How To Choose A GNSS Receiver?

-05-23

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With the development and popularity of GNSS technology, there are many GNSS brands emerging in the market, providing plenty of different models of GNSS Receivers. This is a good trend, but on the other hand, it can be confusing for users when you want to buy one GNSS receiver from so many choices.

In this blog, our technologist will share his ideas and skills on how to choose the most suitable GNSS receiver for your needs, based on his over a decade of experience in the GNSS field.

1. What are the key points in the specifications?

Show in the table below, it is a typical example of the GNSS Receiver specifications, which can stand for the main specifications of most brands of GNSS receivers. It contains a total of seven parts: Signal Tracking, Accuracy, Data Format, Communication, Electrical, Physical and Environment. 

Faced with so many headache parameters, actually we only need to pay attention to a few key parameters.

Satellites Tracking

The first is satellite tracking ability, which is fundamental to the performance of a GNSS device. It is very important whether all working constellations are supported. Except for the common GPS, GLONASS and Galileo, the BeiDou Global Signal is also a judgment reference, which greatly improves the satellite coverage density, especially in Latin America. 

For other regional satellite systems, Japanese users can pay attention to whether QZSS is supported, and for Indian users is the Navic (IRNSS).

Accuracy

The second is accuracy, this part is very important, but most receiver models will not have much difference in specifications, which is the accuracy under statistically value. You just have to do a quick check to confirm the value is around 1 cm.

Working Hours

Then there is the continuous working time, the battery capacity is only a reference, it also depends on the power consumption of the device. So how many hours the receiver can work continuously on site is what we need to care about.

The hot-swap battery is also a good feature, which can allow us to replace the battery without interrupting the work of the device.

Weight & Size

In addition, the size and weight of the device in actual use are also important factors that affect the user experience. A smaller-sized device with a small package can be more portable during commutes. The light weight will relieve a lot of the burden of daily work. 

Environmental 

Last but not least we should care about is the environmental feature. No one wants to risk accidental damage to equipment. 

IP67 waterproof and dustproof and 2-meter drop protection are the most basic requirements to protect your device from accidental damage. You can also check it based on your actual work environment, like what is the temperature of your working environment.

2. What Functions Do You Need?

Except the basic parameters, we should also concern how many functions a device has. 

4G, radio, IMU, NFC, WIFI, display, memory... What functions do you need according to your work scenario? Here I will list some common functions that surveyors care most about.

4G & Radio

For 4G and radio, it is easy to understand. It&#;s just a way to establish communication between Base and Rover. If you are in a city with a good network, 4G is always the first choice for RTK. Then for areas with poor network coverage, the radio working range is key feature we need to focus on. If the radio working range is too short, it will increase the workload of surveyors.

IMU For Tilt

IMU used for Tilt survey can largely improve our survey efficiency. But for some very rigorous projects, you also need to consider the tilt accuracy issue.

NFC Connection

For more Multi-GNSS Timing Antennainformation, please contact us. We will provide professional answers.

NFC is used to simplify the connections of GNSS receivers to data collectors. One touch without searching and pairing. Although it's just a small feature, some surveyors say it really improves their user experience. 

Learn more about the NFC 
https://www.singularxyz.com/358.html

WIFI & WEB UI

WIFI and WEBUI functions may not be very familiar to surveyors. The main advantage of which is that no cables are required, and static data can be downloaded by connecting to the WIFI of the receiver.

Front Panel

The front panel of most receivers includes the power button and status indicators for your basic needs. If you have further demand, you can choose receivers with display and configuration buttons on the front panel. It is easy to check the detailed status and do some configuration, such as setting up base mode without any controller.

So, in the face of a dizzying array of features advertised by various manufacturers, we need to ask ourselves what these features can bring us. Is it really helpful? Do I really need it?

3. Is the Price Reasonable

When talking about price, it should be compared under the same specs and features.

