GPS Question
I received a few questions recently, and the most interesting ones regarded the filing of flight plans with non-certified GPS units. A reader said he has been flying with a non-IFR certified GPS unit, and filing flight plans indicating GPS equipment with remarks stating "non-IFR certified GPS". ATC in his area had been issuing clearances to fixes that were not contained in the GPS database, and the reader was confused about why ATC would be relying on his GPS equipment, as evidenced by the clearances issued, and whether he should be accepting them. Here's the best answer I can give.
If you're talking legalities, then no, a non-certified GPS is not a legal method for navigation for IFR. To get certification of an IFR GPS, certain minimum requirements must be met, though to be honest, I don't know what they are. They are contained in documents, though, and the certification level is coded as TSO-C129a or higher. There are different levels of certification, including those that make use of WAAS. If it doesn't meet that, the GPS receiver may be not be used as a method of navigation. If a unit meets the standard, then it is legal and may be used. The reason for certification is to ensure the unit meets standards of redundancy and standards of reporting to a pilot if it isn't operating up to standard for accuracy. For example, if your GPS stops receiving good satellite geometry and the navigation solution has a high HDOP (Horizontal Dilution Of Position), does your receiver let you know? If it doesn't meet TSO129C, then it may not report such a situation at all, or the reporting method may not be adequate to let you know you have a problem. If you can't be certain about your position then what happens if ATC clears you on a routing and separates other traffic from you, only to find out that you're not where you thought you were?
I have seen a couple of situations in which my basic GPS receivers showed in error, and one in particular that was dramatic, and inexplicable. It showed me 35 km away from where I knew I was, out of 21,000 feet climbing at 65 km/h when I was stopped on the surface nearly at sea level. There was excellent geometry with the satellites, but no indication on the unit that something was going wrong. 35km is almost 20NM, and if ATC is using your G as coding for a capable GPS and something like this were to happen, you could end up anywhere between where the unit says you are and 10NM on the other side of the traffic ATC is separating you from. I think you can see how this could be bad.
Another issue with GPS is the database. For example, some people believe it's fine to enter waypoints into a GPS by hand, set up a route including the fixes, in order, associated with a GPS approach, and use the GPS to guide them in. Apart from the issues I just mentioned above, there are other points that need to be made. The approaches, to be legal, may not contain waypoints entered by hand. Instead, the approach must be pulled from a database and run through automatically in sequence, rather fixes that are "hand picked" by the pilot. The reason for this is the possibility of a serious error when a digit is missed by one, or two digits reversed. Image descending on an approach profile you loaded by hand only to pass a step-down fix and suddenly take a 90° left turn. Just before you head off toward the hillside that's beside your approach path, you realize that you inadvertently entered "45 53.760" instead of "45 57.360" translating to an error on the order of 5 NM. By the time you disconnect the autopilot and start a turn back, you may have already progressed to a dangerous position, and you're at a low altitude already having completed most of the approach. Similarly, even if the named fixes are in a database, the approach must be pulled from the database to ensure errors don't occur by choosing the wrong name. A few years ago, a pilot asked for "direct BIMKU" at Moncton (it no longer exists), and took a turn of about 90° to the left. When queried, the pilot realised in accidentally entered "BIMTU", which was at Bathurst, about 90NM north of Moncton. This in itself could have been serious if I had traffic on a parallel vector north of him, but the consequences on an approach could leave very little room for error indeed.
Now to the flight plan. Not all remarks you file in your flight plan will make it to the controller's eyes. If you file as having GPS on board, their flight information may reflect GPS equipped. ATC only sees one equipment suffix, so if you file G, it will show up over S and many others. Also, the remark about it being non-IFR certified may get lost somewhere along the way due to equipment limitations or processing of the flight plan. Hence, if they see a G in the equipment suffixes on your aircraft, you'll quite likely receive clearances direct to VORs that are not in range and to fixes. Also, ATC may separate you from other traffic relying on your GPS and therefore reducing separation between you and other aircraft.
In short, I'd recommend sticking with your airplane's capabilities, and filing flight plans accordingly. This is the only way to ensure that you get clearances that keep you on routes within the aircraft's capabilities, and that you are capable of continuously and reasonably navigating what you are assigned. Separation, and therefore safety, could be compromised if you're not meeting standards. I know the standards thing can seem constraining at times like this, but those standards are not without their reasons and there is a lot of background data that we just aren't privy to in the general populace. Transport Canada has been slow to react to GPS, but they have their reasons for going cautiously. I understand completely the issue made about the long way around on some airways. If the issue is really that important, getting an IFR certified GPS installed in the aircraft might be worth it. I'm sure, with the price of gas, there can be a business case made for the long term fuel savings vs. the cost of purchasing and installing a regulation unit. Not to mention the peace of mind that comes with knowing you're doing it right.
