GNSS antennas
GNSS antennas
Elevation angle and errors
Let's take a closer look at the photo. The path from the satellite to the car roof and then to the antenna is 4 cm longer than the direct path from the satellite to the antenna. If the strengths of the direct and reflected signals were equal, their combination would extend the path by 2 cm. However, the car roof is not flat, and the direct and reflected signals are not equal. As a result, some satellites are received more by the direct signal, and others are received more by the reflected signal. This is different for all three antennas, causing the vector between the antennas to move as it pleases. An even worse situation occurs when the direct signal is not captured, but the reflected signal is. For example, if the satellite is low in the sky and the direct signal is blocked by a building, the reflected signal arrives from a building opposite. In this case, the error in measuring the signal is already in the hundreds of meters, and the error in measuring coordinates across all satellites can be tens of meters. That's why receivers use an "elevation mask", which blocks the reception of satellites located low above the horizon. For high-precision receivers, the typical mask is 15 degrees. This value is chosen because signals from low satellites travel a much longer path through the troposphere, and their tropospheric distortions are too large and unpredictable. © Eltehs SIA 2023
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Big antennas, Small antennas, caseless antennas
Now it's clear what a "big" antenna is - it's a small (usually caseless) antenna assembled together with a groundplane in a large housing. As you can see, there's a caseless TW2405 antenna and a board that works as a groundplane. In fact, the picture shows not just a big antenna, but also a smart antenna, meaning there is a receiver and processor on the backside of the board. See the photo." Big antennas are ready-to-use devices, they are used independently, as the character Carlson, who lived where the antennas are, would say. Small antennas are designed to be used with an external groundplane, for example, any antenna will show its maximum performance right on a car roof. The smallest antennas are either antennas for drones, where low weight is essential, or antennas for less accurate autonomous solutions, where half of the day, the accuracy (CEP-50) is less than 3 meters, and the other half, even larger (up to 9 meters). As for caseless antennas, they are semi-finished products for making big antennas or embedding them in devices. Regarding the shape of antennas - it's just a matter of design and... mounting. Square, round, octagonal - all of this is not important. What matters is that the antenna is easy to attach. An antenna torn off by the wind and left hanging by its cable is a heartbreaking sight. © Eltehs SIA 2023
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Rain? No, wind!
When installing an antenna, don't forget that the satellite signal is very weak and comes from above. For example, trying to protect the antenna from rain with a tarpaulin leads to almost complete loss of reception.Sometimes it can be even worse. Once, we were called for diagnostics because a "satellite compass was not working." It turned out that the crew couldn't find a better place for the antennas than under a metal platform. Of course, as soon as the antennas were given access to the sky, everything started working. Now let's talk about climate. Few people consider antennas as consumables. Most would like an antenna to last at least 10 years. That's why antennas are divided into classes, depending on the operating conditions. Marine antennas. This is very tough. For example, during a storm, a couple of cubic meters of salty water can be poured onto an antenna mounted on the roof of the wheelhouse. In high latitudes, not only water but also a mixture of snow and ice. Coaxial cables for marine antennas are made as thick as a thumb, but I've been told of situations when even such cables were broken by a snow-ice charge. Interestingly, the antennas themselves survived. No wonder - marine antennas are designed for 20 years of service in very harsh conditions.A separate topic is marine connectors. They are expensive, reliable, and waterproof. The degree of protection for a marine antenna is IP68, which means the antenna is actually submerged in a bathtub of water, and it is checked that no bubbles enter, and the antenna works after 24 hours underwater. The marine antenna is not 100% hermetic; there is a hole for balancing pressure outside and inside the antenna, but the hole is covered with a filter so that water does not pass through it.These antennas are usually only L1, meaning they are not suitable for most applications. Mounting is typically on a threaded inch-sized peg, sometimes on a "geodetic" 5/8 inch peg. The main thing to remember here is that if you are sold an antenna in a "marine housing," the housing will be excellent. But the connector may be ordinary, not