GNSS Technical Specifications Explanations

Q. Why is it that when using GNSS RTK the vertical accuracy is worse than the horizontal accuracy
A. The reason that vertical accuracy is worse than horizontal is because the GNSS receiver cannot see below the horizon. With horizontal measurements, the receiver has satellites visible across all quadrants of the horizontal plane which produces strong geometry for horizontal calculations. However, in a vertical sense, the GNSS receiver can only track satellites in two quadrants of its vertical plane which does not provide the same strength of geometric calculations.

Q. In most cases when buying an RTK there are technical data about the position accuracy that I always don’t understand. I bought Pentax G6 and it had the following written about the position Accuracy

               Horizontal............................8 mm + 1 ppm RMS

               Vertical...............................15 mm + 1 ppm RMS

               Initialization time.................. typically < 10 seconds

               Initialization reliability..................typically > 99.9%

What does it mean? Thank you
A. When a manufacturer writes a specification about the accuracy of their GNSS receiver, they generally want to impress the potential buyer with the highest degree of accuracy possible. So,1 Part Per Million (PPM) means that for every kilometer the rover goes away from the base, another millimeter of given error is added to each measurement. For instance, let’s imagine a situation where the rover is 16km from its base station. There is already 0.8 centimeter horizontally and 1.5 centimeters vertically given plus the PPM accumulation. At 1 PPM, this contributes another 1.6 centimeters of given error both horizontally and vertically making the accuracy specification at 16km in perfect conditions 1.28 cm horizontally and 2.4cm vertically.

Q. I saw Comnav T300 GNSS RTK specification has 256 channels, is it necessary we have many channels in a RTK

There are a number of reasons that having more channels in a receiver is beneficial. The primary reason (and most obvious) is to track all of the available (all in view) satellites. In this case, when a satellite is being tracked, one channel will be devoted to following that satellite.
Secondly, when satellites are just coming into view (or the receiver has just started), they must first be acquired. This process requires varying Doppler shift (as a result of the satellite's speed) and estimated satellite location, until the signal is acquired. Multiple channels speed up this process (Time-to-first-fix) by allowing multiple channels to look for the same satellite.

So, in short channels serve more than one purpose. They:

• Speed up satellite acquisition
• Reduce power consumption
• Reduce the likelihood of losing a 3D fix even in harsh environments
• Provide better sensitivity, allowing fixes in places where previously we would struggle
• Provide better positioning accuracy

 Q. what does IP67 in most of GNSS RTK specification means?

An IP (Ingress Protection) rating tells users exactly how waterproof a product is. Each IP rating includes two digits, for example, IP67 or IP68, the first of the two digits (the 6 in IP67) refers to protection against solids, including dust. Products are rated using a 0-6 scale, with 6 being the most well protected. Here's what each level means:

0 - No protection.

1 - Protection against objects up to 50mm. Also, protection against any large surface of the body, such as the back of the hand, but no protection against deliberate contact with a body part.

2 - Protection against objects up to 12.5mm, along with fingers or similar objects.

3 - Protection against objects up to 2.5mm, such as thick wires or tools

4 - Protection against objects up to 1mm, including most wires and screws.

5 - Not entirely protected from dust, but enough to significantly affect performance significant damage

6 - Totally protected against dust.

The second digit in the IP rating (the 7 in IP67) refers to the level of protection against liquids - here's what they mean:

0 - No protection.

1 - Minimal protection against vertical drops of water, such as very light rain.

2 - Protected by vertical drops of water when the device is tilted at 15° from its normal position.

3 - Protection against sprays of water when the device is tilted at 60° from its usual position, including rain.

4 - Protection from sprays and splashes of water from any direction.

5 - Protection against water sprayed directly from a low-pressure nozzle measuring 6.3mm, from any direction.

6 - Protection from high-powered water jets with a 12.5mm nozzle, from any direction, such as a shower.

7- Protected for up to 1m of immersion for up to 30 minutes, including splashing from a shower or an accidental (and brief) dunking.

8 - Protection against submersion beyond 1m. May mean that device is hermetically sealed or merely that any water that can get inside will cause no damage.