Learn about GNSS antennas, cables, splitters, and ...
Discover GNSS Antennas, Cables, Splitters, and More
The Global Navigation Satellite System (GNSS) utilizes radio-frequency (RF) waves to broadcast navigation information. It's essential to grasp how devices capture these waves, as this knowledge is crucial for effective system utilization. A GNSS satellite's signal is transmitted and must be caught by a device equipped with an antenna. Numerous types of antennas featuring diverse characteristics, cabling, connectors, and various accessories are available.
GNSS Antennas
Three primary antenna types are commonly found in GNSS systems: patch, helical, and choke-ring antennas. The main distinction among these antennas lies in the shape of the sensing element responsible for capturing RF waves. Antenna design also influences the gain, which measures how efficiently an antenna captures intended RF energy. The efficiency of receiving antennas in capturing incoming signals from a specific direction is known as gain, typically expressed in decibels (dB). Generally, larger antennas house larger sensing elements, resulting in higher gains.
Each antenna has a designated location for tracking the signal, referred to as the phase center, which can vary with temperature and may be crucial for certain applications.
Active vs. Passive Antennas
Passive antennas consist solely of an RF receiving element and do not draw power from the receiver. In contrast, active antennas comprise a low-noise amplifier and require power from the receiver through the RF connector. Active antennas typically introduce a current draw of 3-50 mA to a system. The low-noise amplifier in active antennas counteracts cable loss and enhances gain closer to unity.
Patch Antennas
Patch antennas, known as microstrip antennas, feature a metal sheet acting as the sensing element, separated by an insulator from a larger ground plane. This unique design allows for a low-profile shape suitable for flat surface mounting. Although the ground plane minimizes multipath effects, it also renders patch antennas directional. These antennas can be inexpensively fabricated on printed circuit boards, making them prevalent in mobile electronics.
Helical Antennas
Helical, or helix antennas, are constructed from one or more wires wound into a helical form, creating a cylindrical shape. While they are usually mounted over a ground plane, omnidirectional designs can be created without it. Helical antennas are adaptable, capable of operating in either normal mode or axial mode, depending on the circumference of the helix in relation to the intended wavelength. Normal mode is utilized for transmitting or receiving waves perpendicular to the helical axis, whereas axial mode is for waves traveling along the helical axis. These antennas can vary in size, accommodating mobile applications as well as larger setups. However, their design renders them more susceptible to multipath interference.
Choke-Ring Antennas
Choke-ring antennas feature a unique directional design comprising a central receiving element surrounded by concentric hollow rings that eliminate multipath signals. The remarkable multipath rejection and enhanced phase-center stability of these antennas provide the millimeter-level accuracy necessary for surveying applications. However, their large size may be a disadvantage in mobility-focused scenarios.
Ground Planes
GNSS satellites transmit RF waves that are vulnerable to multipath interference, which occurs when signals reflect off solid surfaces like buildings before advancing to the GNSS antenna. This reflection can lead to multiple signal paths, resulting in navigation errors. To combat multipath interference from beneath the antenna, a ground plane is commonly installed under the GNSS antenna. Ground planes can be any thin metal piece that blocks multipath interference from reflecting upward to the antenna's base. It’s noteworthy that ground planes need not be electrically grounded.
RF Connections
GNSS signals register below the noise floor and require specialized processing algorithms to interpret the signal effectively. Hence, reducing potential losses is crucial. The characteristics of antennas, cable loss, and connector loss each play a role in signal reception quality. Choosing the right cable, connector, and, optionally, a splitter can significantly impact GNSS signal strength.
Cables
With the high frequencies associated with GNSS signal reception, coaxial cables are utilized. These cables consist of a central conductor encased in dielectric material, surrounded by an outer conductor and an outer insulator. Keeping cables short minimizes GNSS signal strength loss, and employing larger-diameter cables also contributes to this reduction.
Connectors
Common RF connectors include U.FL, SMA, and MMCX, as illustrated in the accompanying figures. Each connector varies in size, latching mechanism, and force, making them suitable for different applications.
U.FL connectors are the most compact, primarily used for directly connecting antennas to exposed PCBs situated close to GNSS chips. Due to their design, they are intended for minimal connections, typically rated for around ten connects and disconnects. They excel in board-to-board applications rather than panel mounts.
SMA connectors come in both male and female forms, along with reverse polarity (RP) versions that maintain identical electrical connections but alter the center pin's positioning on the female connector. Designed for performance up to 18 GHz, they feature a low insertion loss of approximately 0.17 dB.
MMCX connectors are smaller than SMA connectors, weighing about one-third less and possessing a 360-degree swivel mechanism, making them popular in consumer electronics. They can operate up to 6 GHz, allowing insertion loss around 0.3 dB.
RF Splitters
RF splitters enable multiple devices to share the same GNSS signal by dividing the signal into two or more outputs. A 1-to-2 splitter typically incurs a 3 dB decrease (50% power) for each output. Caution is advised not to over-split the signal, as excessive division may hinder recovery for the GNSS receiver without the addition of another amplifier, which can also introduce noise. Most splitters include DC blocks on all but one antenna to prevent multiple powered antenna sources from operating in parallel, safeguarding them from potential damage.
Understanding GNSS Functionality
The typical GNSS setup consists of two main components: the antenna, which captures the satellite signals, and the processing unit or receiver, which interprets the incoming information into comprehensible measurements, such as latitude and longitude.
What is a Dual Antenna?
In dual antenna systems, the antennas are generally labeled as the "primary" and "secondary." The RT unit features two GNSS receivers integrated into it, responsible for processing. While the GNSS receivers perform all functions, the measurements produced correspond to the antennas' positions.
It's critical to remember that the length of the antenna cables may position a receiver far from the output position measurements. In everyday GPS devices, this typically doesn't pose a problem, as such devices rarely achieve accuracy surpassing several meters.
What are GNSS Segments?
Understanding GNSS necessitates an acknowledgment that calculations about position, speed, and altitude correlate to the antenna itself, not the receiver. To elucidate how GNSS functions, we can categorize it into three segments, focusing on GPS, the most familiar system:
- The space segment
- The control segment
- The user segment
1) The Space Segment
The space segment encompasses the satellites orbiting the Earth. The GPS constellation comprises 32 non-geostationary satellites in medium Earth orbit, though not all satellites are active at all times. Each satellite completes an orbit approximately every 11 hours, 58 minutes, and 2 seconds, positioned at an average altitude of 20,200 km (orbital radius of 26,571 km).
These satellites are arranged in six orbital planes, ensuring a minimum visibility of four satellites above 15° on the horizon from virtually any location on Earth, although in practice, more satellites are usually detectable.
What is the Function of the Space Segment?
Despite variations in satellite age and design, the core operational principle remains consistent. Each satellite features four highly precise clocks operating on a fundamental frequency of 10.23 MHz, continuously transmitting two carrier waves in the L-Band, traveling back to Earth at light speed. These carrier waves, referred to as L1 and L2, are key to relaying satellite information to Earth, allowing receivers to calculate geographical positions.
- The L1 carrier operates at a frequency of 1.57542 MHz.
- The L2 carrier operates at a frequency of 1.2276 MHz.
The role of the carrier waves is fundamental, as they convey crucial information from satellites back to terrestrial receivers, enabling accurate location determination. For more details, please visit the GPS signal page.