J. R. VETTER AND W. A. SELLERS TEST AND EVALUATION Differential Global Positioning System Navigation Using High-Frequency Ground Wave Transmissions Jerome R. Vetter and William A. Sellers Since 1992, the Strategic Systems Department of the Applied Physics Laboratory has been investigating the use of high-frequency (HF) ground wave transmissions in the upper HF band for sea-based communications. This effort has examined the optimization of shipboard antennas for two-way communications with ground stations. This article describes early tests using differential Global Positioning System navi- gation for investigating the accuracy of determining a ship’s position from a base station using HF ground wave transmissions. (Keywords: Differential GPS, HF signal transmissions, Kalman filtering, Navigation.) INTRODUCTION In late 1990, the U.S. Coast Guard undertook a reference and remote stations into the system was comprehensive data link study to determine which RF costly. Although the system could provide broadcasts broadcast systems could be used to support differential beyond LOS, it was affected by poor performance due Global Positioning System (DGPS) correction trans- to propagation variations in spite of a predicted 700- missions over the continental United States (CO- km sky wave mode range. NUS) and to determine which radio systems were The purpose of APL’s early high-frequency ground appropriate for a given operating area. The broadcast wave (HFGW) experiments at sea was to improve constraints for the system design had to meet accura- two-way voice and data communications between cies of ±3 m at data rates of 100 bits/s with data submerged submarines and surface ships; previous latencies of less than 4 s. The frequencies finally se- experience with HFGW on land can be found in Ref. lected were in the LF (285–325 kHz) and MF (405– 1 and will be discussed in an upcoming article in the 415 kHz) bands; existing Marine radio beacons were Technical Digest by Vetter et al. The experiments pro- used since they could broadcast beyond line of sight vided an opportunity to investigate differential GPS (LOS) and propagated via ground wave signals. The broadcasts transmitted over an upper HF band to entire radio spectrum from LORAN-C (long-range surface ships. The higher accuracy onboard the ship navigation) to HF, including FM subcarrier and cellu- was necessary to assess antenna patterns from a sub- lar radio, was addressed for reliably providing differen- marine-deployed cable when more precise real-time tial corrections to the transmitting stations using plain navigational ship resources were unavailable. Previous old telephone system (POTS) modems and sending theoretical and empirical measurements in this area the broadcast messages to coastal users within 100 km supported the use of ground wave for communications of CONUS. The HF system proposed for the data link in the HF and VHF regimes.2,3 The advantages of was a DGPS sky wave mode of propagation that using HFGW rather than the other options were that was not compatible with the GPS receivers available it provided a non-LOS mode of propagation with over- in early 1993. More importantly, incorporating the the-horizon capabilities, allowed the use of reasonably 340 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 3 (1998) DIFFERENTIAL GPS NAVIGATION USING HFGW TRANSMISSIONS sized resonant antennas, and was based on extensive military system and because of the effects of SA im- APL test experience in Europe with ranges over land posed on the precision code L1 broadcast signals, the to 100 km. best accuracy obtainable is currently limited to about Using HFGW to send differential corrections reli- ±100 m using the GPS standard positioning service ably over sea was an attempt to show that its capabil- (SPS) receivers. Differential techniques achieve accu- ities could meet the accuracy levels desired with cur- racies comparable with those obtained with the best rently achievable HFGW baud rates and system receivers nearly anywhere on Earth within a reason- latency effects. A series of DGPS tests of opportunity able distance of a reference site. were undertaken from March 1993 through May 1994 Two techniques are commonly used for deriving a to evaluate the capability and show the use of HFGW DGPS solution from coarse-acquisition code (C/A- for real-time differential navigation in a low dynamic code) GPS data. The first and simplest uses latitudinal environment. DGPS methods have been shown to and longitudinal corrections. For this method, a re- significantly reduce the effects of selective availability ceiver at a known location (reference) calculates a (SA), ionospheric errors, and ephemeris errors on the position from a given number of satellites. The remote positional solution. Post-test analysis of the GPS data receiver is configured to use the same satellites and collected in the early test phase showed that a ship- compute a position. The differences in latitude and board C/A-code receiver could achieve accuracy