Alternatives to Cat III ILS Autoland Recoveries

as might be applied to RoboLander

The FAA (in 1993) was considering three types of differential GPS service for aviation use:

(1) local area DGPS (LADGPS), which would be located at each airport or closely grouped airports to support instrument approaches to current CAT I weather minimums;

(2) wide area DGPS (WADGPS), which would provide GPS integrity broadcast (GIB) and accuracy improvements for all of North America; and

(3) use of kinematic carrier phase positioning for instrument approach and landing.

All three types of DGPS service are still under development; however, WADGPS/GIB is in the FAA budget for procurement and installation. The basic concept for WADGPS/GIB is to have several GPS ground monitoring stations (about 20 for North America) with two master control stations where differential corrections and integrity for each satellite are determined. This information will be sent to two communications satellite earth stations and relayed to the aircraft via a satellite signal that is similar to a GPS signal with unique codes. This signal may also be suitable for ranging providing improved navigation availability.

General principles

Differential GPS (DGPS) is a method of eliminating errors in a GPS receiver to make the output more accurate. This process is based on the principle that most of the errors seen by GPS receivers in a local area will be common errors. These common errors are caused by factors such as clock deviation, selective availability and changing radio propagation conditions in the ionosphere. If a GPS receiver is placed at location for which the coordinates are known and accepted, the difference between the known coordinates and the GPS-calculated coordinates is the error. This receiver is often called a "base station".
The error, which the base station has determined, can be applied to other GPS receivers (called "rovers"). Since the sources of the error are continuously changing, it is necessary to match the error correction data from the base station very closely in time to the rover data. One way of doing this is to record the data at the base station and at the rover. The data sets can be processed together at a later time. This is called post processing and is very common for surveying applications. The other way is to transmit the data from the base station to the rover. The error calculation is made in the rover in real time. This process is called real-time DGPS.

Definitions of accuracy

Circular Error Probable (CEP)

The CEP is the radius of a circle in which the true horizontal coordinates will be located 50% of the time.

Spherical Error Probable (SEP)

The SEP is the sphere in which true position fixes will be located 50% of the time.

Attainable accuracies

300 meters - 100 meters

This is the accuracy range that the Department of Defense guarantees from the Standard Positioning Service (SPS), the only service commonly available to civilian users.

25 meters - 10 meters

Cheap handheld receivers in the $300 to $3000 range with basic DGPS can usually achieve accuracies in this range.

5 meters - 1 meter

Better handheld receivers, mapping grade receivers and aeronautical receivers can get down to this level of accuracy. They will cost from $500 to $5000.

1 meter - 10 cm

Better quality mapping receivers and low-end surveying equipment can get this accurate. Such receivers will generally use carrier phase measurement techniques instead of code-based solutions and will cost more than $3000 per unit. Users requiring better accuracy can get it by taking long observations (between 20 seconds and 2 hours) and by surveying multiple points as a network and using network adjustment routines. Sub-centimeter accuracies are possible using these techniques.

10 cm - sub-centimeter

High end surveying receivers and geodetic receivers are used to reach this level of accuracy. Receivers of this class, costing more than $15,000 per unit, will always (correct me if I'm wrong) use carrier phase measurement techniques and will usually use both of the GPS frequencies. A relatively new technique known as ambiguity resolution on the fly (AROF) allows receivers to start producing high-quality solutions very quickly (within 40 seconds) without complicated initialization procedures.
Global Positioning System (GPS)

GPS is a satellite-based global navigation system created and operated by the United States Department of Defense (DOD). Originally intended solely to enhance military defense capabilities, GPS capabilities have expanded to provide highly accurate position and timing information for many civilian applications.

An in-depth study of GPS is required to fully understand how it works, but simply stated: Twenty four satellites in six orbital paths circle the earth twice each day at an inclination angle of approximately 55 degrees to the equator. This constellation of satellites continuously transmit coded positional and timing information at high frequencies in the 1500 Megahertz range. GPS receivers with antennas located in a position to clearly view the satellites, pick up these signals and use the coded information to calculate a position in an earth coordinate system.