very waterproof, and afraid of water. So check the protection class of the entire antenna, not just the housing. Aircraft antennas. These are antennas according to the ARINC-734A standard. They are very reliable, but their high price is due more to the lengthy certification process according to aviation standards rather than reliability. For use outside of an aircraft, they are unsuitable, as their mounting and connectors are very specific. Such an antenna looks like a patch on an airplane. In the photo - a typical aviation antenna for a large passenger aircraft. In terms of characteristics, there's nothing special about them, only L1, GPS, and sometimes GLONASS. Geodetic antennas. This is another monster of antenna construction, but in a completely different sense. There is a concept called the phase center of antennas, roughly speaking, the very point where the signal is received. So, in the documentation for a severe geodetic antenna, it is written something like, "the phase center for the L1 frequency is located at 68 mm ±0.2 mm from the bottom edge of the antenna, horizontal error ±0.1 mm. The phase center size is 1.3 mm ±0.1 mm vertically and 1.1 mm ±0.2 mm horizontally." And similarly for L2. In the most advanced cases, the measurement results for a specific antenna sample are provided, with graphs showing the dependence of the phase center location on the satellite's elevation above the horizon. Why is this necessary? For heavy geodetic tasks such as monitoring mountain movements, continental drift, or measuring a first-class geodetic network on a country scale. There, every millimeter of error counts. Moreover, they calculate both the maximum and average error. Significant part of geodetic antennas are "smart" antennas, meaning that there is at least a receiver inside the antenna housing, and sometimes even a battery and radio modem. Understandably, such antennas are quite expensive. They usually have a 5/8-inch geodetic peg mount. Timing antennas. Another interesting class. To receive very accurate time signals and even more precise synchronization of two events occurring in different places, it is necessary to receive signals exclusively from satellites, eliminating all possible reflections from below and from the sides. And even more so - interference.Even more importantly, for determining position, we need relatively low (close to the horizon) satellites. But for determining time, it is better to use only high satellites (from 30 degrees above the horizon), as they are less delayed by the troposphere, resulting in fewer errors in the solution. Therefore, timing antennas are antennas that poorly receive both low satellites and interference and reflections coming from below and the sides.Another trick of timing antennas is polarization. Satellite signals are RHCP, meaning they have right-hand circular polarization. With each reflection, the polarization changes to the opposite, i.e., from right to left, from RHCP to LHCP, and vice versa. Timing antennas are designed to receive RHCP polarization much better than LHCP. This feature is called anti-jamming (our anti-jamming solutions). Photo: anti_Jam.jpg In the photo, you can see how the RHCP satellite signals (black) are received much better than the reflected LHCP (red) signals. Additionally, it is noticeable how the reception level decreases as the angle from the vertical increases. Another feature of timing antennas is the precise time signal (called 1PPS) that needs to be transmitted to the equipment. A signal with a one-nanosecond leading edge is a signal with a spectrum ranging from a gigahertz to hundreds of gigahertz, depending on the required edge steepness, and it is difficult to transmit it through a regular cable; a very good coaxial cable is needed. Plus, it's challenging to ensure proper impedance matching so that the leading edge doesn't reflect off the cable's end and create a secondary peak. Therefore, timing receivers are placed closer to devices that use precise time, and they are connected to the antenna with a long (up to hundreds of meters) cable. This results in another feature of timing antennas – a high gain (up to 40 dB, which is 10,000 times) of the built-in low-noise amplifier.The fact that timing antennas are usually mounted on metal masts leads to three more interesting features. Firstly, the antenna is designed in the shape of a rocket to prevent birds from perching on it. The red color is a bird-repellent paint. For a regular antenna, an interruption of a few seconds caused by a bird landing on it may be somewhat acceptable, but for continuous process synchronization, it can be disastrous. Secondly, a tall pole is an attractive target for lightning. Therefore, timing antennas are the only ones with some lightning protection. Some protection is provided because it is usually designed to withstand 5-6 strikes. It should be the last line of defense against lightning, not the first.The third feature is clearly visible in the photo – it's the side mounting to the pipe from which the mast is