levels longitude between the reference location and the of 2–5 m in position. position computed by the receiver at the reference This article presents the results of using DGPS location are applied to the position computed by the navigation techniques to improve the accuracy of a remote receiver. The greater the distance between the C/A-code GPS receiver for tracking ships using radio reference location and the remote receiver, the less signals transmitted via a ground wave technique in the similar the satellite geometries are relative to the two HF radio band. An inexpensive, commercial off-the- receivers, and the less accurate the differential correc- shelf (COTS) C/A-code GPS receiver (Trimble tions will be. SVeeSix) was placed onboard the USNS Range Sen- The second and more accurate DGPS method is to tinel (T-AGM 22), which acted as the surface ship for determine corrections to the range and range rate a series of tests, and at the APL office at Cape Canav- measured for each satellite. The remote user is inde- eral, Florida. A precision code (P-code) GPS receiver pendent in the selection of satellites to compute po- was also used onboard the ship to collect positional sition, provided that corrections for all satellites in data and was used as a scoring reference. The GPS view can be generated from reference receiver data. A receiver at the APL Cape Canaveral office was used reasonable limit in the separation between the refer- to calculate pseudo-range and delta-range differential ence and remote receivers with this method is on the corrections. The differential corrections were applied order of 1000 km. Also, additional processing of the to the pseudo-range and delta-range measurements remote receiver data is required to compensate for the collected onboard the surface ship, and a Kalman filter error from user clock bias. was used to process the ranging measurements and For this evaluation, the range-correction method calculate a state vector solution for post-flight evalu- was used to compute the DGPS solutions. Corrections ation. A COTS software package (LabWindows) was to the range rate measured for each satellite were also used to generate real-time differential corrections. computed for the reference receiver data and then applied to the remote receiver data. Common-mode clock biases for range and range rate were removed, DGPS THEORY AND PROCESSING and the measurements were input to an 8-state Kalman SCHEME filter to output state vector position and velocity es- Differential navigation using satellites has been timates. Figure 1 illustrates the DGPS concept and practiced since the mid-1960s using translocation and shows flowcharts of the data processing for both the Transit satellites.4 With this method, a precisely locat- real-time and non-real-time modes. The real-time ed fixed site receives data from a satellite at a known differential corrections used the COTS LabWindows location in space from which calculations or correc- package integrated into the HFGW system. tions are made and sent to a remote site. This early work was done using a reference station on the APL GPS REFERENCE RECEIVER campus and a remote ground site 75 km away. Earlier than this, differential navigation was also used with COMPARISONS the Omega and LORAN ground-based navigation sys- To evaluate receivers for DGPS at-sea testing, three tems. With the advent of GPS, the use of precision types of receivers (Table 1) were used to collect data navigation anywhere on the Earth’s surface became a at the APL Cape Canaveral facility during a period reality. However, because GPS is still considered a when GPS was being used for experiments by the Air JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 3 (1998) 341 J. R. VETTER AND W. A. SELLERS Pseudo-range and delta pseudo-range data HF discone antenna SVeeSix receiver HFGW HF discone SVeeSix antenna receiver Real-time differential USNS Range Sentinel corrections HFGW APL reference site Time Measurement Measurement update preprocessing update HF HF transceiver transceiver Raw x measurements Terminal Terminal y Determine node node z Differential + Kalman controller controller xÇ corrections gains x = Ç Latitude/longitude Latitude/longitude y corrections corrections zÇ Satellite Compensated ephemeris measurements c GPS LabWindows LabWindows GPS cÇ receiver calculates applies receiver latitude/ latitude/longitude latitude/longitude latitude/ longitude corrections corrections longitude x0 Time Residuals Measurement – x update prediction Predicted state Display Display vector measurements State vector corrected ship’s track corrected corrections and differential ship’s track corrections + Reference Ship station system configuration configuration Figure 1. DGPS navigation concept (top) and flowcharts of the data processing for the non-real-time mode (bottom left) and real-time mode (bottom right). The state vector is composed of inertial Earth-centered frame of position and velocity components and user clock bias (c) and clock rate (cÇ).