Radar VS ADS-B
As a pilot I’ve been following the FAA NGATS (Next Generation Air Transport System) effort to modernize the air traffic control system. There are a number of projects involved but ADS-B is one of the early ones. ADS-B is short for “Automatic Dependent Surveillance - Broadcast”. It’s a GPS-based system that reports the altitude and location over a broadcast radio link both to other aircraft and to Air Traffic Control (ATC). This will provide the surveillance of aircraft necessary.
Currently ATC uses radar to see aircraft. Actually, there are two kinds of radar - primary and secondary. Primary radar is the reflection of the radar beam from the aircraft itself. Secondary radar uses a transponder in the aircraft. When it receives a signal from the ATC radar, it transmits back its code. The code is set by the pilot at the direction of ATC. This code then shows up on the ATC radar screen so that the controller knows which plane is which. Most of the time ATC uses secondary radar.
So I decided to compare a radar track with a GPS track. I used GPS to simulate ADS-B. My plane is located in Leesburg VA inside the ADIZ. Without going through all the detail about that, this means that I have to have a discrete transponder code rather than a generic one. That means sites like Flight Aware will track me when I’m in the pattern.
I used my hand-held GPS to provide the GPS tracking. Unfortunately, it’s a hiking GPS so didn’t track altitude well. But, it did a nice job of tracking my path over the ground with 4 or 5 second updates. ADS-B is planned to have one-second updates. Since my plane isn’t fast, 4 second updates worked fine.
This is the GPS track in the purple on the right as shown by Google Earth from the KML file I created. My company helped make this display possible by supporting some of the data translation. The radar track is shown by Flight Aware. In both tracks, my landing direction is to the south.
The radar track on the left below is truncated. I have no way to know, but it’s possible that ATC decided to ignore my track since they knew I was just in the pattern. So, below right is another link to some previous pattern work. This one is using the opposite direction runway so I’m landing to the north. But it still serves to give you an idea of what a radar track looks like.
The radar tracks look like modern art. I can’t get the actual radar fixes as I can with my GPS, but Flight Aware’s line fitting shows enough. My low altitude (pattern altitude 1200 feet) was probably the cause of the major errors in the radar track. But, my feeling is that the radar isn’t nearly as accurate as the GPS.
The GPS track is clear enough to make one friend ask why I was steering away from the runway (the two “bumps” in the upper right). Once was for bird avoidance. The other was why partly why I was practicing landings I suppose. The close in track was for a simulated engine failure landing.
It’s clear for this case which is more accurate. But I like to consider the source of the errors. Errors can occur in the GPS location that feeds ADS-B as well as the radar.
Radar errors are from two major sources: errors from being out of the main beam, and errors from being far away from the radar antenna. Since the radar works by sweeping a beam across my plane then if I’m too low or too high for the beam to hit me well then some positions are either missed or possibly calculated incorrectly. The farther I am from the radar antenna the greater the lateral distance the beam sweeps per unit of time. So, the less accurate my lateral position is.
GPS, on the other hand, is quite accurate. But, the GPS signals are very small signals as they come from satellites in orbit. This is a long way so the signal can be distorted to some extent. Also, being such a small signal, it’s more easily disrupted or interfered with.
There’s also the issue that ADS-B relies on the aircraft’s equipment to report its position accurately. Primary radar can work independently of the aircraft. So for some purposes, ADS-B cannot replace radar. However, for normal ATC use ADS-B offers high accuracy and the potential for many planes to have in-flight traffic avoidance.