Now in the GNSS market, the promotion of many models does not talk about performance and functions, but only boasts low prices. That's why we should compare specs and features before comparing prices.

We have made a comparison table for the popular GNSS receivers. If you are interested, you can contact us to get the table.

4. Warranty and After-sales 

Finally, when you get the right receiver with proper specifications you need at a reasonable price, you still need to care about warranty and brand aftermarket capabilities.

It is recommended to purchase GNSS equipment from the original manufacturer or their local distributor. Otherwise, the warranty and after-sales service cannot be well guaranteed.

We, SingularXYZ Intelligent Technology Ltd., is a professional GNSS equipment manufacturer. Our Y1 GNSS Receiver is an all-around device with an affordable price. Our vision for Y1 is to provide surveyors a high-spec and fully-functional device but with reasonable price. Thus, you don't spend too much time comparing and headache choosing.

Learn more about Y1
https://www.singularxyz.com/Y1.html
https://www.singularxyz.com/Y1_Land_Surveying_Solution.html

It can be a surprise to learn that the quality of a GNSS receiver&#;s performance is largely limited by its antenna. Ken MacLeod looks at the factors you need to consider when picking an antenna

The antenna is the front end of the GNSS signal processing chain. To avoid signal degradation, the antenna must filter out interference from multipath and near- frequency signals to capture a clean and pure right hand circular polarised (RHCP) signal. A common analogy is comparing the GNSS antenna to the lens of a high- end digital camera. If the lens does not capture enough light or has distortions, the image captured and subsequent processing will be sub-optimal.

Unlike a few years ago, inexpensive high- precision multi-band GNSS receivers are now available. As a result, antenna designs have had to keep pace and now provide wideband coverage. The current GNSS frequencies and signals are grouped into two bands. The upper band, spanning from - MHz, was the first GNSS band available and included GPS-L1, GLONASS-G1, Galileo-E1 and BeiDou-B1 signals. New bands from all constellations are received in the lower- frequency band (- MHz). If the antenna supports L-band correction services, the upper band ranges from - MHz (67 MHz bandwidth). The key features and characterisation parameters of full-band GNSS will be described in this article.

GNSS broadcast signals

GNSS signals are broadcast from medium Earth-orbiting (MEO) satellites. For example, GPS satellites are 20,200km above the surface of the Earth, but have low transmit power levels ranging from only 50W to 240W. As a result of the low transmit power and the distance travelled, the GNSS signals received on the Earth are very weak (as weak as -155dBW). As such, the receiving antenna must be able to capture and amplify received signals from the horizon to the zenith and from all azimuths.

Antenna designs

GNSS antennas have evolved over the years. Currently, many models meet the accuracy and precision required for most applications. Tallysman has designed and currently manufactures the following patented models: Accutenna, which uses a ceramic patch, Helical, VeroStar and, lastly, the cross-dipole based VeraPhase and VeraChoke antennas.

A ceramic patch Accutenna antenna is a general-purpose PNT antenna. It has a low profile and is approximately 70mm in diameter with a tight phase centre variation (PCV) (<10mm). A 10cm ground plane is required to achieve the best performance.

Helical antennas, on the other hand, do not require a ground plane and are small and light (~8g embedded and ~42g housed). They are ideal for handheld and UAV applications, as they are light and have a radiation pattern suitable for dynamic applications. The PCV is typically less than 5mm.

The VeroStar antenna&#;s key features are excellent element gain and radiation pattern. Both of which lead to excellent low-elevation angle tracking, resulting in an excellent antenna for land-survey or machine-control applications. PCV is in the ±2 mm range.

Lastly, the VeraPhase and VeraChoke antennas are ideal for reference stations, as they have very precise phase centre characteristics. The VeraPhase antenna&#;s PCV is ±1mm, while the VeraChoke antenna is approximately ±0.5mm.

