How Does GPS Work?

What is GPS?

There are currently 30 operational (December 2019) US Military owned Satellites orbiting the earth on 20,200 km orbits, giving 100 % world coverage (24 is considered the minimum required for full global service). The free signals from these Satellites can be used in different ways to determine a number of measured parameters of movement and position.

To calculate the Longitude, Latitude and Height position, a GPS receiver precisely measures the different speed of light (299,792 km/s) delays in the signals coming from 4 or more satellites. The distance to each satellite is calculated, and then using trilateration, the 3D position of the GPS antenna is calculated.

Each satellite transmits messages that include:

• The time the message was transmitted.
• Precise orbital information, known as the Ephemeris.
• Approximate orbits of all GPS satellites, known as the Almanac.
• General satellite health, etc.

All of this information takes 12.5 minutes to transmit, meaning that VBOX products that are brand new, haven't been used for a while or have moved location since last use may take up to this time to acquire an accurate location lock

Here's an example of a 24 GPS Satellite constellation moving as the Earth rotates:
 

By Paulsava - Own work, CC BY-SA 4.0
 

The GPS engines used in VBOX’s also calculate the Doppler shift in the carrier frequency of each satellite transmission to build an accurate measurement of the speed of the GPS antenna in the X, Y and Z planes. X is north/south, Y is east/west and Z is vertical velocity.

The X and Y velocities are combined to give ‘course over ground’ speed and heading data.
 

The velocity and heading channels are the accurate channels from which most VBOX data is derived: velocity, heading, accelerations, radius of turn, deviation, braking distance etc.
 

The less accurate positional channels, longitude, latitude and height are only used for plotting the vehicle path.

How accurate is GPS?

Velocity

The use of GPS in automotive testing is now well established, and Racelogic has been a market leader and innovator for over a decade. GPS systems have revolutionised the accuracy and convenience of testing, development, and validation within the industry. Why and how is it so effective?
 

There is a common misconception that speed measured via GPS is done so as a function of position against time. If this were the case, GPS velocity would be just about unusable, because GPS position relies on precise measurements of the distance from the receiver to the satellite, and therefore suffers from numerous effects - such as atmospheric interference - which delays the signal.
 

Fortunately, velocity isn't measured like this: instead, the Doppler shift in the signals coming from the satellites is captured and this leads to an incredibly accurate measurement of speed.
 

If you are tracking seven satellites, then this is like having seven police radar guns aimed at you: the measurement is made by taking them all into account.
 

 

Other crucial factors involved in producing useful velocity measurements for vehicle testing are sample rate, frequency response, latency and noise. See 'GPS Factors' for further explanation.

Distance

GPS satellites are equipped with an atomic clock, which ensures timing stability to less than one-millionth of a second. By integrating Doppler-derived speed with this level of time signal reliability, an extraordinarily accurate distance measurement is achieved.   There are a number of tests which we have carried out over the years to verify and improve our measurement algorithms for applications in vehicle testing.

One such test is to place two reflective strips on the road at a known distance apart. Using a laser sensor connected to the trigger input of a VBOX, the vehicle is then driven between these two points a number of times, and the distances compared. In such tests, the VBOX 3i will always be within 3cm in 1000m, which is about the same as the measurement uncertainty due to the slight deviation of the vehicle during the driving.

Position

GPS positional accuracy is subject to ionospheric interference, which causes a satellite signal to change in length as it travels towards the Earth's surface. This means that, depending on the quality of the receiver, accuracy can vary by several metres.

VBOX data loggers are fitted with superior GPS engines that give better positional accuracy than those found in mobile phones and satellite navigation devices. By using additional signals other than those from the standard US GPS satellites, this can be enhanced further. The free SBAS services augment the positional capabilities of a survey-grade receiver to as good as 1 m without the use of a Base Station.

However, by adding another GPS engine at a fixed position, the majority of errors can be calculated and removed from the final result. This is the function of a Base Station: because it remains stationary, it is able to broadcast correction messages to a roving VBOX, resulting in much improved positional accuracy.

By incorporating signals from the Russian GLONASS satellite constellation and employing a Real-Time Kinematic (RTK) algorithm, the VBOX 3i Dual Antenna RTK with suitable Base Station achieves a positional accuracy of +/-2 cm.
 

This image shows a scatter plot of samples taken over a period of 24 hours demonstrates just how accurate the RTK + GLONASS really is:  Normal GPS in red, RTK in blue.   The advantage of using both sets of constellations is that there are almost twice as many measurements available to use, and they are on two slightly different frequencies. This allows sophisticated algorithms inside the GPS engines to eliminate all of the major errors normally associated with position estimation, leading to the highest accuracy GPS engine on the market today. See this video for a demonstration.

 

 

Dynamics

By employing multi antenna systems, it becomes possible to measure vehicle body attitude parameters. When placed in line down the length of the vehicle roof, a dual antenna setup can be used to capture slip angle and pitch; when placed across the width of the roof it can record roll.
 

This works by configuring the VBOX with the antenna separation distances, and with an RTK lock operating between them, a very accurate angle measurement is produced, achieved by comparing the heading taken from the primary antenna against the relative position of the others.
 

Multi antenna VBOX data loggers are used not just in the automotive sector but also in Marine applications, where they are perfect for measuring trim, heel, and lee of the vessel.
 

GPS Factors

Crucial factors involved in producing useful velocity measurements for vehicle testing are sample rate, frequency response, latency and noise.