Differential GPS DGPS is a technique for reducing

Differential GPS DGPS is a technique for reducing the error in GPS-derived positions by using additional data from a reference GPS receiver at a known position. The most common form of DGPS involves determining the combined effects of navigation message ephermeris and satellite clock errors. Differential position accuracies of 1 -10 m are possible with DGPS. 1

Introduction • Until 2000, civilian users had to contend with Selective Availability (SA). The Do. D intentionally introduced random timing errors in satellite signals to limit the effectiveness of GPS and its potential misuse by adversaries of the US. These timing errors could affect the accuracy of readings by as much as 100 meters. • With SA removed, a single GPS receiver from any manufacturer can achieve accuracies of approximately 10 meters. To achieve the accuracies needed for quality applications such as GIS records— from one to two meters up to a few centimeters—requires differential correction of the data. • The majority of data collected using GPS for GIS is differentially corrected to improve accuracy. 2

Why Differential GPS? • Basic GPS is the most accurate radio-based navigation system ever developed. And for many applications it is plenty accurate. But it is human nature to want more! • As each GPS receivers use timing signals from at least four satellites to establish a position then each of those timing signals is going to have some error or delay depending on what sort of problems have occurred it on its journey down to Earth. • Since each of the timing signals that go into a position calculation has some error, that calculation is going to be a compounding of those errors. 3

• Using a modified form of GPS called Differential GPS (originally initiated by the U. S. Coast Guard to counter the accuracy degradation caused by Selective Availability) can significantly reduce various inaccuracies in the GPS system, pushing its accuracy even better. • Even with SA eliminated, DGPS continues to be a key tool for highly precise navigation on land sea. DGPS can yield measurements accurate to a couple of meters in moving applications and even better in stationary situations. • DGPS can yield measurements good to a couple of meters in moving applications and even better in stationary situations. 4

• That improved accuracy has a profound effect on the importance of GPS as a resource. With it, GPS becomes more than just a system for navigating boats and planes around the world. It becomes a universal measurement system capable of positioning things on a very precise scale. • Differential GPS involves the cooperation of two receivers, one that is stationary and another that's roving around making position measurements. The stationary receiver is the key. It ties all the satellite measurements into a solid local reference. • Since GPS receivers use timing signals from at least four satellites to establish a position. Each of those timing signals is going to have some error or delay depending on what sort of perils have befallen it on its trip down to the object. • Since each of the timing signals that go into a position calculation has some error, that calculation is going to be a compounding of those errors. 5

• The sheer scale of the GPS system comes to our rescue. The satellites are so far out in space that the little distances we travel here on earth are insignificant. For example, if two receivers are fairly close to each other, say within a few hundred kilometeres, the signals that reach both of them will have traveled through virtually the same slice of atmosphere, and so will have virtually the same errors. • So if two receivers are fairly close to each other, let us say within a few hundred kilometers, the signals that reach both of them will have traveled through virtually the same slice of atmosphere, and so will have virtually the same error. • That is the idea behind differential GPS: We have one receiver measure the timing errors and then provide correction information to the other receivers that are roving around. That way virtually all errors can be eliminated from the system. 6

• The idea is simple. Put the reference receiver on a point that is been very accurately surveyed and keep it there. This reference station receives the same GPS signals as the roving receiver but instead of working like a normal GPS receiver it attacks the equations backwards. • Instead of using timing signals to calculate its position, it uses its known position to calculate timing. It figures out what the travel time of the GPS signals should be, and compares it with what they actually are. The difference is an "error correction" factor. • The receiver then transmits this error information to the roving receiver so it can use it to correct its measurements. 7

• This means that you could use have one receiver to measure the timing errors and then provide correction information to the other receivers that are roving around. This allows virtually all errors to be eliminated from the system. • The reference station operates by receiving the same GPS signals as the roving receiver but instead of working like a normal GPS receiver it uses its known position to calculate timing, rather than using timing signals to calculate position. • Essentially determining what the travel time of the GPS signals should be, and compares it with what they actually are. The difference is an "error correction" factor. The receiver then transmits this error information to the roving receiver so it can use it to correct its measurements. 8

• Since the reference receiver has no way of knowing which of the many available satellites a roving receiver might be using to calculate its position, the reference receiver quickly runs through all the visible satellites and computes each of their errors. Then it encodes this information into a standard format and transmits it to the roving receivers. The roving receivers can then apply the corrections for particular satellites they are using. • Error transmissions not only include the timing error for each satellite, they also include the rate of change of that error as well. That way the roving receiver can interpolate its position between updates. 9

• In the early days of GPS, reference stations were established by private companies who had big projects demanding high accuracy groups like surveyors or oil drilling operations. And that is still a very common approach. Users buy a reference receiver and set up a communication link with roving receivers. • The US Coast Guard and other international agencies are establishing reference stations all over the place, especially around busy harbours and waterways. 10

• These stations often transmit on the radio beacons that are already in place for radio direction finding (usually in the 300 k. Hz range). • Anyone in the area may receive these corrections and radically improve the accuracy of their GPS measurements. Most ships already have radios capable of tuning the direction finding beacons, so adding DGPS will be quite easy. • Many new GPS receivers are being designed to accept corrections, and some are even equipped with built-in radio receivers. • Not all DGPS applications are created equal. Some do not need the radio link because they do not need precise positioning immediately. The roving receiver just needs to record all of its measured positions and the exact time it made each measurement. 11

WAAS • WAAS a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy (of up to five times better). • A WAAS-capable receiver can provide a position accuracy of better than three meters 95 percent of the time. And you do not have to purchase additional receiving equipment or pay service fees to utilize WAAS. • The Federal Aviation Administration (FAA) and the DOT are developing the WAAS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA’s navigation requirements for accuracy, integrity, and availability. • WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite. 12

