1. Introduction

1.4. High Resolution Aviation System S-band Radar Subsystem Introduction And Processing Effect

The multi-dimensional synthetic aperture radar of the Chinese high resolution aerial system quantifies the target in multi-band, multi-polarization, multi-temporal, elevation, multi-angle, and other multi-dimensions, which can obtain more complete target characteristics for scientific research and quantitative remote sensing applications in various fields. The S-band radar subsystem is one of the six single-band radar subsystems designed and deployed in the multi-dimensional SAR system, which can provide quantitative measurement information such as full polarization, multi-temporal phase, and scattering coefficient.

1.4.1. System composition and main functions

There are two support states for the S-band radar subsystem. The first support state is a multi-dimensional radar system subsystem that works jointly with one or more other band radar subsystems under the centralized control of the multi-dimensional radar system. The second support state is that the S-band radar subsystem works independently as a radar.

When working jointly, the multi-dimensional radar provides a unified mission management computer, data logger, real-time imaging processor, and motion attitude measurement equipment with a unified mounting cabinet. In this matched state, the S-band radar subsystem consists of three components: an antenna unit, an integrated electronics unit, and a power distribution unit. The system composition, the interconnection relationship between the units and the external interfaces of the system are shown in the following figure. The antenna unit is installed under the belly of the wheel loader, and the power distribution unit and the integrated electronics unit are installed in the cabin of the wheel loader.

Figure 1. System composition of multi-dimensional joint work.

Figure 1. System composition of multi-dimensional joint work.

The S-band radar is a complete synthetic aperture radar when the single band works independently. The system composition includes antenna unit, integrated electronics unit, power distribution unit, mission management unit, data recording unit, signal processing unit, motion attitude measurement equipment, and cabinet. The system composition, the interconnection relationships between the units, and the external interfaces of the system are shown in the following figure. The antenna unit is installed under the belly of the wheel loader, and the other equipment is installed in the wheel loader cabin.

1.4.2. System main technical requirement

The following table shows the technical indexes achieved by the S-band radar subsystem.

Table 1. Technical indexes.

Table 1. Technical indexes.

Parameters	Value
Operating frequency	S, Center Frequency 3.2Ghz
Operating pattern	Full polarimetric stripmap mode
Polarization	HH,HV,VH,VV
Resolution/ Swath width / Operating distance	1m/15km/20km
Operating range	1000～10000m
Polarization isolation	≥ 28dB(Antenna) ≥ 30 dB (Corner reflector)
Noise equivalent sigma zero	-30dB（20km Operating distance, 1m Resolution） ≤-33dB（30km Operating distance, 3m Resolution） ≤ -35dB（40km Operating distance, 10m Resolution）
Side-lobe level and ambiguity	ISLR ≤ -13Db; PSLR: ≤ -20dB; Azimuth ambiguity≤ -20dB； Range ambiguity≤ -20dB
Quantitative measurement index
Geometric measurement accuracy	Horizontal accuracy ≤ 5m（1σ） ( Flatland DEM)
Relative radiometric calibration	≤ 1dB（3σ）
Absolute radiometric calibration	≤ 2.5dB（3σ）
Polarimetric imbalances	Phase ≤ 5°（1σ） Amplitude：≤ 1dB（1σ）

Note: ISLR: Integral sidelobe ratio; PSLR: Peak side lobe ratio.

2. Materials and Methods

2.1. Route planning

The white area in the figure refers to the area to be surveyed, and the yellow mark represents the airport where the aircraft takes off and lands.

Figure 2. Route planning.

Figure 2. Route planning.

The flight task is divided into two tasks, i.e., calibration flight and survey area flight. The introduction to each task is shown:

Table 3. Introduction to the calibration flight.

Table 3. Introduction to the calibration flight.

Index	Calibration flight	Task introduction
Task date	2020.0524	2020.0524\0527
Survey area	Jilin Yushu	Jilin Yushu
Side visual angle	45° (Left view)	45° (Left view)
Flight altitude	2200m	4200m
Flight speed	250km/h	250km/h
Breadth	1456m	4000m
Lateral overlapping degree	\	30%
Lateral extension	\	20%
Mean height of survey area	\	200

The planned route is as follows:

Figure 3. Route diagram.

Figure 3. Route diagram.

In the diagram, the routes from 1 – 10 are used to cover the survey area, and the average length of each route is between 40 km and 41 km. The length of the straight line needed by a flight to cover the survey area is 410 km, and the covered area is 1600 square kilometers (including the overlapping part). Route 11 is used for calibration flight, and X, Y is the framework route. The pilot data table is as follows:

Table 4. Route diagram.

Table 4. Route diagram.

