Diagram of the XPNAV-1 satellite X-ray instruments

* Mission Overview

The X-ray Pulsar Navigation (XPNAV-1) satellite is small satellite designed and developed by the China Academy of Space Technology (CAST). It aims to observe a number of X-ray pulsar sources and develop a database of timing profiles in order to use them for navigation throughout the solar system.

XPNAV-1 was launched on November 10, 2016 from the Linquan satellite launch centre aboard a Long March 11 rocket. It was placed in a 500 km altitude low earth polar orbit (with an inclination of 97.4° and a descending node equator transit at 6 am local time). The satellite, which weighs 270 kg, is three-axis stabilized, can perform up to 90 minutes of observation on a source with one or the other of the two X-ray instruments. The mission goal is to observe as many as 26 different sources, including both isolated and binary system pulsar targets, over a five to ten year span.

* Instrumentation

The XPNAV-1 payload includes:

  • Time-resolved Soft X-ray Spectometer (TSXS) which consists of a grazing incidence Wolter-I type lens of four nested mirror shells with a collection area of 30 cm2 that focus X-rays onto a 15-arcmin field of view silicon drift detector (SDD). The effective detection area is 2.4 cm2 at 1.5 keV. The energy range of the SDD is 0.5–10 keV with a spectral resolution of 180 eV at 5.9 keV. Timing was tracked with a GPS calibrated Rb clock with timing accuracy of events of 1.5 μs.
  • High Time-resolution Proportional Counter (HTPC) consists of a collimator to narrow the field of view to 2 degrees, a microchannel plate (MCP) X-ray detector sensitive to 1–10 keV with a 1200 cm2 collecting area and 100 ns timing accuracy.
  • Support systems for ground communication, operations, power supply, and solar panels.
Operations occur in one of four modes: self-test, scanning, default target, and arbitrary target operations. In the self-test mode, a hood covers the instrument to allow measurement of instrument noise. The scanning mode views a swath of sky to assess background contributions from the sky. The two operations mode consistent of up to 90 minutes of observation by one or the other of the two instruments (power limits prevent operating both at the same time and they are not co-aligned), followed by a slew to orient the solar panels to recharge the batteries. The default target mode works with a set of eight pre-selected bright targets (include the Crab pulsar, a bright and extremely well observed source useful for calibration and instrument verification). The arbritrary mode uses targets uploaded from ground systems.

Operational constrains include turning off instruments during South Atlantic Anomaly passage to prevent damage, and keeping instruments pointed at least 45 degrees off the positions of the Sun and Moon to avoid interference.

* Science

The XPNAV-1 science objectives includes (Zang, X. et al., IJAE, 8561830 (2017)):

  • Test the functionality of the instruments in space;
  • Detect photons from X-ray pulsars and acquire pulsar profiles;
  • Perform observations of an extended period of time to measure pulsar parameters via X-ray instruments.
XPNAV-1 is the first of a series of planned stepping stone missions: XPNAV-1 demonstrates fundamentals of the technology, to be followed later with a medium sized mission to time 3–5 pulsars suitable for developing a navigational database, followed ultimately by a constellation of satellites to fully develop the use of X-ray pulsar timing for solar system navigation.

[XPNAV-1] [Bibliography]

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