In the previous issue, Ken MacLeod looked at the factors you need to consider when picking an antenna. In this article, he describes the main characteristics of the broadcast GNSS signals, describes the key features of various antenna element types and suggests which type of antenna is suitable.
Early antenna designs were directional and typically linearly polarised. To understand what a linearly polarised signal is, think of a sinusoidal wave transmitted either vertically or horizontally. A directional radiation pattern means that the antenna radiates or receives high power waves in a single direction with a narrow beamwidth. This type of transmission is generally suitable for communication between fixed points where the signal is expected to arrive from a specific direction.
However, GNSS signals are broadcast from many points in space and it is impractical to expect the user to point the antenna at each satellite. For this reason, a GNSS antenna must be less directional, which implies that its gain will be spread across a wider beamwidth to “see” various satellites from different angles.
Moreover, another important characteristic of GNSS signals is that they are broadcast as a circularly polarised waveform – think of a spiral wave travelling through space. Spiral waves turning clockwise are ‘right-hand circular polarised’ (RHCP), those turning anticlockwise are ‘left-hand circular polarised’ (LHCP); GNSS signals are RHCP.
An important concern of GNSS antennas is ‘operational frequency bandwidth’ – the bandwidth over which the antenna operates efficiently – as GNSS signals are spread over a wide frequency range. For example, the GPS L1 signals are centred around 1575.42 MHz with a frequency bandwidth of ±15MHz; considering a multi-constellation system, the full GNSS L1-E1-B1-G1 bandwidth plus the L-Band corrections is around 67MHz (1539- 1606 MHz), while the L2-L5-G2-B2-E5-E6 signals cover an even wider, approximately 134 MHz, range extending from 1,166 to 1,300 MHz. A GNSS antenna must maintain consistent performance over all GNSS frequency bands to operate properly.
GNSS ANTENNA TYPES
In antenna design, size matters. Tallysman’s Accutenna ceramic patch antennas are small, ~60mm in diameter, with a low profile, and are excellent for general purpose positioning, navigation and timing antennas. A dual-pin feeding technique allows ceramic patch antennas to cover a wider frequency bandwidth. Since the ceramic patch is relatively small, a ground plane is always recommended: a 10cm and 12cm ground plane are ideal for L1 and L2 GPS signals respectively.
Helical antennas are small and lightweight (8g embedded and 40g housed) and are ideal for UAVs. They have a radiation pattern that is ideal for dynamic applications where the vehicle pitches and rolls, as the antenna has good low- to mid-elevation angle gain. Another key feature is that the helical design does not require a ground plane to provide excellent gain.
Tallysman’s VeroStar antenna design supports excellent element gain over the full GNSS, Iridium and L-Band Correction bandwidth. Additionally, the design supports excellent low elevation angle gain, making it an ideal antenna for land survey rovers (small 15cm diameter and lightweight) and machine control applications (L-Band Correction reception).
The VeraPhase and VeraChoke antennas are ideal for land survey and geodetic reference station applications. They support excellent antenna element gain over the full GNSS, Iridium and L-Band Correction bandwidth. The VeraPhase antenna element strongly mitigates multipath, with an excellent axial ratio. Lastly, the VeraChoke adds a choke ring to the VeraPhase antenna element that minimises the near-field multipath.
GNSS has become the worldwide standard for computer, electrical and communication network time and synchronisation. Until recently, only GPS L1/CA was used for these applications. However, as new GNSS constellations (GLONASS, Galileo and BeiDou) became available, the timing industry started adopting multi- constellation and multi-signal GNSS.
At the same time, new 5G cellular networks have been installed. 5G networks require precise timing, typically in the range of 5-10ns. The key features of a modern GNSS timing antenna include multi-constellation and multi-signal support, eXtended Filtering (XF), strong multipath mitigation and high low noise amplifier (LNA) gain as long antenna cable runs are sometimes required. Ceramic patch antennas are commonly used for timing applications as they support multiband GNSS and strong multipath mitigation.
Navigation and positioning
Mobile phone navigation and positioning applications provided positions accurate to 5m. Self-driving cars and precision agriculture require an accuracy of 10cm or better. To achieve this level of accuracy, a high-quality GNSS antenna and receiver are required. In addition to high precision equipment, a GNSS corrections service is also required. Two of the most common correction methods are real-time kinematic (RTK) and precise point positioning (PPP). Both techniques rely on the antenna phase measurements.
To achieve positioning accuracy better than 10cm, the selected antenna must have a stable phase centre and have strong multipath mitigation to minimise phase noise and maintain phase lock. Ceramic patch and cross dipole antennas provide the accuracy and precision required for modern high precision positioning and navigation.
UAV-use has exploded over the past few years. What was once the domain of governments and large corporations is now accessible to land survey and remote sensing professionals. GNSS antennas for use with UAVs must support multi- constellation and multiband, have low weight, low power consumption, small size, and strong radio-frequency (RF) filtering.
Helical antennas are ideal for UAVs as they meet all the requirements listed above and do not require a ground plane to perform optimally. They have a radiation pattern that provides strong mid to low elevation angle gain, ensuring that the GNSS signal remains phase locked when the UAV rolls and pitches.
For larger, high-speed UAVs, a more aerodynamic profile is required. A housed or embedded ceramic patch antenna is ideal for this. To achieve optimal performance, a ceramic patch antenna should have a 10cm or 12cm ground plane. The ground plane can be metalised plastic, and if possible, it should be symmetric around the antenna element.
Land and geodetic survey
This demands the most precise and accurate GNSS antennas. Land survey rover antennas should have a phase centre variation (PCV) less than 5mm, and be physically small and lightweight. This form factor makes them ideal for RTK and PPP rover applications.
Geodetic grade antennas are used for continuously operating reference stations (CORS) and International GNSS Service (IGS) stations. Geodetic antennas are typically larger than rover antennas and have a 1mm PCV. As with all the antennas mentioned above, support for multi-constellation and multiband and strong RF filtering is required.
Iridium communication antennas are commonly classified as active or passive. An active Iridium antenna is used for receive-only applications, while a passive antenna can be used for active and passive applications. Ceramic patch, helical and crossed dipole antennas can be used for Iridium communication.
GNSS signals are broadcast RHCP and ground-based antenna should strongly receive RHCP signals and mitigate reflected multipath signals (LHCP). At the same time, since GNSS signals are spread over a wide band, a GNSS antenna must have wideband support. An antenna’s local environment must also be considered as signal reflections and near band and in- band radio frequency noise can severely affect the quality of the received signal.
Ken MacLeod is product line manager, antenna products at Tallysman (www.tallysman.com)