifts to the pulses which it transmits so that the Doppler of the clutter patch at which the RSTER receive beam is looking is shifted to 0 Hz. All other clutter patches' Dopplers are then shifted accordingly.
The pulse-to-pulse gain stability of the IDPCA is not as good as would be the case for an airborne radar which transmits all pulses out of a single antenna. At the time the clutter data described below was collected, there was a periodic component to the IDPCA's element-to-element gain; the effect of this is that large clutter scatterers will be replicated periodically in the Doppler dimension, with a period of 1/3 of the total Doppler dimension, but with a reduced gain of about -25 dB relative to the strength of the true clutter return. These artifacts will look like additional clutter scatterers to the beamforming signal processor.
14 transmit/receive elements/record channels
Antenna array is horizontally oriented with respect to the ground
Uniform linear element spacing of 0.333 meters
Fixed vertical beam, pointed horizontally, beam width 6 degrees (3 dB)
Horizontal polarization
Element gain 17.5 dBi
Operating frequency range 400 to 500 MHz
Transmitted pulse is 500 kHz Linear Frequency Modulated (LFM) pulse
Peak transmit power 100 kW
Transmit duty cycle 6% (max.)
Coherent Processing Interval (CPI) is 16 coherent pulses
PRF capability 250 to 1500 Hz
Receiver IF bandwidth 200 kHz (3 dB)(Gaussian filter)
Receiver noise figure 6 dB (including plumbing)
Receive system causes 3 phase reversals in the data; thus approaching
targets appear to have negative Doppler and vice versa.
Noise floor in recorded data ~48 dB (with respect to 1)
Calibration: 1 watt at a receive element gives 193 dB in the recorded
data (with respect to 1)
Effective range sampling interval (after demodulation) is 1 microsecond
16 transmit subarrays
Horizontally oriented with respect to the ground
Uniform linear subarray spacing of 12.2 inches
Horizontal beam width 32 degrees (3 dB)
Vertical beam width 28 degrees (3 dB)
Horizontal polarization
Subarray gain 16 dBi
Operating frequency range 430 to 440 MHz
Peak transmit power 2400 watts
Transmit beam steering capability +30, 0, -30 degrees wrt broadside
Transmitted pulse, duty cycle, CPI, PRF same as above
A CPI of data has 3 dimensions: range sample, pulse, and receive element.
Because MATLAB can only support 2-dimensional matrices, two of the data
dimensions are packed into the matrix row dimension. The 14 columns of the
data matrix correspond to the 14 receive channels. The number of range
samples may be found from
azxmit: transmit/receive beam azimuth of the RSTER array, in degrees
relative to true north. When IDPCA is in tracking mode, this is the
azimuth at which the clutter will be shifted to 0 Hz Doppler.
cpi1 (cpi2, etc.): data matrix containing a CPI of data.
fxmit: vector containing the transmit frequency (Hz) for each CPI
npulses: vector containing the number of coherent pulses for each CPI
(usually 16).
pri: vector containing the Pulse Repetition Interval (PRI) (seconds) for
each CPI
tpulse: vector containing the transmit pulse width (seconds) for each CPI
trecord: time delay (microseconds) from pulse transmission to beginning of
recording. Since the range samples are collected at 1 microsecond
intervals, the range corresponding to range sample S is approximately
given by
Range = 0.5*C*(trecord - 1 + S)
where C is the speed of light (in meters per microsecond).
Jamming signal was pseudo-random noise with a bandwidth of 600 kHz: this
signal is broadband relative to the radar's bandwidth, so it appears
as broadband barrage jamming.
The jammer was sited on a mountain at a range of 65 km from the radar,
and at an azimuth of 302 deg relative to true North, with respect to
the radar.
The jammer transmit frequency was 435 MHz
The jammer transmit power was 44 watts
Horizontal polarization
Jammer's antenna was pointed at the radar
Vertical beamwidth 70 degrees (3 dB)
Horizontal beamwidth 22 degrees (3 dB)
Jamming antenna gain 11 dBi
Radar system parameters specific to this data:
RSTER array broadside 260 degrees relative to true North
Pulse width N/A
Pulse Repetition Interval (PRI) 1600 microseconds
Range window 897 to 1300 microseconds (134 to 195 km)
The IDPCA transmit beam was steered to -30 degrees to illuminate a particular large clutter scatter (a mountain range), at a range at which clutter returns in most other directions were minimal due to shadowing by nearer-range mountains. Thus this data contains ground clutter primarily from a single large scatterer which is isolated in angle. The IDPCA was not in tracking mode (there was no artificial Doppler shift), so the clutter at -30 degrees off IDPCA broadside had a Doppler of (PRF/2)*sin(30 deg) = (PRF/4). The simulated target was placed at the same range and Doppler as this scatterer, but at a different azimuth. With nonadaptive beamformer processing, the target will be partially or completely obscured by sidelobe leakage from the clutter scatterer (unless a very low sidelobe taper is used). Adaptive nulling can be used to mitigate the clutter interference and clearly reveal the target.
Note that jamming may be included in this scenario by adding the jamming-only data described above to this data. The jamming data may be scaled to adjust its relative power level, and a linear phase progression may be applied to the element dimension to change the apparent direction of the jammer.
RSTER array broadside 260 degrees relative to true North
IDPCA array broadside 275 degrees relative to true North
IDPCA transmit beam steered to -30 degrees relative to IDPCA broadside
First pulse transmitted out of southernmost IDPCA element (apparent
direction of motion is south to north)
Transmit frequency 435 MHz
Pulse width 100 microseconds
Pulse Repetition Interval (PRI) 1600 microseconds
Range window 881 to 1283 microseconds (132 to 192 km)
Simulated target parameters:
Target range 152 km
Target Doppler 156 Hz (PRF/4)
Target azimuth 274 degrees relative to true North
Target Radar Cross Section (RCS) ~0 dBsm