Route No.	Start		Destination
	Longitude	Latitude	Longitude	Latitude
1	126°45′18.10″	126°45′18.10″	126°45′18.10″	126°45′18.10″
2	126°43′45.35″	44°48′46.74″	126°43′05.23″	45°11′14.54″
3	126°42′12.60″	44°48′45.92″	126°41′31.86″	45°11′13.93″
4	126°40′39.84″	44°48′45.07″	126°39′58.49″	45°11′13.32″
5	126°39′07.09″	44°48′44.22″	126°38′25.12″	45°11′12.68″
6	126°37′34.34″	44°48′43.32″	126°36′51.76″	45°11′12.02″
7	126°36′01.59″	44°48′42.41″	126°35′18.39″	45°11′11.33″
8	126°34′28.84″	44°48′41.50″	126°33′45.03″	45°11′10.64″
9	126°32′56.09″	44°48′40.54″	126°32′11.67″	45°11′09.90″
10	126°31′23.35″	44°48′39.57″	126°30′38.34″	45°11′08.37″
11	126°25′08.42″	44°51′20.83″	126°27′41.62″	44°50′17.40″
Y	126°27′15.83″	44°48′41.86″	126°42′59.94″	44°49′03.59″
X	126°42′26.27″	45°10′17.20″	126°25′23.49″	45°09′50.42″

Considering that the flight time of general aviation aircraft is about 5 hours per flight, and the effective operation time is 3 hours, the effective flight mileage is 750 km. A single flight can meet the requirements to observe the entire survey area, and two flights are available to complete two cover flights in different time phases for the survey area.

2.2. Design of External Calibration Field and Determination of Quantitative Index

2.2.1. Design of External Calibration Field

Before carrying out agricultural application experiments, external calibration flight inspection is required. The purpose of calibration flight experiment is to exam the system function and performance index, survey resolution, side lobe ratio, radiation calibration accuracy, polarization calibration accuracy and other quantitative indexes by using external calibration method. Therefore, the external calibration equipment used for the flight test is also an important test equipment for this flight test. The calibration field test equipment includes the calibrator and other equipment required to install and test the calibrator at the external field. The test equipment at the calibration field for the S-band radar subsystem flight test is shown in the following table:

Table 5. Test equipment at calibration field.

Table 5. Test equipment at calibration field.

No.	Description	Qty.	Remarks
1	0° dihedral corner reflector (Side length of 0.6m)	3	For polarization calibration
2	22.5° dihedral corner reflector (Side length of 0.6m)	2
3	45° dihedral corner reflector (Side length of 0.6m)	1
7	trihedral angle reflector (Right-angle side length of 0.7m)	13	For resolution measurement and radiation calibration mode
8	Difference measurement equipment	1 set	Measure the position of aerial carrier and the corner reflector

The calibration experiment field site is selected and set at a large area with flat terrain and weak ground-object backscattering. The following figure shows the layout diagram of calibration field angle reflector.

Figure 4. Layout diagram of calibration field.

Figure 4. Layout diagram of calibration field.

In the diagram, T1-T6 are used for radar range radiation calibration, which should be uniformly distributed in the mapping bandwidth, and l1 is calculated accordingly. T7-T10 are used for azimuth radiation calibration, and l2 is not less than 100 meters. T11-T13 and D1-D3(A/B) are used for polarization calibration, and l3 is not less than 200 meters. When deploying the calibrator, it is necessary to overlap the normal line of each calibrator with the connection line between the calibrator and the aircraft in the front and side view.

The ground synchronous data acquisition scheme requires the difference measurement equipment to synchronize, during the flight of the aircraft, the position information of the aircraft and the calibrator, thereby obtaining high-precision relative position relationship.

Relevant radar and flight parameters are shown in Table 6.

2.2.2. Determination of Quantitative Index

• Resolution

The selected calibration field is near the runway in the airport. The ground object of the airport is bare land, and six trihedral corner reflectors are arranged with the interval of 300 m. The following figure shows the SAR image of the calibration field and the two-dimensional profile of the calibrator.

Figure 5. Calibration field imaging result.

Figure 5. Calibration field imaging result.

Figure 6. Mode 1 (1m) resolution point target corresponding (midrange) (a)Azimuth (b)Range.

Figure 6. Mode 1 (1m) resolution point target corresponding (midrange) (a)Azimuth (b)Range.

It can be seen from the measurement resolution index of multiple calibrators navigated that in the whole breadth, both the image distance and azimuth resolution are better than 1 m, the peak side lobe ratio is better than 20 dB, and the integral side lobe ratio is better than 15 dB.

