The GNSS antenna's reception quality measurements are presented on a polar graph, logging the direction and C/No for each data point, creating an overall heat map.
The SkyView™ polar graph (also known as a heat map) is a collection of individual measurement data points, with each dot representing the received signal quality in a particular tridimensional direction. The direction is a vector that can be used to point at where the GPS/GNSS satellites were when their signal quality (Carrier-to-noise) measurements were made. Each of the dots (datapoints) are characterized by:
- Their Azimuth angle, which is counted clockwise from the North (Range: 0º to 359°).
- Their Elevation angle (not to be confused with Altitude), which is counted from the Horizon (0°) towards the Zenith (90°).
- Carrier-to-Noise Density (C/No), which indicates the quality of the RF signals at the GNSS receiver. The signal quality for each data point is represented in a color scale.
As satellites pass across the sky, over the antenna, their signals' quality are continuously measured, color-coded and plotted, leaving a trail. The example shown above is a fast playback of a time lapse recording from an actual test, showing how the polar graph gets painted by all the satellites passing by. These polar graphs are built over a 24 to 72-hour time frame, in order to have the opportunity to capture all supported GNSS satellites as they pass over the antenna under test.
The trails from all the GNSS satellites that the antenna can "see", paint a dome-like map, which is presented as a polar graph. Then, one can simply identify an area of interest, get the direction (angles) and point to the sky in that direction to see/identify what could be blocking or causing RF signals to fade.
Types of Polar Graphs
Linear - The linear elevation scale is commonly used in polar graph representations of the antenna's sky view, because it makes it easy to interpolate elevation angles between its concentric grid lines, making reading them simpler. For example, on 10° or 15° elevation grids, users can easily identify the peak of a poor performing area on the map, roughly estimate its elevation (angle from the Horizon) in between two grid lines. For example, one can identify a point around 23°, do something similar for the Azimuth (angle from the North), then use their arm to point in that direction and see what may be blocking the RF signals.
2D Projection - This alternative view represents a flattened 2D projection of the actual 3D view around the antenna (also referred as cosine projection). It provides a more qualitative and realistic 2D visual representation of the antenna's reception quality map. It is like looking at the top of the antenna's 3D reception dome, from far away. However, its non-linear elevation scale makes it difficult to estimate elevation angles between grid lines.
Here is an example of the same set of GNSS SkyView antenna reception measurement results, presented in Linear (left) and 2D Projection (right). VeEX's Excel template calculates both types of maps.
45 to 52+ dB-Hz, Excellent- 36 to 44 dB-Hz, Good
- 27 to 35 dB-Hz, Fair
- 18 to 26 dB-Hz, Marginal
- 9 to 17 dB-Hz, Poor
- 1 to 8 dB-Hz, Bad
Using other editing tools, such as Microsoft's PowerPoint, the 2D projection can be easily converted back to a 3D representation, if needed (e.g., for illustrative or training purposes, etc.).
For some examples on how the polar graphs are used to validate and troubleshoot GNSS antenna distribution installations, refer to the following article.
How to Determine if an Antenna Installation is a PASS or a FAIL?
There are no simple PASS/FAIL reference masks. It all depends on the expectations set from their local situations, type of installation, applications and (in certain cases) accepted trade-offs. In general, we can consider some commonsense guidelines:
- For roof installations, it is expected that the signal quality (C/No) should be all green (excellent and good) towards the Zenith.
- It is expected that the satellites' signal quality (C/No density) will degrade as they approach the horizon. It is normal to see the heatmap transition from marginal (orange) to bad (red) below elevations of 20°. That angle may be expected to be larger in urban canyon scenarios and lower in tower-mounted antennas.
- Some antenna installations may be designed with narrower apertures, in order to avoid noise sources from the surroundings. You need to know that information in order to set the right expectations.
- Be familiar with unavoidable features around the antenna that may block or degrade the signals, they will definitely show up in that direction on the heat map. In some cases, reception could be improved by positioning the antenna above nearby obstacles.
- It is not just about the GNSS antenna. Consider the whole GNSS signals' distribution system, including cables, amplifiers, splitters, lightning arrestors, terminations, etc.
- Based on the previous points and having a good idea of the environment surrounding the GNSS antenna installation, create your own expectations of how the heat map should look like, to determine if the final results are good (as expected) or bad.
- In certain cases, multipath problems from nearby buildings (e.g., ones with metal-coated reflective windows) may cause darker spots on the opposite direction in the polar heat map. It may look like a phantom obstruction in a direction in which there is none. This could happen due to the reflective angle required by the bouncing signals to create destructive interference with the original (direct) RF signal.
- Some PRTC (Primary Reference Time Clock) vendors promote indoor antenna applications, usually by setting the C/No mask in their equipment to 0 dB-Hz and perhaps relying on GNSS augmentation. Depending on the construction of the building and window coatings, (if tested) you may have to settle for a heat map with lots of orange (marginal) and red (poor/bad).
Related Test Solutions
- MTTplus - Modular Test Platform
- CX41 - Coaxial Time Domain Reflectometer (TDR)
For more information about SkyView, visit our Knowledge Base page.