# An Improved Aeromagnetic Compensation Method Robust to Geomagnetic Gradient
## 4 Field Experiments and Data Analysis  
### Aeromagnetic Survey System  
As shown in Figure 6, the Cessna-208 fixed-wing aircraft was selected as the survey platform, and a 5-m-long probe rod was installed at the tail of the aircraft. The probe rod was equipped with an optically pumped cesium magnetometer and a three-axis fluxgate magnetometer. The data recording and compensation system was installed in the aircraft cabin. All these instruments were developed by the Aerospace Information Research Institute of the Chinese Academy of Sciences (AIRCAS). A DGPS antenna was attached to the top of the plane to better receive satellite signals. The optically pumped cesium magnetometer was the most important sensor in the system, and the sensitivity was as low as 0.3 pT/\(\sqrt{\text{Hz}}\) at 1 Hz. The sensitivity of the fluxgate magnetometer was 6 pT/\(\sqrt{\text{Hz}}\) at 1 Hz. The technical specifications are shown in Tables 1 and 2. \begin{table}
\begin{tabular}{c c c c} \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline Measurement range & 10,000 nT to 105,000 nT & Operating temperature & \(-40\)\({}^{\circ}\)C to +70\({}^{\circ}\)C \\ \hline Orientation range & Not less than 15\({}^{\circ}\) to 75\({}^{\circ}\) and 105\({}^{\circ}\) to 165\({}^{\circ}\) & Power consumption & 12 W (24 V input) \\ \hline Heading error & \(<\)\(\pm\)0.3 nT & Dimensions & Sensor: \(\Phi\)45 mm \(\times\) 130 mm; Electronics: 38 mm (H) \(\times\) 50 mm \\  & & & (W) \(\times\) 250 mm (D) \\ \hline Noise & \(\leq\)0.3 pT/\(\sqrt{\text{Hz}}\) & Weight & Sensor: 0.5 kg; Electronics: 0.5 kg \\ \hline Gradient tolerance & 40,000 nT/m & & \\ \hline \end{tabular}
\end{table} 
Table 1: The technical specifications of the optically pumped cesium magnetometer. Figure 6: The aeromagnetic survey system. The aeromagnetic data recording and compensation system and the Differential Global Positioning System (DGPS) receiver were installed in the cabin in Figure 7. A software application was used to check the status of sensors and DGPS. All of the devices were powered by lithium batteries. The technical specifications of the recording system and the DGPS are shown in Tables 3 and 4. \begin{table}
\begin{tabular}{c c c c} \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline Measurement range & \(\pm\)100,000 nT & Offset error & \(\pm\)5 nT \\ \hline Bandwidth & DC-1 kHz (\(-\)3 dB) & Operating temperature & \(-\)40 \({}^{\circ}\)C to +85 \({}^{\circ}\)C \\ \hline Internal noise & \(\leq\)6 pT/\(\surd\)Hz & Supply voltage & \(\pm\)12 to \(\pm\)15 V DC \\ \hline Scaling & 100 mV/\(\upmu\)T & Power consumption & 0.4 W (\(\pm\)12 V input) \\ \hline Scaling error & \(\leq\)\(\pm\)0.5\% & Dimensions & 27 mm (H) \(\times\) 41 mm (W) \(\times\) 110 mm (D) \\ \hline \end{tabular}
\end{table} 
Table 2: The technical specifications of the three-axis fluxgate magnetometer. \begin{table}
\begin{tabular}{c c c c} \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline Magnetometer input & Two optically pumped cesium & Operating temperature & \(-\)40 \({}^{\circ}\)C to 70 \({}^{\circ}\)C \\ \hline System noise & 0.1 pT/\(\surd\)Hz & Input voltage & 24 to 35 V DC \\ \hline Sampling rate & 160 Hz & Power consumption & 10 W (24 V input) \\ \hline Analog input & Three differential inputs for & Receiver size & 110 mm (H) \(\times\) 220 mm (W) \(\times\) 160 mm (D) \\ \hline Data ports and interfaces & Ethernet, TCP/IP & Weight & 1.6 kg \\ \hline \end{tabular}
\end{table} 
Table 3: The technical specifications of the aeromagnetic data recording and compensation system. Figure 7: The equipment installed in the cabin. The block diagram of the aeromagnetic data recording and compensation system is shown in Figure 8. Optically pumped magnetometer data and three-axis magnetometer data were recorded on the rising edge of the pulse per second (PPS), which was obtained from the DGPS.