Troubleshooting GNSS/GPS: Part 1: Real-Time Data Links

Parent Category: Columns

Written by Joe Sass

Created on Wednesday, 02 November 2011 20:06

One of the more expensive costs of doing business in the world of Machine Control is heavy equipment sitting idle with the operators standing around waiting for their gear to be fixed.  Getting the equipment up and running is imperative to staying competitive and profitable.  In a previous article, we discussed techniques to troubleshoot any complex electronic system.  In the realm of electronics, many potential failures are common to a majority of systems.  But as the systems become specifically targeted at an industry, some of the problems that occur are unique to the technology being used.  In part one of this discussion, we will examine the data links that are required for a GNSS Machine Control system and things you can do when they are not working.  In a subsequent article, we will discuss the other pieces of a GNSS configuration and the specific troubleshooting techniques that can be employed to mitigate or minimize problems as they occur.

Effective troubleshooting begins with the knowledge that problems will happen.  Accepting this as fact should compel the system user to prepare in advance for these failures. Preparation includes possessing an overall understanding about how the system works, having spare parts to replace those pieces most likely to fail and keeping resources nearby such as reference manuals and other supporting documentation that describes the system in detail.

A typical Machine Control GNSS configuration contains four major components:  A geodetic antenna is connected to a GNSS receiver.  Connected to the GNSS receiver are also a power supply and some type of communication device.  When these components are setup on a tripod or other mount that keeps the receiver and antenna static, this is known as a base station.  Other sets of these components are then mounted to heavy machinery, surveying poles or mobile units and these are known as rovers.  Base stations provide information to rovers via real-time communication devices. 
<< “Static base station provides correction data to mobile rover units.”

GNSS Machine Control systems rely on a real-time data link between the base and the rover.  There are many techniques employed to establish this link; some are robust solutions while others are more susceptible to having problems.  The most robust method is an actual cable that connects the base to the rover.  Unless the data cable is compromised, the link will be nearly 100% reliable.  But this is not practical when the rover must be miles from the base.  Therefore, radios and cellular services are the most common technologies in use today for this communication link. 

Over-the-air techniques are subject to range limitations, signal interference and coverage areas.  Most radios being deployed for machine control systems are in the UHF band of the radio spectrum.  The Federal Communication Commission (FCC) mandates that broadcasters in this band be licensed, antenna heights are regulated and limits are placed on the output wattage.  Further, the FCC states that the radios must be equipped with voice monitors so that data transmissions are squelched when voice traffic is detected on the same frequency.  Cellular data links are more fragile than UHF radio systems and problems with these links may be out of the user’s control.

What happens when the communication link fails?  The first determination should be whether the problem is at the base station or the rover.  Many times, it is not convenient to check at the base station so troubleshooting usually begins at the rover.  Is correction data being received but not being processed?  Most rover systems have indicators to inform the user about this.  If the broadcast is via UHF radio, a scanner should be used on the same frequency to see if the base station is still sending corrections or to see if there is voice traffic interrupting the data broadcasts.  If there are two rovers being used, is one working and the other is not?  This condition points directly to a hardware or configuration problem.  Is the modem’s antenna in tact?  If cellular services are being used, is there signal available?

Communication failures between the base and its rovers can be complete or partial.  This too is a clue in determination of the problem.  If signal is getting through to the rover occasionally, this indicates bandwidth problems, weak signal or the beginnings of hardware failure.  System configuration is generally not an issue since the communication pipe appears to be open.  However, data modulation techniques vary and this might allow data to be received, but in a format that cannot be processed.  With UHF radios, it is imperative to know that modulation type, frequency, over-the-air baud rate, Forward Error Correction (FEC) and scrambling are all in parity.  If any one of these settings is different between the base and rover, the system will not communicate effectively.

Sometimes, it will be necessary to go back to the base station to troubleshoot the data link.  As you get closer to the base, does the rover start receiving data again?  In a UHF configuration, this might indicate problems with the output wattage or antenna at the base radio.  If using a cellular connection, a restored data link while returning to the base could indicate pockets of coverage gaps or bandwidth limitations from the service provider.  When arriving back at the base, the first area to inspect is the broadcasting radio.  Are the indicator lights blinking normally?  Secondly, look at the GNSS receiver indications.  Is it still tracking satellites?  If it is not tracking satellites, there is no information to broadcast to the rover.  Feel the equipment.  Is it hot?  Broadcasting radios generate heat but it should not be extreme to the touch.  If cellular services are being used at the base, is there still sufficient signal strength and bandwidth?  If the base station is using an Ethernet connection to broadcast its data to the rovers, is the internet connection still working?  These determinations are often difficult to quantify, but they can still point in the right direction.

A data link that is not robust or is beginning to fail can cause several different symptoms.  Some of these do not appear to be directly related to the data link.  A failing data link first exposes itself to the rover with degrading precision.  Whereas an operator may be familiar with working in the range of a couple of inches, precision may start to degrade to a foot or more.  At first glance, this may appear to be a problem with satellite tracking or signal processing when in fact the problem is lack of data from the base station.  Operators learn to recognize these indirect clues and interpret their meaning.

Cellular data links provide few clues about why they are not working.  Many of the details about through-put and bandwidth are in the domain of the cellular service providers and not the users in the field.  If the system has worked in the past, but is not working now, the problem could be related to the service provider or it could be failing hardware.  If the cellular data link has never worked, it is probably related to improper configuration of the modem or lack of proper options within the cellular subscription.  If there are two systems side by side and one of them is working and the other is not, look carefully for the exceptions and try swapping modems, SIM cards or controller.

Books can be written about the problems with these data links.  Anyone that has operated a real-time GNSS for more than a week has likely had to deal with one or more of the issues mentioned in this article.  There is no better teacher than experience to help find a problem and get it fixed.  However, with an open mind, adequate resources and training in advance, any user in the field can be empowered to solve these issues as they arise and get back to work as quickly as possible.  Many times, the solution is plainly in front, but either through assumption, frustration or haste, is often missed.  Start with the basics and work through the details.  In part two of this article, we’ll shift our attention to troubleshooting the other pieces of hardware that make up a modern GNSS Machine Control installation.

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