diff --git a/src/main/lemondronor/advisorycircular.cljs b/src/main/lemondronor/advisorycircular.cljs
index 2c67e3d..78308fd 100644
--- a/src/main/lemondronor/advisorycircular.cljs
+++ b/src/main/lemondronor/advisorycircular.cljs
@@ -110,14 +110,22 @@
 
 
 (defn adsbx-url [{:keys [url lat lon radius-nm rapid-api?]}]
-  (let [url-pieces [url
-                    "lat" lat
-                    "lon" lon
-                    "dist" (.toFixed radius-nm (if rapid-api? 0 1))]
-        url-str (->> url-pieces
-                     (map str)
-                     (string/join "/"))]
-    url-str))
+  (if (or lat lon radius-nm)
+    (do
+      (when-not (and lat lon radius-nm)
+        (throw (js/Error. "Need all of :lat, :lon, and :radius-nm")))
+      (let [url-pieces [url
+                        "lat" lat
+                        "lon" lon
+                        "dist" (.toFixed radius-nm (if rapid-api? 0 1))]
+            url-str (->> url-pieces
+                         (map str)
+                         (string/join "/"))]
+        url-str))
+    (do
+      (when rapid-api?
+        (throw (js/Error. "Must specify :lat, :lon, and :radius-nm with RapidAPI")))
+      (str url "/all"))))
 
 
 (defn adsbx-headers [{:keys [url api-key api-whitelist rapid-api?]}]
@@ -153,6 +161,20 @@
     updated))
 
 
+;; Filters out aircraft that aren't in the keep list--unless the keep
+;; list is empty, in which case we don't do any filtering.
+(defn keep-specified-icaos [live-data keep-list]
+  (if (seq keep-list)
+    (let [keep-list (set (map string/upper-case keep-list))]
+      (update
+       live-data
+       :aircraft
+       (fn [aircraft]
+         (filter #(keep-list (string/upper-case (:icao %)))
+                 aircraft))))
+    live-data))
+
+
 ;; Given a vector of position history, removes old entries.
 
 (defn prune-history [history now config]
@@ -872,6 +894,7 @@
                                   :radius-nm (* (:radius-km config) 0.539957)
                                   :rapid-api? (:rapid-api? config)}))
                 filtered-api-data (remove-blocked-icaos api-data (get config :icao-blocklist '()))
+                filtered-api-data (keep-specified-icaos filtered-api-data (get config :icaos '()))
                 now (current-time)
                 [new-db potential-circles] (-> db
                                                (update-history-db (:aircraft filtered-api-data) now config)
diff --git a/src/test/lemondronor/advisorycircular_test.cljs b/src/test/lemondronor/advisorycircular_test.cljs
index 3ee52b9..f1100a0 100644
--- a/src/test/lemondronor/advisorycircular_test.cljs
+++ b/src/test/lemondronor/advisorycircular_test.cljs
@@ -473,7 +473,21 @@
            :lon 1
            :radius-nm 2
            :rapid-api? true})
-         "http://adsbx/lat/0/lon/1/dist/2")))
+         "http://adsbx/lat/0/lon/1/dist/2"))
+  (is (thrown-with-msg?
+       js/Error #"Need all of :lat, :lon, and :radius-nm"
+       (advisorycircular/adsbx-url
+        {:url "http://adsbx"
+         :lat 0
+         :rapid-api? true})))
+  (is (= (advisorycircular/adsbx-url
+          {:url "http://adsbx"})
+         "http://adsbx/all"))
+  (is (thrown-with-msg?
+       js/Error #"Must specify.*lat.*RapidAPI"
+       (advisorycircular/adsbx-url
+        {:url "http://adsbx"
+         :rapid-api? true}))))
 
 
 (deftest adsbx-headers
@@ -494,3 +508,21 @@
          {:x-rapidapi-key "key"
           :x-rapidapi-host "adsbx"
           :useQueryString true})))
+
+
+(deftest remove-blocked-icaos
+  (let [aircraft {:aircraft [{:icao "abc"} {:icao "ADEF"} {:icao "123"} {:icao "124"}]}]
+    (is (= (advisorycircular/remove-blocked-icaos aircraft '())
+           {:aircraft [{:icao "abc"} {:icao "ADEF"} {:icao "123"} {:icao "124"}]}))
+    (is (= (advisorycircular/remove-blocked-icaos aircraft '("a.*"))
+           {:aircraft [{:icao "123"} {:icao "124"}]}))
+    (is (= (advisorycircular/remove-blocked-icaos aircraft '("a.*" "123"))
+           {:aircraft [{:icao "124"}]}))))
+
+
+(deftest keep-specified-icaos
+  (let [aircraft {:aircraft [{:icao "abc"} {:icao "ADEF"} {:icao "123"} {:icao "124"}]}]
+    (is (= (advisorycircular/keep-specified-icaos aircraft '())
+           {:aircraft [{:icao "abc"} {:icao "ADEF"} {:icao "123"} {:icao "124"}]}))
+    (is (= (advisorycircular/keep-specified-icaos aircraft '("123" "adef"))
+           {:aircraft [{:icao "ADEF"} {:icao "123"}]}))))