In order to get the most out of your RTK system it is important to understand what your receiver needs and what it is telling you. Your GNSS receiver can tell you a lot of information if you know what to look for, and in this article, we will go over what each of these parameters mean to you. When in difficult environments, understanding what these numbers mean can be the difference between a fixed solution and having to break out your total station. Do you know what your PDOP, SDEV, Satellite Number and SNR means about your setup?
Position Dilution of Precision (PDOP)
With modern 7th generation RTK systems, like the Hemisphere S631 and Trimble R12, in an open field you almost always be able to obtain a fixed solution nearly instantaneously. Where they really start to make you, money is in tree canopy and near buildings. In these heavy multi-path areas, there are a multitude of factors that can influence your time to fix (ttf) and whether or not it is even possible to obtain a fixed solution. If you can understand these factors, you can maximize your chances of success.
What makes surveying under dense canopy conditions so challenging is the effect these conditions have on the incoming GNSS signals. In order to maximize your efficiency with RTK, you will need to understand; when and where you should be able to get a fix, when and where you will need to take an offset, and when and where it is time to break out the total station.
When GNSS technology first
Position Dilution of Precision (PDOP) quantifies the effect of satellite geometry on GPS accuracy. In simpler terms, PDOP tells you how close your satellites are to one another. The lower the DOP value, the better the satellite geometry.
In practice, this means the lower the DOP value, the faster your GPS receiver will be in resolving a fix, and will be able to hold onto that fix for a longer amount of time. We consider 1 to be a good PDOP value, at a PDOP of 1, you can see satellites in every part of the sky, and they are well spaced from each other to minimize the triangulation error. The further you get above that, the more the receiver will struggle to get a fixed solution.
In early receivers, low PDOP values were common, simply due to the low number of satellites in the sky. Users would have to do significantly more planning before their surveys, ensuring that the satellite coverage they required was available.
In today’s surveying world, this is no longer the case. Most often, higher PDOP values are caused by canopy coverage, or building obstructions. The higher PDOP value is caused by the receiver losing up a significant portion of satellites to sky that is no longer visible. It is therefore a decent measure of multipath conditions.
When surveying with GPS, especially under a canopy, it’s crucial to understand and monitor the standard deviation (SD) of your position. The standard deviation reported on your handheld is an estimation of the the uncertainty in your GNSS position. This estimated error can be in the range of millimetres in the case of an RTK fixed position, or several meters for a receiver with an autonomous position.
A surveyor entering information using MicroSurvey’s FieldGenius. A live error estimation gives a surveyor an indication of the quality of their measurement.
For the vast majority of RTK equipment used in surveying, the standard deviation will be reported in two different values. The horizontal standard deviation (HDEV) is an indication of the error in the Northing and easting. The vertical standard deviation (VDEV) reports the estimated error in the vertical height. For more information on how standard deviation is calculated, check out this article: Postioning Error: ESA. The reported error is a plus / minus value. When in the field, imagine drawing a circle with a equal to the reported error. This circle is equivalent to the estimated position of your receiver.
In the field, it is important to keep your eye on your Vertical and Standard Deviations. A typical open sky fix with the Hemisphere GNSS S631 will give you a horizontal SDV of 0.008m and a vertical SDV of 0.015m. When you aren’t fixed, these numbers will be much higher. For instance, when in canopy conditions, where you have to wait for a fix to be calculated, the sdev values can tell you a lot about what the receiver is doing. If your horizontal and vertical SD values are gradually decreasing, it suggests that your receiver is acquiring better satellite signals and is likely to resolve the Real-Time Kinematic (RTK) equations soon. This is a positive sign, indicating that a fixed solution is imminent. If your SD values remain high or do not improve, it indicates that your receiver is struggling to track good quality signals. This could be due to dense canopy cover, significant multipath interference, or insufficient satellite visibility.
One of the most common indicators of a good GNSS solution is the number of visible satellites. All RTK receivers will report the number of satellites that the unit is attempting to use in the RTK equation. The lower the number of satellites, the more difficult it will be for your receiver to compute a fixed position.
