Introduction and Overview of GPS

Developed by the U.S. Department of Defense (DoD), the Navstar Global Positioning System was initially intended to protect the security interest of our nation. Broad ranging civilian applications were not initially envisioned, and not until recently has the potential for production agriculture become evident. Today, with the development of yield monitoring capabilities on agricultural grain combines, GPS is being utilized to provide a geographic reference for yield data. Of increasing importance is the application of GPS to real-time navigation for grid soil sampling and variable-rate application of fertilizers and chemicals. Selection, integration and use of GPS in precision agriculture requires a fundamental understanding of the technology.

GPS consists of three major systems; space-based satellites, ground-based control and the user segment. All three components form what is known as a radio-navigation system where the travel-times of radio waves through space and the earth’s atmosphere are used to solve for a position fix of either satellite or user segments. The space segment consists of 24 satellites which orbit the earth. A ground-based control segment has been established to track the orbits of the satellites and broadcast orbit and clock correction information. GPS receivers decode and process radio-navigation signals from the satellites.

Each GPS satellite completes two orbits of the earth in a 24-day. These satellites are positioned in one of six orbital planes, each inclined at an angle of 55^o relative to the earth’s equator. The average altitude of each satellite orbit is approximately 10,900 nautical miles above the earth’s surface. Four satellites are spaced within each orbital plane so that the signals from four or more satellites are always available anywhere on the earth’s surface. Availability is a function of geometry as a line-of-sight is required between the space and user segments, for which the implications will be discussed later.

A master control station in Colorado Springs, Colorado, monitoring stations, and ground antennas around the world compose the control segment. The monitoring stations are used to track satellites and collect what is termed ranging data. This data is transmitted to the master control station facility. Responsibility of the master control stations include satellite maneuvers, configuration of satellite equipment, updating of navigation messages, and other health monitoring and maintenance activities. Any of the various adjustment of satellite data or configuration are accomplished using the ground antennas to upload ephemeris and clock parameters. Ephemeris data refers to the parameters which describe an individual satellite’s orbit.

Accurate location determination by GPS is accomplished by the ranging of at least four satellites, simultaneously. In determining the receiver’s position, three unknowns arise; latitude ("x-position"), longitude ("y-position"), and elevation ("z-position"). For a GPS receiver to determine these three unknowns, it must know the location of three other points (i.e., three space satellites). The need for the fourth satellite is due to small errors between the GPS receiver clock and the satellites’ clocks. Often, more than four satellites are in a receiver’s view at a given time. In this case, position accuracy may be enhanced by choosing one satellite over another due to satellite geometry. The number of satellites a receiver can track simultaneously is a function of the number of channels the receiver possesses. As one might expect, receiver expense is a function of receiver channels, among other things.

Each GPS satellite transmits two radio signals on the L-band (the L-band is the radio spectrum between 1,000 and 2,000 MHz), termed L1 and L2. L1 and L2 broadcast at frequencies of 1575.42 MHz and 1227.60 MHz, respectively. Superimposed on the L1 signal are two pseudo random noise (PRN) codes; Coarse/Acquisition (C/A) and Precision (P). The L2 signal is superimposed with the P-code only. The C/A code is much shorter in duration than the P code and allows a receiver to locate and recognize a satellite quickly. PRN codes contain navigation information.

Two user services are available through GPS; Precise Positioning System (PPS) and Standard Positioning System (SPS). PPS is comprised of both L1 and L2 signals (and PRN codes) and is mainly reserved for the military and authorized users. Two-dimensional positional accuracy of the PPS is approximately 120 ft at a 95% confidence interval. Access to PPS is determined by the U.S. DoD and is controlled by Selective Availability (SA) and Anti-Spoofing (AS) encryption techniques. These intentional errors will be discussed in more detail later. Since nearly all of the civilian population, including agriculture, is denied access to the PPS, discussion of its specifications will be limited.

The SPS is available to all civilian users, is free of charge, and is comprised of accessing the L1 signal and the C/A code. Two-dimensional (2-D) positional accuracy of the SPS is approximately 510 ft at a 95% confidence interval. 2-D is equivalent in meaning to latitude and longitude and does not account for elevation. GPS receivers used in precision agriculture make use of the SPS.

The developers of the GPS were faced with a serious problem. If an accurate positioning system (such as GPS) was developed, what would stop adversarial forces from utilizing the technology against the U.S? Two possibilities of hostile use were considered threatening; military device guidance, and the transmission of false GPS signals. The development of Selective Availability (SA) and Anti-Spoofing (AS) cryptographic techniques addressed these concerns. SA is the falsification of satellite clock and ephemeris data, intentionally limiting positional accuracy. The process introduces pseudorandom noise errors into the satellite signals. The AS technique eliminates potential fake satellite signals, encrypting the P-code into the Y-code. Users should be aware that the C/A code is not protected by AS.

