F-SAR – The New Airborne SAR System

F-SAR identifies the successor of the E-SAR system and is presently flying in several antenna configurations (ATI, fully polarimetric) in X-band. Its maiden flight was in November 2006. This new development was triggered by the strong demand of E-SAR users and customers for data being simultaneously acquired at different wavelengths and polarisations as well as by the demand for very high range resolution. F-SAR is a totally new development utilizing most modern hardware and commercial of the shelf components.

F-SAR concept

• full modular system in X-, C-, S-, L- and P-bands.
• full polarimetric capability in all frequencies.
• single-pass polarimetric interferometry capability in X- and S-bands.
• four recording channels with data rates of up to 2 Gbit/s per unit.
  precise internal calibration (relative accuracy better than 1 dB).
• fully reconfigurable operation modes including capability for digital beamforming on receive.
• additionally, also repeat-pass Pol-InSAR as standard measurement mode for ensuring baseline flexibility.

 

Artist’s view: F-SAR acquiring data simultaneously in X-, C-, L- and P-bands.

F-SAR Technical parameters

 

 

.	X	C	S	L	P
RF [GHz]	9.60	5.30	3.25	1.325	0.35
BW [MHz]	800	400	300	150	100
PRF [kHz]	5	5	5	10	12
PT [kW]	2.50	2.20	2.20	0.70	0.70
Rg res.[m]	0.3	0.6	0.75	1.5	2.25
Az res. [m]	0.2	0.3	0.35	0.4	1.5
Rg cov [km]	12.5 (at max. bandwidth)
Sampling	8 Bit real; 1Gsample/500Msamples selectable; max number of samples 64K per range line; 4 recording channels
Data rate	247 MByte/s (max. per rec channel)

 

The radar is designed to cover an off-nadir angle range of 25 to 55 degrees at altitudes of up to 6000m above sea level, which is the maximum operating altitude with the DO228.

A central computer unit controls the radar via CAN bus and Ethernet. There are 4 modes of operation:

• System configuration,
• System test,
• Internal calibration and
• Radar operation (nominal mode)

The required synchronous timing and clock signals are generated in the main timing unit with less than 6ps jitter and rise times of less than 80ps. A 50MHz ultra-stable Quartz oscillator is the reference. The IGI D-GPS/INS based precision navigation system delivers a GPS 1PPS signal which regularly triggers an absolute time stamp in the raw data header.

In its basic configuration the radar operates with four 1GSample-ADCs. The timing unit allows for two additional ADCs. Each ADC unit has raw data formatting integrated thus the number of recording channels can be easily increased if need be. Via optical fibre high speed data recording units are connected. A second optical fibre links the ADCs to the control computer (monitoring bus) for internal calibration and system monitoring. Quick-look processing will be implemented via dedicated hardware and optical fibre link to the data recording units. On- and/or off-line operation is possible.

 

New F-SAR antenna mount attached to the right hand side of the aircraft behind the wing allowing 7 antennae to be integrated.

Airborne platform & antenna mount

As for E-SAR DLR’s Dornier DO228-212 aircraft was the first choice as platform for the new system. Alternative solutions for a more suitable aircraft with improved performance in terms of flight range and altitude are being investigated in order to better cope with requirements of future campaigns and missions.

A new antenna mount designed to fix planar array antennae to the Do-228 aircraft has been developed. Fully-fledged in multi-frequency configuration it holds 7 right-looking dual polarised antennae: X-band (3), C- band (1), S-band (2) and L-band (1). The P-band antenna is mounted under the nose of the aircraft as for E-SAR.

One important advantage of the antenna mount will be that it makes it easier to change antenna configuration and to mount other antennae avoiding individual airworthiness certification procedures the same time.