GPS is the navigation system of choice for today and many years to come. While GPS is clearly the most accurate worldwide all-weather navigation system yet developed, it still can exhibit significant errors. GPS receivers determine position by calculating the time it takes for the radio signals transmitted from each satellite to reach earth. Itís that old "Distance = Rate x Time" equation. Radio waves travel at the speed of light (Rate). Time is determined using an ingenious code matching technique within the GPS receiver. With time determined, and the fact that the satelliteís position is reported in each coded navigation message, by using a little trigonometry the receiver can determine its location on earth.

Position accuracy depends on the receiverís ability to accurately calculate the time it takes for each satellite signal to travel to earth. This is where the problem lies. There are primarily five sources of errors which can affect the receiverís calculation. These errors consist of (1) ionosphere and troposphere delays on the radio signal, (2) signal multi-path, (3) receiver clock biases, (4) orbital errors, also known as ephemeris errors of the satellite's exact location, and (5) the intentional degradation of the satellite signal by the DOD. This intentional degradation of the signal is known as "Selective Availability (SA)" and is intended to prevent adversaries from exploiting highly accurate GPS signals and using them against the United States or its allies. However, on May 1, 2000, U.S. President Bill Clinton ordered Selective Availability (SA) turned off at midnight (Coordinated Universal Time). Now, civilian GPS users around the world will no longer experience the up to 100 meter (approximate 300 feet) random errors that SA added to keep GPS a more powerful tool for the military. Today, GPS units are accurate to within 20 meters (approximately 60 feet); although in good conditions, units should display an error of less than 10 meters. The combination of these errors in conjunction with poor satellite geometry can limit GPS accuracy to 100 meters 95% of the time and up to 300 meters 5% of the time. Fortunately, many of these errors can be reduced or eliminated through a technique known as "Differential."

Differential Global Positioning System (DGPS) - how it works

DGPS works by placing a high-performance GPS receiver (reference station) at a known location. Since the receiver knows its exact location, it can determine the errors in the satellite signals. It does this by measuring the ranges to each satellite using the signals received and comparing these measured ranges to the actual ranges calculated from its known position. The difference between the measured and calculated range is the total error. The error data for each tracked satellite is formatted into a correction message and transmitted to GPS users. The correction message format follows the standard established by the Radio Technical Commission for Maritime Services, Special Committee 104 (RTCM-SC104) These differential corrections are then applied to the GPS calculations, thus removing most of the satellite signal error and improving accuracy. The level of accuracy obtained is a function of the GPS receiver. Sophisticated receivers like the Starlink DNAV-212 and INVICTA 210 series can achieve accuracy of the order of 1 meter or less.

Differential GPS Broadcast Site


  • The reference station GPS receiver knows exactly the position of its antenna, therefore it knows what each satellite range measurement should be. It measures the ranges to each satellite using the received signals just as if it was going to calculate position. The measured ranges are subtracted from the known ranges and the result is range error. The range error values for each satellite are formatted into messages in the RTCM SC104 format and transmitted continuously.


  • Depending on the transmission format, the modulator encodes the data as necessary for transmission. For example, in the free USCG system, the modulator creates a carrier signal which varies using MSK modulation. A "one" data bit is represented by a advancing carrier phase and a "zero" bit by a retarding carrier phase. The modulated carrier output from the modulator is connected to the transmitter.


  • The transmitter is basically a power amplifier which is connected to an antenna system. The modulated carrier is amplified and driven to the antenna. In the USCG system, the transmitter is 250-1000 Watts and operates in the 300KHz frequency range. The amplified signal is radiated via the antenna to remote DGPS receivers for real-time position correction.


  • A DGPS correction receiver decodes the signals received from a reference site. Data is formatted into a serial RTCM SC104 data stream and provided to the remote GPS receiver. There are many types of DGPS correction receivers.


  • The GPS receiver measures ranges to each satellite, but before the measurements are used to calculate position, corrections received from the DGPS receiver are applied to the measurements. The position is then calculated using the corrected range measurements providing vastly increased accuracy.


  • GPS with SA has an accuracy of about 100 meters. GPS with corrections (DGPS) has an accuracy of between 1 and 5 meters depending on the quality (price) of the GPS receiver.
         OmniSTAR: Worldwide DGPS Service Regions
North and South America Australasia

OmniSTAR is a wide-area differential GPS service, using satellite broadcast techniques. Data from many widely-spaced Reference Stations is used in a proprietary multi-site solution to achieve sub-meter positioning over most land areas worldwide.