GPS Error

Considering the accuracy inherent to the SPS (510 ft, 2-D) the reader may have questions about the usefulness of GPS. The location of a point in a field confined in an area 510 ft x 510 ft (roughly six acres) is of little use to grain producers. Older methods (e.g., dead reckoning) often provide better location accuracy than a six acre resolution. The question becomes; how can the performance of GPS be increased to provide the grain producer an accurate location? To answer this question we must first determine what is meant by accurate location.

Error can be defined as any deviation in position from the true position. Some errors are natural phenomena while others are intentional These errors can combine and become significant. Errors include: satellite/receiver clocks, satellite orbits, prediction of atmospheric delays, multipath, SA, and GPS receivers’ internal circuitry.

Each satellite contains four highly accurate atomic clocks. However, due to the large distance (10,900 miles) the satellites are from the earth and the ranging method used to determine position, very small deviations in the satellite clocks produce noticeable ranging error. For instance, a clock error of 1 microsecond (1 millionth of a second) translates to a distance error of nearly 1,000 ft. An established technique termed differential correction eliminates clock errors from GPS receivers.

The 24 Navstar GPS satellites orbit the earth and communicate their position to the Control Segment regularly. Due to their altitude, the satellites are unaffected by the earth’s atmosphere. However, gravitational forces of the moon and sun, as well as solar radiation pressure, alter their orbits slightly. Therefore, small corrections are made to the satellites’ orbits by the Master Control Station to account for these cumulative errors. Satellite orbit errors range from 3-8 ft with the GPS.

The earth’s atmosphere is comprised of the ionosphere and troposphere. The ionosphere is located 50-250 miles above the surface of the earth and contains electrically charged particles. Lying in an area below the ionosphere, the troposphere is made of mostly water vapor. Satellite signals traveling through these mediums are slowed, ever so slightly, creating delays or errors in ranging. The effect of signals passing through the troposphere has been accurately modeled allowing the GPS receivers to account for troposphere delay. The remaining signal delay can be attributed to ionosphere effect. GPS atmosphere delays are in the range of 17.6-36.3 ft.

Multipath errors are reflected signals that do not travel directly from satellite to receiver. In the case of a receiver located adjacent to a building, a signal may be directly in a line-of-sight with a satellite (a desirable situation). However, the signal may also reflect off the building walls creating a second or multiple signal path from the satellite to the receiver. This has the effect of confusing the receiver and ultimately creating additional error in position. Multipath errors are in the range of 1.6-2.0 ft.

The concept of SA has been introduced earlier. SA is established by the Control Segment to maintain accuracy of 100 m (330 ft) with a 95% confidence interval. The effect of SA can be completely accounted for and removed by the differential correction technique which will be discussed in the next section.

Not all GPS receivers are created equal. The performance of a receiver is directly related to its cost, which in turn directly affects accuracy. Clock errors, electrical noise, and mathematical precision all effect the GPS receiver and its position accuracy. The desired accuracy a grain producer requires should be the deciding factor when choosing a GPS receiver. Conversely, it is generally not desirable to allow the expense of a receiver to dictate the accuracy of a precision farming system.

The accumulation of errors limits the two-dimensional accuracy of GPS to approximately 150 m (500 ft). This resolution is not high enough for yield monitoring, soil sampling, and variable rate application in precision agriculture. Accuracy of the SPS can be improved to a meaningful resolution by the differential correction technique and at a relatively affordable cost

Differential Global Positioning System (DGPS) Overview

A method of differentially correcting GPS receivers, known as Differential GPS (DGPS), greatly improves their accuracy. DGPS is based on the principal that receivers located in close proximity will experience similar errors in satellite ranging signals. The method makes use of two GPS receivers; one stationary and located at a known point (reference receiver), the other is operated as a mobile receiver. Since the coordinates of the reference receiver are known, it can correct satellite pseudorange to the true range. True range less the pseudorange is equal to the differential correction (DC). Through communication between the stationary and mobile receivers, the mobile receiver’s pseudoranges are corrected to true ranges. As can be seen from Table 1, DGPS exhibits accuracy in line with the needs of grain producers.

Table 1. Estimated errors experienced by users of typical
GPS and DGPS receivers

Positional data obtained by GPS receivers may be stored and differentially corrected at a later time. This method of correction is known as "post-processing". Such applications as soil grid sampling and yield monitoring are suitable for post-processing DGPS. However, for "on-the-go" (or real-time) applications, such as Variable Rate Application (VRA) of fertilizer, accurate positional data must be known in the field at the time of application. Real-time DGPS receivers have been developed specifically for these precision agriculture needs. Real-time DGPS receivers operate as their name implies. The reference and mobile receivers track the same satellites simultaneously. Correction data is continuously transmitted from reference to mobile receiver via a radio signal link. For the grain producer, the available technologies of a DGPS are important components of a precision farming system. Typically this is true of a yield monitoring system.

