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Frequently Asked Questions

Q: What factors must be considered when to install the outdoor GNSS antenna?

A: Connecting to the Antenna
A coaxial cable transmits GNSS signals from the antenna to timing receiver. It also conducts 5 volts from the receiver to power the antenna and other elements of the antenna system if necessary. The choice of cable is on the basis of attenuation characteristics, weathering ability, temperature rating, and UV resistance. Type N female connectors are used on the antenna and any other element in the antenna cable system, therefore male connectors terminate cables. While Type N female connectors are used on many receivers, smaller form factor products can require the use of alternative connectors. Therefore a different connector may need to terminate the cable to the receiver or an adapter can be used. The application of weather proofing sealant or tape is a good idea for all outdoor connections.
Locating the GNSS Antenna
The GNSS antenna must have a direct view of GNSS satellites with an unobstructed line of sight to the sky. Rooftops that are clear of other structures or geographic features overhead, with views to the horizon, generally make good installation locations. Such a clear view allows the antenna to track the maximum number of satellites throughout the day. Installations with obstructed views may experience reduced reception quality and may not be able to simultaneously track the maximum number of satellites. An observation angle of 70° from the vertical axis (20° above the horizon) usually offers good performance.
When installing your GNSS antenna, select a site at which the antenna will not become buried in drifting or accumulated snow. It should not be covered by foliage or placed in a position where it could become obstructed in any way. Whenever possible, avoid placing the GNSS antenna in close proximity to broadcast antennas or high power transmitters.
Using Ancillary Products
Several items are available to improve the functionality and reliability of a GNSS antenna system. You can leverage the installation of a single GNSS antenna across several GNSS devices by the use of an RF splitter. A surge suppressor is strongly recommended to protect indoor equipment against lightning damage. Install a surge protector and properly connect it to earth ground at the point where the antenna cable enters the building. It is best to calculate cable distances from the antenna to the surge suppressor and surge suppressor to receiver so two connectorized cables can be ordered at the proper length. This saves time and eliminates the risks of splicing cable and installing connectors in the field.
For installations that require a long cable run, an inline amplifier can be used to ensure signal levels at the receiver. The amplifier should be placed at a point where the antenna gain has been reduced to 10 db. It should also be placed a minimum of 5 ft (1.5 m) from the surge suppressor to protect it from lightning damage.

Q: How to place the GNSS Antenna

A: Appropriate placement of the GNSS antenna is crucial to the quality of the data that is being recorded.
Be aware of objects that can shadow the antenna or block the signal to the antenna. Some objects can also reflect signals which can send weaker GNSS signals to the antenna. This is called multipath, and these reflections can disturb the signal in an unpredictable way.
If an antenna is not mounted on a large enough ground plane, multipath reflections can also come from the ground beneath the antenna.
If you are using the antenna without a sufficient ground plane (such as on a bike or carrying the unit by hand), you can put a sheet of metal underneath the antenna (this can for example be copper or aluminium foil) or use an antenna with strong multipath-rejection properties.
On a motorbike, you should place the antenna as far from the rider as possible to reduce the rider's shadowing effect. The best place is usually at the back of the bike.

Q: How to select GNSS/GPS antenna?

A: Select the right GNSS antenna for your GNSS receiver There are many frequencies broadcasted from satellites depending on the type of constellations and signals. For example, the L1 signal of the GPS constellation is 1.57542 GHz and the L1 signal of the GLONASS constellation is 1.602 GHz. Please check the satellite systems and signal types supported by the GNSS receiver and select a GNSS antenna accordingly.
Select the right antenna according to the environment Satellite signals are very weak. When installing a GNSS antenna close to other antennas ex. radio system, a noise filter built-in antenna is recommended.
Regarding the shape of the antenna, if the antenna is to be installed in an area with heavy snowfall, special shape to prevent snow from accumulating is recommended.
In addition, GNSS antennas are recommended to be installed in an open sky environment, which is often on rooftops. If lightning surge protection is required, please consider installing a lightning arrester as part of the set.

Q: What is an "antenna calibration"?

A: An antenna calibration is the act of determining the point of reception of the Global Navigation Satellite System (GNSS) carrier phase signals. Antenna hardware such as the antenna elements and pre-amplifiers create signal phase advance and delay before passing the signal to the receiver. The phase advance/delay changes the range measurement, and will introduce error to position solutions. The point of signal reception of an antenna is not a physically measurable location, and varies depending upon the direction of the satellite signal being received. Therefore antenna calibrations create a map of phase advance and delay which depends on the satellite position in an antenna-centric frame (based on a specific ARP and NRP).

Q: Why do I need to use an antenna calibration?

A: To obtain accurate coordinates at a point, the signal phase advance and delays (calibration values) that occur at the antenna must be applied when processing collected GNSS data. Non or misapplication of antenna calibration values can introduce errors at the centimeter level. Furthermore, because the calibrations are specific to a reference frame, processing GNSS data with the correct values is an essential step to accurately define and access positions consistent with the National Spatial Reference System (NSRS).

Q: Why are antenna calibration values always given for antenna code+radome code (antenna type)? Why are some antennas calibrated with different configurations?

A: Near field effects are important to the calibration of an antenna. The antenna type explicitly indicates the configuration of the antenna for that calibration. The presence or absence of a radome, RF antenna or offset can change the PCO and PCV of an antenna.

Q: What is a relative calibration?

A: In a relative calibration, all antenna offsets (PCO) and phase center variations (PCV) are computed with respect to a reference antenna which is normally assigned zero PCV values. For NGS relative calibrations, the reference antenna is the Dorne Margolin choke ring antenna, type T (AOAD/M_T NONE). A relative calibration is therefore biased by the phase advance/delay experienced by the reference antenna.

Q: What is an absolute calibration?

A: To conduct an absolute calibration, the antenna being tested is moved via a robot so that a particular satellite is received at different angles by the test and reference antennas. This angular separation enables cancellation of the reference antenna effects, leaving behind only the antenna offsets (PCO) and phase center variations (PCV) of the test antenna.

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