made. However, it's essential to understand that the pipe should not be higher than the border between the upper (white) and lower (red) parts. In the photo on the left, you can see improperly installed antennas – the red pipes are too high and cause significant reflection in the antenna. Additionally, the antenna system on the right is lower than these pipes and also receives a reflected signal. Since we're discussing installation errors, let me tell you about the "piano in the bushes"As you can see, the antenna is placed at the edge of a grove, significantly lower than the surrounding trees. The creators of this wonder complained that they could only receive signals from 2-3 satellites from each system. I'll repeat once again – antennas need a clear sky. Trees, especially wet ones, can significantly attenuate GNSS signals and cause reflections if antennas are placed at the edge of a forest. Stationary antennas. These are a lightweight version of geodetic antennas. For surveyors, they are like fake Christmas tree decorations – they cost ten times less but lack precise figures describing the phase center position. For everyone else who doesn't mind losing a few millimeters of accuracy, they are an excellent working tool. These antennas are typically designed in the shape of a mushroom with a ground plane inside, and they screw onto a geodetic stake. The most crucial role of this stake is to raise the antenna above the water or snow level, which may accumulate on a roof during heavy rain, snowfall, or snowmelt. The connectors of these antennas are standard and can rust underwater. Therefore, I recommend finding out the maximum daily precipitation level before installing the antenna on a "bordered" roof and making the stake slightly higher. In northern regions, learn the maximum snow level that falls during the winter – not in millimeters of water, but in actual snow level. Alternatively, you can clear the antenna buried in the snow a couple of times during the winter, or learn from experience that antennas perform poorly under snow.Another function of the stake is the ability to replace the antenna. This may be necessary to obtain an official document with the antenna coordinates from surveyors.Another purpose of stationary antennas is mounting them on the roofs of truck cabins and cars. The reason is the same – to avoid cleaning the antenna from snow, it's easier to raise it above the roof. This means having a separate ground plane in the antenna and a mushroom shape. Magnetic antennas. These are antennas without their own ground plane, but attached to someone else's using a magnet. For example, to a car roof, a steel sheet, and so on. They are ideal for testing. Since magnetic attachment does not last for years, some of these antennas have tabs for screwing. It is a very convenient option if you want to remove the antenna at night. Sometimes such antennas are used instead of groundless ones and are placed on a ground plane inside a housing.As stationary antennas, they are not very suitable. IP67 allows for immersion in water for up to one minute, but not lying in a puddle for hours. During the spring snowmelt, several antennas on our roof died (reducing the signal strength). It turned out that in this batch, the housing had a hair-thin crack. The amplifier rusted while lying in the puddle. By the way, the manufacturer fully recognized the problem. So it's better to use antennas attached to a stake, which are always above the puddle. Or an explicit IP68, which will not be affected by puddles. Drone antennas. The peculiarity of a drone is that it can tilt significantly during maneuvers, so helical antennas with a radiation pattern that includes the lower hemisphere are used for drones, and a ground plane is not used. The second feature is the very lightweight antennas, weighing 30 grams.To reduce weight, the attachment reliability was sacrificed, and drone antennas are simply screwed onto the CMA connector. This is logical because if the drone crashes and breaks, but the antenna survives, it will hardly console anyone. Therefore, the reliability of the antenna attachment in collisions is approximately equal to the reliability of the other parts of the drone. Groundless antennas. These are semi-finished products for creating stationary antennas and custom devices (smart antennas). They are usually mounted on a multilayer printed circuit board, the ground layer of which represents the ground plane, and the receiver and processor of the device are mounted on the bottom. © Eltehs SIA 2023
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Lightning protection that doesn't exist