Methods of differential correction fall into to two broad categories; Local Area DGPS (LADGPS) and Wide Area DGPS (WADGPS) differential correction. LA differential correction can be accomplished with three methods;

1. FM (Frequency Modulated) radio stations,
2. United States Coast Guard beacon signals, and
3. user purchased base station.

Some companies involved in differential correction have teamed up with local FM radio stations to modulate GPS correctional data onto the radio signal. To use this service, a farmer must possess a FM receiver (obtained from the differential correction service provider), and a compatible GPS receiver. The DC provider will charge the user a subscription fee for the service. Typically, the provider will offer multiple subscription plans with varying degrees of accuracy. Subscription rate increases with accuracy provided.

The strength of FM radio signals limits the range of a FM DC service. FM radio signals are within the frequency range of 88-108 MHz. Radio signals at this frequency range are known as "space waves" and travel in direct line of sight paths between transmitter and receiver, much the same way GPS signals do. Curvature of the earth, geographic obstacles such as mountains, man made obstacles such as buildings, can block the signal from reaching the receiver. In an effort to compensate for this, transmitters will broadcast the FM signal at elevated power levels using tall towers. The range of all FM signals cannot be generalized but a rule-of-thumb range is 60 miles from the transmitter.

The United States Coast Guard (USCG) has established a network of radio beacons used by the shipping industry for navigation. The beacons are located on the coasts of the U.S., the Great Lakes area, and other major waterways. The service is free of charge to all users and available to anyone capable of receiving the correction signal. USCG beacon signals are broadcast in a frequency range of 125-375 kHz. Radio waves at this frequency are known as "ground waves". These radio waves follow a "bouncing" path between the troposphere and the surface of the earth which allow them to follow the curvature of the earth, be received near mountains and other obstacles, and ultimately extend their range to over 300 miles in favorable conditions. Range is considerably longer than the FM DC method. Maps are available illustrating the coverage of this service.

A third and less attractive option of DC is to purchase a second GPS receiver to operate as a reference (or base) station. The base station is operated as described in the fundamental DGPS concept. The reference station is placed at a known location (usually a benchmark) while another GPS receiver is placed on the mobile unit (i.e., combine). Radio communication between the two receivers corrects the location in real time. Although this is an effective means of DC, it requires a large investment (the second receiver) and extra setup time. The availability of other (and more practical) LA methods limits the use of a DGPS reference station in agriculture.

Remembering that differential correction relies on the principal that receivers which receive the same satellite signals will experience similar errors, the accuracy of LA methods decrease with distance. That is to say that a GPS receiver located 20 miles from a USCG beacon (or FM transmitting tower) will likely have better accuracy than the same receiver located 200 miles from the beacon, even if both signals are received clearly by the GPS receiver. This phenomena is mostly due to the deviations in the troposphere with location. Relative humidity has a large effect on signal delay through the troposphere and is known to vary from region to region. The algorithms used to predict this delay are likely to be different from one location to another, thus affecting the accuracy of DGPS. A precision farming system’s location relative to LA systems will certainly have a large impact on the options of local area differential correction.

WADGPS corrects GPS signals in exactly the same manner as LADGPS. The most important difference in the two methods is coverage. As the name suggests, WADGPS covers a much larger area than LADGPS. In fact, the entire continental U.S. is covered by WADGPS and many other world locations. The use of stationary satellites (geo-referenced) accomplishes the massive coverage. These satellites are very similar to television satellites and are not part of the GPS.

WADGPS makes use of advanced techniques to optimize troposphere error algorithms by choosing the same satellites for correction which a receiver "sees" in the sky. The network of geo-referenced satellites tracks all the satellites in view of the coverage area broadcasts the optimized DC signals to specific regions. WADGPS is more expensive than LADGPS do to the higher overhead costs of the system. However, a grain producer may find the reliability and full coverage of WADGPS justifies the added expense.

WADGPS Service Providers

The above material is intended to provide a basic understanding of a precision farming system and associated terminology. The following section details companies and systems most popular in the Differential Global Positioning System. The reader is encouraged to investigate additional literature on precision farming, especially that which relates to a specific need.

RACAL is a WADGPS service provider with international coverage. The satellites used by the company operate in the L-band at one meter accuracy. RACAL offers a flexible system where third party receivers may be used with the DGPS service.

The company Omnistar provides WADGPS and has eleven reference sites located throughout the U.S. One network control center in Texas sends uplink data to three C-band (3750-4250 MHz) satellites, providing coverage over the entire U.S. and some sections of Canada and Mexico.

Satloc is a company offering a WADGPS service. The correction signal operates on the L-band (1551-1556 MHz) from a geo-stationary satellite located approximately at 101 degrees West longitude. Signals are broadcast in three overlapping beams which provide 100% coverage to the U.S. and partial coverage to Canada and Mexico. Fourteen reference stations throughout the U.S. generate correction signals. Network control centers in Virginia and Arizona send DC data to the uplink satellite.