Ardupilot Helicopter loading RTK GPS

Chris, I agree with your statements regarding the ability of your typical UAV to adhere to a specific flight path even if absolute position is a given, however the use cases you refer to seem to deviate somewhat from what most people frequenting this forum would consider ‘normal’. You also seem to like referring to what happened multiple decades ago as though that is some reason to write it off as antiquated, obsolete and without advantage. Do you have any references that would back up your claim that typical WAAS accuracy (for those for whom it is available) is as good as a a typical float solution given a similar environment? Also, why is there a requirement to perform at anywhere near 27m/s?

Working with L1/L2 GPS most every work day, I agree with you but I think that currently, these systems are not so common among those frequenting this forum. As far as I’m aware, you are looking at over $1000 USD at the very least for a single dual band GPS module, often much more. More affordable units are certainly coming but I would consider them pretty exotic currently. My position is that even your cheap single band RTK GPS offers advantages over standard GPS at a price point an order of magnitude less…

Because it’s all about acres per hour in this business. A full section survey at 170 feet separation is 31 passes, which is 31 miles of flight distance, one minute per pass not including turning time. Then the imagery gets dropped into the software and the problem areas identified and mapped. The “precription” gets dropped into the controller in the airplane. And the controller turns the nozzles on and off or varies the rate accordingly. The airplane can cover 640 acres an hour at a cost of $1,500/hr to operate it. The spotting aircraft has to be able to cover that same section in the same time, including setup time, but it only costs about $35/hr to operate with a pilot.

When we first started using UAV’s for spotting aircraft tried various multicopter designs. They aren’t fast enough and don’t fly long enough. Tried fixed wings and they struggle in the wind and get tossed around like a leaf. Helicopters work and can keep up the pace. They have the power to maintain 60 mph ground speed in a 20mph headwind. They have the stability that fixed-wings lack but they still have the flight of a fixed-wing. And they have the multicopter capability to take off and land from anywhere, without having a prepared staging area. The UAV helicopters are also expensive at $10 Grand an example, fully set up and RTF. But a $10,000 aircraft that can get the job done with one pilot is worth more in the big picture than four $2,500 aircrafts that can’t get the job done and require four pilots to fly them.

The FAA’s fact sheet on WAAS provides the information. It is certified at many airports in lieu of the traditional Cat-II ILS for approaches to minimums of 250 feet. A Cat-II ILS is 200 feet ceiling and RVR of 1,000. Cat-III is 100 feet and RVR 600, CAT-IIIb is 50 and RVR 150. Cat-IIIc is no minimums.

So WAAS-corrected GPS, while considered ok for primary navigation and approaches to 250 min, is still not as good as the the 60 year old ILS for precision approaches. GBAS systems, same as RTK GPS, are out there but not TSO-C129 certified. The FAA, at one point, intended to install GBAS (Ground Based Augmentation System, formerly called LAAS) but there was inherent problems with it with reliability that made it no better than WAAS. So the FAA has put GBAS systems on indefinite hold and no certified receivers exist for it for precision approaches in manned aircraft.

It is widely used in the ag industry for precision application. But it’s not quite good enough, no better than WAAS, when human lives are at stake. Therefore the DH for precision approaches for GPS still remains higher than for the 60 year old ILS, no matter what type of augmentation system is used.

There is two GBAS systems in the US - one at Newark Liberty International and another at Houston Intercontinental. These are Cat-I (known as GAST-C) and provide a lateral accuracy of 16 meters **(See Note 1) and vertical of 4 meters. It is the best the RTK system can do in fast-moving aircraft. Both systems currently in use in the US are Honeywell and received FAA design approval in 2009.

Again, there is inherent limitations with a single ground-point correction station in aviation. You have to maintain RTK-fixed to get the claimed accuracy. And it simply can’t be done at any significant range in the highly dynamic aircraft environment. The faster you go the more problems there is with it. And the conditions in aviation where the accuracy is actually required cause the signal to degrade. Plus it can be easily jammed or corrupted by folks with less than honorable intentions with handheld gear.

**Note 1: the poor lateral accuracy of the system is mostly due to horizontal speed.