On-Orbit GPS Receivers: Not Just for Navigation
By Dr. Rebecca Bishop, Senior Scientist, The Aerospace Corporation
Dr. Rebecca Bishop, Senior Scientist, The Aerospace Corporation
Global Navigation Satellite Systems (GNSS), such as the United States’ Global Positioning System (GPS) constellation, providewide-ranging benefits to society as a whole. It is commonly known that by tracking transmitted signals from GNSS satellites, a receiver is able to calculate and report the receiver’s position, velocity, and timing (PVT) information whether on the ground, air, or in space. But those transmitted GNSS signals can be used for much more than PVT. They can also be used to probe the Earth’s atmosphere when hosted on low Earth orbit (LEO) spacecraft.
When any radio frequency (RF) signal travels through the Earth’s atmosphere, there is a possibility that the signals are absorbed, reflected, refracted, or scattered depending on the conditions in the atmosphere and the frequency of the RF signals. For GPS signals, we know that when the signals pass through the lower neutral atmosphere (~0-35 km) and the upper ionized atmosphere (~80-500 km), significant signal refraction may occur. The amount of refraction depends on the atmospheric conditions; thus, by measuring the amount of refraction we are able to measure and characterize those conditions. Radio occultation(RO) is a technique used in space that takes full advantage of signal refraction. It relies on a GPS receiver hosted on a satellite at an altitude above 500 km where it can observe the signals as the GPS constellation satellites pass behind the Earth (i.e., occulted). One of the strengths of GPS RO is its ability to provide global observations including over the ocean; an obvious advantage over ground-based receivers. In the lower atmosphere, the pressure, temperature, and partial pressure of water vapor as a function of altitude are measured. In the upper atmosphere, electron density and very small-scale density structures that scintillate the signals are measured.
RO atmospheric data has become a critical resource for weather forecasting and space weather monitoring.
GPS RO sensors have flown successfully on many smallsats and the current challenge is to miniaturize the sensors in order to host them on CubeSats without compromising capability and data accuracy.
A complete GPS RO sensor consists of a GPS receiver and patch antenna, interface electronics, and cabling. Whether the antenna is pointing forward or aft (rising and setting occultations) is partially dependent on the receiver’s tracking algorithm. Not every GPS receiver can be used as an RO sensor, though. At a minimum, the receiver must measure and output the carrier Signal-to-Noise Ratio(SNR), pseudo-range, and phase at a 50 Hzsampling rate (or 1 Hz for upper atmosphere density)forall transmitted GPS signals in the antenna’s field of view (FOV).The first developed GPS RO sensors were hosted successfully on smallsats with mass between 100 and 500 kg.
With the successful development and popularity of CubeSats along with the simplicity of the RO sensor hardware and associated satellite requirements, GPS RO sensors have become a popular CubeSat payload. However, significant challenges exist to host GPS RO sensors on CubeSats smaller than 6U (1U = 10x10x10 cm). Up to now, satellite resources have been the limiting factor. Depending on the receiver and operating mode, the CubeSat must be capable of 1) supplying continuous power ranging between ~1-2W, 2) have a data downlink capability to support 10s of MB or more data per day, and 3) maintain 3-axis stabilization throughout the mission. It has not been possible for CubeSats to provide all three resources without significant instrument duty cycling, which limits the amount of data collected. The CubeSat paradigm of frequent spacecraft launches can compensate for duty cycling and provide a similar amount of data compared to a single smallsat RO sensor.
In order for GPS RO sensors to become routine, continuously operating CubeSat payloads, the sensors must reduce overall mass, dimensions, power consumption, and on-board processingas well as expand their capability to utilize other GNSS constellations.
To that end, there are three receiver design paths being explored: 1) commercial-off-the-shelf (COTS), 2) academic/laboratory science, and 3) commercial receivers. COTS receivers were the first successful type of CubeSatRO receivers flown. COTS receivers, designed for ground and aviation uses, are mass produced but can be adapted for space. Typically, they are limited to setting occultations. The second development path includes those receivers designed by academic institutions and national labs specifically for science missions. These often consist of software defined radios (SDRs) which are more adaptable allowing reprogramming, mode changes, and tracking algorithms that enable collection of both rising and setting occultations. Current development efforts focus on SDR size and power reduction,both of which are likely achievable in the near future. Commercial satellite companies are the final RO receiver development path. These receivers are designed and built to be hosted on large CubeSat/smallsat constellations to produce routine atmospheric data products for sale.Two companies currently have RO sensors on-orbit routinely producing data.
Signals transmitted from GNSS constellations (e.g. GPS, GLONASS, BeiDou, Galileo) can be used for more than PVT determination. They can also be used to probe the Earth’s atmosphere between ~0-30 km and above 80 km using the radio occultation technique by GNSS receivers hosted on LEO satellites.GPS RO sensors have flown successfully on many smallsats and the current challenge is to miniaturize the sensors in order to host them on CubeSats without compromising capability and data accuracy. CubeSat RO receiver development is well underway and shows great potential for success, with several types of RO sensors currently operating in orbit.
Caption: An example of a COTS receiver and dual frequency patch antenna that will fly on an upcoming CubeSat science mission.