Why Don’t My *bleeping* GNSS Radios Work?
  • Posted on: September 14, 2023
  • Category: Technology
  • Written by:
    Joe Sass

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Why Don’t My *bleeping* GNSS Radios Work?

GNSS RTK base and rover radios can be complex to work with, and sometimes a source of frustration. A new international standard has been developed, and once broadly adopted, should offer some relief.

Single-base, real-time kinematic GNSS operation where radios are often used to send corrections from the base station to a rover or rovers.

Working with GNSS is comparable to a maestro conducting an orchestra.  There are many facets of the system that must work together to produce useful data.  The geometry of the satellites, sky occlusion and multipath signals, hardware and software limitations, mixed brands, expectations and methodologies, interference, biases and other factors all play a role in producing a working system.  For real-time GNSS users seeking the highest levels of accuracy, there must be a data link between the base station and its rovers.  Real-time kinematic (RTK) is the technique whereby a base station sends observation data to rovers.  This data stream is often referred to as corrections.

Working with GNSS is comparable to a maestro conducting an orchestra.  There are many facets of the system that must work together to produce useful data.

Even in this age of I.P. base-rover comms options, NTRIP, and real-time networks (RTN), radio base-rover is a staple for many RTK operations. Historically and still predominantly, this RTK data link uses a UHF transmitting radio at the base station that broadcasts correction information to any number of rovers.  There are other methods as well to establish the data link such as the internet and space-based delivery.  Regardless of the data link in use, this aspect of RTK seems to be the most fragile part of the system.

In the case of UHF radios, the data link is often broken due to other users on the same channel or a lack of commonality in the settings.  Range expectations may be unrealistic.  Power issues and overheating of the radio are also frequent occurrences.  The main reason though for this fragile data link is that there are no standards to describe the connection.  Different manufacturers have developed their own proprietary protocols.  Competitors have come along and tried to reverse engineer these approaches.  The ability for brand A to work with brand B is difficult at best and often impossible.

In an ideal scenario, any job site using RTK will be working with a single brand of radios.  This ideal scenario rarely exists.  It is common for many users from different companies and different disciplines to use a single base station on a construction site.  This base station will generally broadcast RTCM data since this is a messaging format understood by all GNSS manufacturers.  RTCM messages may be a well-known and interoperability-tested standard, but the radio modulations that are being used to deliver these messages are not standardized and are rarely interoperable between brands.

The International Organization for Standardization (ISO.org) has authored over 20,000 standards related to technology and manufacturing.  ISO has representatives in at least 160 countries and hosts over 800 technical committees for standards development.  “Worksite Data Exchange” is one of their technical committees.  It became evident among this committee’s membership that the RTK data link is a big problem that needs to be resolved.  Since the data link is generally transporting RTCM Special Committee 104 messages, it seemed natural that RTCM should also standardize the data link layer.

RTCM Special Committee 135, Radio Layer for Real-Time GNSS Applications (SC135) was established in July of 2018.  The goal of this committee was to develop standards for (GNSS) radios to exchange data efficiently and reliably regardless of the brands.  The road to standardization involves countless considerations and compromises.  Decisions about the following parameters needed consensus: sensitivity, bandwidth, symbols and symbol rates, forward error correction, whitening, cyclic redundancy checks, framing, sync patterns, modulation and demodulation scheme, tail, timing, deviations, FSK error, interleaving and many others.

Standards seek to offer the best compromise for the industries being served.  Assemble a room full of subject matter experts from interested stakeholders and drive technical discussions.  Topics that are non-proprietary are discussed at length and then decisions are made.  Other times, a participating company may divulge their proprietary algorithms.

RTCM SC135 members have made all the technical decisions.  Interoperability testing has been completed.  Radios from four different manufacturers with firmware built compliant to the proposed standard communicated with each other in base/rover modes.  The committee has drafted a 55-page document describing everything a radio firmware engineer needs to know to build firmware compliant with the standard.  This document has passed the plenary committee’s voting process.  The standard was announced and published in September 2023.  All RTCM members have access to this document.  This standard may be the only over-the-air format developed for narrow-band radios in the last ten years and comes with many performance improvements.  Increased efficiency of the bandwidth allows more satellite data to be broadcast.  Forward Error Correction (FEC) messaging will allow a rover to automatically detect if this is turned on at the base station.

If only it were this simple. Sometimes complex RTK radio operations might get simpler with the recent introduction of an international standard for applicable radios.

Early on, SC135 decided to focus its initial efforts on the narrow-band UHF radios spanning the spectrum from 403 to 473 MHz. This decision was made for a couple of reasons.  Firstly, the narrow-band radios dominate the radio-based RTK landscape.  Secondly, standardizing the UHF radios was deemed slightly less complex than spread-spectrum radios that are also commonly used in RTK.  Considering both points, SC135 decided to work on the narrow-band solution first, and once this standard was released, look at other radio protocols, frequencies and scenarios that may need to be standardized.

The vision of SC135 has been to remove the complexity of UHF RTK and simply make it work.  Imagine your favorite radio programming software.  Instead of making decisions about settings that may or may not make sense, you simply choose the RTCM mode.  Nearly all the other parameters will be set according to the standard.  No need to be a radio expert.  No worry about brand of radio.  It just works.  With the release and implementation of this standard, UHF RTK will become a lot more user-friendly which will make job sites more efficient resulting in less expense and increased productivity.  This standardization effort was long overdue.  The glimmer of an easier, more robust and reliable RTK world is on the horizon.