• WAAS consists of approximately 25 ground reference stations positioned across the US that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. • This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal. • Currently, WAAS satellite coverage is only available in North America. 13

• For some users in the US, the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land marine applications. • WAAS provides extended coverage both inland offshore compared to the land-based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment, while DGPS does. • Other governments are developing similar satellite-based differential systems. In Asia, Japan is developing the Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). • GPS users around the world will have access to precise position data using these and other compatible systems. 14

• The WAAS is based on a network of approximately 25 ground reference stations that covers a very large service area. Signals from GPS satellites are received by wide area ground reference stations (WRSs). Each of these precisely surveyed reference stations receive GPS signals and determine if any errors exist. These WRSs are linked to form the US WAAS network. Each WRS in the network relays the data to the wide area master station (WMS) where correction information is computed. • The WMS calculates correction algorithms and assesses the integrity of the system. A correction message is prepared and uplinked to a GEO via a ground uplink system (GUS). The message is then broadcast on the same frequency as GPS (L 1, 1575. 42 MHz) to receivers on board aircraft which are flying within the broadcast coverage area of the WAAS. The communications satellites also act as additional navigation satellites for the aircraft, thus, providing additional navigation signals for position determination. 15

Real Time Differential 16

Local Area DGPS (LADGPS) • LADGPS (Local Area DGPS) typically cover an area up to several tens of kilometers. • LADGPS is a form of DGPS in which the user’s GPS receiver receives real-time pseudorange, and possibly, carrier phase corrections from a reference receiver generally located within the line of sight. • The corrections account for the combined effects of navigation message ephemeris and satellite clock errors, and usually, atmospheric propagation delay errors at the reference station. 17

Wide-Area DGPS (WADGPS) • WADGPS is a form of DGPS in which the user’s receiver receives corrections determined from a network of reference stations distributed over a wide geographical area. • Separate corrections are determined for specific error sources, such as satellite clock, ionospheric propagation delay, and ephemeris. • The corrections are applied in the user’s receiver or attached computer in computing the receiver’s coordinates. • The corrections are typically supplied in real time by way of a geostationary communication satellite or through a network of ground-based transmitters. 18

Wide Area Augmentation System (WAAS) • WAAS enhances the GPS SPS and is available over a wide geographical area. • WAAS being developed by the Federal Aviation Adminstration, together with other agencies, will provide WADGPS corrections, additional ranging signals from geostationary (GEO) satellites and integrity data on the GPS and GEO satellites. • The GEO uplink subsystem includes a closed-loop control algorithm and special signal generator hardware. These ensures that the downlink signal to the users is controlled adequately to be used as a ranging source to supplement the GPS satellites in view. 19

Primary Mission of WAAS • Navigation for all phases of flight in the National Airspace System (NAS) from departure, en route, arrival, etc. • GPS augmented by WAAS offers the capability for both nonprecision approach (NPA) and precision approach (PA) within a specific service volume. • WAAS provides a signal in space (SIS) to WAAS certified aircraft avionics using the WAAS for any FAA-approved phase of flight. • The SIS provides two services: – Data on GPS and GEO satellites. – A ranging capability. 20

WAAS Top Level View GEO subsystem GPS satellites User’s WAAS receiver GEO uplink subsystem Wide-area reference Station-1 Wide-area master station Wide-area reference Station-n 21

WAAS INMARSAT GEO Coverage Starting in Fall 2006 At present there are two geostationary satellites serving the WAAS area (INMARSAT IIIs: POR (Pacific Ocean Region) and AOR-W (Atlantic Ocean Region-West. The European area will eventually be served by two INMARSATS, AOR-E (Atlantic Ocean Region-East) and IOR (Indian Ocean Region) and the European Space Agency satellite, ARTEMIS. The footprints of all but ARTEMIS (Aircraft-Based Augmentation System) is shown below. On the future ARTEMIS satellite, the GPS/GLONASS augmentation is made directly from aircraft based equipment. Japan will be served by the MSAS system. 22

GEO Uplink Systems (GUS) • Corrections from the WMS are sent to the ground uplink subsystem (GUS) for uplink to the GEO. The GUS receives integrity and correction data and WAAS specific messages from the WMS, adds forward error correction (FEC) encoding, and transmits the messages via a C-band uplink to the GEO satellites for broadcast to the WAAS user. THE GUS signal uses the GPS standard positioning service waveform (C/A code, BPSK modulation); however, the data rate is higher (250 bps). • Each symbol is modulated by the C/A code, a 1. 023× 106 chips/s pseudorandom sequence to provide a spread spectrum signal. This signal is then BPSK modulated by the GUS onto an IF carrier, upconverted to a C-band frequency and up-linked to the GEO. It is the C/A code modulation that provides the ranging capability if its phase is properly controlled. 23

What Should you Know Before buying a GPS Receiver? • Before buying GPS equipment, it is important to clearly define your needs in terms of accuracy level required and end results expected. Do you simply want to be able to navigate in the woods, or do you want to map out points, lines and areas that can be differentially corrected and imported into a GIS (a computer mapping system)? Do you need real-time differential GPS for any reason? • Is 15 meter accuracy good enough? If so, you do not have to worry about differential correction. If you want to make a map from your data, is 1 -5 m accuracy sufficient, or do you need sub-meter accuracy for your application? More accurate equipment is more expensive. If you decide you need high accuracy, be sure you can justify the added expense. • Consider your needs for durability and weather resistance, and details such as whether or not an external antenna can be connected to the receiver, and its size, weight and suitability for your method of survey (e. g. , will it be used in a backpack, mounted on a vehicle, or carried in hand? ). 24