• Radiometric Calibration

Relative radiation accuracy is a measure of system stability, which refers to the maximum error of the relative value of target backscatter coefficient or radar cross-sectional area measured at different positions in the SAR image. Usually, the maximum error value is expressed in dB, and its calculation formula is shown:

$${\Delta }_{rad\_r}=\sqrt{{\displaystyle \sum _{i=1}^{N_p}\frac{\left({\widehat{\epsilon }}_{idB}-{\overline{\epsilon }}_{dB}\right)}{N_p}}}$$

(1)

In the formula, ${\widehat{\epsilon }}_{idB}$is the point target energy of the $i$ inspection point expressed in dB, and ${\overline{\epsilon }}_{dB}$ is the average value of the point target energy of $N_p$ inspection points, and its calculation formula is as follows:

$${\overline{\epsilon }}_{dB}=\frac{1}{N_p}{\displaystyle \sum _{i=1}^{N_p}{\widehat{\epsilon }}_{idB}}$$

(2)

Absolute radiation accuracy refers to the error between the measured value of the backscatter coefficient of a uniform target on the ground or the radar cross-sectional area of a point target on the SAR image and the actual value of the target's backscatter coefficient or radar cross-sectional area. Typically, the maximum error value is expressed in dB. The absolute radiation accuracy reflects the absolute difference between the backscatter coefficient or radar cross-sectional area of the ground object corresponding to the image pixel after radiometric calibration and the backscatter coefficient or radar cross-sectional area of the real ground object. Its calculation formula is shown by formula (3):

$${\Delta }_{rad\_a}=\max \left\{\left|{\widehat{\sigma }}_i-{\sigma }_i\right|\right\}$$

(3)

In the formula, ${\widehat{\sigma }}_i$ is the RCS measurement value (dBm²) of the $i$ inspection point, and ${\sigma }_i$ is the nominal RCS measurement value (dBm²) of the $i$ inspection point.

To verify the performance of the system in complex environments, the radiation calibration process only corrects the effects of the cubic term of the antenna pattern and distance. The point target energy of the index inspection point (corner reflector) ${\epsilon }_P$ is calculated using the peak value method. The amplitude of the peak point in the energy sub-image corresponding to the inspection point is taken as the energy of the inspection point ${\epsilon }_P$. It can be seen from the measurement indicators of multiple calibrators that have been voyaged many times, the relative radiation accuracy is better than 1dB and the absolute radiation accuracy is better than 2.5dB in the entire width range.

2.

Polarization Calibration

Polarization isolation refers to the ratio of the energy of the transmitted or received H-polarized (V-polarized) signal to the energy of the serially transmitted or received V-polarized (or H-polarized) signal. The calculation formula of polarization isolation is as follows:

$${\Delta }_{pol1}=20\cdot \lg \left(\min \left\{\sqrt{{\displaystyle \sum _{i=1}^{N_p}\frac{{\left(abs\left(H{H}_i/H{V}_i\right)\right)}^2}{N_p}}},\sqrt{{\displaystyle \sum _{i=1}^{N_p}\frac{{\left(abs\left(V{V}_i/V{H}_i\right)\right)}^2}{N_p}}}\right\}\right)$$

(4)

The polarization channel amplitude imbalance is defined as the ratio between the HH and VV signal energies of the co-polarized channels. The amplitude imbalance calculation formula is as follows:

$${\Delta }_{pol2}=20\cdot \lg \sqrt{{\displaystyle \sum _{i=1}^{N_p}\frac{{\left(abs\left(H{H}_i/V{V}_i\right)\right)}^2}{N_p}}}$$

(5)

Polarization channel phase imbalance is defined as the difference between the HH and VV signal phases of the co-polarized channels.

The phase imbalance calculation formula is as follows:

$${\Delta }_{pol3}=\sqrt{{\displaystyle \sum _{i=1}^{N_p}\frac{{\left(phase\left(H{H}_i/V{V}_i\right)\right)}^2}{N_p}}}$$

(6)

It can be seen from the measurement of the resolution index by multiple calibrators that have traveled many times. The following table shows the calibration results:

Table 7. Polarimetric calibration results.

Table 7. Polarimetric calibration results.

Parameter	Value
Polarization isolation(dB)	-30.67
Phase imbalance(°)	3.41°
Amplitude imbalance(dB)	0.98dB

In the entire width range, the polarization isolation is better than -30dB, the amplitude unbalance is better than 1dB, and the phase unbalance is better than 5°.

3. Results

Figure 7. Quantitative Measurement Effect (Pauli Decomposition) (a) Before the calibration of the elm survey area (b) after the calibration of the elm survey area.

Figure 7. Quantitative Measurement Effect (Pauli Decomposition) (a) Before the calibration of the elm survey area (b) after the calibration of the elm survey area.

4. Conclusions

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