Table of Contents

  • Global navigation satellite systems (GNSS), which were originally designed to provide positioning, velocity, and timing services for terrestrial users, are now increasingly utilized for autonomous navigation in space as well. Historically, most space users have been located at low altitudes, where GNSS signal reception is similar to that on the ground. More recently, however, users are relying on these signals at high altitudes, near to or above the GNSS constellations themselves.

  • The vast majority of Global Navigation Satellite System (GNSS) users are located on the ground, and the GNSS systems are designed to serve these users. However, the number of satellites utilizing on-board GNSS space receivers is steadily growing. Space receivers in the SSV operate in an environment significantly different than the environment of a classical terrestrial receiver or GNSS receiver in low Earth orbit. SSV users span very dynamic and changing environments when traversing above and below the GNSS constellation. Users located below the GNSS constellation can make use of direct line of sight (LoS) signals, while those above the orbit of the GNSS constellations must rely on GNSS signals transmitted from the other side of the Earth, passing over the Earth’s limb. These space users experience higher user ranging error, lower user-received power levels, and significantly reduced satellite visibility.

  • The number and scope of GNSS-based space applications has grown significantly the since the first GNSS space receiver was flown. The vast majority of space users are operating in low Earth orbit (LEO), where use of GNSS receivers has become routine. For spacecraft in the SSV, however, the first demonstrated uses came in the late 1990s. Use of GNSS receivers aboard high-altitude spacecraft remains limited due to the challenges involved, including much weaker signals, reduced geometric diversity, and limited signal availability. By focusing on interoperability, the multi-GNSS SSV will provide numerous benefits, expanding the opportunity for full exploitation of the existing potential.

  • Historically, most space users have been located at low altitudes, where GNSS signal reception is similar to that on the ground. More recently, however, users are relying on these signals at high altitudes, near to or above the GNSS constellations themselves. The availability and performance of GNSS signals at high altitude is documented as the GNSS SSV. While different definitions of the SSV exist and may continue to exist for the different service providers, within the context of this booklet it is defined as the region of space between 3,000 km and 36,000 km above the Earth’s surface, which is the geostationary altitude. For space users located at low altitudes (below 3,000 km), the GNSS signal reception is similar to that for terrestrial users and can be conservatively derived from the results presented for the lower SSV in this booklet.

  • To convey a consistent set of capabilities across all GNSS constellations, an SSV capabilities template has been completed by each GNSS service provider to capture their contributions to each of the parameters identified in section 3.2. The full text of these completed templates, along with appropriate context, is available in annex A. This chapter presents an aggregated subset of the full data so that the individual SSV characteristics of each constellation can be readily compared and contrasted.

  • The Working Group B of the International Committee on GNSS (ICG WG-B), has simulated the GNSS single- and multiple-constellation performance expectations in the SSV, based on the individual constellation signal characteristics documented in chapter 4. As outlined in chapter 3, navigation performance in the SSV is primarily characterized by three properties: user range error (URE), received signal power, and signal availability. The focus of these simulations is on signal availability, which serves as a proxy for navigation capability.

  • GNSS, which were originally designed to provide positioning and timing services to users on the ground, are increasingly being utilized for on-board autonomous navigation in space. While use of GNSS in LEO has become routine, its use in higher orbits has historically posed unique and difficult challenges, including limited geometric visibility and reduced signal strength. Only recently have these been overcome by high-altitude users through weak-signal processing techniques and on-board estimation filters.

  • To promote the multi-GNSS SSV for the purpose of safe robotic or manned missions in SSV as defined in this booklet and beyond including cislunar space it will be necessary to update this booklet, extend efforts on simulation and modelling as well as elaborating further on recommendations for GNSS providers. Some potential evolutions of the booklet could include:

  • This booklet was published by the United Nations Office for Outer Space Affairs in its capacity as executive secretariat of ICG and its Providers’ Forum. Sincere thanks to all who have helped, and who recognize the in-space advantages of the SSV specification and provide leadership in developing an SSV specification for the GNSS constellations.