Low satellite counts can be caused by a number of different factors. However, the most common are caused by obstructions between your receiver and the sky, including tall buildings, large trees, or power lines. These obstacles will block or reflect satellite signals. A good rule of thumb is that if you cannot see a portion of the sky, that means your receiver cannot use satellites in that portion of the sky. When your receiver encounters a situation where signals are obstructed or reflected, the receiver can either filter the bad signal out (As the S631 often does using it’s Cygnus technology) or discount the satellite.
Another common cause of low satellite counts is down to how a surveyor has set their base up. Older broadcast messages may not necessarily support all available constellations. Although once very popular, older message types like CMR only broadcast information from GPS satellites. This is an issue, as although your rover may see all constellations, it can only use constellation corrections supplied by the base in the RTK equation. As a result, your rover may only report that it is using a handful of satellites, even though it is in reality capable of using 30+ different satellites. If you are seeing a consistently low satellite count, then that is often an indication of a configuration issue, but if you are seeing a low satellite count in one specific area, you may want to consider performing an offset instead.
An example of a location a surveyor may encounter low satellite counts. The heavy tree coverage obstructs and reflects signals which can significantly reduce performance.
Signal-to-Noise Ratio (SNR) measures the quality of the GNSS signal. While this is more of an issue with older receivers that do not track as many signals per satellite, it is good to know for when you run into issues with specific constellations. High SNR values indicate a clear signal with minimal interference, while low values suggest the signal is weak or obstructed. Under a canopy, SNR can be significantly reduced. Aim for an SNR of at least 30 dB for reliable positioning.
If you are out in the trees or up against buildings, keeping an eye on these statistics can help you determine whether or not you should wait a minute for a fix on your point, or get the position using a different method. Beginner surveyors can use these to help them up their pace in the field, and more senior users can use them to aid their determination of which process to use. By understanding and keeping an eye on these metrics, beginner surveyors can improve their chances of obtaining reliable positioning despite challenging conditions. Happy surveying!
PDOP quantifies the effect of satellite geometry on GPS accuracy. A lower PDOP value indicates better satellite geometry, which means your GPS receiver can resolve a fix faster and maintain it longer. A PDOP value of 1 is considered good as it means satellites are well-spaced across the sky, minimizing triangulation error. Higher PDOP values, often due to canopy coverage or building obstructions, indicate challenging conditions for obtaining a fixed solution.
Standard Deviations in GNSS surveying indicate the uncertainty in your position. Horizontal Standard Deviation (HDEV) measures error in Northing and Easting, while Vertical Standard Deviation (VDEV) measures error in vertical height. Low SD values, such as 0.008m horizontally and 0.015m vertically, suggest high accuracy, typical of an RTK fixed position. Monitoring SD values can help surveyors assess the quality of their measurements and determine if a fixed solution is imminent or if the receiver is struggling.
The number of visible satellites is crucial for computing a fixed GNSS position. Obstructions like buildings, trees, and power lines can reduce the number of visible satellites, making it harder to achieve a fixed position. Older base station messages that don’t support all satellite constellations can also limit the number of satellites your receiver can use. Consistently low satellite counts may indicate configuration issues, while low counts in specific areas suggest the need for alternative positioning methods.
Signal-to-Noise Ratio (SNR) measures the quality of the GNSS signal. High SNR values indicate clear signals with minimal interference, while low values suggest weak or obstructed signals. An SNR of at least 30 dB is recommended for reliable positioning. In canopy-covered areas, SNR can be significantly reduced, impacting the receiver’s ability to obtain a fixed solution.
Understanding PDOP, Standard Deviations, the number of satellites, and SNR helps surveyors determine the best times and locations to obtain a fix, when to take offsets, and when to use alternative methods like a total station. By monitoring these parameters, surveyors can maximize their efficiency and chances of success, especially in challenging environments like dense canopies or near buildings.
Bench Mark Equipment & Supplies is your team to trust with all your surveying equipment. We have been providing high-quality surveying equipment to land surveyors, engineers, construction, airborne and resource professionals since 2002. This helps establish ourselves as the go-to team in Calgary, Canada, and the USA. Plus, we provide a wide selection of equipment, including global navigation satellite systems, RTK GPS equipment, GNSS receivers, and more. We strive to provide the highest level of customer care and service for everyone. To speak to one of our team today, call us at +1 (888) 286-3204 or email us at [email protected]
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