In-orbit Demonstration of X-ray Pulsar Navigation with NinjaSat

Abstract

This study demonstrated the pulsar navigation capability of the CubeSat X-ray observatory NinjaSat, which is equipped with two Gas Multiplier Counters (GMCs). The GMCs are sensitive to the 2-50 keV energy band and have an effective area of 16 cm^2 per module at 6 keV. We verified the timing accuracy by observing the Crab Pulsar and confirmed stable timing performance within 100 microseconds. To demonstrate pulsar navigation, we applied a method that optimizes orbital parameters to maximize the significance of the pulsar X-ray pulse profile, known as the Significance Enhancement of Pulse-profile with Orbit-dynamics (SEPO) method. We observed the Crab Pulsar with a total exposure of approximately 100 ks at different epochs and analyzed the data transmitted to the ground. By comparing the optimized orbit with the satellite position derived from Global Positioning System data, we quantitatively evaluated the navigation performance. The results show that the position component along the Crab line of sight was consistently constrained within approximately 40 km, and the three-dimensional position error ranged from 27 to 370 km depending on the observation epoch. These results demonstrate the feasibility of applying a CubeSat-class X-ray observatory to pulsar navigation and provide the first experimental verification that the accuracy of the SEPO method depends on the seasonal geometry between the orbital plane and the pulsar direction.

Figures (9)

• Figure 1: (a) A photograph of NinjaSat. (b) A photograph of the GMC.
• Figure 2: A schematic of the X-ray timing measurement system of the GMC and time synchronization between the GMC local clock and GPS time. Blue arrows represent X-ray event data, red arrows represent PPS signals, and green arrows represent GPS data.
• Figure 3: (a, top) Pulse profile of the Crab Pulsar, created by epoch folding in the spin period. Data were acquired by the GMC2 with an exposure time of 80 ks. The pulse phase was assigned to each X-ray photon based on the JBO radio observations. Blue dot lines represent the fitting range by Nelson's formula. The red dot line is a fit result $\phi_0$. (a, bottom) Fit residuals normalized by the fitted values. (b) Primary peak phase offset from JBO derived by the fit results of pulse profiles with Nelson's formula. Red and blue points represent the GMC1 and GMC2 data, respectively. Vertical error bars are 1-$\sigma$ error of the mean of the primary peak fit. $1 {\rm phase} \simeq 33.83~{\rm ms}$. Horizontal error bars are the start and end of each observation.
• Figure 4: Example of cross-correlation of pulse profiles of NinjaSat and NICER as a function of the phase lag $\tau$. Black points are $C(\tau)$ calculated with data observed by NinjaSat and NICER in February 2025. A red curve is a fitted Gaussian.
• Figure 5: Orbital elements for a circular orbit around the Earth. $a$ is the orbital radius, $i$ is the inclination, $\theta$ is the orbital phase at the start of the observation, and $\Omega$ is the right ascension of the ascending node.