Have you ever been warned about speeding or not recording rest properly, based on what the GPS says?
It’s more common than you’d think, so it’s helpful to know how GPS works and when it can be inaccurate.
What is GPS?
We use GPS every day. It’s what helps us with directions, tracks our steps, and shows us where shops are. But what is GPS?
GPS stands for Global Positioning System. It works by sending signals from satellites in space to devices here on Earth, like your phone, a GPS tracker in a truck, or even a smartwatch.
These devices, called GPS receivers, “listen” to signals from satellites. These signals tell the receiver exactly where it is.But for GPS to be accurate, it needs good signal strength. Signal strength is based on two things: Sat-Count and HDOP.
Understanding Sat-Count and HDOP
Let’s start with Sat-Count. Sat-Count is just a fancy way of saying, “How many satellites is your GPS receiver using?” The more satellites a receiver uses, the better the GPS accuracy.
Then there’s HDOP, or Horizontal Dilution of Precision. This is how well the satellites are spread out in the sky and obstacles on Earth, like buildings and hills. When the satellites are far apart from each other, the GPS has a clearer picture of your location. But if they’re too close together, or if there are things in the way, HDOP can move outside the preferred limits, and your GPS result might not be as accurate.
For GPS to be considered accurate, here’s what you need:
Sat-Count should be four or more satellites
HDOP should be between 0.5 and 1.0
Why does this matter for truck drivers?
If you’re a truck driver, your company probably has a GPS receiver in your vehicle. This system lets your company track where the truck is, how fast it’s going, and sometimes even how much weight it’s carrying.
Some companies set up their GPS systems to check if drivers are speeding. For example, here’s how some of these limits might look:
Going over 104km/h but under 109km/h for more than 10 seconds
Going between 109km/h and 115km/h for over 5 seconds
Going over 115km/h for any time at all
In these cases, the GPS receiver will alert the company if you go over those limits. GPS can also track if you’re taking breaks when you’re supposed to. This works when someone checks your speed and location against your work diary to see if you’re resting when you say you are.
Let’s say you’re moving slowly around a worksite at under 5km/h—maybe loading or parking. GPS can still show small movements, therefore recording this as rest will be a breach. This is why it’s helpful to know if the GPS data is correct, so you can clear up any misunderstandings.
Can GPS make mistakes? Yes!
Sometimes, the GPS data isn’t accurate. This happens when the numbers are outside the parameters shown above (Sat-Count – 4 or more; HDOP – between 0.5 and 1.0).
If these numbers aren’t accurate, the GPS might show speeds or locations that aren’t accurate. For example, a GPS receiver sent an alert for a vehicle travelling at 126km/h in a small lane in a city. But the GPS data showed low Sat-Count and a high HDOP, meaning the reading was wrong. In other words, it was unlikely the driver was going that fast in such a tight space!
How to protect yourself with GPS data
If you get a warning for speeding or not taking breaks, it’s okay to ask to see the GPS data. Here’s what to look for in the data:
Latitude and Longitude: These are the coordinates, showing exactly where you are. These can be copied into Google Maps to pinpoint the location.
Street Address: This tells you the closest street to where the GPS thinks you are.
Date and Time: This shows when the reading happened.
HDOP: Check if the HDOP number is between 0.5 and 1.5. If it’s higher, the GPS accuracy might not be reliable.
Sat-Count: 4 or more for good accuracy.
GPS Speed vs. ECM Speed: ECM Speed is data from the truck’s engine. It’s more accurate than GPS speed. If the ECM speed is different from GPS speed, always use the ECM speed.
If the HDOP or Sat-Count isn’t good, the GPS might be showing the wrong speed or location. In cases like this, you can explain to your boss that the GPS data may not be accurate enough to rely on.
What if GPS data shows you’re breaking rules?
Sometimes, people don’t know how GPS data works. This can lead to unfair warnings or disciplinary action for drivers. So, here’s what to remember:
Check the HDOP and Sat-Count: If these numbers aren’t right, the GPS data could be wrong.
ECM Speed is Better: If your truck has ECM speed data, use that — it’s more accurate because it comes from the truck’s engine, not the GPS.
Ask for a Copy of the Data: You have the right to see the GPS data if you get a warning. It’s important to review the data carefully, especially if you think you’re being blamed unfairly.
What else should you know about GPS?
GPS isn’t perfect and doesn’t always work well everywhere. If you’re driving in hilly terrain, in the bush, or through a city with tall buildings, the GPS might not be accurate. This is called “interference”, and it can affect the HDOP reading, meaning the GPS location isn’t as precise.
How can you avoid GPS problems?
The best way to stay out of trouble with GPS is to follow your company’s speed and rest rules. But if you ever get a warning, and you think there’s been a mistake, remember to check the numbers. If the GPS shows errors, it’s your right to ask questions and get help to understand it better. You don’t have to take the blame for something the GPS might have gotten wrong.
Jodie Broadbent is the founder of Know the Road, which provides consulting auditing and training services for road freight supply chain partners.
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