*an important add on for information has been added as a comment to this post. References to the other standards for GPS installation and TC approval have been added. The section of the AIP that deals with GPS is COM 3.16.
If you're talking legalities, then no, a non-certified GPS is not a legal method for navigation for IFR. To get certification of an IFR GPS, certain minimum requirements must be met, though to be honest, I don't know what they are. They are contained in documents, though, and the certification level is coded as TSO-C129a or higher. There are different levels of certification, including those that make use of WAAS. If it doesn't meet that, the GPS receiver may be not be used as a method of navigation. If a unit meets the standard, then it is legal and may be used. The reason for certification is to ensure the unit meets standards of redundancy and standards of reporting to a pilot if it isn't operating up to standard for accuracy. For example, if your GPS stops receiving good satellite geometry and the navigation solution has a high HDOP (Horizontal Dilution Of Position), does your receiver let you know? If it doesn't meet TSO129C, then it may not report such a situation at all, or the reporting method may not be adequate to let you know you have a problem. If you can't be certain about your position then what happens if ATC clears you on a routing and separates other traffic from you, only to find out that you're not where you thought you were?
I have seen a couple of situations in which my basic GPS receivers showed in error, and one in particular that was dramatic, and inexplicable. It showed me 35 km away from where I knew I was, out of 21,000 feet climbing at 65 km/h when I was stopped on the surface nearly at sea level. There was excellent geometry with the satellites, but no indication on the unit that something was going wrong. 35km is almost 20NM, and if ATC is using your G as coding for a capable GPS and something like this were to happen, you could end up anywhere between where the unit says you are and 10NM on the other side of the traffic ATC is separating you from. I think you can see how this could be bad.
Another issue with GPS is the database. For example, some people believe it's fine to enter waypoints into a GPS by hand, set up a route including the fixes, in order, associated with a GPS approach, and use the GPS to guide them in. Apart from the issues I just mentioned above, there are other points that need to be made. The approaches, to be legal, may not contain waypoints entered by hand. Instead, the approach must be pulled from a database and run through automatically in sequence, rather fixes that are "hand picked" by the pilot. The reason for this is the possibility of a serious error when a digit is missed by one, or two digits reversed. Image descending on an approach profile you loaded by hand only to pass a step-down fix and suddenly take a 90° left turn. Just before you head off toward the hillside that's beside your approach path, you realize that you inadvertently entered "45 53.760" instead of "45 57.360" translating to an error on the order of 5 NM. By the time you disconnect the autopilot and start a turn back, you may have already progressed to a dangerous position, and you're at a low altitude already having completed most of the approach. Similarly, even if the named fixes are in a database, the approach must be pulled from the database to ensure errors don't occur by choosing the wrong name. A few years ago, a pilot asked for "direct BIMKU" at Moncton (it no longer exists), and took a turn of about 90° to the left. When queried, the pilot realised in accidentally entered "BIMTU", which was at Bathurst, about 90NM north of Moncton. This in itself could have been serious if I had traffic on a parallel vector north of him, but the consequences on an approach could leave very little room for error indeed.
Now to the flight plan. Not all remarks you file in your flight plan will make it to the controller's eyes. If you file as having GPS on board, their flight information may reflect GPS equipped. ATC only sees one equipment suffix, so if you file G, it will show up over S and many others. Also, the remark about it being non-IFR certified may get lost somewhere along the way due to equipment limitations or processing of the flight plan. Hence, if they see a G in the equipment suffixes on your aircraft, you'll quite likely receive clearances direct to VORs that are not in range and to fixes. Also, ATC may separate you from other traffic relying on your GPS and therefore reducing separation between you and other aircraft.
In short, I'd recommend sticking with your airplane's capabilities, and filing flight plans accordingly. This is the only way to ensure that you get clearances that keep you on routes within the aircraft's capabilities, and that you are capable of continuously and reasonably navigating what you are assigned. Separation, and therefore safety, could be compromised if you're not meeting standards. I know the standards thing can seem constraining at times like this, but those standards are not without their reasons and there is a lot of background data that we just aren't privy to in the general populace. Transport Canada has been slow to react to GPS, but they have their reasons for going cautiously. I understand completely the issue made about the long way around on some airways. If the issue is really that important, getting an IFR certified GPS installed in the aircraft might be worth it. I'm sure, with the price of gas, there can be a business case made for the long term fuel savings vs. the cost of purchasing and installing a regulation unit. Not to mention the peace of mind that comes with knowing you're doing it right.
*an important add on for information has been added as a comment to this post. References to the other standards for GPS installation and TC approval have been added. The section of the AIP that deals with GPS is COM 3.16.
posted by Michael at
05:43