To escape from a bear, you don't necessarily have to run first; the main thing is not to be last. This is the basic principle of lightning protection. It is obvious that an antenna will not survive a direct lightning strike, no matter how well it is protected. Therefore, it is important for the antenna to withstand the induced effects of distant lightning strikes. That is, it is best to install the antenna so that the lightning strikes not it or very close to it, but rather a lightning rod located further away. But what is needed for the antenna to survive a distant lightning strike? Where does the trouble come to the antenna? The trouble comes through the coaxial cable that connects the antenna to the receiver. And, by the way, this trouble also comes to the receiver on the other side of this cable, and it also needs protection. What is the complexity of protecting antenna inputs and high-frequency circuits in general? Telephone lines are protected - varistor assemblies, large spark gaps are installed, and we usually do not hear stories about a phone burning out due to a thunderstorm. But there is a complexity here. A telephone line is a low-frequency device; you can install large protective elements that can absorb many joules of energy without interfering with signal transmission. However, a GNSS signal is high-frequency, and every picofarad of capacitance of the protective element counts and weakens the signal. No, antenna inputs can also be protected. There are protective TVS diodes with capacitance in fractions of a picofarad. And there are special assemblies of serially connected regular fast diodes with low capacitance and more powerful protective TVS diodes, and these assemblies have both low capacitance and can absorb the necessary impulse energy. However, in this armor and projectile competition, one has to be lenient about the fact that the armor is not very thick, and corresponding standards for resistance to high-energy pulses grant some concessions to antenna inputs. There are antennas with some lightning protection, especially among timing ones. Just understand that this protection is usually relative, guaranteed for 5-6 times, no more. And it is the last line of defense against lightning, not the first. The first and main line of defense is a lightning rod located further away from the antenna and higher than it. © Eltehs SIA 2023
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Groundplane and multipath
Table of contents 1. Understanding Signal Reflections and Their Effect on GNSS Reception 2. Groundplane Solutions: From Simple CD Discs to Advanced Choke Ring Antenna 3. The Ideal Size of a Groundplane for GNSS Patch Antennas The Importance of Groundplanes in Improving GNSS Reception and Accuracy How Signal Reflections Affect GNSS Reception and the Role of Groundplanes in Improving Accuracy One of the biggest challenges in GNSS (Global Navigation Satellite System) signal reception is signal reflection, also known as "multipath." This occurs when satellite signals bounce off obstacles such as buildings, trees, people, and the ground before reaching the antenna. The result is that the antenna receives multiple signals: one direct from the satellite and others reflected from different surfaces, such as buildings or the ground. These reflections distort the signal and reduce the accuracy of the reception. Understanding Signal Reflections and Their Effect on GNSS Reception The most significant signal reflection typically comes from the ground below the antenna. To address this issue, engineers developed a solution known as a "groundplane." A groundplane helps prevent signals from the lower hemisphere (the ground and surrounding reflections) from reaching the antenna. In its simplest form, a groundplane can be a flat surface, such as a CD disc, with the antenna placed in the center. More advanced designs use choke ring antenna, a set of concentric rings placed around the antenna. Groundplane Solutions: From Simple CD Discs to Advanced Choke Ring Antenna A well-designed choke ring antenna, weighing about 7.5 kilograms (16.5 pounds), can cost several thousand dollars. However, for less demanding applications, inexpensive groundplanes, like old CD discs, can significantly improve signal reception. The Ideal Size of a Groundplane for GNSS Patch Antennas For instance, when a satellite compass is used to measure the vectors between two antennas, placing the antennas just a few centimeters above a car roof (which acts as a signal reflector) will result in inaccurate measurements. In the photo titled "That Doesn’t Work" this setup causes signal fluctuations of up to 1-2 centimeters, instead of the required 5mm RMS (Root Mean Square) accuracy. However, when the antennas are placed on a proper groundplane, such as a CD disc, the accuracy improves drastically. This is illustrated in the photo titled "That Works" There is some debate regarding the ideal size of a groundplane for patch antennas. Some experts suggest that the groundplane should be slightly larger than half the wavelength, which is about 10 centimeters for L1 and 13 centimeters for L2 frequencies. Further details on this can be found in the "GNSS-Antennas_AppNote_UBX" document. It's important to note that patch antennas generally perform poorly without a groundplane. In contrast, other types of antennas are less sensitive to the absence of a groundplane. © Eltehs SIA 2023
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