Antenna gain

A GNSS antenna has two types of gain. The first type is the radiating gain from the antenna element and the second is provided by the low-noise amplifier (LNA). Antenna element gain at zenith typically ranges from a high of 8dBic for the VeraChoke antenna to ~2.5dBic for a small, lightweight helical or Accutenna antenna.

Low-noise amplifier features

Low-noise amplifiers (LNA) are designed to provide two key functions: filter out-of-band signals and amplify in-band received signals. Every year, the radio frequency spectrum gets more congested. For example, a cellular signal broadcast at 800MHz can double to 1,600MHz, and this frequency lands in the middle of the GLONASS-G1 band.

To prevent out-of-band signals from entering and saturating the LNA, Tallysman uses a front-end filter and a comprehensive filtering strategy. The front-end filter enables the antenna to strongly attenuate out-of- band signals and prevent the antenna LNA from saturating. However, at the same time, a front-end filter will slightly increase the antenna&#;s noise figure, and that will cause the signal received by the GNSS receiver to have a slightly lower signal-to-noise ratio (SNR) than the same antenna design without a front-end filter. Thus, a good filtering system is essential to improve GNSS signal quality and ensure the antenna and GNSS system function in today&#;s crowded and noisy radio frequency environment.

Since the received signal power is very low, both a high-quality antenna element and LNA are required. It is important to note that antenna users should determine the recommended signal strength required by the GNSS receiver and the signal loss in the antenna cable. With this information, the required LNA gain can be selected. More gain than is required is not beneficial, as more amplification will affect both the GNSS signal and the noise in the environment. Typically, if the antenna cable run is short and the cable loss is low, an antenna gain of ~28dB is sufficient. Longer cable runs can require 35-50dB LNA amplification. In the case of a very long antenna cable run, an inline signal amplifier can be used.

Axial ratio

Another important characteristic is the antenna&#;s axial ratio. All GNSS signals are broadcast as RHCP signals. Ideally, the transmitted signal will describe a perfect circle. Problems happen in multipath environments such as urban canyons where delayed signal reflected on surfaces such as building or other structures. In these situations, the circular purity of the GNSS signals will be affected and the signals will start to describe an ellipse instead of a circle. Delayed signals due to multipath will add noise and phase shifts in the GNSS system. Therefore, they have to be attenuated by the antenna. The axial ratio reflects the ability of the GNSS antenna to attenuate multipath noise. A perfect antenna will have an axial ratio of 0dB (perfect circle) over all azimuths, elevation angles, and frequencies. An antenna with a good RHCP axial ratio will be able to capture more pure signals and mitigate multipath (typically elliptical or LHCP signals).

Antenna phase centre

To measure precise code and phase measurements, a GNSS antenna must concentrate the received radio frequency signals at a point. This point exists at both the GNSS satellite and at the user&#;s antenna. A helpful analogy is to think of a standard measuring tape where the zero end is the satellite broadcast phase centre, and the measurement end is the user&#;s antenna. Tallysman&#;s precision antennas have a stable well-defined phase centre and a small phase center variation (measured by a calibration facility) that enable the GNSS system to support precise and accurate real-time kinematic and precise point positioning applications.

Conclusion

In this brief article, I presented an overview of the key design objectives, properties, and characteristics of a GNSS antenna. The goal is to emphasise that scoring only the antenna noise figure and the gain is insufficient. Keep in mind that comprehensive filtering is required, and filtering will minimise the effects of near-band noise and harmonic signals. Users often compare one antenna to another by evaluating the GNSS receiver&#;s SNR values. When this comparison is made, keep in mind that a front-end filter will slightly increase the noise figure and decrease the signal-to-noise ratio of the received GNSS signals, but this will protect the antenna from saturation and improve system robustness. The axial ratio of an antenna characterises the purity of the RHCP signal and is a good indicator of multipath mitigation. Reviewing and field-testing the antenna parameters discussed will enable a non-radio frequency engineer to evaluate and select an ideal antenna for their application.

Ken MacLeod is product line manager, antenna products at Tallysman (www.tallysman.com)

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