GPS System and GPS antenna

I can argue that the GPS system is the most complicated and most difficult system in a modern race car, especially in a racing motorcycle. It is amazing that it even works and how good modern GPS receivers are. Yet, it is a common complain that the simple GPS doesn't work, lap times are not correct. Before starting arguing with me, consider this:

Signal level:
Imagine that there is a 30W lamp, 22000kms away and the GPS antenna needs to "see" that when the bike is leaning 50deg to the other direction. It is not an easy feat.
This signal level is about -125dBm and is below the thermal noise level. GPS antenna needs to be able to pick it, amplify the exact frequency only and minimize the noise while doing that.

Signal Timing:
GPS works by measuring the time it takes for the signal from the satellites to reach the GPS receiver. 
Per the standard, normal GPS system operations must ensure the UTC offset data broadcast by GPS is within 40 nanoseconds to UTC, that is 0.000000004s.

Reflections and multi-path:
Then there are reflections, typically from buildings, trees. In motorcycle racing reflections can come from the riders back, windshield. 
Multi-path reflection is a case where a duplicate signal that is created by reflection. This signal is little bit delayed compared to the original, real and original signal. GPS receiver then needs to decide which one is the correct one. Not easy.

Athmospheric disturbance:
Weather, sun etc... will also affect the accuracy of the GPS. Modern SBAS corrections can eliminate this error. This correction is applied when you see GPS Fix Type 2, Differential GPS. Fix type 1 is normal GPS without this correction.

Antenna installation:
This is the most important thing, think the antenna as the "eyes" of the GPS receiver and need to see the 30W lamp from 20000kms away. Chances are that the receiver can't receive the satellite if the antenna is blocked by some obstacle, rider, bike next to it, building etc... 
I have seen very poor antenna installations, worst one was that the antenna was inside the seat unit, zip tied to the bottom of the sub frame. No wonder why the GPS reception was bad...

Basic instructions for antenna install:
- Always install it so that it has clear 360 view to the sky
- Ground plane is needed for the antenna. 70mm metal, round plate or aluminum tape under the fairing. Ground plane is not needed if installed on the top of the carbon fiber.

Motorcycles and to reduce the high lean angle problems: 
- Install on the side that stays upright most of the time.
- Install to the highest and narrowest location.   

2023 Antenna Test

Below are results from the 2023 GPS antenna test.

Only way to get comparable GPS antenna results is to do a long test, 12 hrs while stationary, at the same time and in open sky. Open sky is not possible for this test but not a big problem since the test is performed at the same time for both.

What comes to GPS antenna performance, simple rule of thumb that I use is:
 More satellites -> Better antenna -> Better result.

Stationary tests

Tested antennas here are GPS L1 band antennas, GPS0010 made by Pulse and Linx, with 2 different GPSL25 SA receivers. Receivers were also swapped for test 2.

PDOP: Dillution of Precision 

No noticeable difference between devices and antennas.

SNR's of all received PRN's

SNR was recorded during the test for each PRN. Average is counted for all PRN's.

GPS L1: 1 - 32 (1575.42 MHz, CDMA)

GLONASS: 38 - 61 (1598.0625 - 1609.3125 MHz, FDMA)

Test1: 12:00, 3hrs

GPS0010 wins GPS, Linx GLONASS.

Test2: 15:00, 3hrs.

Identical GPS performance, Linx wins GLONASS as expected.

Summary, average for all PRN's

As per spec, GPS0010 is GPS L1 only and this is visible in results, it works better at GPS PRN's while Linx works better at GLONASS frequencies. 

Both antennas are good but Linx might work better at areas with mountains due to better GLONASS performance.

Moving tests

Test1

Red	MoTeC L10
Green	L20 SA GPS0010
Blue	RTKCAN uBlox

Test2

Red	L20 SA GPS0010
Green	L20 HA Linx
Blue	RTKCAN uBlox

Test3

Red	L20 SA Linx
Green	L20 HA GPS0010
Blue	RTKCAN uBlox

8hr Drive test

Linx and GPS0010 antennas were also tested on the road for 8hrs. Antennas tested at the same time.

Red	MoTeC L10
Green	L20 SA GPS0010
Blue	L20 SA Linx

Here's the summary of the number of satellites from all 3 logs:

Winner of the drive test is Linx, GPS0010 very close.

Summary

Few words about the consistency value. Measurement is invented by me to try and quantify the accuracy of the antenna. Values are taken from the rounds test and the variation between rounds is calculated together and then divided with the number of rounds.

RTKCAN is just there for reference, there is no way for the normal GPS to beat it, but closer to it, the better.

It is normal for HA to have less satellites than SA due to lack of GLONASS. In case of HA, satellite number is little misleading since there can be double satellites due to 2 bands. If 15 L1 is available, there might be another 15 L5 satellites available making total of 30.

Summary is that GPS0010 is very good GPS L1 antenna and tough to beat, only negative is the long, 5m cable.

It is not recommended to cut the cable since it is optimized by the manufacturer. GPS signal is very weak and below the noise floor. If cut, noise will increase also and receiver might not be able to pick up the real signal from the noise. Also the receiver front end might be overloaded. If you need to cut it, cut in 1m pieces and test.   

Linx looks to be very good also for L1/L5 antenna. It also worked very well with SA device also because supports all systems GPS, GLONASS, BEIDOU and GALILEO. 

 

See the list of recommended antennas here, specifications and where to find those.